Fnergy and
TJesource
TQonsultants, INC
PARTICULATE CONTROL
TECHNOLOGY AND
PARTICULATE EMISSIONS
STANDARDS FOR HEAVY-
DUTY DIESEL ENGINES
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nr energy • engineering • economics -financial analysis- environn
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P.O. Drawer O, Boulder, CO 80306 (303) 449-5515
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PARTICULATE CONTROL TECHNOLOGY AND PARTICULATE
EMISSIONS STANDARDS FOR HEAVY-DUTY DIESEL ENGINES
A Report To:
The U.S. Environmental Protection Agency
Office of Policy Analysis
Washington, D.C.
Final Report
EPA Contract //68-01-6543
Work Order if92
Task 1
Christopher S. Weaver P.E., Task Manager
Craig Miller, Ph.D., Project Manager
Lisa Nelowet
Energy and Resource Consultants, Inc.
P.O. Drawer O
Boulder, Colorado 80306
December 11, 1984
©1984,
Energy and Resource Consultants, Inc.
HEAVY (123)
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EXECUTIVE SUMMARY
INTRODUCTION
The U.S. Environmental Protection Agency has proposed stringent particulate and NOx
emissions standards for heavy-duty engines. These proposals are controversial. The
major points of controversy concern levels of engine-out emissions achievable in the near
and intermediate term, the effects of stringent NOx standards on emissions and fuel
consumption, and the feasibility and technical maturity of trap-oxidizers, which are
devices which filter the particulate material from diesel exhaust. A-major area of con-
cern is the interrelationship between NOx and particulate control — generally, very low
NOx emissions can be achieved only at the cost of high particulate emissions, and vice
versa.
This report provides an independent assessment of these and other issues related to
heavy-duty particulate control. It concludes with recommendations for feasible near-
term and intermediate-term emissions standards, as well as suggestions for further
research. The report is based on a thorough review of the applicable technical literature
and the regulatory dockets, extensive discussions with manufacturers of heavy-duty
engines and particulate control devices, and independent engineering analysis. A draft
version of the report has been subjected to extensive review by the same manufacturers,
as well as by other knowledgeable parties. Appendix A summarizes the reviewer's com-
ments and the degree to which they have been incorporated into the analysis.
INDUSTRY STRUCTURE
Heavy-duty vehicles, as defined by EPA, are highway vehicles with a manufacturer's
gross vehicle weight rating greater than 8,500 pounds. They range from pickup trucks
and vans with GVW only a little over the minimum up to tractor-trailer rigs with a gross
combined weight (tractor plus two trailers) of 150,000 pounds. They are thus a far less
homogeneous group than light-duty cars and trucks. The heavy-duty vehicle manufactur-
ing industry also has a markedly different structure from the light-duty industry — verti-
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cal integration is far less extensive, and customer choice is greater. The purchaser of a
heavy truck generally specifies the make and model of the engine, transmission, rear-
axle assembly, truck body, and other components, choosing from a large menu of selec-
tions compatible with a given truck chassis. A single truck model may be available with
numerous different engine models from several different manufacturers. For this reason,
among others, EPA regulates emissions by heavy-duty engines rather than heavy-duty
vehicles.
Classes of Heavy-Duty Vehicles
The most commonly used classification scheme is that of the Motor Vehicle Manufactur-
ers Association, which defines eight classes of trucks and buses on the basis of rated
GVW. The MVMA classification is unsatisfactory for this analysis, since it lumps
together highly disparate vehicles in some classes (notably the heaviest — Class 8), while
separating basically similar vehicles in the lighter classes. For this reason, the authors
have adopted a scheme separating heavy-duty diesel vehicles into four classes, on the
basis of technical characteristics and usage patterns as well as GVW. These classes are
the following:
1. Light-Heavy Duty Vehicles — This includes all heavy-duty vehicles
from 8,500 up to about 14,000 pounds GVW. These are mostly large
pickup trucks and vans, similar to light-duty trucks. The remainder
are mostly specialized vehicles such as tow trucks and motor homes,
which are built on pickup or van chassis. Diesel engines used in this
class resemble those used in light-duty cars and trucks, so that Light-
duty emissions control technology could be adopted fairly readily.
2. Medium-Heavy Duty Vehicles — This group includes all vehicles
heavier than about 14,000 pounds, except for transit buses and line-
haul trucks. Most medium-heavy vehicles are "straight" (single-unit)
trucks, as opposed to semi-trailers. This class exhibits a very wide
variety of body styles, equipment, and usage patterns, but they are
primarily urban.
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3. Line-Haul Trucks — These are very large, high-powered, very heavy
trucks used primarily for inter-urban freight. Virtually all of these
are semi-trailer and double-trailer combinations.
Transit Buses — These are full-sized buses used for intra-urban
transit, typically in stop-and-go operation. This class does not
include school buses (which resemble medium-heavy trucks) or buses
for inter-urban transport (which should probably be counted in the
line-haul class).
Since the draft of this report was completed, EPA has adopted very similar classifica-
tions in its recent rules defining the full useful life of heavy-duty engines and in its
recent proposals for NOx and particulate regulations.
As will be brought out below, these four classes differ markedly in technical characteris-
tics, operating and ownership patterns, and ability to comply with emissions regulations.
It is recommended that emissions standards for each class be considered separately, and
on their own merits, rather than adopting one blanket standard for all heavy-duty vehi-
cles.
Heavy-Duty Vehicle and Engine Manufacturers
The major manufacturer of light-heavy duty diesel vehicles is General Motors, which
produces its own engines. Ford — using International Harvester engines — will probably
become a major factor in the future. Major engine manufacturers in the medium-heavy,
line-haul, and transit-bus classes are Cummins Engine, Caterpillar Tractor, Detroit
Diesel-Allison Division of General Motors, International Harvester, and Mack Trucks.
Mack Trucks, International Harvester, CMC Division of General Motors, and Ford are the
major truck manufacturers; smaller firms such as PACCAR, Freightliner, and White also
hold significant market shares in the line-haul class. General Motors, Flexible, and Flyer
Industries produce most transit buses in the U.S., with GM supplying most of the en-
gines. Major truck importers are Daimler-Benz (Mercedes), Fiat (IVECO), and Volvo,
each of which manufactures its own engines. Imports are still a very small (but growing)
portion of the heavy truck market.
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EMISSIONS CONTROL TECHNOLOGIES AND FEASIBLE STANDARDS
To establish feasible emissions standards for heavy-duty engines, it is necessary to consi-
der three things: the engine-out emissions levels that can be achieved, the feasibility
and effectiveness of aftertreatment technologies, and the amount of slack or margin
between achievable low-mileage emissions levels and achievable standards. These issues
are discussed separately.
Engine-Out Emissions Controls
Engine-out controls are those which affect the amounts of pollutants in the exhaust as it
leaves the engine, before processing by aftertreatment technologies such as catalytic
converters or trap-oxidizers. The most important consideration in engine-out particulate
control for diesels is the interrelationship between particulate and NOx emissions, which
is known as the NOx/particulate tradeoff. Figure E.l shows the general nature of this
relationship. Generally, decreasing NOx increases particulates, and vice vesa. This rela-
tionship derives from the fundamental nature of the combustion process in diesels. Thus,
although there is some scope for improvement in NOx and particulate emissions, this
scope is limited. Similar fundamental tradeoffs exist between NOx and hydrocarbon
emissions, and between NOx and fuel consumption. The later relationship has been
documented in another ERC report ("Weaver, 1984b).
Techniques which can be used to "trade-off" emissions along the NOx/particulate
tradeoff curve are changing the fuel-injection timing and (for very low NOx levels)
exhaust gas recirculation. Retarding injection timing to control NOx has an adverse
effect on fuel economy, as well as increasing particulate emissions. EGR (in moderation)
has little effect on fuel economy, but may increase engine wear and maintenance costs.
Several techniques are available to improve on the NOx/particulate tradeoff (i.e., shift-
ing the entire curve inward toward the origin, rather than moving along it). Techniques
which wiJJ be available in the near term are turbocharging with improved charge-air
cooling, high-pressure/high-precision fuel injection systems, and incremental engine
improvements. Those which will be available in the intermediate term include those
already listed, plus optimal electronic control of fuel injection timing, the quantity of
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fuel injected, and (at low NOx levels) the EGR rate. All of these technologies have gen-
erally beneficial effects on fuel economy, as well as on emissions levels, and thus are
likely to be adopted with or without a strict emissions standard. Without a standard,
however, they would be adjusted to achieve optimal fuel economy, rather than the lowest
possible emissions.
Line A-A of Figure E.l shows the NOx/particulate tradeoff relationship which is
expected to be achievable in the near term (roughly 1987), using the techniques discussed
above. Lines B-B and B'-B1 of Figure E.l show the approximate emissions levels achieva-
ble in the intermediate term (roughly 1990-1991). Figure E.l also shows the distribution
of NOx and particulate emissions in current engines. The tradeoff curves shown in the
figure are for low mileage engine-out emissions, not feasible emissions standards, and
apply to direct-injection engines such as those used in the medium-heavy, line-haul, and
transit bus subclasses. Small direct-injection engines are also being introduced into the
light-heavy duty subclass, and the curves in Figure E.l would apply to them as well.
Light-heavy duty engines using indirect injection (presently the dominant technology in
the light-heavy subclass) are inherently lower emitting, and could achieve lower levels
than those shown in the near term. Emissions results for the G.M. 6.2 liter engine (a
typical light-heavy indirect-injection engine) are also shown in Figure E.l
Trap-Oxidizers
Trap-oxidizers are presently the only aftertreatment technology which shows much
promise for diesel emissions control. Basically, a trap-oxidizer is a durable filter (the
trap) placed in the exhaust stream to catch and retain particulate material. This must be
accompanied by some means of cleaning (regenerating) the filter by burning off (oxidiz-
ing) the accumulated particulate material, since otherwise the filter would clog within a
few hundred miles. Regeneration and the systems for accomplishing it have presented
the most important difficulties in the development of trap-oxidizer technology.
Trap-oxidizers have been developed first for light-duty diesel vehicles, in order to meet
the EPA light-duty particulate standard which goes into effect in 1987 (the CARB has a
similar standard going into effect in 1986). Light-duty trap-oxidizer technology is well-
developed — Mercedes is now producing a trap-oxidizer equipped cars for sale in
California, and several other manufacturers have reached the fleet test stage. Trap-
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oxidizer technology for heavy-duty vehicles is much less advanced. There are several
reasons for this; the most important ones are the lesser regulatory pressure, the lesser
R&D capabilities of the heavy-duty manufacturers, and the greater difficulty of the
development task. As a result, heavy-duty trap-oxidizers will probably not be available
before 1990 or 1991, except for light-heavy duty vehicles. These vehicles could use
adaptations of light-duty trap-oxidizer technology, and could thus implement trap-
oxidizers as early as if successive light-duty trap-oxidizers are introduced in 13X7.
The heavy-duty manufacturer which appears to have made the most progress in this area
is Daimler-Benz, which has developed a very attractive heavy-duty trap-oxidizer sys-
tem. Daimler states that it is confident that this system could be in production by 1990.
Four general types of trap-oxidizer systems now show promise for use on heavy-duty
vehicles. Three of these were first developed for light-duty use, the other is the one
developed by Daimler-Benz. These systems are described in detail in Chapter Five. At
present, it is too early to state which, if any, of them will ultimately be adopted for
widespread use.
Trap-oxidizer systems would be expensive: estimates of (discounted) life-cycle cost
range from $550 to $715 for light-heavy vehicles, $14M to $1540 for medium-heavy
vehicles, $2^09 to $2973 for transit buses, and $3^62 to $40^7 for line-haul trucks. A
large fraction of this cost, especially in the heavier classes, is due to increased fuel con-
sumption and the cost of maintenance and replacement traps. Users could avoid these
costs by removing the trap after purchase, thus there would be a potentially serious
tampering problem, especially in line-haul trucks.
Feasible Emissions Standards
Feasible emissions standards can be derived from feasible low-mileage emissions levels
by considering the amount of "slack" required in the standard to account for emissions
deterioration, random variation in engines, lab-to-lab variability, and the manufacturer's
margin of safety. Based on manufacturer's submissions to EPA, and the authors' own
analysis, the required slack is approximately 25 percent of the low-mileage level for
engine-out particulates, and 10 percent for NOx. The values assume the use of noncon-
formance penalties for non-compliance, as well as emissions averaging. Without these
assumptions, greater margins than those listed would be required. These values also
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assume zero deterioration for NOx emissions and 15 percent deterioration for particu-
lates.
Line A'-A* on Figure E.2 shows feasible near-term emissions standards for particulates as
functions of the level of the NOx standard. This line was obtained from the low-mileage
emissions curve by multiplying the NOx coordinate of each point by 1.10 and the particu-
late coordinate by 1.25, corresponding to 10 percent and 25 percent slack, respectively.
Figure E.3 shows a preliminary estimate of the feasible intermediate-term non-trap par-
ticulate standards, derived from the midpoint of the range shown in Figure E.l by a simi-
lar process. Feasible trap-oxidizer based standards can be derived from the levels shown
In Figure E.3 by multiplying the particulate coordinate by 0.25, corresponding to a trap
efficiency of 75 percent.
Table E.l lists feasible NOy and particulate emissions standards and the corresponding
average low-mileage emissions rates for a number of different regulatory scenarios.
Achievable standards and emissions levels are shown separately for light-heavy engines
and all other heavy-duty engines, since light-heavy engines would be able to comply with
a stringent standard more quickly by adopting light-duty emissions control technology.
All scenarios share the same initial set of regulations — 6.0 g/BHP-hr NOx in the near
term (1987-88), accompanied by the strictest engine-out particulate standard which is
estimated to be feasible by that time. The scenarios differ in the regulatory philosophy
that is assumed for the intermediate term. One scenario — the relaxed scenario —
assumes no further change from the near term. The other four scenarios assume more
stringent regulations; they differ in the relative emphasis placed on NOx and particulate
emissions, and on whether or not it is considered worthwhile to require trap-oxidizers.
OTHER ISSUES
Emissions Averaging
Emissions averaging would reduce uncertainty due to statistical variations in engine
emissions, and would also allow emissions from inherently higher emitting engines to be
traded off against lower emitting ones, thus helping to maintain a broader product line.
This would increase consumer choice and reduce the cost of compliance, and is thus
desireable. However, care will be needed in designing such a regulation, in order to
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PARTICULATE
(g/bhp hr)
IO
7 -
6 -
, • A
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estimated low-mileage NO /particulate
levels achievable In thp near term
A'-A' feasible NO /particulate standards
derive*! froA A-A.
5 6
NO* (g/bhp hr)
l.O
figure 1L.J: Lstiniates of achievable near-term particulate emissions standards
vs. NO standards for heavy-duty diesel engines.
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PARTICULATE
(9/bhp hr)
l.O
.8 -
7 *
6 -
.5 -
.4 -
.3 -
.2 -
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1 1
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C-C Midpoint of ranye of estimates of
NO /(iartKuldtu levels achievable In
thi Intermediate tenn (see Fig 4 12)
C'-C leasible NO /paniculate standards
derived froA C-C
5 6
NOjj (g/bhp hr )
1
9
1—
l.O
Figure E.3: Estimates of achievable intermediate-term particulate emissions
standards vs. NO^ standards for heavy-duty diesel engines.
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Table E.i
Numerical Standards and Low-Mileage Emissions
Levels for Five Feasible Heavy-Duty Regulatory Scenarios
NO^ Particulate
Scenario Date Standard LMT Standard LMT
1. Moderate Control
Light-Heavy 1986 6.0 5.5 0.62 0.50
All Others 1987 6.0 5.5 0.62 0.50
2. Moderate NO^/Best Engine-Out Particulate
Light-Heavy 1986 6.0 5.5 0.62 0.50
1988 5.0 4. 5 0.56 0.45
All Others 1987 6.0 5.5 0.62 0.50
1990 5.0 4.5 0.56 0.45
3. Moderate NO^/Trap Oxidizers
Light-Heavy 1986 6.0 5.5 0.62 0.50
1988 5.0 4.5 0.14 0.08
All Others 1987 6.0 5.5 0.62 0.50
1990 5.0 4.5 0.14 0.0S
4. Strict NO^/No Trap-Oxidizers
Light-heavy 1986 6.0 5.5 0.62 0.50
1988 4.0 3.6 0.72 0.58
Ail Others 1987 6.0 5.5 0.62 0.50
1990 4.0 3.6 0.72 0.58
5. Strict NO^/Trap-Oxidizers
Light-Heavy 1986 6.0 5.5 0.62 0.50
1988 4.0 3.6 0.18 0.09
All Others 1987 6.0 5.5 0.62 0.50
1990 4.0 3.6 0.18 0.09
I
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guard against unnecessary anticompetitive effects, and to ensure that the emissions
being averaged together are truly comparable. For instance, light-heavy IDI engines are
inherently cleaner (on a g/BHP-hr basis) than DI engines, so that light-heavy engine
manufacturers would gain a substantial competitive advantage if they were allowed to
include them in averaging. Light-heavy engines also generate many times fewer BHP-hr
over their useful liftimes than do medium-heavy, line-haul, or transit bus engines, so that
a simple averaging scheme which counted all of them the same would result in a net
increase in emissions. Another concern stems from the difference in usage patterns
between subclasses — transit buses are almost entirely urban, for instance, while line-
haul trucks operate mostly in rural areas.
A simple solution to the problems of averaging would be to allow averaging only within
the subclasses defined above, but this would reduce flexibility and might have adverse
competitive effects. Alternatively, each engine's emissions could be weighted by its
average level power and expected life, with some adjustment made for different patterns
of urban vs. non-urban operation. Careful design of any averaging regulation will be
regional in order to maximize the benefits to manufacturers without introducing com-
petitive distortions, or jeopardizing air quality.
Subdivision of the Heavy-Duty Class
Heavy-duty vehicles and engines are not all alike, and these differences have important
effects on the feasibility and desirability of emissions standards. As has already been
remarked, light-heavy engines could readily adopt light-duty technology for trap-
oxidizers and engine controls, and light-heavy IDI engines are inherently lower emitting
than the DI engines used in other classes. As a result, light-heavy engines could meet a
stricter standard earlier than any of the other classes. Transit-bus engines are similar to
medium-heavy engines technologically, but transit buses operate in congested areas, on a
cycle which produces very high emissions. Thus the benefits of emissions control for
buses would be greater, and a stricter standard might be justified.
On the other hand, line-haul trucks spend comparatively little time in urban areas, while
they account for nearly half of total heavy-duty fuel consumption. Thus the benefits of
control would be smaller for this group, while the costs of increased fuel consumption
would be very large. For this reason, EPA should consider exempting line-haul engines
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from any standards strict enough to have a significant effect on fuel economy. This
could be done on a case-by-case basis — EPA could issue a special permit, entitling the
issuee to purchase an engine conforming to more lenient standards, upon the issuee's
demonstration that the engine would be used primarily outside of urban areas. The cost
of line-haul trucks is high enough and their numbers small enough that this should not
pose a major administrative burden.
Diesel Fuel Quality
The major indicators of diesel fuel quality are the cetane number, aromatic content,
boiling point range, and sulfur content. Cetane number and aromatic content are
related — increasing aromatic content decreases cetane, although .cetane-improving
additives can be added to recover this. Low cetane and high aromatic content tend to
increase particulate emissions. Most of this effect appears to be due to the aromatics —
adding cetane improvers to high-aromatic fuel does not seem to improve particulates
much as long as the cetane number is within the engines' design range. Due to the grow-
ing scarcity of high-quality crude, average cetane number has been decreasing, and
average aromatic content increasing, over the last decade. This can be expected to lead
to increased particulate emissions, as well as worsened fuel economy and performance in
use. EPA should consider establishing more restrictive standards for diesel fuel aromatic
content and/or cetane number.
There could be a beneficial synergism between improving cetane and reducing the sulfur
content of diesel fuel. Presently, sulfur content is limited mostly by market accept-
ance — too high a sulfur content leads to corrosion and excessive wear. Thus the
increasing trend to high sulfur crudes has required more extensive use of desulfuriza-
tion. Sulfur in diesel fuel is oxidized to SO2, which is itself a regulated pollutant
(although SO2 emissions from motor vehicles are not regulated), as well as being a sig-
nificant contributor to acid deposition. High-sulfur fuel has also been shown to increase
particulate emissions significantly. Secondary particulate formation from SO2 in the
atmosphere is also a significant contributor to urban particulate levels. In addition,
catalytic trap-oxidizers with precious-metal catalysts react with SO2 to produce sulfuric
acid, which can be a major problem at the high loads typical of heavy-duty service.
Catalytic trap-oxidizers have numerous benefits — they would reduce odor, HC, and CO
emissions, as well as particulates, but their future is in doubt because of the sulfur prob-
lem.
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The refining process used to reduce aromatic content and improve cetane also removes
essentially all of the sulfur, thus offering potential double benefits. Sulfur can also be
removed by hydrodesulfurlzation, a less severe (and thus less expensive) version of the
de-aromatization process, but hydrodesulfurization has little effect on aromatics. For
this reason it would be desirable to consider cetane/aromatic standards and sulfur-
content standards for diesel fuel simultaneously.
CONCLUSIONS AND RECOMMENDATIONS
This report has produced a number of important conclusions and recommendations with
regard to heavy-duty emissions control, regulation, and future investigation. The most
important of these are listed below.
1. In establishing regulations for heavy-duty engines, EPA should consi-
der the four major subclasses of heavy-duty vehicles separately. This
does not necessarily mean that different regulations should be
adopted for each subclass, only that different regulations should be
considered.
2. Figure E.2 is a plot of estimated feasible engine-out particulate
standards versus the NOx standard for medium-heavy, transit bus, and
line-haul engines in the near term (roughly 1987 or 1988). Because
the technology involved is not radically different from what is now in
use, this frontier is considered to be fairly well defined. Any emis-
sions standards applying to that period should be chosen to fall on or
above the frontier. Light-heavy duty engines are capable of comply-
ing with a similar standard by 1986.
3. Figure E.3 shows the estimated feasible engine-out particulate stand-
ard as a function of the NOj. standard for intermediate-term (1990 or
1991) application. As for Figure E.2, this figure applies to medium-
heavy, line-haul and transit bus engines. Application of trap-
oxidizers would reduce the feasible particulate standard by approxi-
mately 75 percent. The information in this figure is much more
uncertain than that in Figure E.2, and should be clarified by addi-
tional research before being used as a basis for regulation. Light-
heavy duty engines should be able to comply with a similar standard
(including the use of trap-oxidizers) by 1988.
4. The continuing degradation in diese! fuel aromatic content, cetane
number, and sulfur content is likely to lead to increased particulate
emissions in use. EPA should consider regulations to counter this.
There is a possible beneficial synergism between reduction in the
sulfur content of diesel fuel and improvements in cetane and aro-
matics. This synergism, and possible regulations to promote it, should
receive further study.
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TABLE OF CONTENTS
Page
EXECUTIVE SUMMARY ui
PREFACE xxvii
ACKNOWLEDGEMENTS xxix
1.0 INTRODUCTION 1-1
1.1 BACKGROUND 1-1
1.2 PURPOSE AND SCOPE OF THE REPORT 1-2
1.3 DATA SOURCES 1-4
1.4 STRUCTURE OF THE REPORT 1-5
1.5 LIMITATIONS AND CAVEATS 1-7
2.0 CLASSIFICATION OF HEAVY-DUTY ENGINES AND VEHICLES 2-1
2.1 LIGHT-HEAVY DUTY VEHICLES 2-5
2.2 MEDIUM-HEAVY DUTY VEHICLES 2-5
2.3 LINE-HAUL TRUCKS 2-6
2.4 TRANSIT BUSES 2-7
3.0 COMMERCIAL FEASIBILITY AND HEAVY-DUTY ENGINES; REQUIREMENTS
FOR EMISSIONS CONTROL TECHNOLOGIES 3-1
EFFECTIVENESS 3-2
DURABILITY 3-2
RELIABILITY 3-4
INITIAL COST 3-5
EFFECTS ON FUEL ECONOMY 3-6
EFFECTS ON ENGINE DURABILITY AND RELIABILITY 3-6
EFFECTS ON PERFORMANCE AND DRIVEABILITY 3-7
MAINTENANCE REQUIREMENTS AND TAMPER RESISTANCE 3-3
SAFETY 3-9
WEIGHT AND BULK, EFFECTS ON SYSTEM INTEGRATION 3-9
MANUFACTURABILITY AND INSTALLABILITY 3-10
ENVIRONMENTAL EFFECTS 3-10
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TABLE OF CONTENTS
(continued)
Page
4.0 ENGINE-OUT EMISSIONS CONTROL FOR HEAVY-DUTY ENGINES 4-1
4.1 FUNDAMENTALS OF DIESEL EMISSIONS 4-2
4.1.1 Combustion in the Diesel Engine 4-3
4.1.2 Particulate Emissions 4-5
Soot formation 4-7
Soot oxidation 4-9
Hydrocarbons 4-10
Effects of Engine Variables on Emissions 4-12
4.1.3 Oxides of Nitrogen (NOx) Emissions 4-17
4.1.4 "Hie NOx/Particulate Tradeoff 4-21
4.2 CONTROL TECHNIQUES FOR ENGINE-OUT EMISSIONS 4-22
4.2.1 Engine Modifications 4-24
Optimized airflow and valve timing 4-24
Optimized cylinder pressure and Compression 4-25
Ratio
Optimized Combustion Chamber, Air Swirl, 4-25
and Spray Pattern
Efficiency Improvements 4-26
Status and Prospects 4-27
4.2.2 Fuel Injection Systems 4-27
Higher Precision 4-28
Higher Injection Pressures 4-29
Optimal Injection Timing and Variable 4-29
Injection Timing
Status and Prospects 4-30
4.2.3 Charging Technologies 4-31
Charge-air Cooling 4-32
Advanced Charging Technologies 4-32
Status and Prospects 4-33
4.2.4 Exhaust-Gas Recirculation 4-34
Status and Prospects 4-35
4.2.5 Electronic Cantrols 4-36
Status and Prospects 4-37
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TABLE OF CONTENTS
(continued)
Page
4.2.6 Indirect Injection 4-40
Status and Prospects 4-41
4.2.7 Advanced Engine Technologies 4-42
Adiabatic Diesel Engines 4-42
Turbocompounding 4-43
Organic Rankine Bottoming 4-43
Status and Prospects 4-43
4.2.8 Additives and Alternative Fuels 4-44
Water/diesel fuel and alcohol/diesel ". 4-44
fuel emulsions
Water fumigation 4-45
Methanol fuel. 4-45
Status and prospects 4-46
4.3 PRESENT-DAY ENGINES AND EMISSIONS LEVELS 4-47
4.3.1 Light-Heavy Duty Engines 4-47
Emissions levels 4-48
4.3.2 Medium-Heavy, Line-Haul, and Transit Bus Engines 4-48
Emissions Levels -50
4.4 ACHIEVABLE ENGINE-OUT EMISSIONS LEVELS 4-54
4.4.1 Light-Heavy Duty Diesel Engines 4-54
4.4.2 Medium-Heavy, Line-Haul Truck, and Transit
Bus Engines 4-55
5.0 TRAP-OXIDIZER SYSTEMS FOR HEAVY-DUTY VEHICLES 5-1
5.1 TRAP-OXIDIZER TECHNOLOGY 5-2
5.1.1 Diesel Particulate Traps 5-2
Ceramic monolith traps 5-3
Ceramin fiber traps 5-8
Catalyzed, radial flow wire-mesh traps. 5-12
5.1.2 Regeneration Techniques 5-16
Positive regeneration techniques 5-17
Self-regeneration techniques 5-21
5.1.3 Control Systems 5-25
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TABLE OF CONTENTS
(continued)
Page
5.2 SPECIAL CONSIDERATIONS FOR TRAP-OXIDIZERS IN 5-27
HEAVY-DUTY SERVICE
Light-Heavy Duty Vehicles 5-28
Medium-Heavy Duty Trucks 5-30
Line-Haul Trucks 5-32
Transit Buses 5-33
5.3 POSSIBLE TRAP-OXIDIZER SYSTEM CONFIGURATIONS FOR 5-35
HEAVY-DUTY VEHICLES
5.3.1 Bypass/Burner System with Ceramic Monolith Trap
System Description 5-36
Effectiveness 5-36
Durability and reliability 5-38
Performance and fuel economy effects 5-38
Estimated cost 5-39
Safety and environmental effects 5-44
Other considerations 5-44
Development status 5-45
Overall assessment 5-45
5.3.2 Ceramic Monolith Trap/Self Regeneration 5-45
System description 5-45
Effectiveness 5-46
Durability and reliability 5-48
Performance and fuel economy
Estimated cost 5-49
Safety and environmental effects 5-51
Other considerations 5-51
Development status 5-52
Overall assessment 5-52
5.3.3 Catalyzed Wire-Mesh Trap/Regeneration by HC s-53
and CO Oxidation
System description 5-53
Effectiveness 5-53
Durability and reliability 5-55
Performance and fuel economy 5-55
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TABLE OF CONTENTS
(continued)
Page
Estimated cost 5-55
Safety and environmental effects. 5-57
Other considerations 5-57
Development status 5-58
Overall assessment .. 5-58
5.3A Ceramic-Fiber Trap/Regeneration by Catalyst 5-58
Injection
System description 5-58
Effectiveness .. * 5-59
Durability and reliability 5-59
Performance and fuel economy 5-59
Estimated cost 5-61
Safety and environmental effects. 5-63
Other considerations 5-63
Development status * 5-6^
Overall assessment 5-64
6.0 EFFECTS OF FUEL5 ON DIESEL EMISSIONS 6-1
6.1 DIESEL FUEL PROPERTIES 6-1
6.2 EFFECTS OF CHANGES IN DIESEL FUEL QUALITY ON EMISSIONS 6-6
Cetane Number and Aromatic Content 6-7
Volatility 6-\<+
Sulfur Content 6-17
6.3 POTENTIAL REGULATORY ACTIONS TO IMPROVE FUEL QUALITY6-21
Limitations on Cetane Number and/or Aromatic 6-21
Content
Changes in Permissible "Back-End" Boiling Points 6-22
Desulfurization 6-23
7.0 EM\55lON5 STANDARDS FOR HEAVY-DUTY DIESEL ENGINES 7-1
7.1 PREVIOUSLY PROPOSED STANDARDS 7-1
7.2 ISSUES RELATED TO STANDARDS: EMISSIONS AVERAGING AND 7 4
SUBDIVISION OF THE HEAVY-DUTY CLASS
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TABLE OF CONTENTS
(continued)
Page
Subdivision of the Heavy-Duty Class 7-4
Emissions Averaging Regulations 7-7
7.3 ACHIEVABLE NOx AND PARTICULATE STANDARDS 7-9
7.3.1 Translations from Low-Mileage Emissions 7-10
to Standards
7.3.2 Limits of Feasibility for NOx and Particulate 7-13
Standards
7.3.3 Alternatives for Emissions Standards 7-16
7.4 RECOMMENDATIONS 7-19
8.0 SUMMARY AND CONCLUSIONS 8-1
CHAPTER 2: SUBCLASSIF1CATION OF HEAVY-DUTY ENGINES 8-1
CHAPTER 3: COMMERCIAL FEASIBILITY AND HEAVY-DUTY 8-2
ENGINES - REQUIREMENTS FOR EMISSIONS
CONTROL TECHNOLOGIES
CHAPTER 4: ENGINE-OUT EMISSIONS CONTROL FOR 8-4
HEAVY-DUTY ENGINES
CHAPTER 5: TRAP-OXIDIZER SYSTEMS FOR HEAVY-DUTY 8-7
VEHICLES
CHAPTER 6: EFFECTS OF FUELS ON DIESEL EMISSIONS 8-9
CHAPTER 7: EMISSIONS STANDARDS FOR HEAVY-DUTY DIESEL .... 8-11
ENGINES
APPENDIX A: SUMMARY OF REVIEWERS' COMMENTS A-l
APPENDIX B: BIBLIOGRAPHY B-l
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LIST OF TABLES
Page
Table 2.' Truck Classification as defined by the Motor Vehicle 2-2
Manufacturers Association
Table 2.2 Truck Classifications Used in This Study 2-3
Table ^.1 Engine-Out Emissions Control Techniques ^-23
Table k.2 Light-Heavy Duty Engine Emissions Levels V-49
Table 5.1 Estimated Cost of Ownership for a Bypass/Burner Monohth
System 5-40
Table 5.2 Estimated Cost of Ownership for an Additive/Monolith
System 5-50
Table 5.3 Estimated Cost of Ownership for a Catalytic Wire-Mesh
System 5-56
Table 5A Estimated Cost of Ownership for a Daimler-Benz 5-62
"Candle" System
Table 7.1 Historical and Proposed Federal and California 7-2
Emission Standards For Heavy-Duty Diesel Engines
Table 7.2 Manufacturer's Recommendations for Heavy-Duty 7-11
Emissions Standards
Table 7.3 Numerical Standards and Low-Mileage Emissions Levels 7-17
for Five Feasible Heavy-Duty Regulatory Scenarios
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UST OF FIGURES
Page
Figure 4.1 Diesel combustion chamber arrangements used in automotive
engines 4-4
Figure 4.2 Cylinder pressure vs. crank angle for different fuel injection
timings in a direct injection engine 4-6
Figure 4.3 Soot nucleation and growth stages under diesel-engine
conditions 4-8
Figure 4.4 Soot oxidation rates predicted by the Nagle and Strickland-
Constable theory vs. temperature and oxygen content 4-11
Figure 4.5 Temperature, equivalence ratio, and soot concentration history
along a transect in a direct-injection diesel engine 4-14
Figure 4.6 Correlation between particulate emissions index and stoichio-
metric adiabatic flame temperature for a number of IDI
diesel engines 4-16
Figure 4.7 Correlation between NOx emissions index and stoichiometric
adiabatic flame temperature for diesel engines and gas turbine
combustors 4-19
Figure 4.S Temperature, equivalence ratio, and NOx formation vs. time
for different injection pressures 4-20
Figure 4.9 Fuel economy vs. NOx emissions — effects of
electronic controls 4-39
Figure 4.in Transient-cycle particulates vs. NOx for heavy-duty diesel
engines 4-10
Figure 4.11 NO^ vs. particulate emissions for current heavy-duty diesel
engines, measured on the EPA transient test 4-53
Figure 4.12 Estimates of achievable average low mileage particulate emissions vs.
NOx for near-term and intermediate-term technologies 4-56
Figure s.l Face view of a Ceramic Monolith Trap 5-4
Figure 5.2 Principle of operation of the Ceramic Monolith Trap 5-5
Figure 5."* Ceramic fiber mat trap 5-9
Figure 5.4 Support pipe with silicon dioxide thread windings for the
Daimler-Benz silica-fiber candle trap 5-10
Figure 5.5 Daimler-Benz silica-fiber candle trap 5-11
Figure s.6 Catalytic wire-mesh trap, engine manifold location 5-13
L
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LIST OF FIGURES
(continued)
Page
Figure 5.7 Catalytic wire-mesh trap, under-floor location 5-14
Figure 5.8 System diagram—ceramic monolith/burner trap-oxidizer
system 5-37
Figure 5.9 System diagram—ceramic monolith trap-oxidizer with fuel-
additive self regeneration 5-47
Figure 5.10 System diagram—catalyzed wire-mesh trap-oxidizer and
regeneration system 5-54
Figure 5.11 System diagram—Daimler-Beriz trap-oxidizer system based on
silica-fiber "candle" trap with catalytic regeneration 5-60
Figure 6.1 Typical hydrocarbon modules in diesel fuel 6-3 i
Figure 6.2 Trends in quality indices for diesel (T-T) fuel since
I960 6-5
Figure 6.3 Correlation between cetane number and aromatic content for
diesel it2 fuel 6-8
Figure 6.4 Effect of fuel aromatic content on NOx emissions by light-
duty IDI diesels 6-10
i
Figure 6.5 Effect of fuel aromatic content on hydrocarbon emissions by j
light-duty IDI diesels 6-11 I
Figure 6.6 Effect of fuel aromatic content on particulate emissions by
light-duty IDI diesels 6-12
Figure 6.7 Effect of fuel cetane number on hydrocarbon, NOx and smoke
emissions by heavy-duty DI diesels, as measured on the EPA
13-mode test 6-13
Figure 6.8 Effect of changing 90% boiling point on particulate emissions
from light-duty IDI diesels 6-15
Figure 6.9 Effect of changing volatility on smoke emissions from
heavy-duty diesels 6-16
Figure 6.10 Effect of changes in fuel density on particulate emissions in
light-duty IDI diesels 6-18
Figure 6.11 Actual particulate emissions vs. emissions predicted from fuel
composition using equation 6.1 6-20
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LIST OF FIGURES
(continued)
Page
Estimates of achievable near-term particulate emissions standards
vs. NOx standards for heavy-duty diesei engines 7-l*f
Estimates of achievable intermediate term particulate
emissions standards vs. NOx standards for heavy-duty diesei
engines 7-15
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PREFACE
This report was prepared for the U.S. Environmental Protection Agency, Office of Policy
Analysis, as Task #1 of Work Order it93 under EPA contract number 68-01-6.5^3. EPA
Technical Project Manager for most of this work was Mr. Steve Steepler; due to illness,
he was replaced during the later portions of the work by Mr. "W illard Smith. This report
was prepared by Energy and Resource Consultants, Inc. (ERC) as part of a project to
determine the status and prospects for heavy-duty diesel particulate control technology,
to determine feasible heavy-duty diesel particulate standards, and to evaluate the costs
of meeting such standards and the air-quality benefits which would result from doing so.
It describes the present status and development prospects of heavy-duty diesel particu-
late control technology, and discusses technically feasible particulate standards and the
costs of meeting them. In addition, it deals briefly with several closely related issues:
the interrelationship between NOx and particulate emissions for diesels; the effects of
changing diesel fuel quality on emissions; of the effects of subdivision of the heavy-duty
diesel class; and the effects of emissions averaging.
This report is the result of an intense research effort by ERC's staff, carried out pri-
marily during the Summer of 1983, with further research and report preparation extend-
ing through early Spring of I98
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the report, incorporates changes made in the light of the reviewers' comments. It should
be emphasized, however, that this report is by no means a consensus document. Although
we have carefully reconsidered all of our conclusions in the light of the reviewers' com-
ments, the ultimate judgement in every case has been that o! the authors, and some who
reviewed the report did and would doubtless still disagree. Appendix A discusses the
more significant of the reviewers' comments, the authors' replies to them, and the
changes, if any, made in this final report as a result.
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ACKNOWLEDGEMENTS
Preparation of this report would not have been possible without the help and cooperation
of many people and organizations, both in industry and in government. Special thanks are
due to Rick Rykowski of EPA, John Howitt of Corning, Miles Buchman and Fred Enga of
Johnson-Matthey, Tina Vujovich, and Lou Broertng of Cummins, James Pasek and Charles
Elder of General Motors, Dr. S.V. Yumlu and Charles Salter of Mack Trucks, Charles
Hudson and J J. Egan of International Harvester, Don DowdaLl of Caterpillar, Gary Ros-
sow and Dr. Horst Hardenburg of Daimler-Benz, and Mike Schwarz of Ford for their help-
fulness in providing data and arranging discussions. A special note of appreciation is also
due to Mr. W.R. Wade and Dr. S. Shahed, their staffs, and their respective employers,
Ford Motor Company and Cummins Engine, for the series of superb papers on the tech-
nology of diesel particulate control they have produced.
A note of appreciation is also due to the reviewers of the draft report, who included Dr.
Michael Walsh, and Mr. Tom Cackette of the CARB, as well as most of the people listed
above. Needless to say, none of these people should be considered as necessarily endors-
ing any of our conclusions, nor are they responsible for any remaining errors; responsi-
bility for both conclusions and errors is solely that of the authors.
The authors also wish to express their appreciation to Ms. Jana Cruz and Ms. Julie Sueker
of ERC for their assistance with the graphics and report production, and to the staff of
Document Control. Finally, the authors wish to express their gratitude to the EPA Tech-
nical Project Managers, Messrs. Steve Steckler and Willard Smith, for their support, and
for their patience as due dates and deadlines have repeatedly slipped.
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1.0 INTRODUCTION
The United States Environmental Protection Agency (EPA) has proposed very stringent
standards for emissions of particulate material and oxides of nitrogen (NOx) from heavy-
duty truck engines (EPA, 1984a). These proposals have occasioned a great deal of
controversy. The major points of disagreement concern the technical feasibility of
meeting the proposed standards, and the maturity and reliability of the technology
required to do so. The two central issues in the controversy are the feasibility and
acceptability of filters (trap-oxidizers) to remove particulate material from the exhaust,
and the levels of emissions reduction achievable with in-cylinder control technologies.
Also controversial are the costs and benefits of meeting the proposed standards, even
assuming that they are technically feasible.
Energy and Resource Consultants (ERC) has been commissioned by the EPA Office of
Policy Analysis to study these issues and provide an independent assessment of the
emissions control technology, feasible standards, and the costs and benefits meeting such
standards. This report discusses emissions control technology, feasible standards, and the
costs of meeting them. This study closely parallels an earlier study (Weaver and Miller,
1983; C. Miller et alia, 1983) of similar issues surrounding the EPA proposals for light-
duty diesel particulate standards.
1.1 BACKGROUND
Due to its unmatched fuel economy and great reliability, the heavy-duty diesel engine
has been the primary power source for large trucks for many years. As fuel prices have
climbed in recent years, an increasing fraction of other vehicles, from medium-sized
trucks to passenger automobiles, have been equipped with diesel engines as well. This
dieselization trend has focused attention on the diesel engine and its contribution to air
pollution.
Diesel engines emit a substantially different mix of pollutants from gasoline engines. Of
the major gaseous pollutants, diesels emit only minor amounts of carbon monoxide (CO)
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and gaseous hydrocarbons (HC), but somewhat more oxides of nitrogen (NOx). In addi-
tion, diesels emit substantial amounts of particulate matter, in the form of an oily black
soot. Efforts to reduce either NOx or particulate emissions are greatly complicated by
the fact that most technologies which are effective in reducing one of these pollutants
also tend to increase the other.
The prospect of greatly increased numbers of diesel-engined vehicles, especially in urban
areas, has generated concern over the impact of diesel emissions on ambient air quality.
In response to these concerns, EPA and the California Air Resources Board (CARB) have
adopted regulations limiting particulate emissions from light-duty vehicles, and EPA has
been studying similar proposals for heavy-duty engines for some time. Because of the
relationship between diesel NOx and particulate emissions, EPA is considering a simul-
taneous tightening of the heavy-duty NOx standard as well. In addition, EPA has man-
dated changes in the heavy-duty engine testing procedure, with the substitution of a
transient test cycle for the older 13-mode gaseous emission test.
Although tightened heavy-duty NOx and particulate regulations were were first proposed
by EPA in 1979 (EPA, 1979a; 1979b), hearings on them were not held until July, 1982. By
that time, it had become clear that the standards originally proposed (4 grams per BHP-
hr of NOx and 0.25 grams per BHP-hr of particulate) could not be achieved by the
proposed implementation date of 1986, and there was considerable question as to whether
the NOx standard could be achieved at all without crippling performance penalties. As a
result, EPA has recently re-proposed (EPA, 1984a) significantly less stringent standards
of 0.6 grams per BHP-hour particulate and 6 grams per BHP-hour NOx to take effect in
1987, with a subsequent tightening to the levels of the original proposal in 1990.
Hearings on this proposal were held in November, 1984, and as of this writing, there is
yet no clear consensus as to the feasibility of the proposed 1990 standards. This report is
intended to provide data and analysis to help in deciding these issues.
I a PURPOSE AND SCOPE OF THE REPORT
This report has three primary objectives. The first is to assess what a commercially
viable heavy-duty emissions control system must include, with special attention to the
differences in requirements between heavy-duty and light-duty vehicles. This includes
defining the operating conditions under which it must function, the tasks it must accomp-
lish, and the levels of efficiency, durability, and reliability required.
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The second objective of the report is to characterize the present state of development of
heavy-duty diesel emissions control technology, and to compare this state with the
requirements for commercial viability determined in objective one. This characteriza-
tion includes an analysis of technologies which could have a significant effect on NOx
and/or particulate emissions, with particular attention being paid to those technologies —
trap-oxidizers for particulates and a combination of electronic engine controls, fuel
system modifications, and injection timing retardation for NOx — which show promise
for permitting major decreases in pollutants in the reasonably near future. Another area
of emphasis is the tradeoff relationship between NOx and particulate emissions which is
found in most control technologies (trap-oxidizers and charge-air cooling being the major
exceptions). Work in this area has included estimation of the tradeoff curve between
these two pollutants for technologies which will be available either in the near term (by
1987 or 1988) or the intermediate term (1990 - 1991).
The third objective of this report is to apply the information developed in objective two
to the determination of feasible NOx and particulate emissions standards for heavy-duty
diesel engines. A range of alternative standards, corresponding to five different
emissions-control scenarios are considered. These range from a requirement for what is
essentially good present-day technology to the application of the most stringent controls
achievable. Numerical standards corresponding to each stringency level have been
defined, and the rough costs of meeting the particulate standards have been estimated.
The report also considers the effects of other regulatory policies — such as emissions
averaging, subdivision of the heavy-duty vehicle class into multiple subclasses for
regulatory purposes, and possible regulation of diesel fuel quality — on the cost and
feasibility of meeting the standards.
This report does not directly deal with, and makes no recommendations concerning, a
number of highly controversial issues related to diesel emissions control. Excluded issues
include: the suitability and cost of the EPA transient test cycle for heavy-duty engines,
the issues surrounding the definition of "useful life" in heavy-duty service, the use of
additive vs. multiplicative deterioration factors in determining life-cycle compliance,
and the design of production compliance auditing and enforcement procedures. These
issues are considered in the present report only as they affect the requirements for
emissions control technologies and the numerical standards that are considered achiev-
able.
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1.3 DATA SOURCES
This report has drawn primarily on publicly available data sources, such as the publica-
tions of the Society of Automotive Engineers and other technical societies, U. S.
government reports, and manufacturers' testimony at and submissions to EPA and
California Air Resources Board regulatory hearings. Portions of this report have also
drawn extensively on ERC's previous study of trap-oxidizer technology for light-duty
vehicles. In addition, ERC personnel conducted interviews in person or by telephone with
the staffs of all of the major U.S. engine manufacturers and a number of foreign ones, as
well as with representatives of the major manufacturers of emissions controls. In many
cases, these manufacturers have provided ERC with additional data from their own
studies in the area and with test results for their engines. These, as well as the discus-
sions and exchanges of views, have been invaluable in assisting ERC's-staff to form their
own judgements of technological problems and potentials.
In addition to the publicly available data, the authors have been able to draw upon a
limited amount of proprietary data, provided by manufacturers on a confidential basis.
In some cases, such data have played a significant role in the formation of judgements,
estimates of technical feasibility, etc. Where this is possible, the general nature of such
data has been described, as for instance "confidential data provided by an engine manu-
facturer." A bibliography, indicating the sources of significant publicly available data, is
given at the end of the report.
A word on the limitations of these data sources is appropriate. The heavy-duty diesel
industry is highly competitive, and technological advance is an important competitive
tool. As a result, commercial secrecy is the rule rather than the exception, and many
advances are not reported in the literature until well after they are developed. Thus,
despite the availability of some confidential and proprietary data, the authors are
painfully aware that there is doubtless much development under way with which they are
not familiar. These data, had they been available, might have affected the estimates of
technological feasibility contained in Chapters Four and Five. This limitation should be
borne in mind in interpreting our results.
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1.* STRUCTURE OF THE REPORT
The remainder of the report following this introduction is organized in seven chapters.
Chapter Two deals with the heavy-duty truck industry, with special attention to heavy-
duty engine manufacturers. This chapter is intended to provide the background against
which the specific requirements imposed on engines and emission controls by heavy-duty
service can be discussed. The chapter describes the major types of heavy-duty vehicles,
and presents a proposed classification scheme by which the highly diverse heavy-duty
class can be divided into more homogeneous subclasses.
Chapter Three addresses the requirements which an emissions control system must meet
in order to be considered commercially viable in heavy-duty service. These requirements
are both technical and economic: an emissions control system which is technically defi-
cient will subject its manufacturer to EPA-mandated recalls and repair programs, while
one which is too expensive or which reduces fuel economy by too much will be removed
or defeated in service. Chapter Three defines specific criteria by which commercial
feasibility can be judged.
Chapter Four is concerned with engine-out emissions (those pollutant emissions which are
produced by the engine, without consideration of exhaust treatment). This chapter
begins with a discussion of what is known of the fundamentals of pollutant formation and
emission in diesel engines, then proceeds to a discussion of technologies which can reduce
those emissions. Both present-day emissions levels and those achievable with future
technology are discussed. The development status of new emissions control technologies
is briefly characterized — to the extent that it is known — and the effects of those
technologies with possible near-term applicability are described. Special attention is
paid to the trade-off relationship between NOx and particulate emissions: trade-off
functions for both present day technologies and possible advanced technologies are
defined.
Chapter Five discusses trap-oxidizer technology, with special attention to technology for
heavy-duty vehicles. Since heavy-duty trap-oxidizer technology has lagged considerably
behind that for light-duty vehicles, this chapter devotes considerable space as well to
light-duty technology and its implications. A general summary of technological
developments in the area is given, then the special requirements and problems of trap-
oxidizers for heavy duty vehicles are presented and contrasted with those typical of
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light-duty applications. Several potential heavy-duty trap-oxidizer systems are
described, and rough estimates of the capital and operating costs of each system are
given.
Chapter Six deals with diesel fuels and related issues, concentrating on the effects of
changes in fuel quality on emissions. Issues addressed include the progressive deteriora-
tion in average cetane numbers, sulfur content, and other indices of diesel fuel quality,
the effects of these changes on emissions levels, and possible EPA actions to reduce
these effects. Other issues addressed include the effects of sulfur in diesel fuels on trap-
oxidizer development, and the possible synergetic improvements in acid deposition,
human exposure to SO2 and sulfates, and reductions in diesel odor, hydrocarbon, and
particulate emissions which might be made possible by reducing the fuel sulfur content.
Feasible emissions standards for heavy-duty diesel engines are discussed in Chapter
Seven. This chapter includes a brief discussion of present and proposed heavy-duty regu-
lations, then builds on the data of Chapters Four and Five to derive feasible NOx and
particulate standards corresponding to five different emissions control scenarios, ranging
from requiring good present technology to requiring the most stringent control achiev-
able, regardless of cost. The possible effects of emissions averaging and subdivision of
the heavy-duty diesel class into more homogeneous subclasses are also discussed, with
special attention to their implications for the cost and feasibility of compliance. This
chapter concludes with recommendations for numerical standards.
Chapter Eight, the last chapter, provides a summary and restatement of the major points
of the previous chapters, and lays out in condensed form the major conclusions of the
study. Chapter Eight is followed by an appendix discussing the external reviewer's
comments on the draft of this report, the authors' replies to those comments, and the
changes, if any, made in the final version as a result. Other appendices include a biblio-
graphy of related material and a list of organizations and individuals who provided infor-
mation during the study.
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1J LIMITATIONS AND CAVEATS
No study of an area as broad, involved, and complex as heavy-duty emissions control can
hope to be able to address all of the issues involved adequately, and the present study is
no exception. This section describes a few of the more significant areas in which the
analysis is felt to be incomplete, or where more study is felt to be required.
The limited time and funds available for this study, and the scarcity of the requisite
data, have prevented a complete analysis of the effects of subdivision of the heavy-duty
class and/or emissions averaging on individual manufacturers. The subdivision scheme
proposed is itself rather crude; it is based on the available data and on reasonable esti-
mates of usage patterns, vehicle production, and related information, and is intended as a
"first-cut" approximation. Better classification schemes (such as by actual usage
pattern) are certainly imaginable, and a more detailed study of usage and production
patterns would certainly be desirable before actually implementing such a scheme. More
study of issues such as useful life, average mileage before rebuild or overhaul, and the
division of miles travelled between urban and rural areas for each class would also be
desirable. Some of these concerns are now being addressed in research sponsored by the
EPA and other interested parties (EPA, 1984b; Energy-Environmental Analysis, 1983).
The analysis of engine-out pollutant emissions (emissions as they leave the engine, before
the trap-oxidizer or other external treatment) is necessarily somewhat limited in scope.
Pollutant formation and destruction in the cylinder are closely linked to the overall pro-
cess of diesel combustion — one of the most difficult and intractable problems in com-
bustion science. Despite recent theoretical advances, practical design of diesel combus-
tion systems is still largely a matter of "cut and try." A complete study of this complex
and poorly understood area would require many man-years and volumes of reports, and
would be well beyond the time and effort available for the present study. The analysis
presented in Chapter Four is thus a somewhat superficial and primarily phenomenological
aproach to a very subtle and complex area. The conclusions arrived at should be con-
sidered in that light, and treated with appropriate caution.
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2.0 CLASSIFICATION OF HEAVY-DUTY ENGINES AND VEHICLES
For regulatory purposes, the EPA divides highway vehicles into two major classes: light-
duty vehicles and heavy-duty vehicles. Light-duty vehicles include passenger cars and
trucks with rated gross vehicle weights (GVW) less than 8,500 pounds. Trucks and buses
with a rated GVW of 8,500 pounds or more are classed as heavy-duty vehicles. This
report is concerned only with the latter group.
The most commonly used truck and bus classification scheme is that of the Motor-
Vehicle Manufacturers' Association (MVMA), which divides vehicles into eight classes
based on rated GVW. This classification is shown in Table 2.1. As this table indicates,
MVMA class 2 — covering vehicles rated between 6,001 and 10,000 pounds — is further
divided this class into class 2A (consisting of vehicles from 6,001 to 8,500 pounds) and 2B
(containing vehicles from 8,501 pounds to 10,000 pounds) to separate vehicles classed as
light and heavy-duty by the EPA.
While it is simple and widely used, the MVMA classification is unsatisfactory for this
discussion. At the lighter end, MVMA classes 3 through 5 are almost unpopulated; while
class 8 lumps together many different kinds of heavy trucks, some of which have very
different design and usage characteristics. For the purposes of this analysis, the classifi-
cation scheme shown in Table 2.2 will be used instead. This scheme divides heavy-duty
vehicles into four classes: light-heavy vehicles (generally pickup trucks and vans);
medium-heavy vehicles (which includes most trucks between about 10,000 and ^5,000
pounds GVW); line haul trucks (generally large, powerful tractor-trailer combinations of
50,000 pounds and up); and transit buses, which are included because of their special
significance as particulate emitters. The medium-heavy class can be further divided into
a lighter fraction (roughly MVMA class 6) which has traditionally used gasoline engines,
and a heavier fraction (MVMA class 7 and the lower portion of class 8), which has tradi-
tionally used a large fraction of diesel engines.
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Table 2.1: Truck Classifications as defined by the
Motor Vehicle Manufacturers Association
Class Rated Gross Vehicle Weight (lb)
Light-Duty
1 0-6,000
2A 6,001 -S,500
Heavy-Duty
2B 8,501 - 10,000
3 10,001 - 14,000
4 14,001 - 16,000
5 16,001 - 19,500
6 19,501 - 26,000
7 26,001 - 33,000
8 33,001 and up (to about 80,000 pounds
for a truck-trailer combination,
more for double-trailers)
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Table 2.2: Truck Classifications Used in This Study
Class Description
Light-Heavy Heavy pickup trucks, vans, panel vans, and recreational vehicles from
8,500 to 1^,000 pounds GVW
Medium-Heavy Straight trucks and all other heavy duty vehicles above H,000 pounds
GVW except transit buses and line-haul trucks
Line-Haul Trucks Large, heavy, very powerful trucks used for long-distance freight and
similar applications. Almost all are heavy tractor-trailer and double-
trailer combinations, generally of 50,000 pounds or greater GVW.
Transit Buses Buses used for intra-urban mass transit and related applications
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PARTICULATE CONTROL
TECHNOLOGY AND
PARTICULATE EMISSIONS
STANDARDS FOR HEAVY-
DUTY DIESEL ENGINES
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APPROVED FOR RELEASE
***** FINAL REPORT *****
PROJECT MANAGER
CORPORATE QUALITY CONTROL REVIEWER
c. ^ j
\ -v./
r?t. n/zi/rt
Christopher S. Weaver, P.E. Date
Senior Engineer
Craig Miller, Ph.D. Bate
Principal, Engineering
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Another classification scheme for heavy-duty engines, as opposed to vehicles, has been
promulgated by EPA as part of its regulations concerning the definition of an engine's
useful life. This scheme divides heavy-duty engines into three groups: light-heavy, with
a designed useful life of 110,000 miles; medium-heavy, with a designed useful life of
185,000 miles; and heavy-heavy, with a useful life of 250,000 miles. The EPA-defined
light-heavy and medium-heavy engine classes are used almost exclusively in the class of
vehicles which ERC has defined as light-heavy and medium-heavy duty, respectively, but
the medium-heavy class defined by ERC would also include some larger engines that EPA
has defined as heavy-heavy duty. These would be the engines used in large straight
trucks such as dump trucks, and in those semi-tractor vehicles which are not used in line-
haul service.
It should be noted that the classifications proposed by ERC are primarily on the basis of
vehicle design and function, not weight, although GVW serves as a useful criterion for
separating the different classes. This is because it is the vehicle's design and function
which control the patterns of its use, and thus the feasibility and cost-effectiveness of
emissions standards. The medium-heavy class is intended to include those trucks which
normally operate in stop-and-go driving, delivery service and so on in urban environ-
ments, while the line-haul class includes trucks used primarily for intercity freight haul-
ing and related tasks. In the same way, light-heavy duty vehicles are distinctly differ-
ence both in form and in typical functions from either of the other classes, and transit
buses, of course, have their own distinctive form and operating pattern.
Since this classification scheme was first described (Weaver, 1984a), it appears to have
found favor with a number of analysts. Energy and Environmental Analysis (1983), in
work for the MVMA, used a similar classification (based on the MVMA classification
scheme, but with separate consideration of very heavy (i.e., line-haul)) trucks in develop-
ing estimates of the size and composition of the future heavy-duty fleet. EPA, in its
recent proposals for NOx and particulate regulations (EPA, 1984a), also proposed possible
special treatment for line-haul trucks (defined as those heavier than 50,000 pounds GVW)
and transit buses.
The special characteristics of each of these classes of vehicles are discussed at greater
length below.
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2.1 LIGHT-HEAVY DUTY VEHICLES
This class includes mainly heavy pickup trucks and vans up to about 10,000 pounds
(MYMA class 2B), with a sprinkling of other types such as panel vans. Diesel-powered
recreational vehicles over 8,500 pounds GVW are also included in this class, but the num-
ber of such vehicles is small. Statistically, these vehicles are usually lumped in with the
more numerous light trucks in class 2A, so separate data on specific ownership and
operating patterns for this group are scarce. It appears likely that almost all of these
vehicles are used for commercial purposes, and that they are used primarily in urban
areas. Except for being owned primarily by commercial enterprises rather than by
individuals, the operating patterns in this group are probably rather similar to those of
light trucks.
The technical characteristics of light-heavy duty vehicles also resemble those of light-
duty trucks rather than the heavier vehicles with which they are classed for regulatory
purposes. Diesel engines used in this class are the GM 6.2 liter engine and the Interna-
tional Harvester 6.9 liter, both of which are high-speed, indirect injection engines
derived from light-duty diesel technology. However, a number of small, high-speed
direct-injection diesel engines have recently been developed, and they will probably see
extensive use in this class. Two manufacturers (Cummins and Isuzu) have already intro-
duced small advanced DI engines suitable for use in light-heavy vehicles.
The production process for light-heavy vehicles also resembles that for light-duty
trucks — they are mass-produced in a few standard configurations rather than in the
semi-custom manner typical of medium-heavy and line-haul trucks. The major manufac-
turers of these vehicles are Ford, General Motors, and Chrysler, all of which also produce
light-duty trucks.
2.2 MEDIUM-HEAVY DUTY VEHICLES
This class includes all of what most people think of as "heavy-duty trucks", except for
the large tractor-trailer rigs. It includes all of the "straight trucks", as well as the
lighter tractor-trailer combinations and some special types such as school buses. This
class is characterized by a great diversity of manufacturers, sizes, body styles, and
engine and powertrain options. This diversity is needed because of the widely diverse
applications to which these trucks are put.
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This class can be further separated into a heavier fraction in which diesel engines have
been commonplace for many years, and a lighter fraction which has traditionally used
gasoline engines. Engines used in the heavier fraction are typically large direct-injected
medium-speed engines very similar to those used in line-haul trucks. These engines
would fall into EPA's "heavy-heavy" category; they are extremely durable, efficient, and
powerful, but also quite costly. Recent years have seen the introduction of a number of
lines of engines which are designed to compete directly with gasoline engines in the
lighter fraction of this class. These engines (which EPA classes as "medium-heavy") are
typically smaller, less powerful, and less durable — matching the lighter weight and
lower lifetime mileage of the smaller trucks — and significantly lower in cost. The
technologies used, in the two classes are rather similar, however, and for many purposes
the two groups of engines — and the trucks they are sold in — can be discussed together.
The manufacturing process for these trucks (and also for line-haul trucks) is sharply dif-
ferent from that for light-duty vehicles. A light-duty vehicle is sold as a package of
body, chassis, engine, and drivetrain — at most, the consumer may be able to select from
two or three different standard engines or transmissions. In the larger trucks, however,
the common American practice is for the purchaser to specify which engine, transmis-
sion, rear axle unit, and body he desires, choosing from a wide selection of types
produced by many different manufacturers. It is for this reason that EPA regulates the
pollutant emissions of heavy-duty engines rather than heavy-duty vehicles. This "mix and
match" approach to truck specification greatly complicates the introduction of new
emission control technologies, especially trap-oxidizers.
2.3 LINE-HAUL TRUCKS
Line-haul trucks are the largest and most powerful highway vehicles. Essentially all are
large tractor-trailer and double-trailer combinations. These trucks are used primarily in
intercity freight trucking. Typical lifetime mileage such a truck is in the range of
500,000 miles — much greater than that for any other class of highway vehicles. Unlike
the other classes of heavy-duty vehicles, these trucks accumulate most of their mileage
outside of urban areas, a fact which has important implications for the cost-
effectiveness of emissions controls.
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The production process for these trucks is similar to that of the medium-heavy trucks
discussed above- The purchaser normally specifies the make and model of engine,
transmission, rear axle, and other major equipment to be packaged with a given cab and
chassis assembly. Engines in this class are almost universally large turbocharged diesels,
designed for maximum fuel economy, power output, and durability. Due to the long dis-
tances they travel and the very large loads they haul, these trucks account for a large
fraction of the total heavy-duty fuel consumption. Fuel-economy is very important —
the lifetime cost of even a one percent increase in fuel consumption for one of these
trucks is more than $1,000.
One notable feature of this class is that many line-haul trucks are owned by individual
owner-operators, rather than by commercial enterprises and governments as most
smaller trucks are. Most line-haul trucks which are not owned by individuals are owned
by large fleets. Both groups of owners — but especially the individuals — are highly
independent and quite sensitive to their own economic interests, especially as these are
affected by government regulations. Given the high cost of even a slight degradation in
fuel economy, and the inevitable effects of forseeable emission control technologies in
reducing fuel economy, this can be expected to pose a major enforcement problem.
2A TRANSIT BUSES
Transit buses, because of their numbers and the circumstances of their operation, are
major contributors to urban particulate levels. In addition, their ownership and use pat-
terns make them especially good targets for emissions control. For these reasons, they
are considered here as a separate class, rather than being lumped in with medium-heavy
vehicles.
The operating and ownership patterns for transit buses are almost polar opposites to
those for line-haul-trucks. They operate almost exclusively in urban areas, generally in
the most congested portions. Furthermore, the typical transit-bus operating cycle is one
of the worst conceivable from the standpoint of particulate emissions. Because of this,
transit buses have been estimated to account for nearly forty percent of the total diesel
particulate measured in some cities (Chock et alia, 198
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In addition, although there is still considerable customer choice as regards engines and
equipment, transit buses overall tend to be much more uniform than are, say, the
medium-heavy duty trucks. At present, only one configuration — with a basically boxy
body and the engine in the rear — is common, and most U.S. buses are made by a few
manufacturers and equipped with one of only a few engine models, this relative homo-
geneity would greatly simplify the implementation of advanced control technologies.
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3J0 COMMERCIAL FEASIBILITY AND HEAVY-DUTY ENGINES:
REQUIREMENTS FOR EMISSIONS CONTROL TECHNOLOGIES
In order to be considered feasible for use in actual heavy-duty trucks, potential emissions
control technologies and devices must be able to meet very stringent criteria for com-
mercial feasibility. As it is used here, commercial feasibility is a much more restrictive
criterion than mere technical feasibility. In order for a technique or device to be consi-
dered commercially feasible, it must not only work, it must also have characteristics
such that manufacturers are willing to include it and guarantee it in or on their engines,
and such that the engine's purchasers are willing to tolerate it in or on the engines that
they buy. Customer acceptance is particularly important in the heavy-duty market,
since—much more than the light-duty vehicle market—it is composed of sophisticated,
economically motivated purchasers. Virtually ail purchasers of heavy-duty vehicles are
profit (or, in the case of governments, cost-effectiveness) oriented, and virtually all have
extensive knowledge of and experience in the heavy-duty field. Many purchase tens of
trucks or buses at a time, and can afford to carry out extensive pre-purchase compari-
sons. Under these circumstances, any attempt to market immature, ineffective, ineffi-
cient, overpriced or unreliable technology will result in an immediate decrease in the
manufacturer's market share, and in lasting damage to the manufacturer's reputation.
A related issue which needs to be considered in assessing commercial feasibility for
heavy-duty engines is the potential competition from rebuilt and reconditioned older
engines. It is common practice to overhaul and rebuild heavy-duty truck engines, replac-
ing those parts—such as cylinder liners, bearings, and injection nozzles—which are sub-
ject to significant wear or deterioration. Other parts, such as fuel pumps and engine
accessories, can also be replaced as needed. By such means, a properly maintained truck
engine can be kept running indefinitely.
In the past, the steady increase in the efficiency and performance of new engines, cou-
pled with the increasing cost of maintaining an engine as it ages, have reduced the incen-
tives to continue rebuilding an engine beyond the first one or two overhauls. Should
over-stringent or premature emissions regulations result in significant degradation of
new-engine performance, fuel economy, or other features, however, this situation would
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be reversed. The results would be increased rebuilding of present engines, a marked drop
in new-engine sales, and (since older engines have higher emissions levels) a net loss in
emission control from what might be attainable through less stringent regulations. Thus,
in assessing commercial feasibility, it is necessary to consider not only the competition
from other engine manufacturer's new models (which would, after all, be subject to the
same constraints) but competition from rebuilt older models as well.
With respect to emissions control technologies, the major criteria for commercial feasi-
bility are effectiveness, durability, reliability, and cost, and the effects of the tech-
nology on the fuel economy, performance, durability and reliability of the engine. In
addition, it is necessary to consider effects on safety, maintenance requirements, tamper
resistance, weight and bulk, ease or difficulty of integration into diverse styles and
models of trucks, ease of manufacturing and installation, and environmental effects—the
latter including both increases in unregulated pollutants and effects on other regulated
emissions. The important considerations in each of these areas are discussed in the fol-
lowing sections.
Effectiveness
It hardly needs to be stated that any commercially feasible emissions control technology
must be effective—that is, it must enable the engine on which it is installed to meet the
applicable standards for pollutant emissions, with an adequate margin for variation in
production and for deterioration with use. Since the numerical levels of future NOx and
particulate standards have not yet been set, no specific numerical criteria for effective-
ness can be defined. The next two chapters of this report, however, will deal extensively
with the relative effectiveness of potential emissions control technologies, alone and in
combination with each other. Rather than define the needed effectiveness by reference
to the standards, this report will, in Chapter 7, attempt to define feasible standards by
reference to the attainable effectiveness of control.
Durability
Durability requirements for heavy-duty engines vary widely. Typical mileage to
replacement or overhaul for light-heavy duty engines and vehicles is in the range from
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100,000 to 150,000 miles. Most light-heavy engines are probably replaced (along with the
rest of the vehicle) at the end of this time rather than being overhauled. In contrast,
engines for medium-heavy, line-haul, and transit bus applications are normally over-
hauled—with replacement or refurbishing of parts subject to wear—rather than
replaced. The average mileage-to-first-overhaul varies widely, depending on the specific
application. A recent survey of truck owners by the Engine Manufacturers Association
(EMA, 1982a) showed average mileage-to-first-overhaul of 303,000 miles for trucks used
in long-haul fleet service (these would be almost entirely hne-haul engines) and 221,000
miles for class 6, 7, and 8 trucks used in other fleet service (these would be predomi-
nantly medium-heavy engines). For operator-owned trucks (almost all of which are in
line-haul operation), average mileage to first rebuild was 312,000 miles. The total life of
the engine could be expected to be at least twice these values.
In order to be considered commercially feasible, any emissions control device which had
a significant effect on the engine's performance would need to be able to last for at least
the typical mileage before first overhaul, while requiring only routine maintenance and
service during that period. Preferably, of course, it should be able to last the entire
lifetime of the engine without requiring rebuild or replacement. This is especially true
for any "add-on" emissions control devices. Since these would not add to the perfor-
mance of the engine, they would be unlikely to be rebuilt or replaced voluntarily.
It is instructive to consider the acceleration smoke-hmiter as an example of an add-on
pollution control device which is in present use. These are commonly found on turbo-
charged engines, to compensate for turbocharger lag during low-speed acceleration.
They limit fuel flow to the engine in order to reduce the opaque black smoke on initial
acceleration which is among the most noticable aspects of truck operation in traffic.
This increase in sociability yields little direct benefit to the owner, and is achieved at
some cost in acceleration and driveability. An EMA study of maintenance practices
(EMA, 1982a) indicates that only 53 percent of truck fleet operations routinely inspect
and maintain this device, and that only 17 percent of owner operators (who are presum-
ably less concerned about public relations) do so. A device controlling a less visible
pollutant could be expected to receive even less maintenance. For this reason, it is
essential that such a device should be highly durable.
In addition to the time and mileage between overhauls, it is also necessary to consider
the effects of EPA in-service deterioration and warranty regulations on durability
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requirements for commercial feasibility. EPA issued final rules on these issues in late
1983 (EPA, 1983a). These rules require emissions compliance over the full useful life of
the engine, defined as 110,000 miles or eight years for light-heavy duty engines, 185,000
miles or eight years for medium-heavies, and 290,000 miles or eight years for line-haul
("heavy-heavy") engines. These correspond closely to the actual expected life of a light-
heavy duty engine, and to the expected mileage until first overhaul for the medium-
heavy and line-haul groups. Engines are required to be certified for, and would be sub-
ject to recall for, the entire period. However, manufacturer's emissions warranties are
only required to extend to five years/50,000 miles for light heavies and five years/
100,000 miles for medium-heavy and line-haul engines. In addition, while emissions com-
pliance is required over the entire period, EPA will not test for compliance any engine
which has expended more than 75 percent of its expected life.
In addition to the overall useful life requirements, EPA has established minimum required
maintenance intervals for a number of emissions-related components, including EGR
valves, fuel injectors, and turbochargers. A commercially feasible emissions control
device would need to be durable enough to meet these requirements if they were
imposed.
Reliability
Reliability requirements for heavy-duty engine emissions controls will vary, depending on
the nature of the control device, possible modes of failure, and the effects of failure on
the engine and vehicle. In the extreme case, where a failure could destroy the engine
and/or the vehicle, nearly absolute reliability would be required. Some designs of trap-
oxidizers (where a regeneration failure might lead to a fire) would fall in this category,
as would some designs of exhaust-gas recirculation systems. A number of California
model medium-duty engines were recently destroyed when failure of the EGR system led
to overloading of the lubricating oil with soot, destroying the oil's lubricating proper-
ties. Given reasonable design, however, such drastic effects should be few.
In the more common case, where failure of the emission control system would cause the
engine either to run very poorly or not at all, a slightly lower level of reliability could be
tolerated. Electronic engine controls and some trap-oxidizer designs would fall into this
category. Failure of this type of device would cost money and time, but would not have
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catastrophic effects. However, frequent failures of this type would not only cost large
amounts of money, they would probably lead to extensive tampering to remove the
offending devices, thus negating their beneficial effects on air pollution.
A third category of devices consists of those which would have minor effects on the
engine, either positive or negative, should they fail. Good reliability will be required for
these devices in order for manufacturers to be able to meet in-service emissions
requirements, since the devices are unlikely to be repaired if they fail. EGR systems
which fail closed rather than open and transient smoke limiters would fall into this cate-
gory. The infrequency of maintenance of in-service smoke limiting devices has already
been discussed above.
Initial Cost
The tolerable level of increase in initial engine costs due to emission controls will vary
depending on the characteristics of the service for which that engine is designed. For
light-heavy and the smaller of the medium-heavy engines, the allowable increase in cost
is fairly small. This is because these engines compete directly with cheaper gasoline-
fueled engines, and their shorter lifetime mileage means that the owner has less oppor-
tunity to recover the increased purchase cost in the form of savings on fuel. The cost
premium for a light-heavy duty diesel engine over a comparable gasoline model was
approximately $2,000 in early 1984. The fuel savings over 120,000 miles (assuming 10
MPG for the gasoline version, 13 MPG for the diesel version, and both diesel and gasoline
at $1.30 per gallon) is approximately $3,600, for a net (undiscounted) life-cycle savings of
$1,600. Increasing the base cost of the engine by as much as $1,000 would significantly
reduce these savings, and raising it by $2,000 would remove essentially all motivation to
purchase a diesel.
On the other hand, it is almost impossible to imagine a cost increase which would make
gasoline engines competitive with diesels in line-haul service. The increased fuel costs
for such an engine, over its full 500,000 mile life, would amount to more than $50,000.
The constraint on increasing costs in engines for line-haul service lies in the potential
competition from rebuilt engines. Assuming no significant fuel-economy penalty for the
new emissions control, the added cost of controls could probably reach $4,000 to $8,000
before new engines ceased to be competitive. This is not to say, however, that such
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increases would be desirable, or that they might not cause considerable economic hard-
ship in the truck, truck engine, and trucking industries.
Effects on Fuel Economy
From the engine purchaser's standpoint, fuel economy is probably the most important
characteristic of the diesel engine. This is especially true in line-haul service and in the
heavier end of the medium-heavy duty class. The additional fuel cost over a vehicle's
lifetime due to the one percent loss of fuel economy is of the order of $1,000 to $2,000
for a line-haul truck. Thus, the potential economic loss from even a slight fuel-economy
penalty is very large. In addition, given the enormous economic incentives, it would be
surprising if any emissions control device which resulted in a significant fuel economy
loss—and which could be tampered with—were not to undergo extensive tampering. This
potential exists with trap-oxidizers and EGR valves, and possibly with injection timing
controls as well.
For light-heavy and the lighter medium-heavy engines, the constraints of fuel economy
are not so binding. The major competition for these engines is with heavy-duty gasoline
engines, over which they enjoy a fuel economy advantage of 25 to 35 percent (Jambekar
and Johnson, 1981). These engines generally travel about 100,000 to 150,000 miles over
their lifetimes, and are not usually designed to be rebuilt. The cost of a one percent loss
in fuel economy over one of these engine's lifetimes is of the order of $150—much less
than that for the heavier trucks. In the case of the light-heavy duty engines, the manu-
facturers have already decided to accept a significant fuel economy penalty—of the
order of 5 to 15 percent—in return for the better performance, compatibility with gaso-
line-engine drivetrains, and lower emissions offered by the prechamber (indirect injec-
tion) design. Thus a fuel-economy penalty of the order of 10 percent for the lighter DI
engines, and perhaps a few additional percent for ID1 engines could be tolerated. It
should be borne in mind, however, that these penalties—while tolerable—are by no means
desirable, and that they would result in significant increased costs to the nation, both
directly and in the form of political risks due to increased oil imports.
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Effects on Engine Durability and Reliability
After fuel economy, the most important characteristics of the diesel engine from its
purchaser's viewpoint are its durability and reliability in heavy-duty service. This is
especially true of line-haul engines, and of the heavier portion of the medium-heavy
class. Heavy-duty engines are quite expensive, ranging from $5,000 to $15,000 or more,
and the vehicle owner expects to get his money's worth. Truck downtime due to failures,
or to preventive maintenance needed to avert failures, is also very expensive, and
failures can result in missed deadlines, spoiled goods, and heavy penalties as well. Thus
any emissions control modifications which are perceived as significantly reducing either
the expected lifetime or the overall reliability of the engine will be strongly resisted, and
are likely to undergo extensive tampering and/or removal in service.
The major durability concerns associated with emissions control technologies are with
EGR, which is widely perceived as increasing abrasive wear in the cylinder and increasing
soot loading of the engine oil, reducing its ability to lubricate. Severe injection retarda-
tion could also harm durability by increasing exhaust gas (and thus exhaust valve)
temperatures. Major reliability concerns exist with electronic engine controls, trap-
oxidizers, and EGR systems.
Effects on Performance and Driveability
Performance and driveability are very important considerations for all classes of heavy-
duty diesel engines, from the light-heavies to the very largest. The relatively poor per-
formance in these areas exhibited by many diesel engines (compared to their gasoline-
fueled counterparts) makes their further degradation especially problematic. As with
fuel economy, durability, and reliability, any emissions control technologies leading to
significant degradation in these areas can be expected to meet with widespread tamper-
ing and/or removal, or—failing that—with widespread competition from engines such as
older rebuilt diesels or gasoline engines which are not burdened with such devices.
"Performance" in a heavy duty engine generally translates as "rated power". "Drive-
ability" is a somewhat fuzzier term: "good driveability" generally means both high
steady-state torque at low engine RPM and good transient acceleration characteristics
(i.e., minimal turbocharger lag). Performance is a major concern throughout the heavy-
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duty engine market, but especially in the heavy classes, since vehicles in these classes
generally have little or no power to spare. Driveability is of great concern for light-
heavy and medium-heavy duty trucks, and for transit buses, since their duty cycles
involve a great deal of starting, stopping, and acceleration. Engines designed for this
service generally have a fairly wide speed range, and good low-speed torque characteris-
tics. Driveability is less of a concern for line-haul trucks, since these vehicles are sel-
dom operated in stop-and-go mode. However, there has been an increasing trend toward
sacrificing driveability for fuel economy in line-haul engines, so these engines also have
less driveability to give up, if that were necessary.
Both driveability and performance are closely related to the combustion system in the
engine, so that any emissions control technologies affecting combustion (i.e., all engine-
out technologies) would be expected to affect driveability and performance as well.
Injection timing retardation (especially static retard systems), exhaust-gas recirculation,
and transient smoke limiting devices all have negative effects on driveability, and the
former two can also reduce maximum power. With present-day mechanical controls for
these systems, the manufacturer must walk a fine line between violating emissions
standards and degrading performance and dnveability to an unacceptable extent. Elec-
tronic controls for the engine governor, EGR modulation, and dynamic injection timing
controls are expected to improve this situation significantly—making possible both
improved driveability/performance and decreased emissions.
Maintenance Requirements and Tamper Resistance
Maintenance requirements and tamper resistance are really two sides of the same coin.
Almost ail heavy-duty vehicles and engines are subject to planned regular maintenance at
moderately frequent intervals, in order to protect the owner's investment and minimize
operating costs due to failures, increased fuel consumption, etc. Any emissions-related
device which is important to engine operation or efficiency can reasonably be expected
to receive regular inspection and servicing as needed, as long as "as needed" is not un-
reasonably often. On the other hand, any emissions control device which substantially
harms the vehicle's performance, fuel economy, or driveability—or which poses a real or
perceived threat to its durability or reliability—can be expected to receive active "dis-
maintenance", i.e. tampering and/or removal. The major concerns in this regard are for
trap-oxidizers and EGR valves, both of which would degrade performance while providing
no perceived benefit to the owner or operator.
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I
Safety
^ Any significant saiety risks due to emissions controls would, of course, be highly undesir-
able—the more so as many trucks are used to transport hazardous materials, and thus an
I accident might have very far-reaching consequences. The major concerns in this regard
are with trap-oxidizers. Depending on the regeneration system used, these might present
| either a slight or a significant fire hazard. The most serious concern is with the burner
type of regneration system, which would inevitably result in some increased risk, due to
| the close proximity of the fuel and fuel piping to the hot trap and exhaust system. Addi-
tive reservoirs for on-board storage of orgaaometaLlic fuel additives (tor a self-
regenerating trap-oxidizer system) would also pose significant safety questions. Same
' candidate orgar.ometallic additives are quite toxic, and many are highly flammable as
| well.
Another, less urgent safety concern would lie in the possibility of sudden engine failure
| or power loss due to the failure of an emissions control device. This concern is closely
tied to the questions of reliability discussed above. Perhaps the major worries in this
1 regard would be with electronic control systems. These systems characteristically fail
suddenly, unlike mechanical control systems, where steady degradation rather than sud-
den failure is more common. Design solutions to this probiemj incorporating a "limp
home" capability, have been reported in the literature, and such approaches presently
seem to be favored.
I
Weight and Bulk. Eliects on System Integration
The weight and bulk of an emissions control device are important, both because of their
J
direct effects in reducing the payload carriable by the vehicle and because of the prob-
lems they can introduce in system integration. This problem is greatest for engine-
mounted or engine-compartment-located technologies; the engine compartments of most
trucks and buses today have very little room to spare- More room is generally available
elsewhere on the truck or bus, but locating an emissions-control device of! ol the engine
greatly increases the problems in system integration. This is because the engine manu-
facturer normally supplies only the engine itself; the rest of the truck is provided by the
vehicle manufacturer. In the common case where a vehicle manufacturer offers four or
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five alternative engines for the same truck, he might find himself having to find room for
and install four or five different off-engine control devices.
The emissions control technologies causing the greatest concern in these areas are trap-
oxidizers (because of their necessarily large size), charge-air cooling (which requires
either a larger radiator or a separate heat-exchanger), and electronic controls (which
may need to be cab-mounted, due to the hostility of the environment in the truck's
engine compartment).
Manufacturability and Ins tall ability
The ease or difficulty of producing and installing a given pollution control device, or in-
corporating modifications for pollution control in the engine, will clearly have a signifi-
cant effect on the ultimate cost of the pollution control, as well as on the lead-time
required to bring it into production.
A number of possible emissions control technologies raise significant questions of manu-
facturabiiity. Perhaps the most serious questions are raised by advanced fuel-injection
systems. Present-day fuel injection pumDs are marvels of precision manufacture — con-
suming significant power and producing pressures of 500 to 1,000 atmospheres, with
millisecond timing, for extremely lengthy periods in a hot, vibrating environment.
Future designs will also need to incorporate provisions for dynamic timing control, fast-
response electronic control of fuel quantities, and even higher pressures. The Drecision
tooling required to manufacture such devices is extremely expensive, and must be spe-
cial-ordered as much as three years in advance. This will obviously have a major effect
on the production lead-time.
Environmental Effects
It need hardly be stated that the application of a control technology for one pollutant
should not result in the production of other, worse emissions. The major concerns in this
regard are the tradeoffs between NOy emissions and particulates and/or gaseous hydro-
carbon emissions. This report deals with those tradeoffs in considerable detail in Chap-
ters and 7. In general, the very presence of the standards minimizes the potential con-
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cerns in this area for regulated pollutants, since a standard-meeting engine will by
definition not emit excessive quantities of regulated pollutants.
Among unregulated emissions, the major concerns raised by NOx and particulate control
technologies are potential increases in sulfate emissions due to the oxidation of SO2 in
catalytic trap-oxidizers. Although sulfates are measured as particulates, and thus regu-
lated to some extent by the particulate emissions regulations, they are probably much
more damaging than other diesel particulate materials. Some forms of exhaust catalysts
used for HC and CO control have also been shown to increase mutagenic activity in par-
ticulates significantly (Scholl, et alia, 1982), which could conceivably be a problem. EPA
regulations require that pollution control techniques for regulated pollutants should not
increase emissions of specified unregulated pollutants, of which sulfate is one. If it were
strictly interpreted, this provision could prevent the application of catalytic trap-
oxidizers.
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~.0 ENGINE-OUT EMISSIONS CONTROL FOR HEAVY-DUTY ENGINES
Pollution-control techniques for motor vehicles can generally be divided into the "engine-
out" and "aftertreatment" approaches. The engine-out approach (also called "in-cylinder"
control) attempts either to prevent pollutants from being formed in the first place or to
increase their destruction by chemical processes inside the engine. The aftertreatment
approach uses a separate processing system in the exhaust pipe to remove or destroy
pollutants before they are emitted to the atmosphere.
"Engine-out" pollutant emissions are those pollutants which exist in the exhaust as it
leaves the engine, before passing into the exhaust pipe and the purview of any after-
treatment technologies which may be in use. At present, engine-out emissions for diesel
engines are the same as tailpipe emissions, since aftertreatment technologies are not
used.
A number of promising techniques for reducing engine-out NOx or particulates are
known. Unfortunately, most techniques which reduce particulate emissions increase NOx
emissions, and vice-versa, so that it is difficult to achieve a significant reduction in both
NOx and particulates by engine-out techniques alone. For this reason, tcap-oxidizers (an
aftertreatment technology for reducing particulate emissions) are of increasing interest
for heavy-duty diesel emissions control. These would be used in conjunction with an
engine-out technique to reduce NOx, since there is presently no feasible aftertreatment
technique for NOx control in diesels.
This chapter discusses engine-out pollution control techniques — beginning with the
fundamental science and proceeding to a discussion of emissions control technologies.
Both technologies presently in use and the advanced technologies now under development
are discussed. Finally, the potential for emissions reduction using these techniques is
assessed, and rough estimates of the NO^/particuIate tradeoff relationships achievable
using these technologies are developed. The next chapter contains a complementary
discussion of aftertreatment technologies — specifically trap-oxidizers.
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Caveat — Engine-out control techniques affect the conditions in the cylinder during
combustion, and thus affect almost every other important aspect of the diesel engine.
These include its efficiency, durability, drivability, maximum power, and torque curve,
among other considerations. Changes in these characteristics can strongly affect the
ultimate saleability of the engine, so that engine manufacturers are continually striving
to improve the multiple tradeoffs between these different considerations. The truck
industry is highly competitive, with technological advance a major competitive tool. In
order to retain the competitive benefits of technological advance, each manufacturer
shrouds its progress in these areas in secrecy, and technical data are generally either not
available at all or available only on a confidential basis.
Because of the difficulty of obtaining data, the complexity of the interactions between
the many different characteristics, and the limited time, funds, and manpower for this
study, the discussion which follows is somewhat limited. In particular, quantitative data
on the effects of many of the technologies are lacking, and estimates of the time
required for introduction may be seriously in error. There is also a greater-than-
de sir able reliance on engineering judgment — as opposed to hard data — in estimating the
effects of these technologies on emissions levels. Despite these limitations, however,
the authors are reasonably confident of the ultimate conclusions as to achievable
emissions levels in the near term (up to about 1988), since the effects of technologies
which can be introduced in this time frame are fairly well known. Prediction of tech-
nological capabilities beyond 1989 — and especially those associated with electronic
controls — is considerably less certain.
4.1 FUNDAMENTALS OF DIESEL EMISSIONS
Pollutant formation and destruction in the diesel engine are determined by the combus-
tion process, which, in turn, is controlled by the nature of the fuel and the oxidizer, the
ambient temperature and pressure, and the process of mixing between the fuel and
oxidizer in the cylinder. The effects of all in-cylinder pollution control technologies can
be understood {at least qualitatively) in terms of their effects on these variables. This
section briefly describes the combustion process in the diesel engine, and discusses the
mechanisms of NOx and particulate formation, and the effects of changes in the combus-
tion process on these mechanisms. This discussion is necessary in order to establish a
theoretical basis for the discussion of practical emission control technologies which is
given in Section ^.2.
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~.1.1 Combustion in the Diesel Engine
From the emissions-control standpoint, the interesting portions of the diesel cycle are
the compression, fuel-injection, combustion, and power/expansion segments. In the com-
pression process, air is taken into the cylinder and compressed by the rising piston to
between about 1/15 to 1/22 of its original volume. This raises the pressure in the
cylinder to about 35 - 50 atmospheres, and the temperature to abut 700 - 800 K. Near
the end of the compression process, liquid fuel is injected from a nozzle into the hot
compressed air in one or more high-speed jets. This fuel jet entrains some of the air in
the cylinder and is heated by it, so that it evaporates rapidly. (The cylinder pressure is
generally above the fuel's critical pressure, so it is not clear that "evaporate" is the right
description — no change of phase per se is possible. The process can, however, be
thought of as the quasi-liquid supercritical fluid becoming more of a quasi-gas with the
absorption of heat.)
Two broad categories of diesel engines are defined on the basis of where the fuel injec-
tion takes place in the combustion chamber. In direct-injection (DI) diesel engines, there
is only one combustion chamber — the space between the top of the piston and the
cylinder head (this space is mostly hollowed out of the top of the piston). Fuel is injected
directly into this space, hence the term "direct" injection. In indirect-injection (IDI)
engines, fuel is injected into a separate pre-combustion chamber or prechamber, which is
connected to the main combustion chamber by a passage. This chamber is designed to
provide very rapid air motion, resulting in rapid mixing between the fuel and the air.
Figures 4.1a and 4.1b show these two arrangements.
The choice of direct or indirect injection has a profound effect on the mixing process. In
IDI engines, the rapid air motion and turbulence due to expansion through the connecting
passage produce rapid mixing. In the DI engine, the connecting passage is absent, and the
air motion is either reduced or nearly absent (for the so-called "quiescent chamber" com-
bustion system). This means that the fuel injection process must supply most or all of
the energy required for mixing, with the result that fuel injection characteristics are far
more important in DI than IDI engines.
After the beginning of fuel injection, there is a short induction time (the "ignition delay")
during which some of the fuel jet mixes with the air and undergoes chemical reactions
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prior to burning. Ignition occurs at the end of this induction time. The initial phase of
combustion is very rapid, as the fuel which had mixed with the air during the delay period
now burns as a premixed flame. After this initial "premixed burning" phase, combustion
of the remainder of the fuel is limited by the rate at which fuel and air can mix. During
this period, combustion takes the form of a highly turbulent diffusion flame. At least
75% of the fuel is burned in this "diffusion burning" phase (Plee and Ahmad, 1983). This
phase of combustion can last 40 to 50 crank-angle degrees into the power/expansion
stroke, although most combustion usually occurs between top-dead-center (TDC) and
about 20 degrees afterward.
In a diffusion flame, combustion is limited by the rate at which fuel and oxidizer can mix
to within combustible limits. Thus, conditions at the flame front are determined by the
inherent properties of the fuel and the oxidizer — not by how much fuel or oxidizer is
present in the cylinder (these quantities do, however, affect where "the flame front is
located, and how long combustion continues). The limits of combustion are determined
by the chemical nature of the fuel, the concentration of oxygen in the oxidizer, the
specific heat of the mixture, and the starting temperature and pressure. These para-
meters thus determine the conditions — especially the local temperature — at the flame
front.
Due to the cooling effect of expansion in gases, the starting temperature is itself a func-
tion of the pressure, which is a function of the crank-angle (as the piston moves
downward, the gases expand) and the amount of the fuel which has already burned in the
cylinder. Thus conditions during combustion are strongly affected by the rate of mixing
in the cylinder, and by the time at which the fuel is injected. Figure 4.2 shows the
pressure-crank angle traces for a number of different injection timings in a typical diesel
engine.
4.1.2 Particulate Emissions
Diesel particulate material has two main components — a solid core of soot and a cover-
ing layer of adsorbed or condensed heavy organic molecules (the "soluble organic
fraction"). The relative proportions of these two components differ, depending on the
engine and the operating conditions. Under most conditions, soot is the dominant com-
ponent — accounting for 50 to 90 percent of the total. The hydrocarbon content of the
particulate matter tends to be greatest at low load conditions, and decreases sharply
with increasing load (Bergin, 1983; Kageyama and Kinehara, 1982). Since these two com
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Figure 4.2: Cylinder pressure vs. crank angle for different
fuel injection timings in a direct injection engine.
(Source: Taylor and Taylor, 1961)
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ponents are formed by somewhat different processes and are to some degree indepen-
dent, they are discussed separately below.
Soot formation — Soot consists of small spherical particles of solid carbon, about 50 to
1500 Angstroms in diameter, which are often linked together in chains and clusters to
give soot its characteristic "fluffy" appearance. The size and characteristics of the soot
particles depend on the conditions of combustion; soot from diesel engines is mostly
made up oi spheres about 100 Angstroms in diameter, which may or may not be linked
into chains.
Soot forms as the result of very rapid gas-phase reactions which occur during the com-
bustion process. The first step in hydrocarbon combustion is pyrolysis — the breaking up
of the hydrocarbon molecules into smaller, more reactive fragments when exposed to
heat. If sufficient oxygen is present, these reactive fragments then oxidize rapidly to
water and CC>2 — the final products of combustion. In the absence of sufficient oxygen,
however, these fragments can undergo rapid recombination and polymerization to form
very large polynuclear aromatic molecules. These molecules then continue to grow by
coalescence and further polymerization into soot particles. The initial stages of this
process are comparable in speed to the combustion reactions themselves, so that soot can
be formed as an intermediate product even in the presence of a fair amount of oxygen.
Figure ^.3 shows the major stages and time-scales an the process of soot formation in the
diesel.
The nature of mixing-controlled flames such as that in the diesel engine is that there is a
fuel-rich mixture on one side of the flame front, an oxidizer-nch mixture on the other
side, and a high temperature at the flame front itself, where the two mixtures are diffus-
ing together. This implies that there will always be an oxygen-deficient mixture on the
fuel side of the flame front, and that this mixture will be exposed to temperatures high
enough to promote pyrolysis. The formation of soot is thus inherent to diffusion flames
in hydrocarbons. However, the specific conditions of temperature, pressure, chemical
composition of the fuel, the length of time the fuel is exposed to high temperatures, and
the volume of mixture exposed can all affect the amount of soot formed, and thus the
amount emitted.
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0.10
0.01
a ODi
CRANK ANOŁ OUNCE
I 10 100
T
Coalescent Coagulation
& Surface Growth
Spherule Identity Established
free Motacul* Regime (Kfl > 10
Continuum Regime (K < 1)
n
10
Chain-farming Coagulation
fc Surface Growth
Formation Complete'
10" 10' 10' 10
TIME AFTER LOCAL HUOEAHQN W
Figure Ł.3: Soot nucleation and growth stages under diesel engine
conditions (Source: G.W. Smith, 1981).
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Because the soot-forming reactions occur very rapidly and close to the flame front, soot
formation is primarily affected by changes which affect conditions at the point of com-
bustion. Data on these effects came primarily from laboratory studies at low pressures,
so their applicability to the diesel engine is not completely clear. The laboratory studies
show that increasing the stoichiometric flame temperature (by increasing the oxygen
concentration in the oxidizer or using a higher-energy fuel) increases soot formation,
while decreasing it (such as by EGR) reduces the soot yield. There is probably a limit on
this effect, however, since theoretical calculations (Amann et alia, 1980) indicate that,
at chemical equilibrium, soot should not be found above a certain (pressure-dependent)
temperature. This is borne out by observations in diesels (Uyehara, 1981). High
pressures should also promote soot formation in theory, and increasing pressure does
increase the soot yield in most laboratory systems (Wagner, 1980). Premixing the fuel
with a small amount of oxygen also increases the yield, possibly due to the effect of
oxygen in promoting pyrolysis.
Taken together, these observations indicate that soot formation in the the diesel engine
should be very rapid, and that soot yield (the fraction of carbon converted to soot) should
be high. These are inherent in the diesel engine. Since the soot-formation reaction is
comparable in speed to the combustion reaction, there is little that can be done to
reduce the fuel's exposure to soot-forming conditions. The high pressures and high
temperatures in the diesel engine will also promote soot formation. Attaining a high
enough temperature to inhibit the soot formation is impossible, since a temperature
gradient will always exist between the flame front and the inner core of the fuel jet.
Soot oxidation — Engine-out soot emissions are equal to the difference between the
amount of soot formed in combustion and the amount which is subsequently oxidized. As
discussed above, soot formation is very rapid, and cannot readily be reduced. Soot oxida-
tion, on the other hand, is much slower, and much more under the designer's control. The
major variables affecting the soot oxidation rate are the temperature and the partial
pressure of oxygen (which can be approximated by the product of total pressure and oxy-
gen concentration).
Soot is a form of pyrolytic graphite, and it has been found that correlations and kinetic
data developed for larger masses of pyrolytic graphite also work well for soot. The most
commonly used correlation is a semi-empirical one developed by Nagle and Strickland-
Constable (Wagner, 1980; Amann et alia, 1980) Figure UAa shows the rate of reaction at
the carbon surface predicted by this correlation as a function of temperature and oxygen
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partial pressure, along with some experimental data which are seen to agree rather
well. Figure 4.4b shows the predicted lifetime of a 100 Angstrom soot particle under the
same conditions of temperature and oxygen pressure as in Figure 4.4a. As these figures
indicate, the oxidation rate increases exponentially with temperature, and nearly inde-
pendently of oxygen pressure, up to some limiting value. After that limiting value is
reached, the reaction is controlled by the availability of oxygen, with only minor tem-
perature effects.
The actual process of soot oxidation is affected as much by the mixing process as by the
oxidation kinetics. The initially oxygen-deficient gases containing the soot must be
brought in contact with sufficient oxygen to react while remaining at a high enough
temperature to promote reaction. The rate at which oxidation occurs in the flame is
thus determined by the pattern of mixing, and thus — for turbulent conditions such as
those in diesel combustion — by the intensity and scale of the turbulence. Theoretically,
there should be an optimimal level of turbulence — with too low a level resulting in
inadequate oxygen for complete reaction, and too high a level cooling the combustion
products by mixing them with too much air. Under practical conditions in the diesel
engine, the first consideration usually dominates — so that increasing the level of turbu-
lence increases the amount of soot oxidized (Amann et alia, 1980).
Hydrocarbons — Emissions of the organic fraction of diesel particulate matter are
related to emissions of gaseous unburned hydrocarbons. It is believed that the hydro-
carbon coating on the particles comes from the adsorption and/or condensation of heavy
hydrocarbons on the soot particles. This is borne out by recent data from Bergin (1983),
who found that with a sufficiently effective filter, the soot particles could be removed
from the exhaust. Heavy hydrocarbons then condensed independently to form a fine
aerosol. Major factors affecting the amount of organic material in diesel particulate are
the exhaust temperature (which affects hydrocarbon condensation) and the level of
gaseous hydrocarbons in the exhaust.
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•U*
I I r
cm. TiwmrwM , iO*'T, '
I
(a) Predicted oxidation rate vs. temperature and oxygen partial presure.
¦ (Source: Appleton, 1973, cited in National Research Council, 1982}
10*/T
(b) Predicted scoc-particle lifetime vs. temperature and oxygen, partial
pressure. (Source: Wagner, 1980)
I
Figure 4.4: Soot oxidation rates predicted by the Nagle and
Strickland-Constable theory vs. temperature and
oxygen content.
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Diesel engine emissions of unburned hydrocarbons appear to be derived from two major
sources: (1) segments of fuel spray which are quenched and diluted below the combustion
limit by rapid mixing or by contact with the cylinder walls; and (2) fuel which enters the
cylinder late in the combustion process due to secondary injection, dribbling from the
injecton nozzle, or vaporization of the fuel contained in the nozzle's sac volume. Hydro-
carbons from this second source can be (and have been) greatly reduced by improved
design of the fuel injection system. So, similarly, can some of the hydrocarbons resulting
from quenching against the cylinder walls. Hydrocarbons which are quenched by mixing
in the interior of the cylinder are more problematic. Reducing the mixing intensity
would reduce this quenching, but would also reduce soot oxidation and increase fuel con-
sumption.
Hydrocarbon emissions, and the hydrocarbon fraction of the particulate material, are
highest at low loads and high speeds. These combine to produce intense mixing together
with low temperatures in the cylinder. The hydrocarbon fraction of the particulate is
correlated with the gaseous hydrocarbon emissions, although this correlation is itself a
function of load {Bergin, 19S3). Both gaseous hydrocarbon emissions and the dependence
of particulate hydrocarbons on the gaseous hydrocarbons tend to decline with increasing
load. This is presumably due to the increased temperature in the cylinder, which both
promotes the oxidation of gaseous hydrocarbons and prevents their condensation onto
particles.
Effects of engine variables on emissions — The processes of soot formation and oxida-
tion, and the oxidation of unburned hydrocarbons, are all controlled by the local tempera-
ture, pressure, and the ratio of fuel to air. However, since oxidation is a slow process, it
is much more affected by the time-history of these variables thoroughout the combustion
and expansion stroke. Soot formation, in contrast, is affected mostly by the values of
these variables at the actual flame front during combustion. In general, high pressure
promotes both soot formation and oxidation, as does high temperature. Since particulate
emission represents the difference between these two processes, the effects of changes
in pressure and temperature on emissions are not readily predictable.
On the other hand, reducing the local fuel-air ratio in the vicinity of the flame front (as
by improved mixing) should both decrease soot formation and increase oxidation, and thus
could be expected to lead to an unequivocal decrease in emissions. However, more
intense mixing also results in more combustion taking place near TDC, and thus at higher
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temperature and pressure. Intense mixing could also be expected to increase the quench-
ing of unburned hydrocarbons. Increased mixing later in the cycle also reduces the
temperature of the burned products, by diluting them with cold air. This effect may
outweigh the increase in available oxygen due to mixing.
To help clarify the soot formation and oxidation process, Figure 4.5 shows three-
dimensional plots of temperature, equivalence ratio, and soot concentration vs. time
along a transect through the cylinder, measured with an in-cylinder sampling apparatus in
a direct-injection engine. This sampling procedure necessarily involves some space and
time-averaging of the results, so what is shown are volume-average properties near each
sampling point. The location of the fuel jet is near the left side of the plots.
Figure 4.5 has a number of important features. Notice that the average equivalence
ratio peaks sharply near the fuel jet shortly after injection, then falls off rather slowly
due to large-scale mixing. The temperature distribution is much more uniform, indicat-
ing that combustion is taking place more or less evenly throughout the space. Notice
also the very sharp increase in soot concentration in the area of high equivalence ratio,
following the peak temperature by about 5 degrees of rotation (equivalent to 1 milli-
second). This sharp increase is followed rapidly by an almost equally sharp decline, indi-
cating that soot oxidation is very rapid at that time. Soot concentration near the end of
combustion is a small fraction of the maximum value, indicating that total soot forma-
tion and oxidation over the cycle are nearly the same, and thus that a small change in
either one might greatly change the emissions level.
Given all of the interacting variables in diesel combustion, it is necesary to fail back on
experimental measurements to determine the effects of a given change in parameters on
particulate emissions. Fortunately, a large number of such measurements is available.
Some of the more important of these studies are discussed below.
It has been found that — other things being equal — increasing turbulent intensity in the
cylinder (and thus increasing the mixing rate) almost always reduces particulate
emissions. Dent (19&0), has reported that changes in soot emissions with turbulence can
be correlated using a parameter which is related to the turbulent intensity on the very
smallest scales, suggesting that the effects of small-scale mixing on the oxidation rate
are predominant. The rapid drop in soot concentration in Figure <*.5, compared to the
much slower drop in average equivalence ratio, also suggests that small-scale mixing of a
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(c) Sooc
Figure 4.5: Temperature, equivalence ratio, and soot concentration
history along a transect in a direct-injection diesel
engine. (Source: Aoyagi et alia, 1980)
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scale too small to be resolved by the sampling probe — is dominant. A trend toward
lower emissions with increasing turbulence level is also clear from extensive practical
development work in diesel engines.
The effects of flame temperature on particulates have been investigated by researchers
at General Motors (Plee et alia, 1980; Ahmad and Plee, 1983) and at Cummins engine {Yu
and Shahed, 1981). In the GM work, the flame temperature was varied by adding oxygen
or nitrogen of the engine intake air. It was found that total particulate emissions can be
correlated very well with the calculated flame temperature at TDC, using the Arrhenius
relation (an exponential function of temperature). This correlation is shown in
Figure The slope of this correlation line does not vary much from engine to engine,
although the intercepts are different for different engines, due to variations in engine
design.
The negative slope and exponential form of the correlation line in Figure suggest that
the predominant effect of temperature on particulate emission comes through increasing
the rate of oxidation of the soot. This increase must outweigh the increase in soot
formation due to the higher flame temperature. A possible contribution to this effect
could come from the fact that changes in flame temperature were achieved by adding
nitrogen or oxygen to the intake air, thus changing the oxygen partial pressure. This is
also suggested by the work of Yu and Shahed (1981), who found that oxygen content,
rather than flame temperature, was the more important variable in determing soot
emissions.
Particulate emissions are strongly affected by engine load, especially at conditions near
full load. This is widely ascribed to the effect on the overall fuel-air ratio, which
increases as increasing amounts of fuel are injected. On a grams-per-kilogram of fuel
basis, most engines show a rather broad minimum in particulate emissions at part load,
with an increase (especially in the hydrocarbon fraction) near zero load, and a much
sharper one (dominated by soot) as full load is approached. This behavior can be
attributed to the competing effects of temperature and oxygen availability on the soot
and hydrocarbon oxidation rate — at low loads, the low temperature attained in the
cylinder dominates, while at high loads, the lack of oxygen results in little soot oxidation
and thus greatly increased emissions.
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Figure 4.6: Correlation between particulate emissions index and
stoichiometric adiabatic flame temperature for a
number of IDI diesel engines. (Source: Plee et alia, 1980)
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To sum up, it appears that the dominant process affecting diesel-engine particulate emis-
sions is oxidation rather than formation. Changes in variables which tend to increase
both soot formation and soot oxidation have a greater effect on oxidation, while those
which would have opposite effects on the two processes seem to work mostly through
oxidation. This is partly attributable to the fact that soot formation is a very rapid
process, which takes place very close to the flame front. The conditions (and especially
the concentrations) near the flame front are determined more by the basic stoichiometry
of the reaction than by any variables under the control of the designer. In contrast, soot
and hydrocarbon oxidation are somewhat slower processes, and are thus more affected by
mixing and other variables under the designer's control. In order to minimize particulate
emissions, it is necessary to maximize the degree of oxidation, by maximizing the length
of time that the particulate is exposed to high temperatures and adequate oxygen.
Oxides oi Nitrogen (NO^) Emissions
The major constutuent of NOx is nitrogen monoxide (NO). Most of the rest is made up of
nitrogen dioxide, which is formed from NO by further oxidation. In combustion, NO
itself is generally considered to be formed through two processes. "Prompt" NO is
formed at the flame front, through mechanisms which are still not well understood, while
"thermal" NO is formed more slowly in the hot products of lean combustion. Thermal
NO is formed via the extended Zel'dovich reaction, which is given by the following
mechanism.
O + N2 = NO + N
N + O2 = NO + O
N + OH = NO + H
The equilibrium constant for this reaction is roughly proportional to the square root of
the oxygen concentration, and increases exponentially with temperature. At tempera-
tures above about 2200 K, equilibrium shifts strongly toward NO. The reaction rate also
increases dramatically at high temperatures, due to the greater dissociation of nitrogen
and oxygen as well as the temperature dependence of the rate constants. Thus at high
temperatures, near-equilibrium concentrations of NO can be formed. As the tempera-
ture drops, equilibrium shifts back toward oxygen and nitrogen, but the reaction rate
slows as well, "freezing" the NO concentration in the exhaust gases.
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In the diesel engine, most NOx is formed in or near the flame front, during the high-
pressure portions of the combustion process. This is when the flame temperature is at its
maximum (Shahed et alia, 1978). The amount of NOx formed is quite sensitive to the
flame temperature at top-dead-center, following am expression of the Arrhenius (expo-
nential) form. Figure 4.7 shows the correlation obtained by Plee and co-workers (1980)
at General Motors for a number of different IDI engines. Later work (Ahmad and Plee,
1983) indicates that the same correlation also applies well to DI engines. Similar con-
clusions for DI engines were reached by Yu and Shahed (1981).
Mixing and injection-timing effects are very important in NOx formation. Injection tim-
ing has an obvious effect on the amount of fuel burned near TDC, and thus on the total
amount of NOx generated. The effects of mixing seem to work primarily through the
same mechanism — more rapid mixing increases the amount of fuel burned near TDC,
and this effect apparently outweighs the additional cooling of the burned gases later in
the cycle. Figures 4.8a and 4.8b give an idea of the magnitude of these effects. These
figures show the calculated NOx formation rates as functions of crank angle for: (a) an
engine using a lower pressure injection system, and (b) one using a higher pressure system
(note that high injection pressure implies high mixing rate). The NOx formation curve is
a sharp spike in either case, but a significantly sharper and higher one for the higher
injection pressure, due to the fact that combustion occurs more rapidly and closer to
TDC, and thus the flame temperature is higher.
To sum it up then, it appears that the conditions for maximal oxidation of nitrogen are
very similar to those for maximal oxidation (and thus minimal emission) of soot and
hydrocarbons. NO emissions are promoted by high flame temperatures, high mixing
rates, and combustion conditions which tend to maximize the amount of fuel burned near
TDC. A high overall oxygen content is also needed, but the effects of oxygen on NO
formation are much less pronounced than for soot. At high loads, for instance, NO for-
mation tends to be greater, due to the higher temperatures and pressures attained, rather
than being reduced by the lower overall oxygen concentration.
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100
Equilibrium
fr*nd
Figure 4.7: Correlation bacween NO emissions index and stoichiometric
adiabatic flame temperature for diesel engines and gas tur-
bine combustors. (Source: Plee et alia, 1980)
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1400
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(a) High mie^tion pressure
rp*
TOC 10 10
(b) Moderate injection pressure
Figure 4.8: Temperature, equivalence ration, and NO formation rate
vs. time for different injection pressures (Source:
Shahed, et alia, 1978)
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$.1.$ The NOK/Particulate Tradeoff
From the discussion above, it is clear that the NOx/particuIate tradeoff is due to very
fundamental conflicts between the combustion conditions required for maximal oxidation
of soot and those required for minimal oxidation of nitrogen. At some point, then, any
combustion strategy which seeks to minimize soot emissions must result in increased
NOx emissions, and vice versa. However, this conflict is not absolute — some changes in
combustion conditions can be arranged so that they decrease one pollutant without dras-
tically increasing the other. By appropriate use of such techniques, it may be possible to
shift the overall NOx/particulate tradeoff curve nearer to the origin (the point of zero
NOx and particulates).
Six different generic approaches to improving the NOx/particulate tradeoff can be
imagined. These are the following:
1. Increase engine efficiency — pollutant emissions are directly related
to the amount of fuel burned. Reducing the amount of fuel burned
without changing combustion conditions (by reducing friction, for
instance) will reduce emissions.
2. Take advantage of the difference in sensitivity to overall oxygen con-
centration between NOx formation and particulate oxidation, by
increasing the overall air/fuel ratio without increasing the oxygen
concentration or temperature of the intake air. This can be
accomplished by turbocharging and intercooling, and by improvements
in engine "breathing".
3. Optimize combustion temperature to stay as low as possible on each of
the exponential curves of pollutant emission versus flame temperature,
and control other conditions in the engine to maintain this optimum
temperature as closely as possible.
4. Load-dependent control of combustion variables — brake-specific par-
ticulate emissions increase sharply at high loads (high fuel/air ratios),
while brake-specific NOx is much less affected. By changing the con-
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trol strategy from one of minimizing NOx at moderate loads to one of
minimizing particulates at high loads, it would be possible to improve
the overall effectiveness of control.
5. Selectively increase soot oxidation by increasing the radiant tempera-
ture in the cylinder. Soot particles are good radiation absorbers and
emitters, while gases are very poor ones. Increasing the radiant
temperature should decrease the cooling of soot particles by radiation
without affecting NOx.
6. Find a way to minimize soot formation. This is obviously the most
desirable approach, since if the particles are never formed they need
not be oxidized. Unfortunately, the only obvious way to do this is to
change fuels.
Due to the stringency of the presently proposed NOx and particulate regulations,
emissions control techniques based on all of these approaches are now being developed.
Since engine efficiency is already close to the best achievable, the most important short-
run gains are likely to come from the second, third, and fourth approaches. However, the
adiabatic engine (which combines the first and fifth techniques) may offer the potential
for dramatic gains in the longer term. The sixth approach - changing fuels - has also
been proposed as a solution for transit-bus emissions (Toepel et alia, 1983).
4.2 CONTROL TECHNIQUES FOR ENGINE-OUT EMISSIONS
Engine-out emissions are determined by the combustion process, which is central to the
operation of the diesel engine. Virtually every characteristic of the engine affects com-
bustion in some way, and thus has some direct or indirect effect on emissions. As a
result, the number of potential emission control techniques is very large. Table V.l lists
the major engine-out NOx and particulate control techniques which are presently either
in use or being studied for use in heavy-duty diesel engines. As the table indicates, diesel
emission control techniques can be grouped into a number of categories, such as engine
modifications, charging technologies, and so forth. The remainder oŁ this section
discusses each of these categories in turn, and describes the potential, present status,
and prospects for future development of each.
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Table 4.1
Engine-Out Emissions Control Techniques
Technology
Status
Engine modifications
Optimized airflow
Optimized cylinder pressure
Optimized combustion chamber
Optimized air swirl and spray pattern
Efficiency improvements
Fuel injection system
Higher injection pressure/injection rate
Improved precision of injection
Optimized injection timing
Variable injection timing
Charging technologies
Turbochargmg
Turbocharger/engine characteristic matching
Charge-air cooling
Advanced charging technologies
Exhaust-gas recirculation
Electronic controls
Open-loop injection timing control
Electronic EGR modulation
Electronic engine governor
Integrated electronic engine control
Closed-loop (feedback) control systems
Indirect injection
Advanced engine technologies
Adiabatic diesel engine
Turbocompounding
Organic rankine cycle power recovery
Additives and alternative fuels
Water-in-fuel emulsions
Alcohol-fuel emulsions
Fumigation with water or alcohol
Methanol fuel
Ongoing
Ongoing
Ongoing
Ongoing
Ongoing
Ongoing
Production
Production
Ongoing
Production
Ongoing
Ongoing
Development
Ongoing (one manufacturer)
Development (most manufacturers)
Application
Application
Application
Application
Development
Production
Early development
Early development
Research
Research
Research
Research
Development
Ongoing:
Production:
Application:
Development:
Research:
In use on production engines, ongoing fine-tuning/optimization.
In use on production engines, little additional potential.
In the process of being adapted for production.
Technical feasibility established, prototypes in development.
Technical feasibility not yet established, under investigation.
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ft.2.1 Engine Modifications
The phrase "engine modifications" is used here to indicate changes in the design of the
basic engine (the block, pistons, cylinder head, etc.), as distinct from changes in external
engine equipment such as the turbocharger and fuel injection system. In general, these
constitute minor improvements on designs which are already highly developed. As a
result, only incremental improvements in emissions are to be expected from this source.
Some truly radical engine design changes — such as the uncooled or "adiabatic" engine —
are discussed in a separate section on "Advanced Engine Technologies" below. These
more radical changes in engine design could result in correspondingly greater improve-
ments in emissions.
Engine design involves a series of complex tradeoffs between different variables such as
emissions, fuel economy, durability, manufacturing cost, and ease of service and repair.
To make matters worse, engine design is as much an art as a science; the effects of
individual design changes on any of these variables are only incompletely understood.
This is especially true with respect to particulate emissions, for which even the theoreti-
cal understanding is not yet complete. As a result, optimization of engine design varia-
bles is a slow process, with much cut-and-try experimentation needed. Optimization for
minimum emissions is also closely related to optimization for best fuel consumption,
which has been going on for more than M years. Thus the incremental improvements in
emissions through engine modifications are unlikely to be either great or rapid.
Optimized airflow and valve timing — Optimization of the combustion air flow path to
minimize pressure losses and equalize air distribution between cylinders can help to
reduce particulate emissions. Equalizing the air distribution makes possible more precise
control of the fuel-air ratio in the cylinder, while reducing pressure losses means that
more air actually gets into the cylinder. This increases the partial pressure of oxygen in
the cylinder, and thus decreases particulate emissions at any given power output. In
addition, increasing the amount of air in the cylinder and equalizing its distribution can
increase the maximum power obtainable from the engine.
Improvements in air flow can be accomplished by minimizing pressure losses and turbu-
lence due to friction, by optimizing valve timing, by increasing the number of valves used
in four-stroke engines from two per cylinder to four, and by optimizing the scavenging
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process in two-stroke engines. Air flow can also be increased by minimizing the need for
turbulence and swirl in the combustion chamber, since the energy required for this must
come from the incoming air. Such changes will also reduce the fuel-air mixing rate,
however, so they must be offset by increases in other mixing parameters such as fuel
injection pressure.
Optimized cylinder pressure and compression ratio — The pressure in the cylinder during
combustion is affected by fuel injection timing, compression ratio, intake air pressure,
and engine load. In turn, this pressure affects the flame temperature during combustion,
the average temperature of the gas in the cylinder, the partial pressure of oxygen, and
the overall efficiency of the engine. High pressures tend to improve fuel economy,
power output, and particulate emissions, but also increase NOx. The NOx increase is due
mostly to a higher flame temperature, and can be offset by charge-air cooling and/or
EGR. The maximum usable pressure is limited by the mechanical strength of the engine,
and by noise and durability considerations, while the requirements for cold-starting
impose a minimum value on the compression ratio. In practice, the compression ratio in
a specific engine model must be carefully optimized to comply with these numerous con-
straints.
Optimized combustion chamber, air swirl, and spray pattern — Together with the fuel
injection system, these variables collectively determine the nature and timing of the
fuel-air mixing process, and thus determine where and when combustion will take place
in the cylinder. This, in turn, has major effects on both NOx and particulate emissions.
Unfortunately, the mixing process and its effects on combustion are still only partly
understood, so that it is seldom clear a priori what effects a given change in any of these
variables will have. Interactions between the three variables are very important — a
change in any one of them will generally require re-optimizing the other two. Air swirl
also has an important and competing interaction with optimal airflow — increasing swirl
requires increasing the pressure drop, and reduces the amount of air that can be gotten
into the cylinder.
Combustion chamber design is of greatest importance in indirect injection engines. In
these engines, the design of the prechamber and of the connection to the main
combustion chamber must combine to create rapid air motion in the prechamber, and
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rapid mixing between the jet from the prechamber and the charge in the main chamber.
This rapid and intense mixing is primarily responsible for the very low emissions typical
of these engines. The fuel spray pattern in these engines is comparatively less
important, since the energy required for rapid mixing is provided almost entirely by air
motion.
In direct-injection engines, the situation is somewhat different. Dl engines place much
more reliance on the energy of the fuel jet and less reliance on air motion than do IDI
engines. The extreme form of this is the so-called "quiescent chamber" engine, in which
essentially ail of the mixing energy is supplied by the fuel jet. These engines use very
high pressure injection systems, with as many as nine holes in the injection nozzle in
order to provide better atomizing and distribution of the fuel. This type of combustion
system is most commonly found in line-haul engines, due to the expense of the high-
pressure injection system. An intermediate system, using moderate air swirl and mod-
erate injection pressures is more commonly found in engines in the medium-heavy
range. In these engines, combustion-chamber designs offering measurable benefits are
still being developed.
In general, more rapid mixing between fuel and air is desirable, both in order to improve
fuel economy and to improve the NOx-particulate tradeoff and thus reduce pollutant
emissions. As indicated in Section (>.1, both NOx and particulates are heavily dependent
on the conditions of combustion, which change continuously during the combustion
cycle. More rapid mixing and combustion result in less variation in conditions, and thus
make it possible to determine conditions for minimal emissions more clorely. Achievable
mixing rates are limited by the greater energy requirements for rapid mixing, the
pressure limitations of the engine, and noise limitations — more rapid mixing produces
more combustion noise.
Efficiency improvements — Other things being equal, any improvement in the efficiency
of the engine which does not affect the combustion process will tend to decrease emis-
sions, since less fuel needs to be burned to generate the same amount of work. Effi-
ciency improvements due to changes in combustion would exhibit a similar effect, but
this effect is normally small compared to the direct effects of combustion changes on
emission levels. Some non-combustion-related efficiency improvement technologies
include thermostatic control of oil temperature for optimal lubrication, use of lower-
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friction lubricants, and improvements in engine design which reduce friction. Reductions
in parasitic loads — such as by fluid-drive or electrically driven fans and improved design
of pumps and other auxiliaries — can also generate marginal improvements in efficiency
and emissions. Efficiency improvements of this sort are constantly being tested and
developed, due to their importance in reducing fuel consumption. Although the effects
of each individual change are generally small (changes with large results have been intro-
duced long ago), their aggregate effect may be a significant 10-15% reduction in emis-
sions. A study performed for the MVMA (Energy and Environmental Analysis, 1983)
estimated that engine efficiency could improve by 7.3 percent between 1982 and 1987,
and another 7.k percent between 1987 and 1992.
Status and prospects — Almost without exception, the heavy-duty diesel engines now in
production have been subjected to extensive optimization studies, so that they probably
represent about the best that can be achieved with current technology, given other con-
straints such as manufacturing cost. Improvements in the area of engine modification
will most likely come about as a result of re-optimization following the introduction of
some other new technology (e.g. high-pressure fuel injection, charge-air cooling), or as
the result of new technological developments. Improvements of this latter type are
likely to involve significant increases in cost. Except for the case of re-optimization
following a change in some other technology, the prospects for emissions improvements
due to engine modifications do not appear to be very great.
b.2.2 Fuel-Injection Systems
The fuel injection system in a diesel engine is the system by which fuel is transferred
from the fuel tank to the engine, then injected into the cylinders at the right time for
optimal combustion and in the correct amount to provide the desired power output. The
fuel injection system normally consists of a low-pressure pump to transfer fuel from the
tank to the system, one or more high-pressure fuel pumps which create the pressure
pulses that actually send the fuel into the engine, the injection nozzles through which
fuel is injected into the cylinder, and a governor and fuel-metering system, which
determine how much fuel is to be injected on each stroke, and thus the power output of
the engine.
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Three generic types of fuel injection system are in common use. These are: (1) systems
with distributor type fuel pumps, (2) systems with unitary in-line fuel pumps, and (3)
systems incorporating unit injectors. Distributor-type fuel pumps have a single pumping
element, which is mechanically switched to connect with fuel lines running to the injec-
tion nozzles on each cylinder. The pump generates a pressure pulse in each fuel line as it
is connected to it, causing fuel to spray out of the associated nozzle into the cylinder.
Because there is only one pumping element, the pump must operate at high speed (in an
N-cylinder four-stroke engine, the pump must generate N/2 pulses per revolution of the
crankshaft). This rapid operation and the mechanical switching process limit the
injection pressure and the precision of the injection pulse that cam be achieved with this
kind of pump.
The in-line and unit injector systems are conceptually similar: each one incorporates a
single pumping element per cylinder. In the in-line system, these pumping elements are
located in a separate fuel purnp, which — as with the distributor pump — is connected to
the injection nozzle by a high-pressure fuel line. Because each pumping element
operates only once every other revolution, and because all of the mechanical connections
are permanent, higher injection pressures and greater precision of injection are possi-
ble. However, very high pressures can still cause problems with fuel-line cracking, and
sharp pressure pulses still lose some of their definition in being transmitted through the
fuel line. For this reason, some manufacturers use unit injectors, in which each cylinder
has — in effect — its own individual fuel pump, directly connected to the injection
nozzle. These can provide very high injection pressures and very sharp pulses, but the
distributed location of the injectors and their close coupling to the engine can make it
more difficult to control the injection timing.
Regulatory pressure on particulates, the need for acceptable performance at low NOx
levels, and increasing fuel prices are all generating significant changes in fuel-injection
system design. These changes include increased precision in fuel metering and pulse
generation, higher injection pressures, changes in fuel injection timing, and/or the use of
variable injection timing (controlled by engine speed and/or load), and — finally and most
significantly — development of digital electronic control systems to replace the engine
governor, and for control of injection timing and rate of injection. Electronic control
systems are discussed separately in Section ^.2.5; the remaining changes are discussed
here.
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Higher precision — There has been a general trend in the heavy-duty industry toward the
use of more expensive, higher precision fuel-injection pumps and governors. Most com-
monly, this has involved substituting an inline fuel pump for the cheaper distributor-type
pump. The benefits of this substitution are better control of the fuel injection process
and the use of greater fuel-injection pressures. Improved control of the fuel injection
process can reduce both emissions and fuel consumption by eliminating secondary injec-
tion (a significant source of hydrocarbon emissions) and by making possible much more
precise control of injection timing. The widespread use of retarded injection timing for
NOx control has made precise control of injection timing very important, since the
engine becomes more sensitive to changes in timing as timing is retarded. At very low
NOx levels, a variation of as little as 1 degree in timing can result in significant
degradation in emissions and fuel economy.
Higher injection pressure — High fuel-injection pressures contribute to more rapid mixing
and more precise control of the combustion process. Fuel injection requires a finite
amount of time, so that the fuel injected near the end may burn under different condi-
tions from that injected near the beginning. Increasing the injection pressure can shorten
this interval, or, alternatively, can allow for finer atomization of the injected fuel.
Increasing the injection rate and atomizing the fuel more finely both help to speed the
fuel-air mixing process, and thus the rate of combustion. By carefully optimizing other
engine settings such as the injection timing, this increased combustion rate can be made
to yield a better tradeoff between fuel economy and emissions, especially NOx.
Optimal injection timing and variable injection timing — Fuel-injection timing involves a
multi-dimensional tradeoff between fuel economy, noise and engine durability, NOx
emissions, particulates, and hydrocarbons. Thus, some compromise between different
objectives will always be necessary. To make matters worse, the optimal injection tim-
ing to achieve any given compromise varies as a function of the engine speed, load, and
fuel cetane rating, among other variables. Because of this, a single, static setting of the
injection timing — even when carefully selected to minimize emissions over a given
cycle — cannot possibly perform as well as one which is dynamically adjusted to match
the current operating conditions. This is especially true when injection timing is
retarded to produce low NOx levels, since performance, particulate emissions, drive-
ability, and fuel economy are all very sensitive to injection timing in that range.
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Despite this fact, dynamically adjusted injection timing is not yet common on high-
powered diesel engines. The major reason for this is the difficulty in implementing it~
the fuel injection pump on a diesel engine is generally geared to the crankshaft, and
absorbs several horsepower. Developing a control mechanism which is sensitive enough
to adjust accurately the phase relationship between crankshaft and fuel pump, and robust
enough to deal with the forces involved, is not an easy task. Because of this,
mechanically-variable injection timing control has — with some exceptions — mostly
been limited to engines with distributor-type fuel pumps. These have design features
well-suited to this type of control. However, such pumps are limited to fairly low injec-
tion pressures, and the recent trend has been to higher pressures using in-line fuel pumps
or unit injectors, for which dynamic timing is more difficult.
In response to regulatory pressure and the need for improved fuel-economy, dynamic
timing mechanisms have been developed for some in-line fuel pumps. An outstanding
example of this is the new Caterpillar fuel pump, used on the Caterpillar 3406B engine
(Connor and Stapf, 1983). This mechanism uses a mechanically-actuated hydraulic servo
to vary the injection timing advance in response to changes in engine speed. The system
has aJso been designed to be used with digital electronic rather than mechanical controls,
when these become available.
Status and prospects — Higher precision, higher pressure, and more flexible (i.e.
dynamically adjustable) fuel-injection systems are among the engine manufacturers' most
important tools in attempting to meet stringent NOx and particulate standards while
retaining acceptable performance, drivability, and fuel economy. For this reason, fuel
injection systems have been an area of very active development, and a number of up-
graded and improved injection systems have been introduced in recent years. To date, all
such systems in the United States have incorporated mechanical controls. However, a
number of manufacturers of fuel injection systems have recently announced the availa-
bility of digital electronic controls for use with their products and Isuzu (Wakabayashi et
alia, 198*0 has just introduced such a system in Japan. CM has also indicated that elec-
tronic controls will be introduced into its product line on a limited basis by 1986, and
Cummins plans to introduce them on its new line of light-heavy diesels. Thus it appears
certain that such systems will appear on production engines in the United States within a
few ) ears.
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4.2.3 Charging Technologies
Most heavy-duty diesel engines are now turbocharged, primarily because of the increased
power output turbocharging provides. Turbocharging can also increase the engine's effi-
ciency slightly, with a consequent decrease in fuel consumption. Turbocharging increases
the temperature and oxygen partial pressure in the cylinder. These tend to increase both
the soot formation and the oxidation rate, but the effect of increased oxidation appears
to dominate, so that the overall effect is a decrease in particulate emissions at any given
power level. Finally, turbocharging reduces the incentive to operate near the engine's
smoke limit in order to obtain maximum power, which also improves particulate emis-
sions. This reduction in particulates is achieved at some cost in increased NOx emis-
sions, however, due to the higher temperatures and pressures reached in the cylinder.
The increase in NOx can usually be recovered by retarding the injection timing, but with
some loss in the advantages of turbocharging. It can also be recovered by cooling the hot
compressed air from the turbocharger before it enters the engine. This practice — vari-
ously known as intercooling, aftercooling, and charge-air cooling — has become
extremely common.
One disadvantage of turbocharged engines is that they can exhibit very high transient
particulate emissions. This is because it takes time for the turbocharger to speed up and
increase the airflow in response to an increase in engine load, while the increase in fuel
flow is very rapid. Thus the increased fuel flow to the engine is not, at first, matched by
sufficient air, resulting in excessive particulate production and a dense black "puff" of
smoke. Modern engines deal with this problem through the use of a "puff limiter" — a
device incorporated in the governor which reduces the fuel flow to the engine until the
turbocharger is up to speed. These devices inevitably compromise acceleration and
driveability, however.
The acceleration smoke problem can be alleviated by improving the match between the
engine and the turbocharger characteristics, and by improving the design of the turbo-
charger itself. Current interests in this area are focussed on development of variable-
inlet area turbochargers (e.g. Arvin and Osborn, 1983), incremental improvements in
current fixed-area turbochargers, and on some advanced technologies which are discussed
below. The incremental optimization procedures are not likely to result in any spectacu-
lar gains, but their potential for reducing transient-test particulate emissions is by no
means negligible. Of equal importance is the fact that these changes can improve
driveability, reducing the incentive for the driver to tamper with the puff limiter.
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Charge-air cooling — Compressing air, as in a turbocharger, increases its temperature.
Charge-air cooling is the practice of cooling the hot compressed air from the turbo-
charger by passing it through a heat-exchanger before allowing it to enter the engine.
This has two beneficial effects: it decreases the maximum temperature reached during
combustion (thus reducing NOx emissions); and it decreases the specific volume of the
air, so that a greater mass can enter the cylinder. This increases the partial pressure of
oxygen in the cylinder, which usually decreases particulate emissions. In addition,
charge-air cooling increases both the efficiency and the maximum power of the engine.
The net result is a decrease in fuel consumption and in NOx emissions, an increase in
available power, and generally some decrease in particulate emissions as well.
Because of these advantages, charge-air cooling is rapidly becoming-universal on line-
haul truck engines, and it is found on many transit-bus and medium-heavy diesel engines
as well. Present production charge-air coolers use the engine cooling water as a heat
sink. This limits the degree of cooling possible — the minimum temperature reachable is
about 100° C. Advanced technologies which would use either a separate cooling circuit
or an air-to-air heat exchanger could attain significantly lower temperatures, with a
consequent decrease of up to 20 percent in NOx emissions (Cummins, 1982).
Advanced low-temperature charge-air cooling systems would decrease fuel consumption
and NOx emissions still more (Henriksen, 1983), but at some cost in other drawbacks.
These drawbacks would include the requirement for more heat-exchanger surface on the
vehicle, with a consequent increase in bulk and drag. In addition, the greater volume of
air in the heat exchanger might exacerbate the problem of turbocharger lag, and the
lower exhaust temperature would reduce the attractiveness of turbocompounding. Low
exhaust temperature would also make a self-regenerating trap-oxidizer system less
attractive, and might rule it out altogether. There is also the possibility of over-cooling
of the intake air, with a consequent sharp increase in hydrocarbon and possibly
particulate emissions, and possible difficulties with cold starting. Such problems could
be prevented, however, by the use of a thermostatically operated bypass valve.
Advanced charging technologies — A number of advanced charging technologies are
under investigation. The technology which has drawn the most regulatory attention to
date is the so-called "three-wheel" turbocharger or TWT (Timoney, 1983). In addition to
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the traditional turbocharger's two "wheels" (the gas turbine and the compressor), the
TWT incorporates a small Pelton hydraulic turbine on the same shaft. This turbine,
which is driven by lubricating oil from an engine-driven pump, is used to provide addi-
tional power to the air-compressor during transient operation. This eliminates the turbo-
charger lag, and thus permits improved acceleration along with a decrease in particulate
emissions and visible smoke. Depending on the specific engine, the reduction in particu-
late emissions on the certification test might be as much as 20 percent. The reduction in
in-use particulate emissions would be even greater, since puff limiters are often not
maintained and occasionally tampered with, and this technology would eliminate the need
for a puff limiter.
The TWT is particularly attractive for use with two-stroke engines, since the presence of
an independently controllable air-compressor would eliminate the two-stroke's present
need for a separate blower. Eliminating the blower would reduce manufacturing cost,
parasitic losses, and crowding in the engine compartment. Although the TWT has
received the most attention in this regard, the same result could conceivably be
accomplished by other technologies. Three possibilities in this regard are the so-called
"Comprex" or gas-dynamic supercharger, an electric-motor assist for the turbocharger
(controlled by a variable-speed electronic drive), and the use of an engine-driven super-
charger (with turbocompounding to avoid wasting the energy now recovered by the turbo-
charger).
Status and prospects — Moderate-temperature charge-air cooling (with heat rejection to
the engine cooling water) is now almost universal on line-haul and other very heavy-duty
engines, primarily because of the improved fuel economy and power output it makes
possible. For the same reasons, an increasing number of premium medium-heavy duty
engines are incorporating this technology as well. Advanced charge-air cooling systems,
using either low-temperature liquid-to-air or air-to-air heat exchangers are now in the
late stages of development, and can be expected to appear in production in the 1986 to
1988 time frame.
Among the advanced charging technologies, the gas-dynamic supercharger is in the most
developed. Such devices have been commercially available for several years, but they
have not yet been accepted by engine manufacturers and users. The TWT is still a devel-
opment project at present, although the early results of this development appear very
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promising. The authors are not aware of any development efforts involving either super-
chargers or electric-motor assist for heavy-duty engines. It is still much too early to
predict when or if engines incorporating any of these devices might appear in production.
**.2A Exhaust-Gas Recirculation
EGR is a time-proven NOx control technique for light-duty gasoline and diesel vehicles,
and it has been proven effective in heavy-duty engines (Yu and Shahed, 1981; GM, 1982a;
Daimler-Benz, 1982). EGR works by recycling a portion of the exhaust gas to mix with
the intake air. This dilutes the air in the cylinder with inert gases and thus increases the
total amount of gas that must be heated by each unit of burning fuel. The recycled
exhaust gases also have a higher heat capacity than does air. Both effects reduce the
peak local temperature in the flame, and thus decrease NOx formation. Unlike retarded
injection timing, properly applied EGR should have only minor effects on performance
and fuel economy at moderate loads. However, the recycled exhaust gas decreases the
total amount of oxygen in the cylinder, which decreases the maximum power output of
the engine. If the engine is not derated to account for this, EGR can cause it to operate
above its smoke-limited power level, with a consequent enormous increase in particulate
emissions.
The effects of EGR on maximum power and particulate emissions can be alleviated
through the use of "modulated" EGR, in which the degree of exhaust gas recycling is
changed to match the load on the engine. In this way, it is possible to obtain reasonable
control of NOx emissions at low and intermediate loads without compromising maximum
power. However, the reduced flame temperature and reduced oxygen content of the
charge still lead to an increase in particulate emissions, which becomes more severe as
NOx levels are reduced further by this means. EGR can be modulated either by
mechanical means or by an electronic control system. The latter arrangement, which is
much more flexible and effective, is discussed in the next section.
Aside from its effects on particulate emissions, EGR has a number of detrimental effects
on the engine. The most important are the effects on durability, maintenance require-
ments, and fuel consumption.
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The effect of exhaust gas recirculation on durability and maintenance requirements is
due primarily to the fact that soot particles in the exhaust are recycled through the
engine. These particles increase abrasive wear on the cylinder liners. They also get into
the engine oil, where they absorb and neutralize some of the additives, and can also lead
to abrasive wear in the other moving parts, necessitating more frequent oil changes.
Soot particles can also form cakes and deposits in the air intake system and on parts such
as turbochargers and blowers. These problems — especially the increased wear — are
much more severe on large heavy-duty engines than in light-duty service, due to the
much greater expected lifetime of the heavy-duty engine. If not successfully mitigated,
they could be expected to lead both to increased economic costs and to significant tamp-
ering with the EGR valve by truck owners in an effort to protect their investment.
These problems could be eliminated by the use of trap-oxidizers, with the EGR return
taken from the exhaust downstream of the trap. They could probably also be alleviated
to some degree by careful design, improved lube-oil additives, and more frequent oil
changes. These (especially the last) would be expensive, however.
EGR is especially problematic for two-stroke engines, since the recycled soot, water, and
acid gases can foul and damage the scavenging blower. On the other hand, these engines
could use a form of "internal" EGR by modifying the scavenging characteristics to retain
more exhaust in the cylinder between strokes. GM (the only U.S. maker of two-stroke
diesels) has reported the development of a very similar system to control the cylinder
temperature in a Methanol-fueled bus engine (Toepel et alia, 198*0.
A number of manufacturers have reported that EGR increases fuel consumption signifi-
cantly — by as much as 8 percent (Daimler-Benz, 1982). However, there is no funda-
mental reason to expect such an increase — at most, a slight decrease in the rate of
combustion near TDC and a consequent slight (1 percent) increase in fuel consumption
would be expected. One possible explanation for the reported increase is that the test
engines had not been completely optimized for use with EGR. Another is that the
recirculated exhaust gas is hot, thus increasing the temperature of the charge and
decreasing both fuel efficiency and maximum power, in the opposite effect to that of
charge-air cooling. This would also increase the flame temperature, partially offsetting
the dilution effect of the exhaust gas. These problems could be overcome by cooling the
recirculated gas, with a consequent increase in both fuel efficiency and NOx control.
Some manufacturers who are considering EGR appear to be planning on this approach.
Design of such a cooler would not be easy, however, since it would tend to be clogged
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with soot, water, and oily deposits from the exhaust, and would suffer from corrosion due
to the acid gases (NOx and SC^) in the exhaust.
Status and prospects — At present, only two heavy-duty engines use EGR to meet NOx
regulations, and only one of these (the Caterpillar 3208 for California) is in widespread
use. However, a number of light-duty diesel engines (including the light-duty version of
the GM 6.2 liter engine) now use EGR, and most heavy-duty engine manufacturers have
experimented with it. Thus there is a well-developed technology base, and EGR could be
implemented fairly readily if necessary. Because of its potential for effective NOx con-
trol with comparatively minor effects on fuel consumption and driveability, EGR seems
likely to play an important role (along with charge-air cooling, injection timing, and elec-
tronic controls) in meeting future NOx emissions standards. However, unlike charge-air
cooling and electronic controls, EGR does not produce any fuel economy or performance
benefits on its own, so that it would not be adopted unless it were necessary to meet an
emissions standard.
<>.2.5 Electronic Controls
As the discussion above has indicated, the major problem with EGR and retarded injec-
tion timing as NOx control techniques is the resulting increase in particulate emissions,
especially at high loads. This problem can be minimized by adopting a control strategy
which reduces NOx emissions in those ranges where doing so does not greatly increase
particulates, and which relaxes the NOx controls at high loads. However, the optimal
injection timing and EGR setting vary continually as functions of the engine speed, the
engine load, and (especially in transient operation) the intake air pressure. Traditional
hydromechanicai control systems are unable to apply a sufficiently sophisticated control
response at a reasonable cost, and are also subject to wear and degradation which can
impair their control response over time.
Another hydromechanicai control system which is subject to these problems is the engine
governor — the device which is responsible for regulating the amount of fuel fed to the
engine. The governor is a complex mechanical device which responds not only to the
position of the accelerator pedal, but also to built-in limitations for minimum idle speed,
maximum engine speed and maximum fueling rate, and cold-starting fuel flow, among
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others. Governors in turbocharged engines also include some form of "puff limiter" to
reduce the maximum fuel flow to the engine during acceleration until the turbocharger is
up to speed. Although the governor is not usually thought of as an emissions control
device, it does in fact have a very important effect on emissions, especially particulates
and hydrocarbons. Some recent research (Reams et alia, 1982) on light-duty diesels has
indicated that the limitations of the mechanical governor (especially the problem of
overshooting during acceleration) may be responsible for a significant fraction of total
light-duty particulate emissions. Reams and coworkers found that an electronic governor
with a fully optimized control program was able to reduce particulate emissions by 37
percent on the light-duty Federal Test Procedure, while reducing fuel consumption by
three percent. Heavy-duty governors are somewhat more expensive and more precise, so
the possible savings would probably be less, but they are still probably quite significant.
Because of the difficulties with present hydromechanical controls, microprocessor-based
digital electronic control systems for injection timing, EGR regulation, and the engine
governor are being developed by virtually all manufacturers (Voss, 1981; Day and Frank,
1982; Lucas et alia, 1983; Toepel et alia, 1983). One such system has already been intro-
duced in Japan (Wakabayashi et aha, 198V). As they are now envisioned, these control
systems will accept input from a number of sensors for accelerator pedal position, engine
speed, engine load, ambient temperature, boost pressure, and other variables, and calcu-
late optimal settings for the injection timing, fueling rate, and EGR valve position.
These calculations will be based on a "map" of optimal settings stored in the micro-
processor's read-only memory. In many cases, this "map" may also include externally
specified limits such as road-speed governing (overriding the driver), which is desired by
many fleet operators as a fuel-saving technique.
The microprocessor will then send appropriate signals to a set of electromechanical
actuators to adjust the fuel injection timing, fuel metering, EGR valve setting, and so
on. Ultimately, a closed-loop control system, in which the microprocessor would con-
tinuously optimize control settings based on feedback from engine sensors, will probably
be adopted. A system of this type for setting injection timing has already been demon-
strated in the laboratory (Pipho and Kittelson, 1983).
Status and prospects — A number of fuel-injection system manufacturers have already
announced the availability of digital electronic control systems for their products,
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although many of these have been for light-duty diesels. Because of the improvements in
fuel economy and driveability of low N0X levels that electronic controls would make
possible, all heavy-duty engine manufacturers (and almost all light-duty diesel manufac-
turers) are now heavily engaged either in developing their own systems, or in applying the
commercially available systems to their engines. The competitive importance of elec-
tronic controls is such, however, that very little information on development status or
the results achieved is available. Several engine manufacturers refused to discuss testing
results even confidentially.
In other work for the EPA (Weaver, 1984b), one of the authors has examined the effects
of electronic controls on the relationship between NOx and fuel economy. The conclu-
sion of this work was that optimal electronic timing controls were most effective at
moderately low NOx levels. As Figure 4.9 shows, for most manufacturers, the electron-
ics make little difference at high NOx levels, where precise injection timing is less criti-
cal. The exception is manufacturer "C", in which the production (static) timing is
apparently sub-optimal. As injection timing is retarded to achieve lower NOx levels,
precise timing becomes increasingly important. Under these conditions, the electronic
timing controls can significantly reduce NOx emissions with less degradation in fuel
economy than static injection retardation. At very low NOx levels (below 4 g/BPH-hr),
this advantage is lost, and the fuel-economy penalties with and without electronics
increase rapidly.
Both fuel economy and particulate emissions are determined by the quality and timing of
the combustion process. Thus, it appears likely that improvements in particulate emis-
sions due to electronic timing control would more or less follow the pattern in Figure
4.9 — being greatest between about 5 and 7 g/BHP-hr NOx and dropping off on either
side. Electronic governor control would improve particulate emissions still further, and
should improve fuel economy slightly as well. Overall, the authors estimate that a fully
developed electronic engine control system whould make it possible to achieve 5.0
g/BHP-hr NOx at low mileage, with a small improvement in brake-specific fuel consump-
tion over present levels, and with particulate emissions 30 to 40 percent lower than
present-day engines at the same NOx level. These estimates are based on the limited
published data available (Ring, 1984; Komiyama et al, 1984; GM, 1984b), and on confiden-
tial information provided by manufacturers.
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z
o
p
a
3
3
CO
z
o
o
-I
til
3
UJ
0)
<
UJ
a
a
3.0 4J) SjO OjO 7j0 BJ)
LOW MILEAGE N0X EMISSIONS (G/BHPHHR)
11.0
Figure 4.9: Fuel economy vs. NO emissions — effects of electronic controls
(Source: Weaver,198^b)
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The economic importance of these improvements, especially the improvement in fuel
economy, is such that electronic engine control systems can be expected to be introduced
as quickly as possible. It is expected that integrated electronic engine control systems
will appear in a significant number of light-duty diesel engine models by 1985, and that
some should be introduced for heavy-duty service in 1986 or 1987. By the end of the
decade, virtually all heavy-duty engine families should be equipped with this technology,
even in the absence of a strict NOx limit. The presence of such a limit would, of course,
tend to accelerate this trend.
4.2.6 Indirect Injection
The indirect-injection (IDI) or prechamber diesel engine is commonly used in light-duty
and light-heavy duty vehicles. It differs from the direct-injection (DI) engine used in
virtually all medium-heavy and larger diesel vehicles in having two combustion chambers
per cylinder — the main combustion chamber above the piston, and a separate pre-
chamber into which fuel is injected. These two arrangements have been discussed in
Section *».Ll, and the different types are shown in Figures
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IDI engines by means of other technologies such as retarded injection timing or high-
pressure fuel injection is rather slight — these technologies have most of their effect
through mixing, which is already nearly optimal in the IDI engine. Other, non-mixing-
related techniques such as EGR, turbocharging/aftercooling, and electronic controls
would, however, be as applicable to these engines as to the direct-injection design.
The significant emissions benefits of IDI engines are attained at a significant price — a
loss in fuel economy of about 8 to 12 percent from that attainable with direction injec-
tion. This loss results from the effective "injection retard" due to the dual combustion,
from pressure losses due to expansion through the orifice between the prechamber and
the main chamber, and from the increased heat loss to the greater area of the combus-
tion chamber walls. This reduced efficiency has generally rendered prechamber diesels
unmarketable in heavy-duty highway applications, in which fuel economy is at a
premium. In addition, the increased heat rejection to the engine also requires a larger
radiator area, which in some cases would make it impossible to mount a prechamber
engine in existing trucks, and in any case would increase the drag and aerodynamic
losses. The increased heat rejection also makes the engine run hotter, impairing its
durability.
Status and prospects — Due to their higher speed capabilities and the fact that they do
not require as expensive fuel-injection systems, IDI diesels are now the only type of
engine used in light-duty or light-heavy duty service. They are likely to maintain this
dominance in light-duty vehicles for the forseeable future. However, in medium-heavy
and especially in line-haul service, the increased fuel consumption, increased heat rejec-
tion, and impaired durability of the IDI engine are likely to continue to rule it out. Only
one manufacturer (Caterpillar) has seriously attempted to market IDI engines in these
classes, and Caterpillar has stated that it plans to abandon that attempt (Caterpillar,
1982). A number of manufacturers are also introducing small high-speed DI engines into
the light-heavy duty class in order to obtain the DI's lower fuel consumption there as
well. In transit buses, where the willingness to pay for reduced emissions may be some-
what greater, the IDI engine could conceivably have a future. However, there does not
presently appear to be any move to develop or market such an engine for this class.
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4.2.7 Advanced Engine Technologies
A number of advanced heavy-duty engine technologies are now in various stages of
development, and many of these — such as the Stirling and the gas-turbine engine —
would emit substantially less pollution than does the diesel engine. However, to date,
these technologies have not been shown to be able to compete with the efficiency and
durability of the diesel, and in any case, they are not themselves diesel engines, and are
thus beyond the scope of this report. The three advanced diesel engine technologies
which are attracting the most attention at this time are the uncooled or adiabatic diesel
diesel engine, turbocompounding, and the use of an organic rankine "bottoming" cycle.
Adiabatic diesel engines — A substantial fraction of the energy consumed by a diesel
engine is lost through heat transfer to the cylinder wails. With metal cylinder walls, this
heat must be continuously removed in order to prevent damage to the engine — thus the
need for cooling channels, water pump, radiator, and other components of the cooling
system. By substituting a high-temperature ceramic for all or part of the cylinder walls,
it is hoped to be able to eliminate both the heat loss and the cooling system. Eliminating
the heat loss to the walls will directly increase the engine efficiency, by several percent,
with a further increase due to eliminating the parasitic loads of the water pump and the
radiator fan. A further increase in vehicle efficiency will be made possible by eliminat-
ing the need for the radiator, which will allow for much better aerodynamic design of the
front of the truck.
Other things being equal, an increase in engine or vehicle efficiency will result in a cor-
responding decrease in pollutant emissions (although engine manufactorers do not receive
credit for vehicle efficiency improvements under the current regulatory structure). In
addition, the adiabatic engine should result in a substantial improvement in the NOx-
particulate and NOx-hydrocarbon tradeoffs, since the higher average exhaust tempera-
ture and higher radiant temperature in the cylinder should increase HC and particulate
oxidation. NOx emissions should not be significantly increased, since the higher average
temperature would have only a minor effect on the adiabatic flame temperature. The
effect of increasing the radiant temperature on particulates should be especially marked,
since particles — unlike gases — are highly efficient absorbers and emitters of radia-
tion. Recent data from Cummins (Sudhaker, 198*0 confirm these expectations for the
NOx/fuel consumption and NOx/hydrocarbon tradeoffs. To date, the authors are unaware
of any publicly available data on particulate emissions by adiabatic diesel engines.
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Turbocompounding — This technique makes use of the residual energy in diesel engine
¦ exhaust to generate additional work. Diesel exhaust, especially in turbocharged engines,
1 is emitted at temperatures and pressures well above atmospheric. Turbochargers use
some of this additional energy to turn a turbine, which in turn drives the air com-
j pressor. By using a more efficient turbine, additional energy beyond that needed by the
air-compressor can be generated, and used (through an appropriate linkage) to drive the
J wheels. Turbocompounding is most attractive when combined with the adiabatic engine,
due to the greater energy content of the exhaust.
The effects of turbocompounding on emissions would be minimal ~ a small decrease due
to increased engine efficiency is all that could be expected. However, turbocompounding
' (along with the adiabatic-engine concept) would be beneficial in reducing the fuel con-
sumption penalty due to injection timing retardation, since the turbine would then
1 recover some oi the energy invested by late combustion (Toyama et alia, 19S3). On the
other hand, turbocompounding might also greatly complicate other exhaust'System
I related emissions control technologies, especially EGR and trap-oxidizers.
Organic rankine bottoming — Like turboco impounding, organic rankine bottoming makes
. use of the residual energy in diesel exhaust to generate additional work. Unlike turbo-
I compounding, however, the rankine engine would require a separate fluid loop and heat
exchanger, with a consequent increase in size, weight, expense, and maintenance. It is
j doubtful whether the increased efficiency of this approach, compared to turbocompound-
ing, would justify these increases in size and weight. If ORC bottoming is ever imple-
mented, it will probably be in line-haul trucks, ior which it can provide a decrease in fuel
consumption of up to 12.5 percent (DiBella et alia, 1983). This would result in a direct
| decrease in NOx and particulate emissions, of a magnitude similar to that of the
decrease in fuel consumption.
Status and prospects — Adiabatic engines and turbocompounding are presently the sub-
I jects of intense research and development by a number of manufacturers — primarily
because of their potential for increasing fuel efficiency. Turbocompounding would use
( relatively well-understood technology, but its value is reduced without the adiabatic
engine, so it is not clear whether turbocompound engines will appear before adiabatic
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ones. Adiabatic engine development, in contrast, may require a substantial extension of
the state of the art in industrial ceramics and high-temperature lubrication. Developing
ceramics and ceramic bonding and fabrication techniques which can survive for hundreds
of thousands of miies in a diesel engine without faiiing or developing excessive wear is a
formidable task. The industry appears to be fairly confident that this can — even-
tually — be done, but it would be premature at this point to predict when (or even whe-
ther) this will occur. Given the present state of the art, and the lengthy lead times for
development, it seems highly unlikely that production adiabatic diesel engines will be
available before the 1990s.
Research on organic rankine bottoming cycles appears to have peaked during the late
70s, under the influence of DOE funding. While development work on this approach is
continuing, it is too soon to predict when — if ever — it may be implemented in practice.
4.2.8 Additives and Alternative Fuels
Fuel additives such as barium have long been used to reduce visible smoke from diesels.
The effect of barium on particulates, however, is to substitute barium sulfate particles
for carbon particles, with little net effect on particulate mass emissions. For this
reason, present interest in these areas is focussed on the use of alternative, cleaner fuels
such as ethanol and methanol, and on the use of water and/or alcohol along with diesel
fuel to affect combustion. The three most highly developed approaches in this regard are
the use of water-diesel fuel and alcohol-diesel fuel emulsions, fumigation of the intake
air with water, and the use of methanol as a substitute for diesel fuel in the diesel
engine.
Water/diesel fuel arid alcohol/diesel fuel emulsions — A number of researchers have
experimented with the possibility of mixing water and/or alcohol with diesel fuel in order
to reduce NOx and particulate emissions. The primary interest in this regard has been
for underground mining machinery, for which the economics of pollution control are
much more favorable than they are in highway applications.
Since alcohols and water are not miscible with diesel fuel at ordinary pressures, it has
been necessary to mix them in the form of an emulsion of finely divided water or alcohol
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droplets in the diesel fuel matrix. This emulsion can either be produced separately and
substituted for diesel fuel in the fuel tank, or it can be produced on the vehicle by an
emulsifier fed by separate diesel fuel and water/alcohol supplies. The former approach
now seems to be preferred due to its greater simplicity. A number of workers have
recently reported the results of tests using this type of fuel (Johnson and Stoffer, 1983;
Callahan et alia, 1983; O'Neal et alia, 1983). The results reported in these papers were
not conclusive — under steady-state conditions, use of a fuel containing a small amount
of water or alcohol seems to reduce both NOx and particulates, by as much as 20% and
50% respectively. On the other hand, transient testing using the EPA cycle showed a
sharp increase in particulates with a water-in-fuel emulsion which had given good results
in steady-state tests, together with increases in HC and CO emissions.
Water fumigation — Research has shown that mixing water into the intake air for a
diesel engine can significantly reduce NOx emissions (GM, 1982a). This is probably due
to several effects, of which the most important are a reduction in the temperature of the
intake air due to evaporative cooling, and a reduction in peak flame temperature due to
dilution and the greater specific heat of water. However, the water flow rates required
for significant benefits are comparable to the fuel flow rate, which would introduce
major questions concerning the willingness of truck users to maintain a continuous water
supply in return for no perceived benefit. Water injection also has a slight negative
effect on fuel consumption, and there is concern that large water flowrates might
increase engine wear. Finally, there would be a serious problem in protecting any on-
board water tanks and piping from freezing during cold weather.
Methanol fuel — Methanol is a relatively inexpensive fuel which burns with a compara-
tively low-temperature flame and without soot. It is also readily available in reasonable
quantities, and is fairly easy to produce from a wide variety of feedstocks, ranging from
coal to natural gas to biomass. Because of these characteristics, it has attracted consi-
derable attention as a possible alternative fuel for motor vehicles. Methanol-burning
heavy-duty diesel engines have been developed by General Motors (Toepel et alia, 1983)
and by M.A.N. A number of spark-ignited or spark-assisted diesel engines using methanol
have also been developed, and there is considerable interest in applying methanol engines
in service — especially in transit buses, where the reduction in visible smoke and odor
would be most valuable, and the increased inconvenience of a non-standard fuel would be
minimized.
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Spark-ignition engines are beyond the scope of this report, and will not be discussed
here. Compression-ignition methanol engines, however, are within its purview, and they
have also attracted considerable attention as a possible means of reducing NOx and par-
ticulate emissions by transit buses. Thus a brief discussion of their characteristics and
prospects is in order.
The GM methanol engine is the one for which the most complete information is availa-
ble. This engine is a modified version of GM's standard 6V-92TA transit coach engine.
The engine is a turbocharged, aftercooled, two-stroke model designed for low-smoke
operation in buses. In converting it to use methanol, GM made a number of minor
changes, including alterations in the compression ratio, using a different turbocharger
and blower drive ratio, addition of glow plugs, and substitution of an advanced elec-
tronically-controlled fuel injection system for the mechanical fuel injection system on
the standard engine. The basic structure and design of the engine were unchanged, how-
ever, and except for the fuel injection system it was practically all made from produc-
tion parts. Dynamometer tests using methanol fuel indicate that this engine has slightly
lower peak power, and lower fuel consumption (measured on an energy-equivalent
basis). It also had much lower particulate and NOx levels than the corresponding diesel-
fueled engine. Thirteen-mode NOx and particulates were 0.20 and 0.17 g/BHP-hr
respectively, or about a third of the levels of each pollutant emitted by a "good tech-
nology" engine burning diesel fuel.
Status and prospects — The prospects for fuel/water or fuel/alcohol emulsions in highway
diesels are difficult to assess, because of the inconclusive nature of the testing results to
date. There would be enormous practical problems in carrying out a widespread switch
to a different fuel of this nature, including difficulties with fuel separation during stor-
age, possible freezing of the water, and possible corrosion in the fuel system. The prob-
lems with water injection into the intake air would be similar — with the additional dif-
ficulty of ensuring that a reliable supply of water is provided by the user. Because of
these difficulties, neither of these approaches presently appears very promising.
The use of methanol-fueled engines — especially in transit coaches — appears to be a
much more promising approach. Such engines could drastically cut particulate and NOx
emissions from transit buses. Since bus emissions result in much greater human exposure
per unit of pollutant than do emissions from most other types of vehicles, this could have
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a disproportionate effect on increased health and welfare. Buses are also well suited to
| using methanol, since they have adequate spare space to carry the larger fuel tanks
required, and they are virtually all supplied with fuel from a separate, central fueling
| facility rather than purchasing it at retail. However, all of the data concerning methanol
engines are not yet in — it will be necessary to know their long-term durability and per-
I formance characteristics before a fully realistic assessment of their potential can be
made.
fc.3 PRESENT-DAY ENGINES AND EMISSIONS LEVELS
!
For discussion purposes, it ts convenient to divide heavy-duty diesel engines into two
groups on the basis of their technical characteristics. These groups are the light-heavy
duty IDI engines on the one hand and all other heavy-duty engines on the other. The
former group are small, high speed, indirect-injected designs derived from passenger car
diesel technology, and bear little resemblance to the large, medium-speed, and almost
entirely direct-injected designs which are used in heavier trucks. Although this latter
group displays some strong internal differences of its own, these are outweighed by the
underlying similarities. The new small, high speed DI engines being introduced for light-
duty services bear a greater resemblance to the larger DI engines than to the IDI engines
in light-heavy duty service, and are included in the former group for discussion.
4.3.1 Light-Heavy Duty IDL Engines
Light-heavy duty diesel engines are very new — until 1981 no such engines existed. At
present, only three models of light-heavy duty IDI engines are manufactured for highway
use. These are the International Harvester 6.9 liter engine, a similar capacity engine
produced by Onan, and the General Motors 6.2 liter. The 6.2 liter engine is a modified
version of GM's successful 6.2 liter light-duty truck engine. The technology for these
engines is derived from passenger-car and light-truck diesels, and they closely resemble
light-duty engines in their durability characteristics, performance, operating characteris-
tics, and emissions levels. These characteristics have been designed with a view to mak-
ing these engines close substitutes for gasoline engines of similar size.
The most important emissions control technology in this group is indirect injection.
Although emissions control was probably not the major consideration in choosing this
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injection mode (other advantages such as high speed capabilities, power output, and a
tolerance for lower-cost fuel injection systems were probably more important), it is
nonetheless a highly effective emissions control technique. Other emissions controls
used in present production versions of these engines include mechanically-variable fuel
injection timing and extensive engine optimization. The close resemblance between
these engines and light-duty diesels also means that light-duty emissions control
techniques could be adopted very quickly. These techniques would include EGR (the
light-duty version of the 6.2 liter already uses mechanically modulated EGR), electronic
control of injection timing and EGR modulation, and possibly an electronic eovernor.
Due to the comparatively low cost and limited durability of these engines, heavy-dutv
emissions control techniques such as turbocharging/aftercooling, turbocompounding, and
adiabatic engine design would probably not be feasible.
Emissions levels — Table 4.2 shows the 1983 model year EPA certification data for the
6.9 and 6.2 liter engines, together with some unofficial transient-test NOx and particu-
late data for the 6.2. Transient test data for the 6.9 liter engine are not yet publicly
available. As the table indicates, both engines are very clean — with NOx emissions
below 4.0 g/BHP-hr and very little smoke on the EPA test. The earlier GM transient test
data for the 6.2 liter engine are even more remarkable in this respect — they are far and
away the lowest measured emissions levels from heavy-duty diesel engine of which the
authors are aware. Changes in the compression ratio and variations in production timing
apparently increased emissions somewhat — the later data for this engine, although still
very good, are not significantly different from what the best DI diesels are now achiev-
ing. The light-duty version of the GM 6.2 liter (which differs from the heavy-duty ver-
sion in having EGR) is also extremely clean. Despite the use of EGR with a mechanical
control system, FTP particulate emissions for trucks using this engine are 0.35 g/mi.
This would be quite respectably low for a mid-size passenger car; it is phenomenally low
for a 6000 pound truck.
fr.3.2 Medium-Heavy, Line-Haul, and Transit Bus Engines
Medium-heavy and line-haul engines represent more of a continuum between two
extremes than they do two separate classes. Transit-bus engines fall toward the lighter
end of this continuum, but with some special features of their own. At the lighter (or
medium-heavy) end, engines in this group tend to be naturally aspirated, to have
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Table 4.2
Light-Heavy Duty Engine Emissions Levels
13-Mode NOx Smoke Opacity (%)
(g/BHP-hr) Accel. Leg Peak
EPA Certification Data (Model year 1983)*
GM 6.2 liter IDI 2.8 6.3 4.9 6.7
IH 6.9 liter IDI 4.3 3.2 4.3 7.7
Transient-test data NOx Particulate
for the G.M. 62 liter IDI (g/BHP-hr) (s/BHP-hr)
Pre-production version^
Setting 1 4.1 0.46
Setting 2 2.8 0.52
Production Version^
6 degrees BTC 4.16 0.66
4 degrees BTC 3.60 0.62
* Source:
^Source:
¦'Source:
EPA, 1983a
General Motors, 1982a
General Motors, personal communication, 1984
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moderate power output and limited duty cycles, and to have design lifetimes in the
vicinity of 150,000 to 250,000 miles. Examples of this class include the International
Harvester 9.0 liter engine and the naturally aspirated versions of the Caterpillar 3208. A
step further along the continuum are the "premium" medium-heavy engines such as the
International Harvester DTI ^66 and the GM 8.2 liter engine. These are generally
turbocharged, may have aftercooling, and tend to have better fuel efficiency and fewer
duty cycle limitations. A step further yet are the engines designed to be used in heavy
trucks in stop-and-go service, such as garbage trucks and dump trucks. These are
turbocharged, may have aftercooling, and usually have rather steep torque curves (a
rapid increase of torque with RPM) in order to improve driveability. Finally, at the other
end of the continuum are the true line-haul engines, which are virtually all turbocharged
and aftercooled, designed to run at rated speed and power for very long periods, and
optimized for best performance and fuel economy under those conditions.
The emissions control technologies presently in use for heavy-duty engines are turbo-
charging, aftercooling (using the engine cooling water as the heat sink), engine modifica-
tions, and optimized or mechanically-varied injection timing control. High precision and
high pressure fuel injection systems are very common in line-haul and similar heavy
engines, and are coming into increasing use in the lighter groups. Mechanical puff-
limiting devices are also used in turbocharged engines in order to reduce visible smoke
emissions during acceleration. In addition, one California-model Caterpillar engine uses
indirect injection, and another uses EGR. Neither of these latter technologies has been
generally accepted for use in engines in this group, however.
Emissions levels — Figure ^.10 is a plot of transient-cycle NOx emissions against
transient particulate emissions for those heavy-duty engines for which these data were
available. For a few engines, data were available at more than one calibration (i.e. at
different points along that particular engine's NOx/particulate tradeoff curve). Data
points for these engines are shown linked by solid lines. As indicated by the symbols,
almost all of the data available are for direct-injected engines, most of which are
turbocharged and aftercooled. However, data for two Caterpillar prechamber engines
(one of which does not appear to be offered any longer) are also shown, along with one
data point for the Caterpillar engine with EGR. For comparison, the four data points for
the GM 6.2 liter light-heavy duty engine have been plotted as well.
^-50
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PARTICULATE
g/fchp M
l-O
.8 -
.7 "
6 -
5 -
.3 -
2 -
1 -
O
G H. t i I
(production)
C M. 6 I I
(pre-production)
lul 01
nnmullv asoirated - ^ A
lurbocttdryrd | Q
turbochdrged/ • O
dfLercooleO
unkfiOMii *
1 1
I 2
3
5 6
NO* (g/bhp-hr)
-r-
8
~i 1—
9 l.O
m
n
0)
tO
-<
Q>
3
Q_
jD
fD
(/>
O
c
o
(D
O
O
3
in
c_
&
D
3
O
Figure 4.10: NO versus particulate emissions for current heavy-duty diesel engines,
measured on the EPA transient test.
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Several points about Figure ^.10 should be noted. The most important point is the large
range in emissions performance among engines in this group. For a given NOx emissions
level, particulate emissions levels may vary by a factor of more than two. There is also
little systematic variation — NOx and particulate emissions seem to be only vaguely
correlated from engine to engine, and they are not strongly correlated with either the
use or non-use of aftercooling. The three naturally-aspirated DI engines, however, are
all relatively high in emissions, while the three prechamber engines are among the lowest
emitters. Significantly, most of the lowest emitters on the chart are turbocharged and
aftercooled, but so, too, are some of the highest.
Perhaps the most important point about Figure 4.10 is the fact that three fairly distinct
groups of engines are visible. Figure 4.11 outlines these groups. Group I, toward the
right of the chart, contains engines using more or less standard technology from a few
years ago, calibrated to meet the present Federal emissions standards. These engines are
relatively low in particulates, but high in NOx. Group II, in the upper left, consists of
engines using the same level of technology but calibrated to meet California emissions
standards, with lower NOx levels but much higher particulates. Group III, contains those
engines which — either by fortunate accident or by careful design -- are able to minimize
both NOx and particulate emissions. This group includes all of the prechamher engines, a
number of advanced-technology low-emission DI engines (many of which are not yet in
production), and a few older direct-injection engines with exceptionally good emissions
characteristics.
In some cases, the characteristics which separate the older engines in Group III from
those in Group I are not completely evident, even to their designers. Probably, they
include a better match between the engine and the turbocharger, more closely optimized
fuel-air mixing, and possibly better design of the combustion chamber. High-qualitv,
high-pressure, high-precision fuel injection systems are also somewhat more common in
this group. However, high-pressure injection per se does not produce lower emissions —
it must be carefully matched to the combustion characteristics of the engine. It is
hypothesized that this has been done more effectively in those older eneines which fall
into Group III.
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PARTICULATE
(g/bhp hr)
I.O
I
OJ
.8 -
.7 "
.6 -
.4
.3 -
ID! 01
normally asmraUiI A A
lurlioc harmed ¦ Q
turbnrharyed/ • O
aftercooled
unknown ^
T
1 2
m
u
O
o
D
(/)
C_
5T
3
o
NOv (g/bhp hf)
Figure 4.11: Heavy duty N0x versus particulate emissions — engine groupings.
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4.4 ACHIEVABLE ENGINE-OUT EMISSIONS LEVELS
As discussed above, present heavy-duty engines can be divided rather naturally into two
ciasses: light-heavy engines and all the others. It is convenient to discuss the future
emissions levels achievable by these engines in the same format.
4.4 J Light-Heavy Duty Diesel Engines
The present light-heavy IDI engines are already very low emitters, thus additional reduc-
tions cannot be expected to come easily. The major near-term improvement is likely to
result from the application of electronically-controlled injection timing and possibly an
electronic governor. Both of these improvements will probably be introduced as quickly
as possible in order to reduce fuel consumption. Other feasible near-term control tech-
nologies such as EGR are unlikely to be introduced without regulatory pressure, due to
their deleterious effects on fuel economy and cost. Any significant near-term regulatory
pressure in this regard would require a special standard aimed specifically at light-heavy
engines, since these engines could readily comply with any reasonable near-term standard
set for the larger direct-injection diesels.
Several manufacturers, including Isuzu and Cummins, have developed small, high-speed
direct-injection diesel engines for use in the light-heavy duty class. Because of their
superior fuel economy, these engines are expected to claim a significant share of the
market. However, D1 engines generally have greater emissions than IDI engines, so that
total emissions levels in this class could be expected to increase as a result. On the
other hand, ail of the engines being introduced are technologically quite advanced (the
Cummins engines, for instance, use full electronic controls), and could thus achieve a
stringent emissions standard more readily than the larger DI engines in the heavier
classes. Overall, then, both the DI and IDI engines in the light-heavy class should be able
to attain any emissions levels which are feasible for engines in the heavier classes, and
could do so at least two years earlier.
In the longer term, only marginal improvement in emissions is to be expected in this class
unless regulations are tightened significantly. Electronic governors and closed-loop con-
trol of injection timing, possibly combined with turbocharging, would probably lead to
some improvements in particulate emissions, but these technologies would probably be
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adopted for improved fuel economy anyhow. If small, high-speed direct-injection engines
are successfully introduced in the light-heavy class, the total emissions could even in-
crease somewhat, since such engines are likely to be higher emitters than the IDI models.
Medium-Heavy. Line-Haul Truck, and Transit Bus Engines
Unlike the light-heavy duty engines, engines in this group are likely to be seriously con-
strained by proposed emissions standards. Thus, a strong and continuing effort to perfect
and introduce emission control technologies is under way, and would be further acceler-
ated by adoption of a definite standard. In the near term, a combination of improved
engine/turbocharger matching, higher precision and higher pressure fuel systems, charge-
air cooling, and careful recahbration and optimization will probably be able to bring all
but a few engines down into the emissions range occupied by Group III (the best present-
day engines) by 1987 or 1988 — which is the time frame for which standards are being
discussed. Introduction of methanol-fueled engines for transit buses (and possibly other
vehicles such as garbage trucks) would bring emissions for those engines down to very low
levels, but it is too early to predict whether that will occur. Electronic engine controls
will definitely be introduced in a few models during 1985-86, and should become fairly
common by 1988. It is unlikely, however, that they could be applied across the entire
product line before about 1990.
Those engines which cannot be brought into compliance by means of the technologies
listed above will probably be dropped, or produced in very limited numbers under an
averaging scheme. It appears certain that at Least some such engines would be dropped
entirely — both Caterpillar and Mack have indicated a possible need to trim their product
lines in response to a strict standard.
Curve A in Figure 4.12 shows the approximate average NOx-particulate tradeoff rela-
tionship which is estimated to be achievable by 1987, assuming vigorous application of
the technologies discussed above, and assuming that definite, achievable, numerical
standards for NOx and particulates are adopted in the near future. It should be carefully
noted that this curve shows the estimated average level of new-engine emissions, not the
level of the numerical standard which is estimated to be achievable by 1987. Derivation
of an estimated achievable standard is done in Chapter 7, and results in higher standards
for NOx and particulate emissions than are shown here as estimates of actual emissions.
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I
LP
PARTICULATE
(g/bhp hr)
l.O
.9 -
.8 -
.7 H
.5 -
.4 -
.3 -
.2 -
G M 6 2 1 —
(pre-production)
JDI
normally tsplraled - ^
C url*ic ¦
lurfoof har
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Figure <*.12 also shows two GM estimates of average engine-out emissions levels from
new engines in 1985 and 1990. As can be seen, the GM 1985 estimate agrees fairly well
with the authors' estimate for 1987.
In the medium term — beginning about 1988 and extending to about 1991 — it can be
expected that an entire new generation of sophisticated emissions control technologies
will come into production. The most important of these will be fully integrated elec-
tronic controls for injection timing, the engine governor, and (if needed) EGR modula-
tion. Exhaust gas recirculation is expected to be widely adopted in heavy-duty engines
only if a very stringent NOx standard is imposed. Electronic controls, in contrast, would
be introduced in most models quite independently of any emissions standards, due to their
beneficial effect on fuel economy, performance, and driveability. Other advanced tech-
nologies which may be introduced on a few models in the 1988-1991 timeframe include
the Comprex supercharger, the three-wheel turbocharger, and uncooled (or adiabatic)
engines.
These technologies — especially the electronic control systems — can be expected to
produce significant improvements in particulate emissions at constant NOx, as well as
better engine performance and fuel economy. The cost of a tightened engine-out par-
ticulate standard would thus be fairly small, since in most cases the emissions control
equipment would already have been installed for other reasons. Alternatively, some of
the benefits of improved technologies could be sacrificed to obtain lower NOx emis-
sions. In this case, the additional first cost attributable to the regulations would be
moderately small (since, except for EGR, the emissions control equipment would already
be in place), but the regulation should be charged with the full cost of lost performance,
increased fuel consumption, and impaired driveability due to altering control system
calibrations to minimize NOx> These issues have been discussed at greater length else-
where (Weaver, 1984b).
In addition to their direct effects on emissions, improved control techniques should result
in lower variability and more accurate prediction — especially of particulate emissions —
thus making possible a more stringent standard even in the absence of any change in
average emissions.
The precise degree of improvement in emissions that can be expected and the exact
shape of the NOx-particulate tradeoff curve with these new technologies are both highly
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speculative — there is simply not enough information publicly available to judge. Curves
B and B' in Figure 4.12 give some idea of the range that might be expected. As noted
above with respect to curve A, these curves represent estimates of actual new-engine
emissions levels which may be achievable — assuming vigorous application of the tech-
nologies discussed — not achievable standards. As can be seen, the GM estimate of
average new-engine emissions for 1990 falls near the middle of this range.
Curves B and B' have been constructed using engine manufacturers1 projections of capa-
bilities (e.g., GM, 1984b), and limited test data provided on a confidential basis, as modi-
fied by the authors' engineering judgement. Given the general lack of hard data and
reportable results, however, these curves should be understood as being little better than
educated speculation, and used with appropriate caution. These curves are believed by
the authors to be reasonable, but it is very easy to imagine future developments which
would reveal them as either over-optimistic or over-pessimistic. Because of this, any
thought of basing actual standards on these estimates would be premature. Further
research to clarify the technological issues and capabilities in this area is urgently
needed before final standard-setting for the 1990 timeframe takes place.
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5.0 TRAP-OXIDIZER SYSTEMS FOR HEAVY- DUTY VEHICLES
Trap-oxidizers are presently the only very promising aftertreatment technology for
reducing diesel particulate emissions. A trap-oxidizer system consists of one or more
particulate traps (filters) which remove particulate material from the exhaust, together
with a system for cleaning the traps by burning off (oxidizing) the collected particulate.
Interest in trap-oxidizer technology was initially stimulated by the introduction of diesel-
powered light-duty cars and trucks in the late 70's, which led EPA to DroDose stringent
particulate emissions standards for such vehicles. In response to these standards, trao-
oxidizer technology for light-duty vehicles has been developing rapidly. The first mass-
produced trap-oxidizer systems were introduced on model-year 1985 Mercedes-Benz
sedans sold in California, and a number of manufacturers appear to be in the late stages
of development and testing for model years 1986 and 1987.
Trap-oxidizer technology for heavv-duty vehicles has undergone much less development.
The reasons for this include: less regulatory pressure (no final heavy-duty oarticulate
standard now exists), the lesser R and D capacities of many heavy-duty engine manufac-
turers, the depressed state of the heavy-duty vehicle and engine market between 1980
and 1983, and the greater difficulty of the development tasks for heavv-duty vehicles.
The little heavy-duty trap-oxidizer develoDment that has been done has been aimed pri-
marily at the adaptation of light-duty trap-oxidizer concepts to heavy-duty applica-
tions.
Because of the lack of trap-oxidizer development for heavy-duty vehicles, this chapter
draws extensively on the accumulated experience and testing in the light-duty field. The
applicability of this experience to heavy-duty vehicles, and the problems involved in
adapting light-duty technologies to heavy-duty vehicles are discussed as well. The dis-
cussion is based in large measure on the authors' previous report on heht-dutv trap-
oxidizer technology (Weaver and Miller, 1983), and a subsequent paper by one of them
(Weaver, 1983a). Up-to-date information on current developments in the field and spe-
cific data on the development of heavy-duty trap-oxidizers have also been included
where applicable.
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T
This chapter is organized into three main sections. Section 5.1 discusses the present
status of trap oxidizer technology, with special attention being given to the major
components of a trap-oxidizer system: the trap, the regeneration system, and the
controls. Since much of the discussion in Section 5.1 concerns trap-oxidizers for light-
duty applications, Section 5.2 focuses on the special considerations involved in applying
trap-oxidizers to heavy-duty vehicles — either by adaDting light-duty technologies or by
developing new approaches suited to the special problems and opportunities of heavy-
duty vehicles. These considerations include both technical and institutional barriers to
trap-oxidizer deployment. Section 5.3 examines some of the more promising potential
configurations for heavy-duty trap-oxidizer systems. Where Section 5.1 focuses on the
technologies of the individual components, 5.3 concentrates on the systems aspects of
trap-oxidizers, including the synergistic interactions between the components, and
between the system and the vehicle.
5.1 TRAP-OXIDIZER TECHNOLOGY
Trap-oxidizer technology can be divided into three separate sub-areas: the traps them-
selves, regeneration techniques, and the controls and sensors necessary to initiate and
control the regeneration process. These three areas are discussed separately in Sections
5.1.1 through 5.1.3 below. It should be borne m mind, however, that the interactions
between these areas are also very important. The choice of a trap defines manv of the
requirements for the regeneration system, and may make possible different approaches
to regeneration. Similarly, the requirements which the control system must meet are
largely determined by the characteristics of the regeneration system it is controlling and
the tolerance limits of the trap. These synergetic effects are discussed at greater length
in Section 5.3, in which several possible combinations of trap, regeneration technique,
and control system are described.
5.1.1 Diesel Particulate Traps
A great number and variety of filter materials have been investigated for use as diesel
particulate filters (traps). At the present time, attention is focused on four major
classes of traps: ceramic monolith filters, ceramic foam traps, ceramic-fiber based
traps, and an alumina-coated catalytic wire mesh design. Each of these types of traps
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except the ceramic-fiber type (which was developed specifically for heavy-duty service)
has seen extensive development in light-duty vehicles. This discussion will focus on the
ceramic monolith, ceramic fiber, and catalyzed wire-mesh traps, since the available data
indicate that ceramic foam traps have given poor results in heavy-duty service.
Ceramic monolith traps: The ceramic monolith is the most widely tested and developed
type of particulate trap. Virtually every manufacturer of light-duty diesels has at least
one trap-oxidizer system based on this trap under development, and it has been tested by
most heavy-duty diesel manufacturers as well. The monolith has found favor because of
its high trapping efficiency, temperature tolerance, demonstrated durability, and com-
paratively tow cost. These have apparently outweighed its drawbacks — high
backpressure, rapid increase of backpressure with filter loading, susceptibility to crack-
ing due to thermal stresses and (in heavy-duty service) possible clogging due to ash reten-
tion. The monolith trap is also fairly adaptable — the porositv, cell density, and wall
thickness of the ceramic monolith can be altered in order to trade off filtration effi-
ciency for backpressure. The monolith trap is also relatively easy to coat or impregnate
with either a precious metal catalyst (like the ones used in catalytic converters) or with
a base metal catalyst. The use of such catalysts could make possible a simpler form of
regeneration system.
Figure 5.1 is a face view of a ceramic monolith trap, of a size intended for light-duty
service. Figure 5.2 shows the principle of filtration of this trap. The basic unit is a
cellular ceramic monolith of the type used as a catalyst support in the catalytic con-
verters of many gasoline-fueled automobiles. The monolith has been modified by block-
ing the upstream and downstream ends in alternating cells with a ceramic material.
Because each cell is blocked at one end or the other, the particulate-laden exhaust *as is
forced to flow through the porous cell walls. Most Darticulate matter is retained on the
cell walls, where it builds up in a carbonaceous layer. This layer is also an effective
particulate filter, which means that the trapping efficiency of these traps can actually
increase as they become more heavily loaded.
Monolith traps are manufactured by extruding the square matrix of the trap in the same
manner as for catalytic converter substrates. Alternate holes in this matrix are then
blocked with a ceramic cement, and the whole assembly is fired. At present, the capa-
bilities of the extrusion equipment limit the maximum trap diameter to about 6 inches —
a size suitable for light-duty but not for heavy-duty applications. Prototype heavy-duty
traps of up to 12 inches diameter have been produced by cutting light-duty traps aoart
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«..2^ŁSSŁK3:;-R.
Z^JSSSSSS^v^fSg&
ffl«Hsr#»«rairfflrBrHr«-:Qn«^)S-y)g''a*^5i^
ammm v «r mtr armr ar
mmmmm*mrmrmnrGtt~iBnBrm^mmm-m't
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^i558BaSte.%g
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Figure 5.1: Face view of a ceramic monolith trap (courtesy of Corning Glass).
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ENGINE
EXHAUST
LINE POROUS WALLS
EXHAUST BUIID-UP
Figure 5.2: Principle of operation of the ceramic
monolith trap.
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into squares, gluing the squares together using a high-temperature ceramic cement, and
then trimming the resulting block to shape (Howitt et alia, 1983). Corning Glass had pro-
duced approximately 50 such prototypes as of late 1983: all, apparently, for research and
development on heavy-duty trap-oxidizer systems. Facilities for direct extrusion of
heavy-duty sized monoliths are being developed by both Corning and NGK. Such facili-
ties will be needed for commercial production to be feasible; the expensive handwork
involved in building the prototype traps makes them too expensive for production use.
The monolith trap's major drawbacks are high backpressure, rapid increase of back-
pressure with filter loading, susceptibility to cracking due to thermal stress, and possibly
excessive backpressure increase due to ash buildup in heavy-duty service. The high ini-
tial backpressure can be traded off against filtration efficiency, and can be reduced by
increasing the size of the trap. In the same way, the backpressure rise rate can be
reduced by increasing the trap size, or it can be compensated for by more frequent re-
generation. The problem of cracking due to thermal stresses is much more severe — it is
presently one of the the major barriers to the successful development of light-duty trap-
oxidizer systems based on the ceramic monolith. Curiously, however, thermal cracking
has not been reported to be a major problem in the larger heavy-duty monoliths produced
to date. This may be a result of the stress relief provided by the cemented joints in the
prototype units (3.H. Howitt, personal communication).
Thermal-stress cracking is caused by the interior of the trap being hotter than the
exterior, and thus expanding more. This occurs during the regeneration process, when
the intense heat generated by the burning particulates can raise the temperature of the
trap interior above 1000° C if sufficient cooling gas flow is not maintained. The
differential expansion results in the exterior layers of the trap being "forced apart"
(placed in tension) by the expanding interior, while the interior portions are "squashed
together" in compression by the force of the exterior. The trap cracks when the tension
in the outer layers exceeds the strength of the material.
Two approaches to solving the thermal cracking problem are being pursued. One is to
carefully define those areas of trap operation (trap loading, gas flow rate, and gas
temperature) at which cracking does not occur, and then to design the regeneration sys-
tem to operate always within those areas. The second approach is to improve the struc-
ture and materials of the trap in order to reduce the probability of cracking—in effect,
to expand the area of safe operation to include a wider range.
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Energy and Resource Consultants, Inc.
Considerable progress has been made in defining the areas of safe operation for light-
duty ceramic monolith traps (Gulati, 1983; Higuchi et alia, 1983), so that it now appears
to be possible to design a positive regeneration system of the burner type which will not
destroy the trap through overheating and thermal stress. It is not yet clear, however,
whether the trap's resistance to thermal stress is adequate for self-regenerating systems,
in which the regeneration frequency and trap loading during regeneration are subject to
much less control by the designer. Recent statements from knowledgeable sources in the
industry, however, indicate that this is not a major problem at this time.
One area of considerable concern with ceramic monolith traps for heavy-duty applica-
tions is a possible long-term increase in backpressure due to ash buildup in the trap.
Researchers at Cummins (Sachdev et alia, 1983) have reported that a significant back-
pressure increase was obtained by simulating the long-term production of ash by means
of a diesel-fuel burner and fuel "doped" with the ash's metallic constituents. Their
results indicated that ash-loading would double the "clean-trap" backpressure after about
90,000 (simulated) kilometers. This effect, if it were actually experienced in practice,
would require replacement of the trap every 100,000 km or so. Daimler-Benz (DBAG,
1984) has also indicated that its traps (which are of a different design) also suffer from
ash buildup and plugging after 100,000 to 150,000 miles.
Other evidence, however, indicates that the problem may not be as severe as the
Cummins study would indicate. Perhaps the most persuasive evidence to date is from an
EPA-sponsored 50,000 mile (80,000 km) durability test of a ceramic trap, carried out at
Southwest Research Institute. At the end of this test, the "clean trap" backpressure had
increased by only about 20 percent (Urban, 1982). Furthermore, in a test involving the
use of metallic fuel additives (Wade et alia, 1983), it was found that a very large amount
of metalliferous ash (enough to substantially block the passages in the filter) could be
accumulated without major increases in backpressure. Volkswagen (Wiedemann et alia,
1983) has reported similar results. This would seem to indicate that the much smaller
amount of ash accumulated without fuel additives would have little effect. On the other
hand, the use of fuel additives reduces the regeneration temperatures, which could have
affected the degree of trap plugging experienced.
Another possible reason for the discrepancy between the Cummins study and other data
is the difference in the conditions of combustion and ash formation, which could
conceivably have led to differences in the structure or composition of the deposited
ash. Sachdev et alia used a low-pressure continuous-combustion burner system. Actual
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Energy and Resource Consultants, Inc.
vehicle combustion, however, occurs intermittently, and at very high pressures and
temperatures. This could lead to different sizes and structures of ash particles. In addi-
tion, Sachdev and co-workers used a much greater rate of ash deposition with time than
would be observed in a real engine, which could conceivably have affected the results.
Because of the discrepancies between Sachdev and his co-worker's model and the actual
diesel engine, it presently appears more reasonable to assume that the other data availa-
ble (indicating lesser effects from ash accumulation) are closer to being correct. Using
this assumption, a trap in heavy-duty service could be expected to last for 150,000 to
250,000 miles before needing to be cleaned or replaced. However, this is an issue which
should be resolved by further research.
Ceramic fiber traps: A number of manufacturers have reported tests of ceramic-fiber
based traps. These include General Motors (Ludecke and Dimick, 1983; GM, 1982a),
Caterpillar (Caterpillar Tractor, 1982), and Daimler-Benz (DBAG, 1982; 1984). Cater-
piller has reported experimenting with a trap utilizing "ceramic yarn", but provided no
details as to configuration or results. The experiment was apparently dropped due to
difficulty in developing a space-efficient support system for the yarn. GM has experi-
mented with a ceramic fiber mat or felt supported by a perforated metal tube. An
example of this type of trap is shown in Figure 5.3. GM's results with this trap have been
generally discouraging — disintegration of the mat, cracking, and separation of the mat
from its supports have been the major problems. Daimler-Benz also experimented with
and abandoned a similar trap.
Daimler-Benz (or one of Daimler-Benz's suppliers — the submission is not clear) has also
developed another, much more promising type of trap based on ceramic fibers. This trap
consists of strands of woven silica-fiber yarn, cross-wound on a porous metal substrate as
shown in Figure to form cylinders which Daimler-Benz refers to as "candles". These
candles are supported in a cannister, as shown in Figure 5.5, so that the exhaust gas must
flow through the candle wall to escape. The silica fibers are roughened and impregnated
with a heat-resistant inorganic substance to improve filtration.
Daimler-Benz has reported excellent results for this trap, including very high efficiencies
(above 90%) at back-pressure levels below those of the ceramic monolith. The trap has
been tested on a number of heavy-duty vehicles, and for a total of at least 200,000 kilo-
meters, including 132,000 km on a single trap (H. Hardenburg, personal communication).
The durability of the traps is fairly good. In city bus service (a fairly demanding applica-
tion) the traps are reported to last between 100,000 and 150,000 miles. Failure occurs as
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TRAP MATERIAL: CERAMIC FIBER WOOL
WOVEN CERAMIC FIBER MAT
Figure 5.3: Ceramic fiber mat trap. (Source: DBAG, 1982)
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Figure 5.4: Support pipe with silicon dioxide thread windings for the Daimler-Benz
silica-fiber candle trap. (Source: OB AG, 1982)
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>»»>*»)>} j win
WA'AsVW \^w\\\\VV *A k
Ą}>?»)>»*>>>>»»}}»»>>>>
Figure 5.5: Daimler-Benz silica-fiber candle trap (Source: DBAG, 1982).
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the result of the trap plugging up with non-combustible ash, resulting in unacceptable
back-pressure increases (DBAG, 1984). Much of this ash is due to the sulfur content of
the fuel, so that trap lifetime would be strongly affected by the use of high-sulfur or
low-sulfur fuels.
With fair durability, no thermal cracking problems, very high temperature tolerance, and
low back-pressure, this type of trap appears (at least on the basis of the limited data
available) to be very nearly ideal. The trap is also well suited to a catalytic regeneration
system developed by Daimler-Benz, whicn is described in Section 5.1.2 below. Its most
significant drawback may be its size — it seems likely to require significantly more space
than the ceramic monolith.
Catalyzed, radial flow wire-mesh traps: This type of trap has been developed and pro-
moted by Johnson-Matthey Incorporated; thus it is commonly referred to as the
"Johnson-Matthey trap". The operation and development have been described by Enga
(1982) and Buchman and Enga (1983). Its application to various off-road and heavy-duty
applications is described by Budd and Enga (1984).
Figure 5.6 shows a typical trap of this type, designed for installation in place of the ex-
haust manifold on a light-duty engine, while Figure 5.7 shows an underfloor design. Ver-
sions intended for heavy-duty use are very similar in overall design, differing mainly in
the number and length of the cylinders. The trap consists of cylindrical sections of knit
stainless-steel mesh, with the density of the mesh increasing toward the center of the
section. Exhaust gas flows from the periphery of the cylinder radially inward toward the
hollow space in the center, from which it leaves the trap. The fact that the density of
the mesh increases toward the center of the trap helps to prevent a thick layer of par-
ticles from building up around the periphery, and also helps to prevent problems with
particles blowing off of the mesh with sudden increases in speed. The mesh is coated
with a layer of alumina, to which a precious-metal catalyst washcoat is applied.
The precious-metal catalyst is the key to the wire-mesh trap's operation. Like a gaso-
line-engine catalytic converter, the catalyst oxidizes gaseous hydrocarbons and CO. In
addition to reducing the emissions of these two pollutants, the oxidation of the gaseous
hydrocarbons also cuts down significantly on odor emissions. The presence of the
catalyst also appears to result in the oxidation of much of the hydrocarbon fraction of
the particulate material. Most importantly, the catalyst reduces the light-off tempera-
ture of the particulate from about 500-600° C to about 350-400° C, reducing the amount
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3 C
Figure 5.6: Catalytic wire-mesh trap, engine manifold location. (Source: Johnson-
Matthey, Inc.)
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Figure 5.7: Catalytic wire-mesh trap, under-floor location. (Source: Volkswagen
A G, 1981)
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of energy required for regeneration. The use of the oxidation catalyst also makes possi-
ble a simplified regeneration scheme, in which the exhaust is enriched in HC and CO,
which then oxidize exothermically on the catalyst, generating the heat required for
regeneration. This scheme is discussed in the section on regeneration techniques.
Some of the primary advantages of this type of trap as compared to the ceramic mono-
lith are a slightly lower initial backpressure and a slower rate of backpressure increase,
which can be made slower yet by the trap's ability to self-regenerate under many driving
cycles. This would be especially true in heavy-duty service, where high exhaust tempera-
tures are more common than in light-duty vehicles. An additional advantage is that the
stainless steel wire-mesh filtering material is not subject to thermal cracking, and its
melting point is comparable to that of the cordierite ceramic which makes up the
ceramic monolith. Because of this, the wire-mesh trap is able to tolerate a greater
range of particulate loadings during regeneration. This simplifies both the regeneration
process and the task of the control system.
The primary disadvantages of the wire-mesh trap are lower trapping efficiency and a
tendency to increase sulfate emissions. In addition, the cost of the trap itself is higher
than that of an uncatalyzed ceramic trap, although the simpler regeneration system it
makes possibly may offset this. The lower trapping efficiency is probably not crippling —
typical efficiencies in light-duty service are in the 50 to 70 percent range, but efficien-
cies as high as 80 percent have been demonstrated in light-duty service (Buchman and
Enga, 1983). These efficiency levels are sufficient to meet the proposed 0.25 g/BHP-hr
particulate standard.
The problem of increased sulfate emissions is, however, a major drawback. These emis-
sions are due to the presence of the oxidation catalyst, which oxidizes some of the SO2
normally present in diesel exhaust to SO^. The SO3 then combines with water to form
sulfuric acid (h^SO^). This process also occurs naturally in the atmosphere, so the effect
of the catalyst is not to increase the total environmental load of sulfate but simply to
change its distribution — resulting in a greater concentration near roads and urban
areas. These areas are, however, the areas in which humans are more likely to be found,
so that total human exposure to sulfates is increased. Since sulfuric acid is a strong
irritant as well as being highly corrosive, any significant increase in human exposure to it
would be of great concern.
L
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Johnson-Matthey has reformulated its catalyst in order to minimize sulfate conversion in
light-duty service, while still attaining low regeneration temperatures. Using the re-
formulated catalyst, between one and five percent of the sulfur emitted under typical
light-duty operating conditions is emitted as sulfate (compared to values from about .2 to
.6 percent for a vehicle without a catalyst). For the higher temperatures achieved in
heavy-duty operation, however, sulfate conversion is still so great as to rule out the use
of this trap in its present form. The results of tests on the present catalyst formulation
carried out at Michigan Technological University (Scholl et alia, 1982) indicate that at
temperatures typical of high-power heavy-duty operation, from 50 to 100 percent of the
sulfur in the fuel is converted to sulfate. Similar experiences are reported by Pattas et
alia (198*0 and by Wong and coworkers (1984). Since the sulfate is also a particulate
material, this can increase the total particulates leaving the trap to several times the
level seen without the trap in place. EPA transient tests conducted by several manu-
facturers have also indicated a several-fold increase in particulates oyer the engine-out
emissions, with virtually all of the particulate being sulfuric acid.
3ohnson-Matthey has formulated a new version of its catalyst for heavy-duty use, which
it believes will reduce sulfate conversion to a much lower level (M. Buchman, personal
communication). This is achieved at some cost in increased regeneration temperature,
since the catalyst's efficiency at oxidizing sulfur dioxide cannot be completely separated
from its efficiency at oxidizing HC and CO. Since high exhaust temperatures are more
common in heavy-duty than in light-duty service, this should not present a significant
problem. Preliminary laboratory tests of this formulation (Budd and Enga, 1984) appear
promising, but no in-use data are available. Thus, it cannot presently be stated whether
this type of trap appears workable for heavy-duty vehicles.
Johnson-Matthey is presently planning to test one of its heavy-duty traps on a Southern
California Rapid Transit District bus in the near future, as part of a cooperative program
with the SCRTD and the California Air Resources Board. This system will include an
automatic regeneration and control system, as well as the trap itself. If successful, this
program will give a good indication of the overall performance of the Johnson-Matthey
trap in heavy-duty service. It will not, however, indicate anything about the sulfate
problem, since the South Coast Air-Quality Management District — where the test will
take place — is presently the only area in the U.S. which requires low-sulfur diesel fuel.
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5.1.2 Regeneration Techniques
Regeneration is the process of burning off (oxidizing) the particulate material trapped on
a particulate filter, thus restoring the "clean trap" filtering efficiency and backpres-
sure. In order to oxidize the particulate completely, it is necessary to raise its tempera-
ture to the ignition point while providing a steady flow of oxygen-bearing gas to support
combustion and to carry away the excess heat generated. With sufficient oxygen, and in
the absence of catalysts, diesel particulate matter ignites at a temperature between
about 500° and 600° C (900° to 1100° F). The actual ignition temperature is dependent
on both the design of the trap and the specific operating conditions under which the
particulate was collected. The required ignition temperature increases sharply at low
oxygen concentrations, and decreases slightly at higher ones.
Once ignited, the particulate material will continue to burn at gas temperatures some-
what below the ignition temperature. The tolerable reduction in gas temperature below
the ignition point is fairly small, however, and heavily dependent on the particulate load-
ing of the trap and the oxygen content and flow rate of the gas. If the temperature of
the gas is too low, or the flow rate is too high, too much heat is removed from the burn-
ing particulate and combustion ceases. On the other hand, if the flow rate is too low or
the temperature of the gas supplied is too high, too little heat is removed from the trap,
which leads to overheating and cracking or melting. In order to attain complete re-
generation while minimizing the possibility of destroying the trap, it has been found best
to maintain a substantial flow of gas through the trap, with the temperature of the gas at
or somewhat above the ignition point.
Diesel exhaust gas normally contains adequate oxygen to support combustion, since
diesels operate at low fuel-air ratios, but generally does not attain the temperatures
required for ignition. Thus, regeneration does not usually occur spontaneously. In order
to cause regeneration to occur, it is necessary either to raise the temperature of the
collected particulates to the ignition point or to lower the ignition point of the particu-
lates to match the available temperatures.
There are two approaches to initiating regeneration. The first approach — self-
regeneration — relies on attaining the conditions required for regeneration during the
normal operating cycle. Generally, this has led to attempts to reduce the ignition temp-
erature of the particulate to within the range of normal exhaust-gas temperatures, so
that regeneration will occur during normal operation. The alternative approach to initi-
i
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ating the regeneration process is called positive regeneration. In this approach, a deci-
sion is made to regenerate the trap at a specific point in time, and positive actions are
taken to ensure that regeneration occurs. These actions may include turning on a burner,
activating a throttling device, or injecting fuel or a catalyst into the exhaust stream,
among others. The decision to regenerate may be made on the basis of some measure of
trap loading (such as increased backpressure) or regeneration may be scheduled to occur
every so many miles or engine revolutions.
The self-regeneration approach is attractive because of its potential simplicity, but it
places greater demands on the trap. This is because the designer has little control over
the degree of particulate loading or the gas flow rate at which regeneration will occur.
Given the somewhat random nature of driving patterns, it is necessary to ensure that
regeneration will occur fairly frequently on the average, in order to be certain that the
trap will not be overloaded under unusual driving conditions. This is especially problem-
atic for heavy-duty vehicles, since a much wider variety of operating patterns is found in
heavy-duty service than in light-duty applications. On the other hand, self-regeneration
might be a very attractive option in those heavy-duty applications (such as transit buses
and garbage trucks) where the operating pattern is well known in advance.
Positive regeneration systems give the designer much more control over the regeneration
conditions, and thus place fewer demands on the trap itself than self-regeneration
systems. However, they require more or less elaborate systems of controls, sensors, and
actuators in order to carry out the regeneration process. These add complexity and ex-
pense to the system, and may present reliability, durability, and maintenance problems as
well. Despite these drawbacks, most heavy-duty engine manufacturers seem to be lean-
ing toward positive regeneration systems for their trap-oxidizer development programs.
This is in strong contrast to the situation in the light-duty field, where many of the major
manufacturers have abandoned positive regeneration systems for self-regeneration.
Positive regeneration techniques; Most present positive regeneration systems involve
raising the temperature of the particulate in order to ignite it. Some techniques for
accomplishing this include: throttling the engine to increase exhaust temperature, use of
a diesel oil burner or an electric heater upstream from the trap, and using the exother-
mic oxidation of hydrocarbons and CO on a catalyst to increase the particulate tempera-
ture. The most promising of these techniques are the diesel oil burner and the catalytic
heating approaches. Daimler-Benz has also experimented with a positive regeneration
technique for heavy-duty vehicles which injects catalytic metal additives into the
exhaust stream to lower the particulate ignition temperature (DBAG, 1982).
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The development of diesel oil burners for light-duty applications has been described in
detail by Wade and co-workers (1983) at Ford Motor Company. Similar systems have also
been experimented with by most manufacturers of diesel automobiles, and by several
heavy-duty engine manufacturers (Cummins, 1982; Caterpillar, 1982). The advantages of
the diesel oil burner include the use of a readily available energy source, a high energy
release rate, and a well-understood technology base. The disadvantages include com-
plexity, high cost, and questionable durability and reliability under the hot, sooty, and
intensely oxidizing conditions found in diesel exhaust.
A trap-oxidizer system incorporating an oil burner can be arranged either so that the
trap is isolated from the exhaust system during regeneration (a "byoass system) or so that
exhaust continues to flow through the trap during the regeneration process (an "in-line"
system). The in-line system is less complex, but entails difficult control problems due to
the rapid changes that are possible in the exhaust flow-rates. Fouling of burner and
ignitor surfaces by soot is also more likely in an in-line system. This method also suffers
from a major disadvantage in that it must heat the entire flow of exhaust gas, which is
usually much more than is required to regenerate the trap safely. With the high exhaust
flow rates in heavy-duty trucks, this can waste a significant amount of energy, and it
also requires a larger burner than would otherwise be necessary.
Bypass systems avoid the energy waste and control problems of the in-line systems, and
permit closer control over regeneration conditions. They could be expected to suffer,
however, from higher costs and possibly from lower reliability than the in-line systems,
due to the additional valves, actuators, and exhaust plumbing entailed. Despite these
problems, the bypass/oil burner system seems to be the one that most heavy-duty engine
manufacturers are considering. Both types of systems are under active development in
light-duty vehicles however, and both in-line and bypass burner systems have actually
been installed and tested in light-duty service (Ford, 1982; Oser and Thorns, 1983; GM,
1981). The control technology for in-line burner systems (if it is developed at all) will
probably be developed first for light-duty vehicles, and then transferred to heavy-duty
applications. The only burner regeneration systems known to have undergone significant
testing in heavy-duty vehicles were of the bypass type.
A second promising class of positive regeneration systems makes use of the exothermic
oxidation of HC and CO to heat the exhaust and/or particulate to the light-off point.
This approach is applicable only to catalyzed traps. The triggering step in this process is
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to greatly increase the HC and CO content of the exhaust at a time when the exhaust
temperature is above the catalyst's light-off point. The excess HC and CO are then oxi-
dized by the catalyst in the same manner as in a catalytic converter, and the heat given
off by the oxidation reaction ignites the particulate. This approach has significant limi-
tations — it will not work at low loads or at idle, for instance, because the catalyst must
be fairly hot in order to function. However, it has the major advantage that it does not
require as complex or expensive a control and actuation system as most other positive
regeneration techniques.
Four techniques have been developed for increasing the HC and CO level in the exhaust.
The first such technique is to inject fuel into a cylinder during the exhaust stroke. At
this point, the exhaust gas has cooled enough to prevent the fuel from igniting, but is still
hot enough to vaporize it and to crack the heavier hydrocarbons into lighter species.
These hydrocarbons are then oxidized by the catalyst. A crude version of this svstem
was used on the 50,000 mile durability test vehicle sponsored by Johnson-Matthey, with
considerable success (Enga and Bykowski, 1982). Due to the difficulties in adapting this
technique to high-pressure injection systems, it is unlikely to see much application in
heavy-duty vehicles.
Three other techniques for increasing the HC and CO levels have been described by
Buchman and Enga (1983). Two of these require the presence of a digital electronic con-
trol system for injection timing and/or EGR. These control systems continously optimize
the parameters under their control in order to provide the best combination of emissions
control and driveability. It is a relatively simple matter, given such a control system, to
have it deoptimi2e the appropriate setting instead of optimizing it, thus greatly increas-
ing the pollutant concentrations in the exhaust. There is some doubt as to whether EGR
(and thus electronically controlled EGR) will ever see much application in heavy-duty
engines, but the technique of over-retarding injection timing would certainly be applica-
ble. Essentially every heavy-duty engine manufacturer is now engaged in development
and testing of electronic injection timing controls, and they can be expected to see very
wide application (due to their beneficial effects on fuel economy and performance at low
NOx levels) during the last half of this decade.
The last technique for increasing HC and CO concentrations is to throttle the air intake
to one or more cylinders, thus causing incomplete combustion and increased HC and CO
concentrations. Oxygen for the catalytic oxidation process is provided by the unthrottled
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cylinders, which continue to operate with excess air. Throttling of the entire engine
would be as effective at producing HC and CO, but would not result in regeneration,
since insufficient oxygen would be present in the exhaust gas to support oxidation.
Another, strikingly different, catalytic technique has been developed by Daimler-Benz
for use in its heavy-duty truck and bus line (early research along these lines was also
carried out by Battelle (Hillenbrand and Trayser, 1981)). This technique is to inject a
catalyst containing copper and chlorine into the exhaust when a decision is made to re-
generate. This catalyst lowers the light-off temperature of the collected particulate to
about 200° C (DBAG, 1984). Successful regeneration requires that the exhaust temp-
erature be at or above this value, but this temperature is low enough to be obtained un-
der most operating conditions.
Daimler-Benz has carried out extensive bench and vehicle tests using this approach with
its "candle" trap, with very encouraging results (H. Hardenburg, personal communica-
tion). In considerable on-road testing, the major problem discovered was that the ash
buildup in the trap raised the trap backpressure to unacceptable levels after 100,000 to
150,000 miles of driving. At least some of these on-road tests were carried out using an
automatic regeneration control system — indicating substantial progress in that area as
well. Although some design problems remain to be solved, Daimler-Benz has expressed
confidence that the system can be fully developed by model-year 1990.
Self-regeneration techniques; Two main approaches to self-regeneration are under de-
velopment. Both of these involve lowering the ignition temperature of the particulate
matter; one by means of a catalytic fuel additive, and the other by means of a catalytic
coating on the trap. The use of catalytic fuel additives to promote self-regeneration is
the best developed self-regeneration technique. A number of light-duty manufacturers,
including Ford (Wade et alia, 1983), Volkswagen (Wiedeman et alia, 1983), and General
Motors (U.S. EPA, 1983b) have announced that they are investigating this approach. Both
Volkswagen and GM have stated that it is presently their preferred technique for light-
duty vehicles.
The additives in question are organometallic compounds containing various metallic
atoms. Compounds containing lead, copper, calcium, and manganese have been used in
the work reported to date, but other metals also appear to be under investigation. The
most complete descriptions of the work to date are those published by Wade and co-
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workers at Ford Motor Company and by Wiedemann et alia, at Volkswagen. Both groups
report that the use of organometallic additives reduced the ignition temperature of the
particulates significantly — to the point where regeneration conditions could be achieved
reliably during the normal light-duty driving cycle. Both groups also reported successful
endurance tests of light-duty vehicles using this system with a ceramic monolith trap.
The Ford test lasted for 16,000 km (10,000 mi.), while two Volkswagen tests lasting
40,000 km (25,000 mi.) and 20,000 km (12,500 mi.) were reported. The last test men-
tioned is especially significant, since it was carried out at a steady 50 km/hr (31 MPH).
That the trap was able to survive and regenerate at this low speed, without the occa-
sional excursions to higher temperatures which are experienced during normal driving,
indicates that self-regenerate on can probably be relied on to occur during almost any
reasonable driving cycle.
In general, since engine load factors (and thus average exhaust temperatures) tend to be
higher in heavy-duty than in light-duty service, one would expect that this technique
might prove especially suited to heavy-duty vehicles. On the other hand, heavy-duty
vehicles may also experience long periods of very light load-factors, which could over-
load the trap without ever getting close to the regeneration temperature. Thus, the
suitability of this type of regeneration system for aU heavy-duty applications is question-
able. Where the operating pattern is reliably known in advance, however, this system
could be a very strong contender. Given the relative sophistication of heavy-duty vehicle
drivers, it is also possible that this sytem, combined with some sort of fail-safe to warn
the driver in the event of a dangerous condition, might be acceptable even where operat-
ing patterns could not be predicted.
The additive/monolith self-regeneration system presently suffers from two major poten-
tial technical problems: the tendency of the ceramic monolith traps to crack due to
thermal stress, and the fact that the metallic additives tend to accumulate in the traps,
eventually plugging them. The ceramic monolith's problems with thermal-stress cracking
have already been mentioned. The use of self-regeneration, with the consequent poor
control of the circumstances and frequency of regeneration, might be expected to
exacerbate these problems. Recent statements by knowledgeable sources have indicated,
however, that contrary to expectation, thermal-stress cracking has not been a significant
problem in the development of self-regenerating systems based on additives. The reasons
for this are unclear. They may involve the effects of the catalyst in controlling combus-
tion rate or low trap loadings due to frequent self-regeneration in use.
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In contrast, the problem of additives accumulating in the trap is a very serious one for
heavy-duty applications. This problem will greatly reduce the economic attractiveness
of the additive approach if it cannot be solved. One published test for which quantitative
data are available was that carried out by Ford (Wade et alia, 1983). This test used a
vehicle with a 2.3 liter engine and a 2 liter trap. The fuel contained 0.75 grams of metal
per gallon for the first 3,500 miles, and 1.25 grams of metal per gallon for the last 6,500
miles of a 10,000 mile durability test. At the end of this test, the trap was found to have
accumulated enough incombustible metal oxides and sulfates to partially block the cells
on the upstream side, although trap backpressure had not increased noticeably. It seems
likely that the backpressure would have begun to increase after the 10,000 mile point,
however, as the remaining open space in the trap was filled up.
Current thinking favors a somewhat larger trap-volume to engine-displacement ratio, and
industry sources indicate that significantly lower additive concentrations (around 0.1 to
0.3 g/gallon) are now being considered. Making allowances for these, and assuming that
the Ford vehicle could have travelled another 5,000 miles before backpressure effects
became so serious as to require replacing the trap, one can estimate a total mileage-to-
trap-replacement in the range from 70,000 to 200,000 miles, or (in round numbers), about
100,000 miles. This is probably tolerable in light-duty vehicles, hut would impose sub-
stantial additional costs in a heavy-duty truck, which could be expected to accumulate
two to six times this mileage during its lifetime. Requiring the trap to be replaceable
would also make it easier to tamper with, and there would certainly be a strong tempta-
tion not to replace it after the first time it filled up.
Recent publications by researchers at Corning (Montierth, 1984) and Volkswagen
(Wiedemann et alia, 1984) seem to indicate that these problems may not be as severe as
was once thought, however, at least for the manganese additive that Volkswagen favors.
Tests indicate that the manganese tends to form a fluffy ash which could be blown out of
the trap, or dissolved out by chemical means. Although both techniques would still
require dismounting the trap, it would not be necessary to replace it.
In addition to these technical problems, the widespread use of organometallic additives in
motor vehicles would raise some very significant environmental and safety questions.
Most organometallic compounds are very toxic, and manv are dangerous to handle. If the
additives were to be supplied in diesel fuel, this problem would be manageable, but then
many diesel vehicles without traps would be burning the fuel as well. This would result in
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significant emissions of metalliferous particles. The emission of large quantities of
metals in particulate form could have significant environmental consequences, the more
so as lead and copper are among the metals being considered. The problems of lead toxi-
city in the environment are well known.
The problems with metalliferous particulate emissions could be resolved by keeping the
additives in a special reservoir on the vehicle rather than mixing them with the fuel-
thus ensuring that only trap-equipped vehicles received the additives. This, however,
would raise important salety questions, given the toxicity and flammability of many
organometallic compounds. The problems of disposing of old vehicles containing such
reservoirs, and of refilling the reservoirs of high-mileage vehicles could be consider-
able. In addition, the crash-safety of such systems would be a major concern. On the
other hand, at least one additive manufacturer, and one auto manufacturer (Volkswagen)
have expressed confidence that these problems can be solved, at least for light-dutv
vehicles.
A second approach to self-regeneration is the use of a catalyst-coated trap. The
precious-metal catalyzed wire-mesh trap, for instance, is able to self-regenerate under
many driving cycles, although this effect is not reliable enough to free it from the need
for a positive regeneration system. The performance of precious-metal catalysts on
ceramic traps, however, has been quite disappointing. For this reason, attention has
turned to alternative catalysts based on a mixture of non-precious metals. Some very
encouraging results from the use of base-metal catalysts on ceramic traps have recently
been reported by Koberstein and co-workers (1983), working at Degussa in Germany, and
by Watabe et alia (1983) at Bridgestone in Japan.
The Degussa researchers worked with catalysts on a ceramic monolith trap, while Watabe
and co-workers worked with ceramic foam. The Degussa catalyst reduced the ignition
temperature of the collected particulate in the trap to 380° C (a reduction of about 100°
C), while the Bridgestone catalyst achieved a reduction of 120° C. These temperatures
would be sufficient to give reasonable assurance of self-regeneration in heavy-duty ser-
vice — the Daimler-Benz catalyst injection approach, which has been successfully tested
in vehicles, gives a similar regeneration temperature.
The Bridgestone catalyst also reduced the sensitivity of the particulate ignition point to
changes in oxygen concentration, which would increase the range of conditions under
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which it could trigger regeneration. There is also some evidence (Oser and Thorns, 1983)
that the regeneration process in catalyzed traps is slower than in uncatalyzed traps.
This would reduce the rate of heat generation in the trap, which in turn would reduce the
problem of cracking due to thermal stresses.
At the present time, the publicly available data on this type of regeneration method are
too scarce and preliminary to permit any realistic assessment of its potential. While the
results reported to date are quite encouraging, they include only very preliminary tests
on dynamometers, not actual on-road vehicle tests, even in light-duty vehicles. Further
testing would have to include on-road tests, and would need to examine the durability of
the catalyst coating, its susceptibility to poisoning by trace metals or other chemicals in
the fuel, its resistance to prolonged high-temperature operation and repeated regenera-
tion, and so forth. When such tests are carried out, and if they appear as encouraging as
the initial reports would indicate, then this approach to regeneration would certainly be a
very strong contender, since it would require no control system, no moving parts and no
fuel additives, and would have none of the operational problems surrounding most other
approaches. A catalytic-trap regeneration system would be very similar in concept and
visibility to the consumer to a gasoline-engine catalytic converter — which is clearly an
acceptable technology.
5.1.3 Control Systems
The requirements for a regeneration control system are heavily dependent on the specific
trap and regeneration technique being used. Self-regenerating trap-oxidizer systems
would — by definition — need no control system as such, although some sort of fail-safe
system would be desirable in order to guard against failure of the self-regeneration
process. Such a device would be highly desirable to guard against failure in positive
regeneration systems as well. This fail-safe might consist of a bypass valve which would
open at a specified backpressure, venting exhaust around the trap and lighting a warning
light on the dashboard.
Positive regeneration systems would require some form of sensing, control, and actuation
system in order to carry out the actions required for regeneration. This would also be
true of "assisted" self-regenerating systems, if it proves to be impossible to develop a
truly reliable self-regeneration technique. The three basic control system functions
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would be to determine when regeneration is necessary, to initiate the regeneration pro-
cess, and to confirm that the regeneration process is complete. These would be common
to all positive regeneration systems.
Other control functions would depend on the specific regeneration process used. For
instance, using an oil-burner type regeneration system, it would be necessary to monitor
the burner in order to ensure ignition—otherwise a failure to ignite might result in
spraying raw fuel into the trap, possibly leading to a fire or subsequent uncontrolled
regeneration. For other types, such as those which rely on the catalytic oxidation of HC
and CO, it would be necessary to ensure that regeneration was possible at a given time,
and to defer it if it were not. These systems require a certain minimum exhaust temper-
ature for the catalytic oxidation to occur, and it would be necessary for the control
system to ensure that the exhaust was at an appropriate temperature before initiating
regeneration. It would also be desirable for a control system for a trap with self-
regeneration potential, such as the catalyzed wire-mesh trap, to detect self-regeneration
when it occurs, thus eliminating unnecessary regeneration cycles.
The development of a control system can be divided into two separate problems: the
development of suitable control algorithms, and the development of a system of compo-
nents which implements those algorithms. The latter proMem is fairly straightforward;
none of the regeneration systems described above appears to require anything beyond the
present state of the art in control equipment or techniques, except possibly for some new
sensor types. The former problem—that of defining the control algorithms to be used or
the set of conditions which the control system must maintain—is also straightforward,
but is likely to be time-consuming and costly to accomplish.
In order to define the operational requirements of a control system, it is necessary first
to understand the characteristics and limits of the system to be controlled very
thoroughly. Once these are known, a prototype control algorithm can be designed to
maintain the controlled system within its allowable limits. This process is well advanced
at the present time. A great deal is known about the performance and safe operating
characteristics of the common types of traps and about the performance of the various
regeneration systems. The frequent reports of trap failures in testing programs indicate
that there is still something to be learned, however. Ultimately, extensive on-vehicle
testing of prototypes will be necessary to ensure that the control algorithm succeeds
under all possible operating conditions. From industry sources, it appears very likely that
this process is going on at some light-duty manufacturers at the present time.
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5.2 SPECIAL CONSIDERATIONS FOR TRAP-OXIDIZERS IN HEAVY-DUTY SERYICE
As Section 5.1 makes clear, most trap-oxidizer research arid development has been
directed at developing systems for light-duty vehicles. There are a number of reasons
for this, of which the following are the most important.
1. Less regulatory pressure — at the present time there are no actual par-
ticulate regulations for heavy-duty engines. Many heavy-duty engine
manufacturers still have lingering doubts as to whether there will ever
be any such regulations, and thus are reluctant to commit resources to a
major R and D program.
2. Less familiarity with regulation — stringent regulatory standards are a
new experience !or the heavy-duty industry, and thus the industry may
be somewhat slow in responding to them. The light-duty vehicle
industry has been stringently regulated since 1975.
3. Industry weakness — the last few years have been disastrous ones for
the heavy-duty truck industry. International Harvester, the largest
manufacturer, has been very close to bankruptcy, and Mack avoided a
similar fate only by being taken over by Renault. With companies slash-
ing overhead in order to survive, new research and development pro-
grams stood little chance of being initiated.
SmaUer engineering and R&D stalls — The major light-duty vehicle
manufacturers all have large research, development, and production
engineering staffs, and relatively short product development times.
This is due partly to the size of these firms (all are very large, and can
afford large corporate staffs) and partly because this staff is needed to
cope with the frequent model changes in light-duty automobiles. Truck
and bus models change seldom, and the basic engine models change even
less often. Thus, heavy-duty engine manufacturers tend to have smaller
engineering staffs and longer product development lead-times than do
light-duty manufacturers.
5. Responsibility for trap development is not well defined — due to the
fragmented nature of the heavy-duty truck industry.
6. More difficult development task — developing a trap-oxidizer system or
systems for the wide range of operating cycles and the very long life-
times characteristic of heavy-duty service is harder than to develop one
which is suitable for light-duty operation.
These conditions are likely to continue to slow heavy-duty trap-oxidizer development in
the future. In particular, given the complexity of the problem, the industry's weakness,
and the relative lack of regulatory impetus, there is a strong tendency on the part of the
independent heavy-duty engine manufacturers to let the light-duty manufacturers solve
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the problem, and then to adapt their solutions. This could have important implications
for competition in the industry, since two light-duty diesel manufacturers (GM and
Daimler- Benz) also make heavy-duty engines, and Ford plans to enter the heavy-duty
diesel market within the next few years. Ford, GM, and Daimler-Benz are among the
leaders in light-duty trap-oxidizer technology, and could be expected to be able to draw
on this expertise to develop heavy-duty trap-oxidizers much more quickly. Indeed, this
appears to be happening already.
Light-duty trap-oxidizer development is well advanced. A previous ERC study (Weaver
and Miller, 1983) concluded that it is highly probable that trap-oxidizer equipped light-
duty vehicles will be in production for model year 1987, and one manufacturer (Daimler-
Benz) is including trap-oxidizers on its 1985 production for California. Heavy-duty trap-
oxidizer development will certainly draw on this development experience. Thus it is
appropriate to consider carefully the degree to which light-duty trap-oxidizer technology
is likely to be adaptable to heavy-duty use. It is especially important to consider what
having light-duty trap-oxidizer systems in production by 1987 would imply about feasible
development schedules for heavy-duty vehicles.
Different classes of heavy-duty vehicles differ greatly in what requirements they would
impose on a trap-oxidizer system, and especially in the degree to which light-duty trap-
oxidizer technology is likely to be adaptable. In addition, the industry structure and
other institutional constraints to trap-oxidizer development and deployment vary greatly
between different classes of vehicles. These variations have important implications for
the feasible deployment schedule for trap-oxidizer technology, and even for the ultimate
feasibility and cost-effectiveness of trap-oxidizers for each class of vehicle. Because of
this, these issues are discussed separately for the four main subclasses of heavy-duty
vehicles.
5.2.1 Light-Heavy Duty Vehicles
Light-heavy duty vehicles generally resemble the heavier end of the light-duty spectrum
much more closely than they do the larger trucks with which they are grouped for regula-
tory purposes. The diesel engines used in this group are presently all high-speed,
indirect-injection, naturally-aspirated engines derived from passenger car technology,
and bear little resemblance in operational characteristics to the medium-speed, direct
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injection engines used in heavier trucks. A prime example is the GM 6.2 liter engine,
versions of which are used in vehicles certified under the light-duty standard, as well as
in light-heavy vehicles. The only other diesel engine currently in this class is the Inter-
national Harvester 6.9 liter engine, which is presently certified only for heavy-duty
operation. International Harvester is considering adding EGR to this engine, however, to
enable it to comply with the light-duty NOx standard as well. Isuzu and Cummins are
also in the process of introducing small high-speed DI engines for this class. These
engines will probably be intermediate between passenger-car IDI engines and heavy truck
engines in their characteristics.
The vehicle characteristics, operating environment, lifetime mileage, driving patterns,
and maintenance habits for light-heavy vehicles resemble those of light-duty vehicles
more than those of the larger trucks. Lifetime mileage, for instance, averages about
110,000 miles for light-heavy vehicles. This is comparable to the 100,000 mile life typi-
cal of light-duty vehicles, but much less than the 185,000 to 500,000 mile lifetimes of the
heavier trucks.
Perhaps more importantly, the manufacturers and the manufacturing process for light-
heavy trucks are quite similar to those of light-duty vehicles. The vehicles in which
these engines are used are also predominantly mass-produced in the manner of light-duty
cars and trucks rather than in the semi-custom manner typical of medium-heavy and
larger trucks. The nature and timing of the development process for these vehicles
should also be similar to that of the light-duty class. It should be noted also that Ford
and GM — the major manufacturers of light-heavy trucks — are also among the most
advanced in light-duty trap-oxidizer development, so that they would have a readily
available pool of expertise to draw on. At the same time, however, it should be noted
that International Harvester and Cummins, the other likely light-heavy engine manufac-
turers are not far advanced in trap-oxidizer development, and it is the engine manufac-
turer who must certify the system. This might place them at a competitive dis-
advantage.
Given the basic similarities between the two types, and assuming that — as ERC has
predicted — trap-oxidizer systems for light-duty vehicles are available in 1987, it would
take comparatively little additional time to develop and certify trap-oxidizers for light-
heavy duty engines. One, or at most two, additional years of delay would probably be
justified in order to minimize the competitive disadvantage suffered by those light-heavy
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engine manufacturers who are not also light-duty vehicle builders, and to enable other,
low-volume users of these engines such as motor-home manufacturers to integrate the
trap-oxidizers into their designs.
This delay would also help to minimize the burden on engineering staffs and certification
facilities by permitting the inevitable last-minute problems in trap-oxidizer application
to be substantially resolved in the light-duty development process before applying them
to light-heavy engines. This would decrease the added costs of trap-oxidizer develop-
ment, and might result in better systems and increased public acceptance. Finally, given
the comparatively small number of light-heavy duty vehicles (compared to the number of
light-duty trucks sold), this delay should not have major effects on air quality.
5.2.2 Medium-Heavy Duty Trucks
This class of trucks includes all of what most people think of as "heavy-duty trucks"
except for the large tractor-trailer combinations used in line-haul service. It contains a
wide variety of truck types and styles, ranging from about 5 tons GVW to more than 25
tons. Most of these trucks are either single or tandem-axle straight trucks (see the truck
typology given in Chapter 2), but they also include some heavy panel vans, some smaller
tractor-trailer combinations, school buses, and other special types. Transit buses —
which are similar to this group in size and weight — are discussed separately below.
The most distinguishing feature of this class of vehicles is its varietv. A large number of
specialized body styles such as dump trucks, garbage trucks, tow trucks, electric-utility
service vehicles, and others are sold as well as the more common "box" van. Often, the
same basic chassis and cab will be offered with many different choices of body styles.
Most trucks in this class are also available with many different choices of power-train
equipment such as engine, transmission, and rear axle. In addition, many are sold with
special equipment such as hydraulic lifts, garbage packers, cherry pickers, winches and
snowplow blades. This equipment may be supplied either by the original manufacturer or
by a third party.
Along with this bewildering array of truck types and styles goes an equally bewildering
selection of operating patterns. Usage patterns include stop and go garbage collection,
short haul deliveries, long-haul deliveries, extended idling while using hydraulic equip-
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ment, over-the-road trucking, and many others. This wide variety of operating patterns
and truck configurations will make the development of workable trap-oxidizer systems
for medium-heavy trucks a difficult and time-consuming task. Given the enormous
number of applications, it would be difficult and probably uneconomical Id develop a
trap-oxidizer optimized for each one. At most, special systems might be developed for a
few common, relatively consistent applications such as garbage packers and school buses,
with the less comon applications having to rely on "generic" systems. For this reason,
the generic system to be developed will need to be able to function correctly and safely
under almost any conceivable duty cycle. This requirement argues strongly for a positive
regeneration system, or possibly some sort of hybrid system which would include a posi-
tive regeneration capability.
Development of a generic trap-oxidizer system for medium-heavy trucks will be sim-
plified somewhat by the fact that the majority of such trucks are built along similar
lines, with similar exhaust system layouts. This means that, in most cases, the same set
of changes to the engine and exhaust system will be apDlicable across a number of
different styles and makes of truck. Offsetting this, however, is the fact that each style
of truck may be offered with several different engines from different manufacturers,
each of which (under the currently proposed regulatory structure) might well have its
own different design of trap-oxidizer. Some attempts at industry standardization can be
expected, but it can also be expected that there will inevitably be body types and/or
accessory packages which will be incompatible with these standards.
Medium-heavy duty engine and vehicle manufacturers typically have smaller engineering
staffs than do light-duty and light-heavy duty manufacturers, and they are less geared
toward frequent model changes. Furthermore, much more extensive testing of new
developments is necessary before they can be marketed, due to the long service lifetimes
found in this class of trucks. For this reason, product development times are generally
one or two years longer than in the light-duty industry — it can take six years or more to
bring a proven concept into production. Because of the longer delay time and the less-
developed state of heavy-duty trap-oxidizer technology, it would appear wise to allow at
least two or three years beyond the effective date of a light-duty trap-oxidizer standard
before imposing a similar standard on these vehicles.
In addition, the smaller engineering staffs and less frequent model changes, combined
with the large number of possible truck chassis, engine, body, and special equipment
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combinations will make it very difficult to implement trap-oxidizers across an entire
product Line in a single year. Thus it would be desirable to consider some sort of "phasing
in" of particulate controls in order to allow time for trap-oxidizers to be engineered into
all vehicle types while minimizing disruption of the market. One way to accomplish this
would be by means of a sales-weighted average standard, which could be tightened each
year for two or three years.
5.2.3 Line-Haul Trucks
The class of lirie-haul trucks includes the largest and most powerful heavy-duty trucks.
These are overwhelmingly devoted to intercity freight commerce and similar activities.
Almost all line-haul trucks are combination units made up of a tractor and one or two
trailers. The large power requirements for this type of operation require large, powerful,
usually turbocharged engines, with very high exhaust flowrates. The high exhaust flow-
rates, in turn, will necessitate very large traps in order to bring the backpressure down to
a reasonably low level. Low backpressure is especially important for line-haul trucks,
since the increase in fuel consumption due to the trap-oxidizer is directly related to
backpressure. Line haul trucks are driven for great distances and consume enormous
amounts of fuel, so even small changes in fuel economy are significant. For a typical
truck in this class, a one percent loss of fuel economy would increase total lifetime fuel
costs by more than $1000.
The large economic effect of even small changes in efficiency could be expected to lead
to widespread tampering and attempts to defeat trap-oxidizers installed in this class of
trucks. This tendency would be reinforced by the well-known mechanical bent of most
line-haul truck drivers, as well as by their notorious independence and sensitivity to
adverse government actions. This sensitivity was dramatically demonstrated in the
independent truckers' strike of a few years ago. If trap-oxidizers were to develop a repu-
tation as being unsafe or unreliable (as seems quite possible, given their complexity and
novelty) this tampering might well become nearly universal. The only obvious counter-
measure to such tampering, given the fact that most of these trucks operate in interstate
commerce, would be a strong Federal or Federally-coordinated inspection and mainte-
nance program.
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Given the difficulty of the tampering problem, the very large economic costs of even
small changes in fuel economy for these trucks, and the fact that the great preponder-
ance of the vehicle miles travelled by this class are in interurban rather than intraurban
driving, it would make sense to consider exempting line-haul trucks from a trap-oxidizer
requirement. Because these trucks spend less time in urban areas, the total effect of
such an exemption on urban air quality should be small, while the economic savings would
be large. Eliminating the need to develop extremely high-capacity and high-durability
trap-oxidizers for these trucks would also free engineering resources to work on the
easier, but still time-consuming, task of integrating trap-oxidizers into the numerous
medium-heavy duty truck models, most of which are operated primarily in urban areas.
The organization of the line-haul truck industry is very similar to that of the medium-
heavy duty truck industry — indeed, many of the major manufacturers in each group are
the same. Line-haul trucks are not qualitatively different from other heavy-duty trucks,
they are simply larger, more powerful, and more intensively used. Thus all of the com-
ments concerning the difficulty of developing feasible traps, the organizational and
engineering manpower constraints, and other difficulties which were made above for
medium-duty trucks can also be applied to this class.
If this class were not to receive a special exemption from a particulate standard, the
leadtime requirements for trap-oxidizer deployment would be even longer than those of
the medium-heavy class — three or four years beyond the implementation of a light-duty
trap-oxidizer standard rather than two or three. The additional year would be required
to develop and test the very large traps and regeneration systems needed, and to carry
out lengthy on-road durability tests. A durability test corresponding to the EPA-defined
"full useful life" of a line-haul truck would need to last for 250,000 miles, and such tests
take a great deal of time.
5.2.4 Transit Buses
The service locations and operating conditions of transit buses, as a group, are almost
polar opposites to those of the line-haul truck class. Transit buses operate almost exclu-
sively in urban areas, and generally in the most urbanized and congested portions of those
areas. Furthermore, the typical transit-bus operating cycle is one of the worst imagin-
able from a particulate emissions viewpoint. Rather than profit-motivated individuals or
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firms, transit buses are owned and operated almost exlusively by service-oriented public
or quasi-public agencies. These agencies are quite sensitive to the problem of smoke
emissions and their offensiveness to the public — mitigatory measures such as derating
bus engines to reduce both maximum power and smoke, use of lower-smoke diesel if 1
(city-bus) fuel rather than diesel ft2, and the use of smoke reducing additives such as
barium are common. Thus a device such as the trap oxidizer, which could greatly reduce
both smoke and particulate emissions is highly desirable from both the human exposure
standpoint and that of the bus fleet owner.
Developing a feasible trap-oxidizer system for transit bus use would be considerably
easier than for most applications. One major reason for this is that transit buses univer-
sally receive regular service, often on a daily basis. Similarly, they almost always
operate near their base, and there is usually at least one other bus on any given route, so
the consequences of a failure are not very severe. Durability and reliability require-
ments would not be nearly as strict as for most other types of heavy-duty vehicles. An
additional advantage comes from the fact that transit buses have a rather predictable
operating cycle, and one which includes a great deal of acceleration. The frequent
occurrence of moderately high exhaust temperatures as a result would help to make a
self-regenerating system feasible.
There are also major organizational and institutional advantages to trap-oxidizer appli-
cation in this class. As remarked before, many bus operators are quite sensitive to the
public offense created by their buses' smoky exhaust, and expend substantial amounts of
money in reducing by such means as derating the engines, buying more expensive //I fuel,
and adding special smoke-reducing additives. Deployment of trap-oxidizers would make
these steps unnecessary, with a consequent savings which would probably outweigh the
cost of the trap-oxidizer. Thus, rather than being forced on unwilling consumers, trao-
oxidizers might well be welcomed with joy.
A final advantage to trap-oxidizer deployment on buses is that buses, at least in con-
gested urban areas, appear to account for a large fraction of the total ambient diesel
particulate. General Motors estimates (Chock et alia, 1984) indicate that as much as 40
percent of the total diesel particulate at urban air-sampling stations is derived from
buses. Since buses generally operate near the sidewalks in the most crowded areas, it
seems likely that an even greater percentage of the human exposure to diesel exhaust in
these areas comes from buses. Thus the cost-effectiveness of trap-oxidizers in this
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application would be very high — so high that it would be worthwhile investigating
whether a program to retrofit existing buses with trap-oxidizers would be feasible.
Despite the attractiveness of trap-oxidizers for transit-bus applications, development of
a feasible trap-oxidizer system will be by no means a trivial task. There is likely to be
considerable difficulty in packaging it in the vehicle, for instance. Transit bus engines
are in the rear, in an area also occupied by much else. Unless the trap-oxidizer could be
successfully substituted for an existing component such as the muffler, it would be
necessary to redesign the back end of the bus.
For this reason, as well as the general problems in developing trap-oxidizer standards for
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This section discusses the four types of trap-oxidizer systems which presently appear
most promising for heavy-duty vehicles. These systems are the following:
Ceramic monolith trap with bypass oil-burner regeneration.
Ceramic monolith trap using self-regeneration, either by means of fuel
additives or a base-metal catalyzed trap.
Catalyzed wire-mesh trap with regeneration by HC and CO enrichment
of the exhaust.
Silica-fiber "candle" trap with catalytic regeneration using CuCl.
Each type of system is described, and its important characteristics — including effec-
tiveness, durability, reliability, and cost — are discussed. The estimated "sticker price"
and life-cycle cost to the vehicle owner of each system in each of the four major classes
of heavy-duty vehicles are also given. Finally, the overall state of development and the
potential for commercial feasibility of each type of system are assessed.
5.3.1 Bypass/Burner System With Ceramic Monolith Trap
System Description: A bypass/burner trap-oxidizer system would consist of one or more
ceramic monolith traps and one or possibly more diesel-oil burners, along with suitable
valves and plumbing to bypass the trap(s) during regeneration and a fairly sophisticated
system of controls, sensors, and actuators. In order to eliminate the extra complexity
and cost of additional valves, sensors, and burners, most systems of this type could be
expected to use a single trap. However, due to limitations on the maximum trap size,
the largest trucks would need at least two traps, and possibly more, in order to provide
sufficient trapping area and to keep the pressure drop through the trap to a minimum. A
conceptual diagram of a one-trap system of the by-pass/burner type is shown in
Figure 5.8. The system shown would be appropriate for a light-heavy vehicle, and for the
majority of the medium-heavy class.
Effectiveness: The ceramic monolith trap is an extremely efficient filter — depending
on the exact material used, its efficiency can be above 90 percent. It is most efficient
at capturing soot — much of the organic fraction of the particulate passes through it, as
does much of the mutagenic activity (Scholl et alia, 1982; McDonald, 1983). These
effects have been discussed in Section 5.1.1. This type of trap would enable manufactur
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Electronic
Contraller
Sensor
Figure 5.8: System diagram — ceramic monolith/burner trap-oxidizer system.
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ers to meet a very strict standard for total mass emissions of particulates, and would
effectively reduce both the total mass of particulates in the air and the degradation in
visibility due to diesel particulate matter. Reductions in human exposure to particulate-
borne organics and especially to particulate-carried mutagens would be much less. The
effects on other pollutants would be minor, except for a possible reduction in odor.
Durability and reliability; These two areas could be expected to present significant prob-
lems for this type of system, especially in the medium-heavy and line-haul truck
classes. The major problems would probably be with the complicated regeneration con-
trol system and with the burner — the trap itself is quite durable unless subjected to high
temperatures during regeneration. The regeneration control system would contain a
great deal of complicated electronics, which would need to survive for very long period
in the hostile operating environment of an on-the-road truck. Similarly, the burner and
associated valves and actuators would need to survive for very long periods in the hot,
sooty, intensely oxidizing environment of the diesel exhaust with minimal service.
Although some inspection and service requirements would probably be tolerable, any such
requirements greatly increase both the risk of tampering with the system and the risk of
system failure (since, inevitably, some systems will not be serviced).
There is also some disagreement over the potential reliability of the bypass/burner
system, even assuming that the regeneration system is working properly. On the basis of
published data, it presently appears probable that a system of this type for light-duty
vehicles can be designed so that (when it is functioning properly) it reliably regenerates
without cracking or melting the trap. The range of tolerances is fairly narrow, how-
ever. In a larger trap, other things being equal, one would expect a greater thermal
stress level, and thus a greater tendency to crack. It is not yet completely clear that a
system of this type, using a truck-sized trap, can be designed to regenerate reliably
without damage to the trap.
Performance and fuel economy effects: The use of any type of trap-oxidizer system will
result in some slight harm to a truck's fuel economy and performance, since power is
required to force the exhaust gases through the trap. In addition, the presence of a trap
can affect turbocharger performance — resulting in significant performance and fuel-
economy losses unless the turbocharger is carefully matched to the altered system. The
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effect of the bypass-burner/monolith system on fuel economy would be somewhat greater
than that of the other systems discussed, due to the additional use of fuel in the burner.
The magnitude of the loss in performance and fuel economy is difficult to estimate, since
the total effects are small compared to test-to-test and engine-to-engine variability.
The magnitude of the effects is also dependent on how well the turbocharger charac-
teristics are matched to those of the trap/engine combination. Most tests of this con-
cept indicate fuel-economy losses ranging from immeasurably small to a few percent, and
one would expect performance losses to be of similar magnitude. The effect on per-
formance would probably not be perceptible, and is thus of little importance, but the
increased fuel consumption would add significantly to the life-cycle cost of the system,
especially in the heavier trucks. This increase is estimated to be about 3.0 percent in
transit buses, 2.5 percent in light-heavy and medium-heavy trucks, and 2.0 percent in
line-haul trucks. These figures reflect the fact that a proportionally larger trap (result-
ing in a lower pressure drop and less frequent regeneration) would be cost effective in
the line-haul trucks.
Estimated Cost: Table 5.1 shows the authors' estimates of the increases in purchase
price and operating and maintenance costs for each of the four major classes of heavy-
duty vehicles which would result from a bypass/burner trap-oxidizer system. The esti-
mated initial cost increase was calculated from the estimated costs of the trap, con-
tainer, and other components and the estimated assembly labor requirements using the
method developed by Fronk (1984) for EPA. This method, in turn, was based on earlier
work by Lindgren (1977) and by EPA. All of these approaches were originally designed
for, and are based on data for, light-duty vehicle manufacturers. They have thus had to
be adapted somewhat to be applicable to the heavy-duty industry.
This report uses the following equation (adapted from Fronk) for the increase in a vehi-
cle's selling price that results from the addition of an emissions control device (such as a
trap-oxidizer) provided by an outside supplier.
RPE = ((SP + AL + AO) MM + RD + TE) DM)
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Table 5.1
Estimated Cost of Ownership For A
Monolith/Burner System
Light- Medium- Line- Transit
Heavy Heavy Haul Bus
INITIAL COST TO MANUFACTURER
Trap $ 72.00 $120.00 $2*0.00 $150.00
Container and Piping 50.00 60.00 120.00 60.00
Regeneration and 170.00 180,00 220.00 180.00
Control system
Modifications to Vehicle $20.00 $40.00 $80.00 $100.00
TOTAL COST TO MANUFACTURER
$312.00
$400.00
$660.00
$490.00
Assembly Labor (hours)
Cost (9 $20/hour
Assembly overhead @ 40%
2.00
$40.00
$16.00
3.00
$60.00
$24.00
5.00
$100.00
$40.00
4.00
$80.00
$32.00
TOTAL COST TO MANUFACTURER
$368.00
$484.00
$800.00
$602.00
Manufacturer's Markup (3 20%
Estimated Tooling Cost Per Unit
Estimated R&D Cost Per Unit
$73.60
5.00
15.00
$96.80
50.00
150.00
$160.00
50.00
150.00
$120.40
100.00
300.00
INCREASE IN DEALER COST
Dealer's Markup @ 8%
$461.60
36.93
$780.80
62.46
$1160.00
92.80
$1122.40
89.79
INITIAL COST TO CONSUMER
$498.53
$843.26
$1252.80
$1212.19
OPERATING COSTS
Vehicle Lifetime (Miles)
Vehicle Lifetime (Years)
Maintenance Costs
Per 100,000 Miles
Discounted Lifetime
120,000
8
$70.00
$56.02
250,000
8
$70.00
$116.70
500,000
8
$100.00
$333.43
250,000
8
$70.00
$116.70
Fuel Consumption
Base Fuel Economy (MPG)
- Reduction Due to Trap
Cost of Fuel ($/Gallon)
Discounted Lifetime Cost
16.20
2.5%
$1.30
$160.54
8.81
2.5%
$1.30
$615.02
6.44
2.0%
$1.30
$1346.16
6.00
3.9%
$1.30
$1083.66
Trap Replacement Cost
Trap Lifetime (Miles)
Trap Replacements Needed
Cost of Replacement
Discounted Replacement Cost
150,000
0
$300.00
$ 0.00
250,000
0
$416.00
$ 0.00
250,000
1
$776.00
$ 530.02
150,000
1
$476.00
$295.56
TOTAL OPERATING COSTS
TOTAL LIFECYCLE COSTS
$216.56
$715.09
$731.72
$1574.98
$2209.61
$3462.41
$1495.92
$2708.11
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Where
SP is the price charged by the supplier to the manufacturer
AL is the direct cost of assembly labor for mounting the device in the vehicle
AO is the manufacturer's assembly overhead cost per unit
MM is the manufacturer's markup percentage
RD is the manufacturer's research and development cost, per unit
TE is the manufacturer's tooling cost, per unit
DM is the dealer's markup percentage
In Table 5.1, the supplier price is estimated separately for each of the major components
of the trap-oxidizer system. These estimates are based on data obtained from suppliers
and vehicle manufacturers, comparison with prices of similar components which are now
in use (e.g. catalytic converters), and a great deal of engineering judgment. The prices
shown for traps represent a very large decrease from present price levels — currently,
large heavy-duty ceramic monolith traps sell for $500 to $1,000. However, these prices
reflect small volume prototype production, and appear to include a substantial premium
for the supplier's R3cD. The prices shown are ERCs estimates of prices in mass produc-
tion.
The labor required to assemble and install a trap-oxidizer system was estimated by ERC
on the basis of engineering judgment and the apparent difficulty of the mounting process
(thus, more labor is required to mount a trap-oxidizer in the confined space of a bus than
is required for a truck). The cost of assembly labor was taken as $20 per hour. Assembly
overhead for light-duty manufacturers was estimated at 40 percent of direct labor costs
by Fronk; for want of a better estimate, that figure is used here as well.
Research and development and tooling costs shown for each category are primarily
guesswork — since no one presently has a fully developed trap-oxidizer system for heavy-
duty vehicles, no one really knows what the development will cost. EPA (1984b) has
estimated the cost of trap-oxidizer research and development as $2.5 million per manu-
facturer, plus $208,000 per engine line. These costs are far too low, however, con-
sidering that General Motors claims to have spent more than $64 million on trap-oxidizer
development already, and that GM has not yet reached the most expensive stages of
development: fleet testing, adaptation to production, durability assurance, and certifi-
cation testing. ERC has Uomewhat arbitrarily) estimated the total cost R&D cost per
manufacturer for the bypass/burner system at $30 million in the medium-heavy and
heavy-heavy classes. Tooling expenses were estimated (arbitrarily) at $10 million.
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Assuming that the typical heavy-duty manufacturer produces M,000 units per year, and
that the tooling and R&D costs are recovered at 20 percent per year, this gives an R&D
cost per unit of $150, with a further $50 for tooling costs. These costs were arbitrarily
doubled for transit buses, reflecting their very small production volume (about 2,500
units per year, in total). For light-heavy duty trucks, these costs were arbitrarily
reduced by a factor of 10, reflecting the much larger volume and the relative ease of
adaptation of light-duty trap-oxidizer technology to the light-heavy class.
The manufacturer's markup term (MM) accounts for both the manufacturer's corporate
overhead and for corporate profits. Fronk, using financial data from the years 1979-
1983, estimates this factor as 1.11 (11 percent markup) for light-duty vehicle manufac-
turers. In the case of heavy-duty manufacturers, however, there are often not one but
two corporate markups to consider — that of the engine maker and that of the vehicle
assembler (light-duty manufacturers generally fill both roles). The duplication of
corporate staffs should result in a higher markup, as should the smaller size and lower
volume (and thus lower economies of scale) of heavy-duty manufacturers. In addition,
the years 1979-1983 are generally regarded as having been disastrously unprofitable ones
for both light and heavy-duty manufacturers. Thus, profit margins estimated from data
in these years would be expected to be too low. Taking all of these factors into account,
the authors consider that a markup factor of 20 percent is probably more representative
than Fronk's value of 11 percent. This value is the one used in Table 5.1.
The dealer's markup term (DM) was estimated by Fronk as being 1.05 for passenger cars,
and 1.06 for trucks. Again, this value was based on data for 1979-1983, and is thus prob-
ably too low to represent the long-term average. There are also major differences
between light-duty vehicles sales and sales of heavy-duty trucks — generally, truck
dealer's technical expertise must be much greater, and their carrying costs are probably
also more. For this reason, the authors regard a dealer's markup of eight percent as
probably more appropriate than the six percent estimated for light-duty trucks.
Inserting the values shown into the equation and calculating through results in the esti-
mated increased cost to the purchaser shown in Table 5.1. Considering the uncertainties
inherent in estimating cost for a system which has not even been designed yet, these
values should be taken as only very rough and approximate. They are also probably
somewhat conservative (that is, they may underestimate the actual cost). One area
where costs may have been underestimated is in provisions for warranty and recall. An
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allowance for warranty and recall costs is supposed to be included in the manufacturer's
markup, but the serious reliability questions surrounding trap-oxidizers in general, and
especially the bypass/burner system, make it questionable whether this allowance is suf-
ficient.
In addition to the initial cost of the system, there would also be significant operating and
maintenance expenses. These expenses and the assumptions going into them are shown in
Table 5.1. The vehicle lifetimes and lifetime mileages shown are considered to be
reasonably representative of vehicles in each class, although of course individual vehicles
would vary. Maintenance cost per 100,000 miles was estimated using engineering
judgment, and taking into account the mechanical complexity of the system. This cost
was then spread over the life of the truck, and discounted to the year of purchase at a 10
percent (real) rate.
Typical fuel-economy values for each class of truck are those estimated for 1992 by
Energy and Environmental Analysis (1983), except for transit buses, for which the
estimate was done by the authors. These represent substantial improvements over
present-day values, and could probably not be achieved in the face of a 4.0 gram N0X
standard. The increase in fuel consumption due to the trap-oxidizer was estimated by
the authors, and may be somewhat optimistic. A two to 2.5 percent penalty is probably
about the best that can be achieved with a reasonable-size trap, considering that the
burner regeneration process also consumes fuel, while a penalty in the three to four
percent range would not be at all surprising. The penalty for buses is estimated to be
somewhat greater, due to the limited space in which to put a trap. The discounted
lifetime fuel cost of the system is calculated by dividing the average mileage per year by
the MPG rating, and multiplying by the fuel-economy penalty to give the average cost
per year. This is then discounted (at 10 percent) to the year of purchase to give the
discounted lifecycle cost.
The trap replacement frequencies shown have been estimated by the authors, using
somewhat optimistic estimates of trap plugging rates and average lifetime. As the table
indicates, only line-haul trucks and transit-buses are expected to require trap
replacements with this system. The cost of a trap replacement was estimated as parts
and labor, with the parts cost taken as supplier's price for the trap, marked up 100
percent to reflect the premium charged in the aftermarket. The labor cost of the trap
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replacement was estimated at two labor hours, at a cost of $28 per hour, and the total
was discounted to the year of purchase at 10 percent per year.
As Table 5.1 indicates, trap-oxidizers would be fairly expensive, especially in heavy-duty
vehicles. The reader is cautioned that these estimates are very crude, however, and are
based on a technology which is still undergoing development. They should be treated
with appropriate caution. In performing these estimates, the authors have tended to err
on the side of optimisim — thus the actual costs would probably not be less than those
shown, but might well be significantly more if unforseen problems occur during
development.
Safety and Environmental Effects: Deleterious environmental effects from this type of
system should be minimal. Some minor increases in emissions of gaseous pollutants
would be generated by the burner and the regeneration process. In addition, the fact that
the trap is more effective at removing soot than condensable hydrocarbons can result in
the formation of very fine particles of condensed hydrocarbon (MacDonald, 1983). The
net effect on condensable hydrocarbons is a reduction however.
In the area of safety, on the other hand, some very serious questions exist. The presence
of a diesel fuel burner, with its associated fuel lines, ignition source, etc. in the exhaust
system would clearly increase the chance of fire, as would the regeneration process in
the trap itself. This risk could probably be reduced to insignificance by careful design, at
least in the majority of applications. However, the acceptability of this (or perhaps any)
type of trap-oxidizer system for trucks hauling flammable or explosive products is very
questionable.
Other Considerations: The complexity of this type of system, and the presence of the oil
burners — which could lead to drastic consequences in case of failure — would probably
tend to encourage tampering and interference in order to minimize the inconvenience
and perceived hazard of the system. Such tampering could be relatively simple — the
trap-oxidizer could simply be bypassed entirely by jamming the bypass valve open. This
problem would possibly be most serious in the larger trucks, and especially those owned
by individual truckers rather than by organizations.
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Development Status: Until about 1982, the bypass/burner system was perceived as the
most promising type of trap-oxidizer system for light-duty vehicles, due primarily to the
fact that it allows complete control of the regeneration process and uses comparatively
well-understood technology. Since that time, a greater appreciation of the serious cost
and reliability problems associated with this type of system and technological progress
(notably in additive self-regeneration) have caused most light-duty vehicle manufacturers
to turn away from it. Nonetheless, many heavy-duty manufacturers (including Cater-
pillar and Cummins) still seem to regard this as the leading system. However, those
manufacturers favoring this system appear to have done very little work in developing
it. The converse is also true — those manufacturers which have devoted significant
effort to developing these types of systems now favor other approaches. At present, the
monolith/burner system appears to be in limbo, awaiting the success or failure of
attempts to develop more attractive systems.
Overall Assessment; The ceramic monolith/burner system is quite unattractive from
both the manufacturer's and the user's viewpoint, due to its complexity and questionable
reliability. However, the basic trapping arrangement should last for a reasonably long
time, unlike the other systems discussed below in which the trap can become plugged
more quickly. For this reason, this type of system might be attractive in very high-
mileage applications such as line-haul trucks and city buses, which otherwise would
require more frequent replacement of the trap. Significantly, those heavy-duty manu-
facturers who are most interested in this system are those whose primary markets are in
large, high-mileage trucks. Even in these applications, however, this system's competi-
tive advantge is not clear-cut.
53.2 Ceramic Monolith Trap/Self Regeneration
System Description: A self-regenerating trap-oxidizer system based on the ceramic
monolith trap would most likely be based on the use of catalytic fuel additives. A possi-
ble alternative approach would be to have the trap itself impregnated with a base-metal
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catalyst, but there is too little publicly available information on the performance of such
traps to realistically assess their potential. For this reason, only the fuel-additive based
system is discussed here.
A self-regenerating system based on catalytic fuel additives would consist of one or more
ceramic monolith traps and some system for getting the additives into the fuel. As with
the bypass-burner/monolith system, use of a single trap would be desirable in order to
minimize complexity and cost, but would probably not be possible for the largest trucks,
due to the size limitations of the trap. In addition to the trap and the additive supply,
some simple sensors to detect trap overheating and some sort of fail-safe bypass in case
of regeneration failure would probably also be required.
Two systems for providing the needed fuel additives have been proposed. These are:
(1) to keep the additives in an on-board reservoir, from which they are metered into the
fuel as required; and (2) to provide the additives in the fuel as it is pumped into the tank,
either by mixing additives into all diesel fuel, creating a special "catalytic" grade of
diesel fuel, or by some sort of at-the-pump mixing. Neither option is fully satisfactory,
for reasons discussed below. Figure 5.9 diagrams a one-trap system based on option one;
a similar system using the second option could be obtained by deleting the reservoir and
the metering pump in the diagram.
Effectiveness: The ceramic monolith trap used in this system would be identical to the
one used in the bypass-burner/monolith system described above. Thus, most of the
comments on the trap's effectiveness are also the same. This trap is extremely efficient
at capturing soot, but less so in capturing particulate hydrocarbons and in reducing the
mutagenic activity of the exhaust. It would thus allow meeting a very strict standard for
total mass emissions of particulates, and would effectively reduce both the total mass of
particulates in the air and degradation in visibility due to particulates. Reductions in
human exposure to particulate-borne organics and especially to particulate-carried
mutagens would be much less. The effects on other pollutants would be minor, except
for a possible reduction in odor due to the surface-catalytic metals in the additives. This
increase should be very slight, however, since almost all of the catalytic metal is
retained in the trap (at least for the tests reported to date). If catalytic additives were
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r'jel Tank
Figure 5.9: System diagram — ceramic monolith trap-oxidizer with fuel-additive self
regeneration.
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to be mixed into all diesel fuel, there could be a significant increase in metal emissions,
since the catalyst-containing fuel would then be used by many vehicles without traps.
Durability and Reliability: Although no tests of this type of regeneration system with
heavy-duty vehicles have been reported, the results reported for light-duty vehicles indi-
cate that reliability should not be much of a problem. The additive self-regeneration
system appears to work quite reliably, even under adverse circumstances such as con-
stant low-speed driving. Since heavy-duty engines are, in general, more heavily loaded
than those of light-duty cars, their average exhaust temperatures are higher. This should
make self-regeneration even more reliable in these vehicles.
The durability of these systems remains somewhat problematic, however. The ceramic
monolith trap retains essentially all of the catalytic metal provided in the fuel in the
form of oxides and sulfates. Over time, these compounds accumulate, and they will
eventually build up to the point where they block the trap. From the currently available
data it is estimated that the traps would need to be cleaned or replaced at intervats
between 70,000 and 200,000 miles. Thus, one trap might well last the lifetime of a light-
heavy duty vehicle, but the heavier and higher mileage trucks might well require three or
four sets of replacements during their lives. The need to replace these traps will add
significantly to the lifecycle cost of the system, and will probably decrease consumer
acceptance and increase the risk of tampering as well. Work is now underway on chemi-
cal methods of removing the additives from the trap without damaging it. If successful,
this would reduce the life cycle cost of the system in high-mileage applications.
Performance and fuel economy: As with any other type of trap-oxidizer system, this
sytem would result in a slight (and probably imperceptible) loss of engine performance,
and a small increase in fuel consumption due to the additional work required to overcome
the pressure drop through the trap. The effects on fuel consumption for this type of
system are estimated as about 2.5 percent for transit buses, 2.0 percent for light-heavy
and medium-heavy trucks, and 1.5 percent for line-haul trucks. These values are slightly
below those for the bypass-burner/monohth system. This is due to the fact that the self-
regeneration process in these traps would occur more often, thus reducing the average
backpressure. In addition, unlike the burner system, this type of system would not use
any fuel directly.
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Estimated Cost: Table 5.2 shows the authors' estimates of the increase in purchase price
and in operating and maintenance costs for each of the four major classes of heavy-duty
vehicles resulting from the use of a monolith/additive trap-oxidizer system. The basis
for this calculation and the general approach used have already been discussed in connec-
tion with Table 5.1, and that discussion will not be repeated here. Table 5.2 differs from
Table 5.1 in that the trap-oxidizer hardware, and thus the hardware costs, are different.
The estimates shown are for a system with the additive reservoir on board; these costs
would be reduced somewhat if the additive were supplied in the fuel instead.
The two systems are expected to require roughly the same amount of labor to install, and
the same amount of tooling, so the estimates of those two costs are the same in Tables
5.1 and 5.2. However, the development of the additive/monolith system is expected to
be much more straightforward than the burner/monolith approach, and thus the esti-
mated RdcD cost per unit for these systems has been reduced by one third.
Maintenance costs for the monolith/additive system are expected to be lower, reflecting
the system's simplicity. The estimated fuel-economy penalty has also been reduced, to
account for the fact that the presence of the monolith results in frequent self-
regeneration, so that the average backpressure is lower. This is expected to more than
offset the effects of ash buildup in the trap in increasing backpressure. Ash buildup will
still occur, however, and will require occasional replacement or cleaning of the trap.
Presently, traps must be replaced, but efforts are now underway to develop a means of
cleaning the trap after it becomes plugged with additives, rather than replacing it. If
these efforts are successful it would reduce the Lifecycle cost of these systems substan-
tially, as Table 5.2 indicates.
The net effect of using fuel additive rather than the burner regeneration system is esti-
mated to be a modest savings in life-cycle cost over the monolith/burner system, except
in the case of line-haul trucks, where the increased trap-replacement costs due to the
additive outweigh the savings from the lower-cost regeneration system. If a practical
method of cleaning rather than replacing the trap is found, the additive system would
become cheaper for line-haul trucks as well. It should be emphasized that these esti-
mates are very crude, and that the system on which they are based is still undergoing
development. The values shown should thus be interpreted carefully, and used with
appropriate caution. As with the estimates for the monolith/burner system, the authors
have tended to err on the side of optimism in developing these estimates — if unforeseen
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Table 5.2
Estimated Cost of Ownership For a Monolith/Additive System
Light-
Medium-
Line-
Transit-
Heavy
Heavy
Haul
Bus
INITIAL COST TO MANUFACTURER
Trap
$ 72.00
$120.00
$ 240.00
$150.00
Container and Piping
50.00
60.00
120.00
60.00
Additive Reservoir and Pump
60.00
80.00
100.00
80.00
Fail-Safe and Sensor
30.00
30.00
60.00
30.00
Modifications to Vehicle
20.00
40.00
80.00
100.00
TOTAL COST TO MANUFACTURER
$232.00
$330.00
$600.00
$420.00
Assembly Labor (hours)
2.00
3.00
5.00
4.00
Cost @ $20 /hour
$40.00
$60.00
$100.00
$80.00
Assembly overhead (9 40%
$16.00
$24.00
$40.00
$32.00
TOTAL COST TO MANUFACTURER
$288.00
$414.00
$740.00
$532.00
Manufacturer's Markup (3 20%
$57.60
$82.80
$148.00
$106.40
Estimated Tooling Cost Per Unit
$5.00
$50.00
$50.00
$100.00
Estimated R6cD Cost Per Unit
$10.00
$100.00
$100.00
$200.00
INCREASE IN DEALER COST
$360.60
$646.80
$1038.00
$938.40
Dealer's Markup @ 8%
$28.85
$51.74
$83.04
$75.07
INITIAL COST TO CONSUMER
$389.4-5
$698.54
$1121.04
$1013.47
OPERATING COSTS
Vehicle Lifetime (Miles)
120,000
250,000
500,000
250,000
Vehicle Lifetime (Years)
8
8
3
8
Maintenance Costs
Per 100,000 Miles
$40.00
$40.00
$50.00
$40.00
Discounted Lifetime
$32.01
$66.69
$166.72
$66.69
Fuel Consumption
Base Fuel Economy (MPG)
16.20
8.81
6.44
6.00
Reduction Due to Trap
2.0%
2.0%
1.5%
2.5%
Cost of Fuel ($/Gallon)
$1.30
$1.30
$1.30
$1.30
Discounted Lifetime Cost
$128.43
$492.01
$1009.62
$903.05
Trap Replacement/Cleaning
Trap Lifetime (Miles)
120,000
125,000
125,000
100,000
Trap Replacements Needed
0
1
3
2
Cost of Replacement
$300.00
$416.00
$776.00
$476.00
Discount Replacement Cost
$ 0.00
$284.13
$1609.37
$626.31
Cost of Cleaning
$84.00
$84.00
$84.00
$84.00
Discount Replacement Cost
$0.00
$194.07
$174.21
$110.53
TOTAL OPERATING COSTS
If Trap Can Be Cleaned
$160.44
$752.76
$1350.54
$1080.26
If Trap Must Be Replaced
$160.44
$842.83
$2785.70
$1596.05
TOTAL LIFECYCLE COSTS
If Trap Can Be Cleaned
$549.89
$1,451.31
$2471.58
$2093.73
If Trap Must Be Replaced
$549.89
$,1541.38
$3906.74
$2609.52
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problems develop, the actual values experienced could well be greater than those shown
in the table, but they are unlikely to be less.
Safety and environmental effects: The major safety concern with this system is with the
on-board storage of the organometallic additive. Most such compounds are highly flam-
mable and some are highly toxic. They could thus be extremely dangerous if released
during a crash. In this regard, it is worth noting that one manufacturer has developed an
additive which is said to be no more toxic than diesel fuel (J.H. Howitt, personal com-
munication, 1983), so the toxicity problem might be avoidable. Flammability, however,
would remain a concern, as would the special problems of additive disposal when the
vehicle is scrapped.
The only significant environmental concerns with this type of system would be the poten-
tial for routine emissions of the catalytic metal in the additive, and for occasional spills
or release of the additive itself. The additives presently under consideration are effi-
ciently collected by the trap itself, so the routine emissions would be a problem only if
additive-containing diesel fuel were to be burned in vehicles without traps. This could be
avoided by using an on-board additive reservoir, or by using some arrangement of special
fuel nozzles similar to the ones for unleaded gasoline. There is, however, considerable
incentive to develop an additive which will not be collected by the trap, in order to eli-
minate the plugging problem. Emissions of such an additive, if it were developed, could
conceivably result in some environmental effects.
Other considerations; If catalytic fuel additives are to be supplied in the fuel, rather
than mixed into it from an on-board reservoir, some EPA action will probably be
required. This will be needed in order to define the appropriate additives and their
amounts, and to ensure that the fuel with the additive will be available. This action
might usefuliy be combined with consideration of overall standards for diesel fuel, such
as are discussed in Chapter 6. A clear policy statement by EPA, defining which additives
would be acceptable under which kinds of circumstances, is also urgently needed in order
to reduce manufacturer's uncertainty and permit additional development in this area.
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Development status: As discussed in Section 5.1.2 and elsewhere (Weaver, 1983a) the
development of additive self-regeneration systems for Light-duty vehicles is well
advanced. A number of manufacturers have reported data from successful tests of this
concept, and industry contacts indicate that a large number of prototype cars using this
approach are now on the road. It appears quite likely that light-duty additive self-
regeneration systems will be offered for sale in California in 1986.
Little development of this approach using heavy-duty engines has been carried out, pos-
sibly because the speed of development in this area has outpaced heavy-duty manufac-
turers R<5cD efforts. However, heavy-duty operation would generally be more favorable
to this type of system than would light-<±ity use (with the possible exception of some
medium-duty trucks), so the apparent success with light-duty vehicles bodes very well for
eventual success in the heavy-duty class as well.
Overall assessment: The rapid and comparatively problem-free development of this type
of system in light-duty vehicles augurs well for a similar development in heavy-duty
applications. Also working in this system's favor are its comparatively low cost, sim-
plicity, and (from the reported development work to date) reliability. On the other hand,
the applicability of any self-regenerating system to all medium-duty trucks is somewhat
doubtful, due to the wide variety of operating patterns found. Some sort of hybrid sys-
tem, using a supplemental heat source for backup, might be workable, however, and this
reservation would apply only to a few models of truck.
The major drawback to this system as it is now envisioned would be the clogging of the
traps with additive residue, which would require replacing or cleaning the traps several
times over the life of a line-haul truck. Since such replacements would be expensive,
they would be unlikely to be carried out (at least in the absence of an inspection and
maintenance program), resulting in increased emissions. Another, less serious disad-
vantage would be the need to supply the additive, either in a reservoir on the vehicle
(reducing payload and possibly causing some safety problems) or in the fuel (which would
generate significant institutional problems). Despite these problems, however, the out-
look for this type of system appears very good.
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533 Catalyzed Wire-Mesh Trap/Regeneration by HC and CO Oxidation
System description; A trap-oxidizer system of this type would consist of one or two
traps, each containing several cylindrical wire-mesh filtering elements and appropriate
manifolding; a system for increasing the HC and CO content of the exhaust; and a set of
sensors, controls, and actuators. The trap(s) might be located either as replacements for
the engine exhaust manifold or further downstream near the mufflers. The former loca-
tion is preferable due to the higher exhaust temperatures there, but would not be practi-
cal in many applications due to lack of room in the engine compartment. Figure 5.10
shows one possible configuration of this system, with a single trap located near the muf-
fler and a regeneration system based on partial throttling of the engine. The type of
system shown would be applicable to most light-heavy and medium-heavy vehicles.
Effectiveness: The Johnson-Matthey trap is somewhat less effective than the ceramic
monolith at reducing the total mass of particulate emitted, due primarily to its much
lower soot-capturing efficiency. On the other hand, this type of trap removes almost all
of the soluble organic fraction of the particulate, and produces a much greater reduction
in total mutagenic activity emitted per mile. Thus, this type of system would have less
effect on total particulate concentrations and visibility than would a monolith-based
system, but would be more effective in reducing human exposure to particulate-bome
organics and mutagenic materials.
A special problem with this type of trap is its tendency to increase sulfate emissions at
high exhaust temperatures, such as are commonly found in heavy-duty trucks. No test
data for traps using Johnson-Matthey's heavy-duty catalyst formulation are available, so
the magnitude of this effect is not known. Test data using J-M's light-duty catalyst for-
mulation on a heavy-duty truck indicte that sulfate production is so high as to rule out
the use of that formulation, except for light-heavy-duty vehicles. This problem and its
implications are discussed further below.
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throttle or intake
.T.ar.i fold
Figure 5.10: System diagram - catalyzed wire-mesh trap-oxidizer and regeneration
system.
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Durability and reliability: The major concern in these areas is with the loss of effective-'
ness of the catalyst coating with increasing time and mileage. Catalysts do deteriorate
with age, and the Johnson-matthey catalyst seems to be no exception. The available
data indicate that the present catalyst should have a useful life of at least 100,000
miles. How much longer it could last is a matter for speculation. A lifetime of 100,000
to 150,000 miles would be adequate for light-heavy vehicles and the lighter end of the
medium-heavy duty vehicle fleet. For the larger trucks, which have longer useful lives,
it would probably be necessary to replace the trapping material at least once during the
truck's lifetime. This would be quite expensive, and it appears unlikely that it would be
done in practice.
Performance and fuel economy; As with any other type of trap-oxidizer system, this
system would result in a slight (and probably imperceptible) loss of engine performance,
and a small increase in fuel consumption due to the additional work required to overcome
the pressure drop through the trap. The effects on fuel consumption for this type of
system are estimated as about 2.5 percent for transit buses, 2.0 percent for light-heavy
and medium-heavy trucks, and 1.5 percent for line-haul trucks. These values are the
same as those for the monolith/self-regeneration system, and slightly below those for the
bypass-burner/monolith system. This is because these traps' ability to self-regenerate
under many driving conditions would reduce the average backpressure, and the regenera-
tion system itself would use only negligible amounts of fuel.
Estimated Cost: Table 5.3 shows the authors' estimates of the increase in owning and
operating costs occasioned by the use of a catalyzed wire-mesh trap-oxidizer system in
each of the four major classes of heavy-duty vehicles. The analytical approach and many
of the assumptions used have already been discussed in Section 5.3.1, in connection with
the cost estimates for the monolith/burner system. That discussion will not be repeated
here.
The estimated cost of the trap-oxidizer and other components shown in Table 5.3 were
derived by scaling up the costs of similar light-duty components, with appropriate
adjustments based on engineering judgment and information supplied by Hohnson-
Matthey. Because of its precious metal content, the catalyzed wire-mesh trap is rather
expensive, but this is offset to some degree by the lower-cost regeneration system the
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Table 5.3
Estimated Cost of Ownership For a
Catalyzed Wire-Mesh System
Light-
Heavy
Medium-
Heavy
Line-
Haul
Transit-
Bus
INITIAL COST TO MANUFACTURER
Trap
Container and Piping
Regeneration and
Control system
Modifications to Vehicle
$240.00
50.00
60.00
20.00
$400.00
60.00
70.00
40.00
$800.00
120.00
70.00
80.00
$500.00
60.00
70.00
100.00
TOTAL COST TO MANUFACTURER
$370.00
$570.00
$1070.00
$730.00
Assembly Labor (hours)
Cost (9 $20/hour
Assembly overhead (d 40%
2.00
$40.00
$16.00
3.00
$60.00
$24.00
5.00
$100.00
$40.00
4.00
$80.00
$32.00
TOTAL COST TO MANUFACTURER
$426.00
$654.00
$1210.00
$842.00
Manufacturer's Markup (3 20%
Estimated Tooling Cost Per Unit
Estimated R&D Cost Per Unit
$85.20
$5.00
$5.00
$130.80
$50.00
$50.00
$242.00
$50.00
$50.00
$168.40
$L00.00
$100.00
INCREASE IN DEALER COST
Dealer's Markup @ &%
$521.20
$41.70
$884.80
$70.78
$1552.00
$124.16
$1210.40
$96.83
INITIAL COST TO CONSUMER
$562.90
$955.58
$1676.16
$1307.23
OPERATING COSTS
Vehicle Lifetime (Miles)
Vehicle Lifetime (Years)
120,000
8
250,000
8
500,000
8
250,000
8
Maintenance Costs
Per 100,000 Miles
Discounted Lifetime
$20.00
$16.00
$20.00
$33.34
$20.00
$66.69
$20.00
$33.34
Fuel Consumption
Base Fuel Economy (MPG)
Reduction Due to Trap
Cost of Fuel ($/Gallon)
Discounted Lifetime Cost
16.20
2.0%
$1.30
$128.43
8.81
2.0%
$1.30
$492.01
6.44
1.5%
$1.30
$1009.62
6.00
2.5%
$1.30
$903.05
Trap Replacement Cost
Trap Lifetime (Miles)
Trap Replacements Needed
Cost of Replacement
Discount Replacement Cost
150,000
0
$636.00
$ 0.00
250,000
0
$976.00
$ 0.00
250,000
1
$1896.00
$1294.99
150,000
1
$1176.00
$730.20
TOTAL OPERATING COSTS
TOTAL LIFECYCLE COSTS
$144.44
$707.33
$525.36
$1480.94
$2371.30
$4047.46
$1666.59
$2973.83
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catalyst makes possible. Assembly labor and tooling costs for the wire-mesh system are
assumed to be similar to those for the monolith/burner approach, but the manufacturer's
R&D per unit would probably be much lower. This is because the wire-mesh system is far
simpler than the monolith/burner approach, and has undergone considerable development
by Johnson-Matthey.
Because of its simplicity, the maintenance costs of the wire-mesh system are estimated
to be well below those for either of the monolith-based systems, and the fuel-
consumption penalty for a reasonably efficient trap (in the 60-80 percent range) is
expected to be comparable to that of the monolith/additive approach. The major uncer-
tainty with the wire-mesh system (aside from the question of technical feasibility, due to
the sulfate problem) lies in the projected lifetime of the catalyst. All catalysts
deteriorate with time, and deteriorate more rapidly when exposed to contaminants such
as sulfur in diesel fuel. No data on how rapidly the Johnson-Matthey catalyst deter-
iorates are available, so the authors have attempted a crude estimate, based on the rate
of deterioration of catalytic converters. The trap lifetimes shown are rather optimistic,
and might require the use of desulfurized fuel to achieve in practice. This greatly
increases the uncertainty in the estimates; if trap lifetime were only half as long as that
shown in the table, the costs of this type of system would increase enormously. This
limitation, as well as the general crudeness of the data, should be borne in mind, and
these estimates should be treated with appropriate caution.
Safety and environmental effects: There are no obvious safety concerns with this type of
system, beyond the slightly increased risk of fire which is common to all trap-oxidizers.
From an environmental standpoint, however, there is an overwhelming concern with po-
tential sulfate emissions. Unless 3ohnson-Matthey can achieve a very great reduction in
sulfate conversion activity at high temperatures, this type of trap will not be usable in
heavy-duty applications except with desulfurized fuel. On the other hand, if the sulfate
problem can be solved or if desulfurized fuel can be provided, this trap will be quite
attractive from an environmental standpoint, due to its activity in reducing gaseous HC,
CO, and odor emissions as well as particulates.
Other considerations; The effect of this trap in reducing odor emissions as well as smoke
might greatly improve the sociability of diesel trucks, and thus help to reduce consumer
L
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resistance to trap-oxidizers, especially among public relations-conscious organizations
such as utilities and other fleet owners.
Development status: Light-duty versions of the Johnson-Matthey system are apparently
in the process of being tested in a production-prototype form, and Johnson-Matthey has
predicted that they will be introduced in commercial production in time to meet the 1986
California particulate standard. Johnson-Matthey also claims to have developed a heavy-
duty catalyst formulation which is now being evaluated by some heavy-duty manufac-
turers. Limited test data published by 3ohnson-Matthey (Budd and Enga, 1984) appear to
support the claim of lower sulphate conversion. Johnson-Matthey has proposed to install
a prototype heavy-duty trap-oxidizer system on a Southern-California Rapid Transit
District bus. This project was stalled until recently by the State of-California's fiscal
problems, but is now in progress. Overall, the development of this system appears to be
very advanced, with the possible exception of the development of a solution to the sul-
fate problem.
Overall assessment: The major advantages of this type of system are that it is well
developed — with its implementation in production light-duty vehicles being predicted
for 1986 — and that it reduces emissions of hydrocarbons and odor as well as parti-
culates. The simpler regeneration system made possible by the presence of the catalyst
is also an advantage. Offsetting this are the higher cost of the trapping medium (this
cost is much more significant in heavy than in light-duty vehicles, due to the much
greater amount required) and the sulfate emissions problem. It appears that the most
promising application of this system is probably in light-heavy duty vehicles, where its
cost disadvantage is smaller and the sulfate problem would be minimized. A shift to
desulfurized diesel fuel (as suggested in the next chapter) could make the system a very
strong contender in every segment of the market.
5.3.4 Ceramic-Fiber Trap/Regeneration by Catalyst Injection
System description: This type of trap-oxidizer system would consist of a number of
"candles" made of woven silica-fiber yarn on a perforated metal substrate and impreg-
nated with an inorganic material to improve filtration. These candles would be arranged
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in one (or two, for a twin-stack exhaust configuration) larger container(s), as shown in
Figure 5.11. In addition, the system would include a backpressure sensor, temperature
sensor, and control logic; a system for injecting the powdered catalyst into the exhaust
stream; and a reservoir of the catalyst. These are also shown in the figure.
Effectiveness: Daimler-Benz reports that this trap's efficiency ranges from about 60
percent to at least 90 percent when clean, and increases rapidly with trap loading
(DBAG, 1984). No data concerning the trap's effectiveness on different components of
the particulate are available. From analogy to the ceramic monolith, however, it might
be expected that the trap would be relatively less effective at capturing the organic
fraction of the particulate, and more effective at capturing the sooty part.
Dirabiiity and reliability: The durability and reliability of this type of system appear to
be extremely good. Daimler-Benz had accumulated more than 280,000 kilometers
(175,000 miles) on five traps as of April 1982 (DBAG, 1982), with some 132,000 kilo-
meters on a single trap. The traps were tested in both highway and city driving, and at
least some of the tests used an automatically controlled regeneration system. Daimler-
Benz does not mention any trap failures in their testing, implying that the reliability of
this type of system, even in the prototype stage, must be reasonably good.
Daimler-Benz describes two types of problems with this system. The first, and probably
less serious problem is that the powdered additive tends to absorb water from the air and
cake together. This greatly complicates the design of the additive metering system. The
second problem is that a significant portion of ash is retained in the trap, resulting in an
increase in backpressure after prolonged operation. Daimler-Benz is presently searching
for a solution to this problem. Unless such a solution could be found, it would be neces-
sary to clean or replace the trap about every 100,000 to 150,000 miles, which would add
significantly to the total cost.
Performance and fuel economy: As with any other type of trap-oxidizer system, this
system would probably result in a slight loss of engine performance, and a small increase
in fuel consumption due to the additional work required to overcome the pressure drop
through the trap. For this type of trap, however, Daimler-Benz has indicated that the
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Figure 5.11: System diagram — Daimler-Benz trap-oxidizer system based
on silica fiber "candle" trap with catalytic regeneration.
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effects on fuel economy are very small, due to its low backpressure. (H. Hardenburg,
personal communication, 1983). It seems unlikely that these effects are completely neg-
ligible, however, so a nominal value of one percent for the fuel consumption increase has
been assumed in calculating the increased operating cost below.
Estimated Cost: Table 5.4 shows the author's estimates of the increased owning and
operating costs due to use of this type of trap-oxidizer system in each of the four major
classes of heavy-duty vehicles. No initial cost data of any sort were available for the
"candle" trap of for its regeneration system, thus, the estimated initial costs should be
treated with great caution. The data shown were estimated by the authors, using
engineering judgment, analogy to other trap-oxidizer systems, and the data-base on
emissions control technology prepared by Lmdgren (1977). They are considered plausible,
but the authors have no indication as to whether they are correct. The assembly labor
and tooling costs for this system are estimated to be similar to those of the other
systems, and thus the same values have been used in the table. The estimated RicD cost,
however, is higher than estimates for all but the monolith/burner system. This reflects
the fact that only one manufacturer (Daimler-Benz) has been developing this approach,
and Daimler would thus be in a position to collect substantial license fees from other
manufacturers if it were used.
Since the overall principal is similar, maintenance costs for the "candle" system were
estimated to be similar to those of the monolith/additive approach. However, based on
Daimler-Benz's statements concerning its low backpressure, the fuel-economy penalty
due to this system should be lower than that of the other candidate approaches. This
would be offset, to some degree, by the trap's tendency to plug with ash at moderately
high mileages, necessitating replacement. According to Daimler-Benz (1984), this occurs
at mileages between 100,000 and 150,000 in transit buses. Since other vehicles would
probably have larger traps, and generally burn less fuel per mile than transit buses, their
traps would be expected to last somewhat longer.
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Table 5.4
Estimated Cast of Ownership For A
Daimler-Benz "Candle" System
Light-
Heavy
Medium-
Heavy
Line-
Haul
Transit-
Bus
INITIAL COST
Trap
Container and Piping
Regeneration and
Control System
Modifications to Vehicle
$ 80.00
50.00
140.00
20.00
$ 130.00
60.00
160.00
40.00
$ 260.00
120.00
200.00
80.00
$ 170.00
60.00
160.00
100.00
TOTAL COST TO MANUFACTURER
$290.00
$390.00
$660.00
$490.00
Assembly Labor (hours)
Cost @ $20/hour
Assembly overhead @ 40%
2.00
$40.00
$16.00
3.00
$60.00
$24.00
5.00
$100.00
$40.00
4.00
$80.00
$32.00
TOTAL COST TO MANUFACTURER
$346.00
$474.00
$800.00
$602.00
Manufacturer's Markup @ 20%
Estimated Tooling Cost Per Unit
Estimated R&D Cost Per Unit
$69.20
$5.00
$15.00
$94.80
$50.00
$150.00
$160.00
$50.00
$150.00
$120.40
$100.00
$300.00
INCREASE IN DEALER COST
Dealer's Markup (9 8%
$435.20
$34.82
$768.80
$61.50
$1160.00
$92.80
$1122.40
$89.79
INITIAL COST TO CONSUMER
$470.02
$830.30
$1252.80
$1212.19
OPERATING COSTS
Vehicle Lifetime (Miles)
Vehicle Lifetime (Years)
120,000
8
250,000
8
500,000
8
250,000
8
Maintenance Costs
Per 100,000 Miles
Discounted Lifetime
$40.00
$32.01
$40.00
$66.69
$50.00
$166.72
$40.00
$66.69
Fuel Consumption
Base Fuel Economy (MPG)
Reduction Due to Trap
Cost of Fuel ($/Gallon)
Discounted Lifetime Cost
16.20
1.0%
$1.30
$64.22
8.81
1.0%
$1.30
$246.01
6.44
0.75%
$1.30
$504.81
6.00
1.25%
$1.30
$451.52
Trap Replacement Cost
Trap Lifetime (Miles)
Trap Replacements Needed
Cost of Replacement
Discount Replacement Cost
120,000
0
$316.00
$ 0.00
150,000
1
$436.00
$297.79
150,000
3
$816.00
$1692.32
100,000
2
$516.00
$678.94
TOTAL OPERATING COSTS
TOTAL LIFECYCLE COSTS
$96.23
$566.24
$610.49
$1440.79
$2363.85
$3616.65
$1197.15
$2409.35
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As noted above, these estimates are based on even cruder and less complete data than
those for the other three candidate systems, and could easily be wrong by a large mar-
gin. As with the other systems, the authors have tended to err on the side of optimism,
although Daimler-Benz's own expressed optimism concerning this system makes this seem
reasonable. These facts should be kept in mind, however, and the estimates shown should
be treated with considerable caution.
Safety and environmental effects: Any trap-oxidizer system will pose a slightly in-
creased chance of fire. For this system, however, that increase would be small enough to
be negligible. There might also be some safety problems associated with keeping a
reservoir of additive powder in the vehicle, but these would be small compared to the
problems involved in keeping liquid organometallic additives on board.
In the area of environmental effects, the only significant concern at present is with the
emission of the catalyst or its chemical derivatives- The amounts emitted would be fair-
ly small. Only a few grams of additive are required for each regeneration, so a typical
heavy-duty vehicle might use five to ten kilograms over its lifetime, and most of that
would be retained in the trap. Thus the lifetime emissions of the additive and its deri-
vatives would be of the order of two or three kilograms. Unless some very toxic or
otherwise harmful product were found among the chemical derivatives, this emission
level would probably be acceptable. The major concern expressed by Daimler-Benz, in
this region, is for possible emissions of dangerous chlorinated hydrocarbons. So far, how-
ever, none have been detected at dangerous levels (DBAG, 1984).
Other considerations; From the available data, this type of system appears to be one of
the most attractive, if not the most attractive, now being considered for heavy-duty
vehicles. The fact that it has been developed and tested exclusively by Daimler-Benz —
a major foreign truck manufacturer which has been making a vigorous effort to penetrate
the U.5. market — could lead to a serious competitive disadvantage for American manu-
facturers. This would be especially serious if the particulate standard allowed too little
lead-time for implementation, since Daimler-Benz's work on this system is well ahead of
developments by any American manufacturers except possibly GM and Ford. Daimler-
Benz has stated that it is confident that it :ould have such a system in production by
1990; no other manufacturer has as much as expressed confidence that a trap oxidizer
system can be built at all.
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Development status: As was stated above, Daimler-Benz has successfully tested a num-
ber of prototypes of this type of system in both city and highway driving for extended
mileages. The system now appears to be undergoing minor modifications and changes
based on the data developed in these tests. Daimler's development schedule (DBAG,
1982) shows vehicle application beginning in early 1984, with testing of prototype vehi-
cles in 1985 and full production for the 1989 model year. This schedule has apparently
slipped some, but a Daimler-Benz spokesman (DBAG, 1984) has indicated that the
company is still confident of production status by 1990.
Overall assessment; Some caution in judging the ultimate potential for this type of sys-
tem is necessary, due to the fact that only one manufacturer appears to be developing it,
and thus all of the available data are from one source. Potential problems could have
been glossed over or neglected, and it might not be equally applicable to other manu-
facturer's designs. From the available data, however, this appears to be among the best
designs for heavy-duty trap-oxidizer systems, except perhaps in very high-mileage appli-
cations. If the problem with trap plugging at high mileage can be solved, this would be
an extremely attractive design in those applications as well. Furthermore, this is among
the most developed types of systems, and the remaining development tasks appear to be
straightforward. Thus it would appear that this type of system will be a very strong
contender in the market.
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6.0 EFFECTS OF FUELS ON DIESEL EMISSIONS
The nature and quality of the fuel being burned is known to affect the emissions from
virtually every class of engine, including both direct injection and indirect-injection
diesels. These effects can be either positive (reduced emissions) or negative, depending
on the fuel and the engine. The recent disruptions in the supply of oil, coupled with the
rapid increase in its cost, have led both to changes in the quality of petroleum-derived
diesel fuels and to considerable interest in the development of alternative, non-
petroleum based fuels. This has generated concern for the possible effects of changes in
diesel fuel composition and/or quality on emissions. This chapter briefly addresses some
of these concerns, and examines some of the problems and opportunities for regulatory
action in this area.
As is true of most topics related to diesel combustion, the area of fuel effects on emis-
sions is highly complex, with much experimental data and little clear theoretical under-
standing. Furthermore, the experimental data themselves are difficult to interpret, and
transient-test data are very scarce. It should be understood that this chapter represents
only a first look at a very complex subject. Firm policy recommendations in this area
would need to be based on a comprehensive study of diesel combustion, diesel fuel
demand, competition from other products such as jet fuel, and the economics and tech-
nology of the petroleum refining industry. All but the first of these topics are well
beyond the scope of this study.
6.1 DIESEL FUEL PROPERTIES
Automotive diesel fuel is a complex blend of hydrocarbons, with boiling points falling
generally in a range between those of gasoline (a mix of light hydrocarbons) and heavy
residual fuel oil. It is thus a "middle distillate" fuel. Other middle distillate fuels in-
clude jet fuel, kerosene, and the lighter grades of fuel oils. The diesel fuel sold in the
United States is of two types: diesel //l, which contains lower molecular weight (lighter)
hydrocarbons, which boil in the range from 180 to 250 C; and diesel //2, which contains
higher molecular weight (heavier) hydrocarbons, which boil in the range from 200 to
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350 C. Diesel //I is also sometimes known as city-bus (C-B) fuel, since it is used pri-
marily by bus fleets, which pay a premium for it because it produces lower smoke emis-
sions. Diesel //2 (also known as truck-tractor or T-T fuel) is used for almost all other
purposes, and makes up the vast preponderance of the diesel fuel sold.
Diesel fuel is made up of three primary types of hydrocarbons: paraffins (straight-chain
hydrocarbons), cycloparaffins, and aromatics. The three groups are distinguished by their
differing molecular structures, which in turn lead to different physical and chemical
properties, some of which affect the combustion process. Aromatic hydrocarbons are
distinguished from the other two classes by the presence of one or more "benzene-ring"
structures, composed of six carbon atoms strongly bonded into a hexagon (they are dis-
tinguished from the cycloparaffins, which also have a ring structure, by the weaker bond-
ing in the cycloparaffin ring). Figure 6.1 shows some typical examples of each group. In
addition to the hydrocarbons, diesel fuel also contains a small amount of organically-
bound sulfur (0.1 to 0.5 percent by mass), and other elements such as vanadium, cobalt,
lead, aluminum and barium, may also be present in trace amounts.
In addition to their differing structures, the hydrocarbons in diesel fuel differ in their
volatilities (boiling points), which are closely related to their molecular weights. Lower
molecular weight hydrocarbons boil at a lower temperature, so that the boiling point of a
fuel sample will increase with prolonged heating as the lighter of its constituent hydro-
carbons boil off first. This provides a convenient way of characterizing the mix of mole-
cular weights in a fuel: one specifies the initial boiling point, the "10 percent point" (the
temperature at which 10 percent of the mass has boiled away, the 20 percent point, and
so forth up to the 90 percent point and the end-point, which is the temperature at which
all of the fuel has evaporated). In general, excessively low "front-end" (initial boiling
point or 10 percent point) and excessively high "back-end" (90% point or end-point)
temperatures are regarded as undesirable; the former because they can lead to vapor-
lock in the fuel system, and the latter because they indicate the pressure of high-
molecular-weight hydrocarbons, which can cause fuel-system plugging and flow problems
in cold weather.
The quality of diesel fuel is most often measured in terms of its cetane number (or its
calculated cetane index, which is an approximation of the cetane number). The cetane
number measures the ease or difficulty of igniting the fuel after it is injected into the
combustion chamber. High cetane-number fuels ignite readily; low-cetane fuels take
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i.tnerj I
< r»uin I % pe i urniul j f Lxanipio Miu<_rure
1 n
I'dTMt fins VI2N+, L16H14 CHj-Cnip^-CHj
CH—- C1U
Monocvc lop.iraf f ins fNH'N C1 bH3'1
1)1 cyt Jopai uf 1 Liih ' n'^N-1 ' 1 fi"30
I r t i_cv( lopar.if f Ins ('n"',N-4 C|ftH'18
CH
'8H17
BcnZfc"°S LNH2N-6 SbH26 CH3CK"l/
Q
P~K»
"b
~;oo'*
V/ r! 11 I M1
lntr3Uns lnh:n-8 rih"24 ,-H
Nap r haienes L,,!!, t,ltlln„ CHJ
"N 2N-12 16 20
Figure 6.1: Typical hydrocarbon molecules in diesel fuel. (fource: Hilden,
et alia, 1982)
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longer to ignite. Rapid ignition reduces the time available for the fuel to mix with air in
the pre-combustion stage, which reduces the amount of energy released during premixed
burning. This reduces noise and stress on the engine, and improves fuel economy (late
ignition has the same effect as injection retard on fuel economy). A cetane number of 50
to 55 indicates a premium quality fuel, while one from about 45 to 50 is good quality.
The ASTM standard for diesel fuel specifies a minimum cetane number of 40, but such a
fuel is considered to be of marginal quality.
Cetane number is measured by testing the ignition qualities of the fuel in a special sin-
gle-cylinder cetane engine. Although there is a growing consensus that the present
cetane measurement does not adequately measure the ignition properties relevant to
today's engines, there is as yet no widely accepted substitute measure. Most of the dif-
ficulty involves the use of special cetane-improving fuel additives, some of which appear
to give good results in the cetane test but poor results in multicylinder engine.
Other common measures of diesel fuel quality are the mass fraction of aromatic com-
pounds, the back-end volatility is seldom of concern, due to the competition by gasoline
and jet fuels for the lighter hydrocarbons. The fraction of aromatic compounds is a con-
cern, because these compounds are more difficult to ignite, which reduces the fuel's
cetane number. A large aromatic content also tends to increase smoke and particulate
emissions, an effect which will be discussed below. A high sulfur content (above about
0.5 percent by mass) is considered undesirable because of the deposition of sulfates as
sulfuric acid in the exhaust, which leads to unacceptable rates of engine and exhaust-
system corrosion. The SO2 and sulfates produced from the sulfur are also significant
pollutants, which makes it undesirable from an emissions standpoint as well. Recent data
developed by Chevron Research (198*0 also implicates fuel sulfur as a major factor in
increasing particulate emissions, a point which is discussed at greater length below.
Figure 6.2 shows the trends in the nationwide averages of these quality indices for diesel
#2 since 1960. As this figure indicates, the last two decades have seen a steady decline
in the average cetane number of the fuel sold, and this decline has become especially
marked since 1976. The average fraction of aromatics in the fuel and the 90 percent
point have increased markedly as well. Of the four quality indices, only the average
sulfur content has remained more or less the same. More recently, average sulfur con-
tent has begun to increase as well, and the amount of fuel with high sulfur content (above
0.5 percent sulfur) has increased sharply (Pless, 1984). This general decline in diesel fuel
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Figure 6.2: Trends in quality indices for diesel #2 (T-T) fuel
since I960. (Source: U.S. Department of Energy, 1982)
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quality is due to the need to process heavier and lower-quality crude oils as more desira-
ble oil resources are exhausted. These lower quality crudes contain more heavy hydro-
carbons and more aromatics than the higher quality oils. Low-quality crudes are usually
also richer in sulfur, but the sulfur content of the refined fuel has been kept in check
somewhat by consumer resistance, due to the destructive effects of high sulfur fuel on
engines. This has required an increasing amount of desulfurization treatment for diesel
fuel and other middle distillates.
The generally heavier nature of present-day crudes, combined with increased competition
by jet fuels and other middle distillates for the lighter fraction of the diesel //2 range has
led to some advocacy (e.g. Barry et alia, 1979) of relaxing the present back-end volatility
requirements on diesel fuel, to permit an increase in the 90 percent point, and thus in the
total amount of heavy hydrocarbons permitted in the fuel. European diesel fuel is
already formulated in this way. Barry estimates that such a change would permit an
additional ^.5 percent of total crude oil to be converted into diesel fuel. This additional
diesel fuel would be obtained at the expense of comparatively low-valued residual oil,
which would help to reduce the price of diesel fuel slightly.
6.2 EFFECTS OF CHANGES IN DIESEL FUEL QUALITY ON EMISSIONS
As was mentioned above, data on the effects of fuel quality and composition on emissions
are, in general, scarce, confusing, inconsistent, and difficult to interpret. Transient-test
emissions and particulate data are especially scarce, due to the recency of the transition
to transient testing and the difficulty and expense of setting up a transient test cell.
These have resulted in transient test cells being reserved for certification and critical
engine-development tests. Fuel effects, since they are not under the manufacturer's
control, have received a lower priority. Thus, most of the data on fuel effects in heavy-
duty engines have been compiled using the older 13-mode testing procedure, and gen-
erally do not include particulate measurements, although smoke and hydrocarbon meas-
urements — from which a qualitative estimate of particulate effects can be made — are
generally provided. In contrast to the heavy-duty situation, light-duty transient-test
data on fuel effects are plentiful. However, it is not clear to what extent these data are
applicable to heavy-duty engines.
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The frequent inconsistency of the available emissions/fuel effects data, and the diffi-
culty of interpreting them, derive from a number of sources. Perhaps the most
important source is the complexity of diesel fuel itself, which is only incompletely char-
acterized by the indices in use. There is also a serious problem with correlation between
variables; aromatic content and cetane number are inversely related in most diesel fuels,
for instance, which makes it very difficult to separate their effects. A third source of
confusion is the fact that today's engines are very finely tuned to produce maximum fuel
efficiency with minimum pollution. A change in fuels may cause the engine to operate
"off-design", and thus increase pollutants quite apart from any inherent effects of the
fuel. The nature and magnitude of this effect may be quite different between different
engines, and different engines may be set to achieve optimal results for different values
of the fuel quality indices. This effect probably accounts for much of the observed in-
consistency in experimental results. This also points up the importance of obtaining test
data in a wide range of engines before making policy decisions.
A final difficulty in interpretation is caused by the fact that fuel effects on emissions
are generally rather small — at least for the range of fuels which could reasonably be
considered for use in diesels — while the random variation and drift in the pollutant
measurements are relatively large, so that the two effects are frequently of the same
order. As a result, statistical techniques are required to extract any clear indication of
trends, and few researchers have accumulated enough data for statistical techniques to
provide much certainty. Three studies which have evaluated large amounts of data are
those of Burley and Rosebrock (1979) with a light-duty indirect injection engine, and of
Wade and Jones (1983), and Bykowsky and coworkers (1983), both of which who combined
the published results from many papers in an effort to discern overall trends. More
recent work by Chevron Research (1984), and ongoing studies by Cummins, Caterpillar,
and other manufacturers should greatly expand our understanding of fuel effects on
heavy-duty engines.
Cetane number and aromatic content: The effects of these two variables are discussed
together, since a fuel's cetane number is closely associated with its aromatic content.
Figure 6.3 shows this correlation.
6-7
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Energy and Resource Consultants. Inc
cm ecftT
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32 96 40 44 ~ 4® 92 96 80 64 66
CETANE NUMBER
Figure 6-3: Correlation between cetane number and aromatic
content for diesel 112 fuel. (Source: Wade and
Jones, 1983)
6-8
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Energy and Resource Consultants, Inc.
A low cetane number means that the fuel takes longer to ignite after it is injected into
the combustion chamber, thus there is more time available for it to mix with the air.
This means that more energy is released in the rapid premixed burning phase of combus-
tion, with a consequent increase in noise and stress on the engine. Premixed burning also
tends to increase NOx emissions, due to the high temperatures reached. Partially offset-
ting this, is the effect of the ignition delay itself, which is similar to the effect of
retarding the injection timing. The increased ignition delay also has many of the ill
effects of injection retard — reduced power, increased fuel consumption, and increased
HC and particulate emissions.
Figures 6.4 through 6.6 show the effects of fuel aromatic content in NOx, HC, and par-
ticulate emissions for light-duty (indirect injection) engines. In order to eliminate the
effects of engine-to-engine variations, the values plotted in this figure have been nor-
malized by the values for the same engines operating on a special, highly consistent,
diesel fuel — Philips diesel //2 control fuel. These figures show a considerable vari-
ability, but there is a general trend toward increasing particulate emissions with increas-
ing aromatic content, and a slight upward trend in NOx emissions as well. Qualitatively
similar effects to those shown could be expected in the indirect-injection engines used in
light-heavy trucks. However, recent work at General Motors (Bergin, 1983) has shown
that the effects of fuels on emissions depend heavily on the engine load, so any quantita-
tive extrapolation from the lightly-loaded light-duty test cycle to the very heavily loaded
heavy-duty cycle is dangerous.
Figure 6.7 shows the effects of cetane number on NOx, HC, and smoke emissions for a
number of heavy-duty diesel engines, obtained using the old EPA 13-mode test for NOx
and HC, and the EPA 3 mode test for smoke. Again, the results have been normalized to
those obtained using Phillips control fuel. As the figure indicates, both hydrocarbons and
smoke increase sharply as cetane number goes below about W, indicating a probably
similar trend for particulates. The few heavy-duty transient cycle particulate data
available (Bunting, 1979; Dietzmann et alia, 1981) show a similar trend to increasing
particulate emissions as cetane decreases. (N.B. — the plots in Figures 6.^ through 6.7
have not been corrected for differences in other, possibly confounding variables. They
should thus be treated with caution, especially where unusual fuels are concerned).
6-9
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Energy and Resource Consultants. Inc
U 2.4
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Figure 6.4:
Effect of fuel aromatic content on N0X emissions
by light-duty IDI diesels. (Source: Bykowski,
et alia, 1983)
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Energy and Resource Consultants. Inc
> 2
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Figure 6.6: Effect of fuel aromatic content on hydrocarbon emissions
by light-duty IDI diesels. (Source: Bykowski et alia, 1983)
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Energy and Resource Consultants. Inc
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Figure 6.5: Effect of fuel aromatic content on particulate emissions by
light-duty 1DI diesels. (Source: Bykowski et alia, 1983)
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Energy and Resource Consultants, Inc
heavy duty DIKE.CT INJECTION DU -I I
PETROLEUM FUELS
l l i i
I
28 32 36 40 44 48 52 5 6 6 0 64 in
CETANE NUMBER
HEAVY DUTY DIRECT INJECTION DIESEL lNGi*l r-
PETROLEUM FUELS
RATED POWER
Figure 6.7: Effects of fuel cetane number on hydrocarbon, NO and smoke
emissions from heavy-duty DI diesel engine, measured on the
EPA 13-mode test (Source: Wade and Jones, 1983).
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Energy and Resource Consultants, Inc.
In addition to their effects on ignition, aromatic hydrocarbons seem to play a direct role in the
formation of soot. At high temperatures, the ring structure breaks down to form acetylene
(Kerns, 1983), which seems to be an important preliminary step in the formation of soot (Weaver,
1983b). It also appears likely that the aromatic structures themselves play an important role in
providing nuclei for the soot formation process. Thus the effects of cetane number on particu-
lates may in fact be due more to the inverse relationship between aromatics and cetane number
than to ignition effects as such. Burley and Rosebrock (1979) and Bykowski et alia (1983) both
found that aromatic carbon content was a better predictor of particulate emissions than was
cetane number at least in light-duty prechamber engines. In particular, Burley and Rosebrock
found that the achievement of high cetane numbers by means of cetane-improvmg additives
increased particulate emissions, which is the reverse of what would be expected if cetane number
alone were the more important property. At cetane numbers below theASTM standard, however,
ignition effects appear to be more important. Wade and coworkers (1984) found that the use of
cetane improvers to bring a 32.5 cetane fuel up to 47.5 reduced particulate emissions substan-
tially, although emissions were still not as low as a 47 J cetane fuel without improvers.
Volatility: Both the "front-end" and the "back-end" volatilities (initial and final boiling points) of
diesel fuel affect emissions. If the initial boiling point is too low (i.e., the fuel contains too many
light hydrocarbons) then HC emissions tend to increase. On the other hand, high 90 percent and
end-points are correlated with increases in smoke opacity, and particulate emissions. Figure 6.8
shows the trends in particulate emissions with changes in back end volatility, from Bykowski's
study of light-duty engines. While in each case there is a great deal of scatter in the data, the
trend does appear to be slightly upward for petroleum-based fuels. NOx does not appear to be
s'.rongly affected by back-end volatility.
Data on the effects of back-end volatility on DI particulate emissions are very scarce. Some
indications of trends can be gotten from studies of the effects on smoke opacity, however. Fig-
ures 6.9a and 6.9b show plots of the trends in smoke emissions with increasing back-end volatility
(a) and total volatility (b). Again, the trend appears to be upward with decreasing volatility,
although the slope of the trend line is heavily dependent on the specific engine. Heavy-duty
engine data obtaned by Chevron Research (1984) also show a small positive correlation between
particulates and back-end volatility.
A possible explanation for both the general increase in particulate emissions with increasing
back-end boiling point and the wide variations in the magnitude of this effect can be found by
hypothesizing that the effect is due to the changed density and viscosity of the fuel, rather than
6-14
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Energy and Resource Consultants, Inc.
N
o i.*
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apm*tiT, •(
Figure 6.8: Effect of changing 90% boiling point on particulate
emissions from light-duty IDI diesels (Source: Bykowski
et alia, 1983).
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Energy and Resource Consultants, Inc
' ' * 11
m «od m * so no
trt imm •»
(a) Smoke opacity vs. 95% distillation temperature (Source: Bykowski et alia,
19 79)
(b) Relative smoke vs. fraction distilling above 140° F. {Source: Gross and
Murphy, 1978)
Figure 6.9: Effect of changing volatility on smoke emissions from heavy-
duty diesels.
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Energy and Resource Consultants, Inc.
to any special combustion characteristics of the hydrocarbons involved. Diesel injection
pumps meter fuel by volume, not by mass, thus an increase in density will result in more
fuel entering the cylinder, a higher equivalence ratio (ratio of fuel to oxygen), higher
temperatures, and greater power. The higher equivalence ratio increases soot produc-
tion. The increased viscosity of the fuel, which reduces the efficiency of atomization
and fuel-air mixing, would also contribute to soot formation to some degree. Finally, a
small increase in the soluble fraction of the particulate might be expected, since the
hydrocarbons in a high-boiling point fuel would condense more easily, even though the
hydrocarbon emissions as a whole might be decreased. Figure 6.10, which shows that
particulate emissions seem to be a strong function of density, lends credence to this
hypothesis.
Again, great caution should be used in extrapolating the emissions effects shown in Fig-
ures 6.8 and 6.10 from light-duty to heavy-duty engines, even to heavy-duty prechamber
engines. Work by Bergin (1983) indicates that even the sign of the change in particulate
emission due to a change in 90 percent point may be different at high load conditions —
at very high loads, he found that increasing the 90 percent point decreased particulate
emissions. This may be the reason for the comparatively slight effect of the 95 percent
point on smoke in direct injection engines, as indicated on Figure 6.9.
Sulfur Content: The sulfur content of diesel fuel has, of course, a direct and linear
effect on an engine's emissions of sulfur dioxide (SO2) and sulfates, but until recently,
was thought to have little direct effect on other pollutants.
However, recent work by Chevron Research (198*0 indicates that fuel sulfur content has
a much greater effect on particulate emissions than was previously thought. The primary
effect of sulfur on particulate emissions is through the formation of sulfates. During
combustion, most of the sulfur in the fuel is oxidized to SO2, and emitted as SO2 gas. A
small fraction, however, is further oxidized to SO^, and is collected as particulate mat-
ter during the emissions test. This material proceeds to react with water in the exhaust
and in the atmosphere to form sulfuric acid, Presently, sulfates make up about
10 to 20 percent of total particulate emissions measured on the transient test. However,
since sulfate emissions are not affected by the combustion modifications that reduce the
hydrocarbon and soot fractions of the particulate, they will make up a larger and larger
fraction of the total as these other sources are reduced.
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Energy and Resource Consultants, Inc.
).2
2,1
Petroleun
H* ill Solid Line
COA- ,)6)
Hi All ST.Mf. tŁ6B(SSI0H LINE KS.lS.
I .ClIftT .04249 !• .00116M* .74464 .00124
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.13770 .03)46 *• . t H 11M ~ . 7 >072 IMf -6
.•3M3 .17I2J T* ).M2)U->.797) .012))
ocniTT, g/ml
Figure 6.10: Effect of changes in fuel density on particulate emissions in
IDI diesels. (Source: Bykowski, et alia, 1983)
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Energy and Resource Consultants, Inc.
In addition to its direct effect in increasing the sulfate component of the particulate
matter, Chevron found that high sulfur content also appears to increase the soluble
organic fraction of the particulate matter. No explanation for this effect has yet been
advanced. Overall, Chevron's data give the following correlation equation for particulate
emissions as a function of fuel sulfur content, aromatic content, and 90 percent boiling
point in steady-state tests on a Cummins NTC 290 engine.
P = 0.00135 A + 0.000232 T + 0.387 S + 0.0225 (6.1)
where P is particulate emissions in grams per BHP-hr
A is the aromatic content in weight percent
T is the 90% boiling point in °F
S is the sulfur content in weight percent
This equation predicts that reducing the sulfur content of diesel fuel from 0.49 percent
by weight (the present ASTM standard for diesel H2 is 0.5 percent) to 0.05 percent should
reduce particulate emissions by .17 g/BHP-hr, or 42 percent. Figure 6.11 shows a number
of particulate measurements obtained by Chevron with different fuels, plotted against
the particulate emissions level predicted by Equation 6.1. As can be seen, the equation
does a good job of predicting emissions over a fairly large range.
In transient testing, overall particulate emissions are higher, and thus the use of low-
sulfur fuel would have a smaller percentage effect, even though the absolute reduction in
emissions .would be expected to be about the same. The directional effects of low-sulfur
fuel on 'emissions have been confirmed by transient tests conducted at Southwest
Research Institute (Chevron, 1984), and by Cummins Engine (L. Broering, personal com-
munication, 1984). The Cummins tests measured a reduction from 0.79 g/BHP-hr with
.54 percent sulfur in the fuel to 0.55 g/BHP-hr at 0.03 percent sulfur, a reduction of 0.24
g/BHP-hr. This compares rather well with the .20 g/BHP-hr reduction predicted by
Equation 6.1.
The Chevron data also cast considerable doubt on the conclusions of previous research
into fuel effects on emissions. Most such research had identified aromatic content and
back-end volatility as the properties most affecting emissions, and related them to emis-
sions changes means of linear regressions. Because of the strong correlation between
sulfur content and aromatics, and between sulfur and back-end volatility in diesel fuels,
it appears that much of the effect of the sulfur was erroneously ascribed to aromatics
and/or volatility in the earlier studies.
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Energy and Resource Consultants, Inc.
TOTAL PARTICULATE EMISSIONS
RELATED TO AROMATICS,
VOLATILITY, AND SULFUR
Calculated from Correlation Equation, g/bhp-Hr
Particulate = (1.35 x 10"3) 9/o A - (2.32 x 10~*) T90 ~ (3.87 x 10'1) % S * 2.25 x 10*2
Figure 6.11: Actual particulate emissions vs. emissions predicted from fuel
composition using equation 6.1 (Source: Chevron, 1984).
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Energy and Resource Consultants, Inc.
Beyond its effect on direct emissions of particulates, fuel sulfur has an important
indirect effect on ambient particulate levels. This is because a portion of the SO2
emitted is converted to sulfate particles by reactions in the atmosphere. Recent esti-
mates by the California Air Resources Board (198^) indicate that, in urban areas, the
secondary sulfate and nitrate particles found due to diesel emissions can be much greater
in mass than the diesel's direct emissions of particulate matter.
6.3 POTENTIAL REGULATORY ACTIONS TO IMPROVE FUEL QUALITY
This section briefly outlines some apparently feasible regulatory actions which might be
taken in regard to diesel fuel, either to prevent further deterioration in diesel-fuel
quality or actually to improve it. Three possible approaches are discussed: regulatory
limits on cetane numbers and/or aromatic content, possible changes in the permissible
"high-end" boiling point for diesel fuel, and stringent limits on its sulfur content. These
approaches are not mutually exclusive — in fact, the synergetic effects between them
make it appear that the optimal strategy might be a combination of ail three. These
points are discussed in detail below.
Limitations on cetane number and/or aromatic content: Given the strong effects of
decreasing cetane and/or increasing aromatic content on emissions, and the trend toward
decreasing average cetane numbers in fuel, there is a strong case to be made for a
standard specifying the minimum permissible cetane number or the maximum permissible
aromatic content for highway diesel fuel. The two are closely related, and for par-
ticulate emissions it appears that the aromatic content and not the cetane number may
be determining. In the absence of cetane-improving additives, a cetane-number standard
would also serve to limit aromatic content. However, such additives do exist, and seem
likely to see increasing use as crude oil quality declines. The most likely response by
refiners to a minimum-cetane-number standard would be to use such additives to meet
it. The additives do not seem to improve the particulate-emission performance of the
fuel as much as would be expected from their effects on cetane. Thus, to control par-
ticulate emissions, a standard setting a maximum permissible aromatic content would be
desirable, in addition to or in place of a minimum cetane standard.
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Energy and Resource Consultants, Inc.
Based on the emissions effects discussed in Section 6.2, a minimum cetane number of
about U2, combined with a maximum aromatic content of around 35 percent, seems like a
reasonable standard. This would place a floor on diesel fuel quality somewhat below the
present average quality, but above the present minimum. This would require quality
improvements by some refiners — about 30 percent of diesel fuels sampled by the MVMA
fell below these limits in 1983 (Pless, 1984). This, in turn, would necessarily reduce the
supply and increase the price of diesel fuel. A precise estimate of the degree of price
increase is beyond the scope of this study, but an approximate indication of its magnitude
is given by a CARB study (CARB, 198V), which reported refiners' estimates of the cost of
meeting a 25 percent aromatics standard. The estimates ranged from zero to $4.09 per
barrel, and averaged $1.74 or about 4 cents per gallon. A reduction to 35 percent
aromatics would have a smaller effect — probably only one or two cents per gallon.
Such a standard might lead to improvements in emissions and fuel economy beyond those
which would be expected from the change in properties alone. This would result from the
fact that, with a firm statutory minimum on fuel quality, engine manufacturers could
adjust the engine parameters for optimal performance on that fuel. As things now stand,
an engine must be able to operate reasonably satisfactorily or very low-quality fuel,
since such fuel is likely to be encountered sometime during the engine's life. The fuel
economy benefits from this readjustment could be expected to partially offset the
increased price of the fuel and there would probably be some savings in maintenance,
repairs and reduced engine wear. Thus the net cost to the truck owner would not be
great, and might even be negative.
Changes in permissible "back-end" boiling points? The relationship between the "back-
end" boiling point and emissions is sufficiently weak that it would be very difficult to
justify any mandatory decrease. Indeed, under some circumstances, it might be justifia-
ble to increase slightly the maximum now set by the standard for ASTM diesel if2. Such
an increase would significantly increase the available stock of diesel fuel, and could be
expected to result in a decrease in its price. Such an approach would be especially
attractive in combination with a stricter limit on cetane and aromatic content, since the
additional fuel made available by increasing the end-point would offset the decrease in
usable fuel occasioned by the more stringent quality regulations. Most or all of the
effect of high-boiling-point fuels on emissions appears to be due to their greater
density. Since reducing the aromatics content also reduces density, a small increase in
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Energy and Resource Consultants, Inc.
boiling point would only return the fuel to where it had been, and would thus have a
minimal effect on emissions. Such a change would also act as a sugar coating on the pill,
making the quality regulations more acceptable to refiners.
Desuifurization: Reducing the sulfur content of diesel fuel is desirable for a number of
reasons. Such an action would reduce total SO2 emissions, and would thus reduce acid
deposition and the acidification of the environment, as well as secondary particulate
formation. In addition, it would reduce human exposure to SO2 and sulfates. Since diesel
exhaust is emitted close to ground level, and frequently in areas of concentrated popula-
tion, the reduction in human exposure would be much greater than for the removal of an
equiva'ent amount of sulfur from a utility stack or a copper smelter. In addition, lower-
ing the sulfur content of the exhaust would make catalytic traps both more attractive
and (since the activity of the catalyst could then be increased) more effective. In addi-
tion to reducing particulate emissions, this would also drastically reduce emissions of
hydrocarbons and odor from the diesels, arid would have a significant effect in reducing
CO as well. Considered alone, any one of these effects might not justify the cost of
desuifurization, but, in combination, the benefits of all together appear to outweigh the
costs (CARB, 198<0.
High-sulfur stocks of diesel fuel already undergo desuifurization, in order to comply with
market requirements lor a noncorrosive exhaust. As Figure 6.2 indicates, the average
sulfur content of diesel fuel changed very little between I960 and 1982; despite a major
shift to high-sulfur crude oil. More recently, however, both the average sulfur level and
the incidence of high-sulfur fuels seem to be increasing. Desuifurization is accomplished
by treating the fuel with hydrogen in the presence of a catalyst, so that the sulfur is
removed as hydrogen sulfide. A more severe version of the same process also decreases
the aromatic content and increases the cetane number of the fuel, since the hydrogen
can also combine with the unsaturated aromatic structures to convert them to saturated
cycloparaffins. A regulatory limit on sulfur would be met by increasing the fraction of
diesel fuel which is treated in this manner, and by increasing the severity of the treat-
ment. According to the CARB (1984), the cost of desuifurization would be about two or
three cents per gallon, which would correspond to a cost of about $2,000 to $3,000 per
ton of SO2 removed. This is about two to four times the cost of SO2 removal from
power-plant exhaust: a cost which society has demonstrated it is willing to pay. Given
the numerous other benefits of desuifurization, this approach deserves very careful
evaluation.
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Energy and Resource Consultants, Inc.
A major synergetic benefit is also possible with widespread desulfurization in combina-
tion with improvements in aromatics and cetane. Since aromatic saturation is only a
more severe version of the processs that removes sulfur from diesel fuel, that the
incremental cost of de-aromatization would be smaller. The reverse is also true; if a
maximum aromatic content standard were in effect, the additional cost of desulfuriza-
tion would be lessened. However, de-aromatization requires higher pressures than
desulfurization, and thus cannot easily be retrofitted to a desulfurization unit (desulfur-
ization, on the other hand, is inherent in the de-aromatization process). Thus it would be
advantageous to consider implementing both such standards simultaneously, in order to
reduce the necessary capital expenditures for compliance.
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Energy and Resource Consultants, Inc.
7.0 EMISSIONS STANDARDS FOR HEAVY-DUTY DIESEL ENGINES
This Chapter addresses a number of questions relating to the establishment of specific
numerical standards for heavy-duty diesel NOx and particulate emissions. In doing so, it
draws heavily on the discussion in the preceding five sections, especially Chapters 4 on
engine-out emissions control technologies and five on trap-oxidizers. This Chapter
begins with a discussion of the regulatory history for heavy-duty diesel engines, including
both those standards now in force and recent studies and proposals. This discussion is
given in Section 7.1. Section 7.2 then moves on to discuss two closely related issues —
subdivision of the heavy-duty class into multiple subclasses, and the effects of a standard
based on emissions averaging on the cost and feasibility of compliance. From there,
Section 7.3 proceeds to address the central issues in this study — the feasible levels of
NOx and particulate standards for both the near term (1987-1988) and the intermediate
term (1990-1991). Section 7A, finally, summarizes ERC's recommendations with regard
to emissions standards.
Caveat — It has been necessary to omit discussion of a number of issues which are
closely related to heavy-duty standard setting from this report. These issues include
EPA's proposals with respect to the assessment of useful life for heavy-duty vehicles,
multiplicative rather than additive deterioration factors, in-use durability testing, and
the requirement to show reasonable likelihood of scheduled maintenance. Each of these
issues Is important in its own right, and each deserves careful evaluation before being
included in a final regulation. All, however, are beyond the scope of the present study.
7.1 PREVIOUSLY PROPOSED STANDARDS
Table 7.1 shows Federal and State of California emissions standards for heavy-duty diesel
engine hydrocarbon + NOx, NOx alone, smoke, and particulate emissions through model
year 1986, as well as EPA's current proposed regulations for 1987 and later years. The
situation in the 1983-1985 period is actually much more complex than that shown in the
table — the establishment and subsequent delays in implementation of transient-cycle
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Energy and Resource Consultants, Inc.
Table 7.1
Historical and Proposed Federal and California Emission Standards
For Heavy-Duty Diesel Engines
Year
California
HC + NOx NOx
HC + NOx
NOx
Federal
Smoke Opacity (%)
Accel/Lug/Peak
Particulates
1973
16
—
—
40/20/—
¦ft*
1974
16
—
16
—
20/15/50
1975
10
—
16
—
20/15/50
ft*
1976
10
—
16
—
20/15/50
¦ft*
1977
—
7.5
16
—
20/15/50
**
1978
—
7.5
16
—
20/15/50
**
1979
—
7.5
10
—
20/15/50
ft*
1980
6
—
10
—
20/15/50
1981
6
—
10
—
20/15/50
-ft*
1982
6
—
10
—
20/15/50
**
1983
6
—
10
—
20/15/50
**
1984
4.5++
5.1++
10++
10.7++
10/15/50
**
1985
4.5++
5.1++
—
10.7+
20/15/50
**
1986
4.5++
5.1++
—
10.7 +
20/15/50
-ft#
1987
4.5++
5.1++
—
6.0*
20/15/50
0.6+*
1988
4.5++
5.1++
—
6.0*
20/15/50
0.6+*
1989
4.5++
5.1++
—
6.0*
20/15/50
0.6+*
1990 and on
4.5++
5.1++
—
4.0*
20/15/50
0.25+++*
All values are g/BHP-hr, measured on the steady-state (13 mode) test, except as noted.
* EPA Proposed standard.
** Not regulated, but controlled to some degree by smoke standards.
— Not regulated.
+ EPA transient test
~+ Manufacturers option: HC + NOx in steady state or NOx alone in transient test.
+++ Transient test, special standards proposed for line-haul and transit buses.
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emissions testing and numerous other regulatory changes have resulted in a complex
patchwork of rules, regulations, extensions and exceptions during that period.
The stringent heavy-duty NOx and particulate regulations which are now proposed for
1987 and later years have a lengthy regulatory history. EPA first issued a notice of
proposed rulemaking (NPRM) for tightened NOx standards, and an advanced notice of
proposed rulemaking (ANPRM) for heavy-duty particulate standards in 1979 (EPA, 1979b;
1979c). The standard levels proposed then were <>.0 g/BHP-hr for NOx and 0.25 g/BHP-hr
for particulates. These were to apply to the 1986 and subsequent model years. The 0.25
g/BHP-hr particulate level was to be attained through the use of trap-oxidizers, which
were expected to be available in 1986. In addition to these very ambitious numerical
standards, the new proposals also included a change in the test cycle to be used in certi-
fication (from the 13-mode steady state test to the present transient test), and changes
in the computation of useful life and deterioration factors. Hearings on these proposals
were not held until July of 1982, by which point it had become clear that the numerical
standards proposed for 1986 could not be achieved, and that more lenient standards
and/or a longer lead time would need to be granted.
Rules requiring the transient test procedure were subsequently promulgated for 1984,
then delayed (at the manufacturer's option) until 1985 due to the manufacturers1 diffi-
culties in implementing the test. No further regulatory action was taken on NOx and
particulate levels until October, 1984, when EPA proposed standards of 6.0 g/BHP-hr
NOx and 0.6 g/BHP-hr particulates for model year 1987, to be followed by 4.0 g/BHP-hr
NOx and,6.25 g/BHP-hr particulates for most heavy duty vehicles. The 1990 proposals
also included two proposals for special treatment of sub-groups of heavy-duty vehicles: a
more lenient particulate standard of 0.4 g/BHP-hr for line-haul trucks, and a more strin-
gent standard of 0.1 g/BHP-hr for transit buses. In outline, at least, these proposals
closely resemble the recommendations of this report.
In connection with the proposed new standards, EPA has begun rulemaking proceedings to
establish non-conformance penalties for manufacturers whose engines are found to be in
violation. Previously, the penalty for non-conformance was denial of the right to sell the
offending engines, or wholesale recalls for repair of engines already sold. The desire to
reduce the chance of such draconian penalties has led manufacturers to allow a substan-
tial margin of safety between the emissions level specified in the standard and the
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engine's projected emissions. With the use of non-conformance penalties, the need for
this margin of safety is reduced.
72. ISSUES RELATED TO STANDARDS: EMISSIONS AVERAGING AND SUBDIVISION
OF THE HEAVY-DUTY CLASS
As the discussion in Chapter Two has indicated, heavy-duty diesel vehicles are by no
means a homogenous group, and the engines that power them are only slightly more
homogeneous. It makes sense to question, then, whether a single standard can or should
be applied to these different groups. A closely related issue concerns emissions averag-
ing: to what extent should manufacturers be permitted to trade-off emissions between
different engine models and types? How should such a system work? What, precisely,
should be averaged: grams per horsepower hour, lifetime emissions, or what? The
answers to these questions could have major implications for completion and survival in
an industry which has already been seriously damaged by recession.
7.2.1 Subdivision of the Heavy-Duty Class
Any system of subclassifying vehicles or engines for regulatory purposes should meet
several basic criteria. First and most important, it should be logical — it should group
vehicles and/or engines with similar characteristics together, and separate those which
differ in. important ways. Secondly, it should be clear — both manufacturers and regu-
lators- should be able to determine rapidly and unambiguously what class a specific vehi-
cle or engine falls into. In addition, the regulations should provide little incentive or
opportunity for a manufacturer to defeat the classification scheme. Finally, such a sys-
tem should be simple — the smallest possible number of subclasses consistent with the
previous requirements should be established, in order to minimize confusion, duplication
of time and effort, and regulatory inflexibility.
In Chapter Two, the authors have proposed a classification scheme for heavy-duty
engines and vehicles which they consider to form an appropriate basis for regulation.
This scheme divides the heavy-duty class into four subclasses: light-heavy, medium-
heavy, line-haul, and transit bus. The division was made on the basis of vehicle and
engine technical characteristics and vehicle operating patterns, with specific attention
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to the effects of these characteristics on emission patterns and on ability to comply with
regulations. Thus each of these four groups is much more homogeneous (with respect to
those characteristics which concern environmental regulators) than is the class of heavy-
rijty vehicles as a whole. A very Similar scheme to this one has largely been adopted by
EPA in its recent rulemaking proposals for heavy-duty NOx and particulate emissions
(EPA, 198i»a>.
The classification scheme used in this report forms a suitable framework within which to
consider subdividing the heavy-duty class. The actual degree of subdivision desirable,
however, would depend on the nature of the regulations being proposed and the costs and
benefits of regulation m each class. The Jjrst and most obvious division, desireabJe in the
case of either a strict NOx or a strict particulate standard, would be between light-heavy
duty engines and those for the other three classes. Light-heavy duty engines resemble
the small high-speed diesels developed for passenger cars and light trucks more than the
larger engines used iti the other classes of fie^vy-tfuty vehicles. Presently, almost aii
light-heavy duty engines use indirect injection, and they are thus able to attain emissions
levels that will not b^ attainable by the larger DI engines for several years. The small,
high-spe&d DI engines now being introduced into this class by Cummins and lsuzu will
probably have higher emissions levels than the current IDI engines, but they generally
employ very advanced technology, and would thus be ablfe to reduce emissions more
readily. The current IDI engines win also be abJe u> take advantage of the rapid
deveiopment of electronic controls for light-duty engines. Ttius, it appears that engines
on the light-heavy class would be able to meet a stringent engine-out emissions control
standard several year* before any of the other Classes.
The same is probably true of trap-oxidizer technology. A trap-forcing standard for light-
duty diesel cars and trucks will go into effect in 1987, and at least some manufacturers
appear to be able to meet jt. If trap-oxidizers are successfully applied to light-duty
trucks in 1987, no more than another year should be needed to apply thern to light-heavy
duty vehicles. A trap-forcing standard lor these vehicles would thus become feasible in
1988, or about the same time at which a more stringent engine-out standard would be
attainable.
Transit buses are another group which should be considered for stricter standards. The
reason in this case is not technological (transit-bus engines are similar in technology to
other medium-heavy duty engines) but operational. Bus emissions result in much more
human exposure per gram of potfutanl than do emissions from most other vehicles. Thus
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a good case can be made for imposing the strictest technologically feasible emissions
standards on buses, even if standards for other classes are relaxed for economic
reasons. In line with this approach, there has been some discussion recently of requiring
the use of methanol engines for transit buses, in order to minimize NOx and particulate
emissions. This, however, amounts to a design standard, rather than the kind of per-
formance standard that EPA has previously used for motor vehicles, and thus could lead
to inefficiencies. A more rational approach would be to specify strict NOx and particu-
late standards, which could be met by use of a methanol engine or by any other feasible
means.
It is worth questioning, also, whether an EPA standard is the best vehicle for achieving
the desired goal of reduced transit-bus emissions. As one alternative, a change in financ-
ing criteria by the Urban Mass Transit Administration could be just as effective, and
possibly more flexible. There is precedent in the use of these criteria to achieve social
goals — the recent requirement of handicapped access for buses is one example. Since
transit buses are purchased almost exclusively by public or semi-public transit agencies,
some sort of program aimed at these agencies might well be even more effective than a
standard for manufacturers, and could also be easier to implement.
A final group that should be singled out for special attention are the line-haul trucks.
These trucks are used primarily outside of urban areas, so the human exposure and envi-
ronmental degradation per gram of emissions is lower for them than for most other
heavy-duty vehicles (they are the exact opposite of the transit buses in this). At the
same timŁ, these trucks consume a large fraction of the total fuel used by the heavy-
duty class, because of the enormous distances they travel. Strict particulate regulations
would increase fuel consumption by several percent, and strict NOx regulations could
increase it by as much as 10-15 percent, so it is worth considering whether such regula-
tions are worthwhile in this class. It is also worth considering whether such regulations
could be enforced — Line-haul truckers are notoriously independent and technically adept,
and the cost of increased fuel consumption due to trap-oxidizers and/or NOx controls
would certainly be adequate motivation for tampering.
A major problem with separate treatment of line-haul trucks is the difficulty in defining
them. The division between line-haul trucks and the medium-heavy group is much
fuzzier than those between medium-heavies and light-heavies, or medium-heavies and
transit buses. Line-haul engines are very similar in size and characteristics to the larger
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medium-heavy engines, and are sometimes used on medium-heavy trucks where addi-
tional power is required. If line-haul engines were subject to a more lenient standard,
there would be a strong incentive for each manufacturer to classify its engines as line-
haul, and it would be difficult to show unequivocally that a given engine was or was not
intended for line-haul service.
Any solution to this problem would need to be structured in such a way as to avoid dis-
rupting the truck engine market, and create a minimum of artificial distinctions. One
approach to this problem would be a case-by-case exemption — anyone seeking to pur-
chase an engine meeting the more lenient standards for line-haul trucks would have to
produce a permit, which would be issued upon his or her demonstration that the vehicle
being purchased was for use in line-haul service. EPA, in its notice of proposed rulemak-
ing (1984a), has proposed to define line-haul vehicles as those gross weights exceeding
60,000 pounds. Other, possibly feasible approaches have been suggested by Cummins
(19S3) and by the EMA (1982b), but all of these proposals have the disadvantage that they
don't discriminate between the large tractor trailers used in interstate service (which
contribute little to urban air pollution) and similar trucks used for heavy hauling within
an urban area (which contribute a great deal).
At this point, there is too little information on the relative costs and benefits of line-
haul truck emissions to make a firm recommendation as to whether to treat them sep-
arately or on the same basis as the medium-heavy group. For the light-heavy group,
however, the differences in technological readiness are quite clear. Thus an earlier
implementation date than 1990 is recommended. For transit buses, the technological
feasibility of control is no better than for medium-heavy trucks, however, the cost-
benefit ratio is significantly better. Thus it is recommended that a strict standard (or
action of some other form to the same effect) for transit buses should be considered even
if a more moderate standard is adopted for other heavy-duty vehicles.
733 Emissions Averaging Regulations
Emissions averaging can, in theory, reduce the cost of environmental protection by
allowing manufacturers to obtain the greatest reductions in emissions from those engines
on which this is technically easiest, while relaxing standards on those engines which are
difficult to bring into compliance. In the area of heavy-duty diesel engines, however,
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this theoretical benefit would be accompanied by some serious practical problems, which
could lead to both disruption of the competitive environment and to circumvention of the
intent of regulations. For this reason, it is necessary to examine the specific structure
of an emissions-averaging regulation with considerable care.
The most commonly conceived form of emissions averaging is one which simply weights
each engine's emissions levels by it annual sales to arrive at an average emissions level
for a given manufacturer. Even if it is assumed that averaging is restricted only to
heavy-duty diesel engines, such a standard would still favor the manufacturer with the
broadest product line, and could be expected to have major deleterious effects on com-
petition in the industry. Such a standard would also result in less protection of the envi-
ronment.
As an example, consider the case of a stringent NOx emissions standard. Stringent con-
trol of NOx emissions in DI engines increases fuel consumption, and results in poor per-
formance and driveability. For manufacturers such as GM and International Harvester,
however, this would be a minor problem, since they would be able to average in the very
low NOx emissions of their indirect-injected light-heavy duty engines. By achieving their
NOx control in these lighter engines, they would be able to set their heavy-duty engines
for higher NOx, better performance, and better fuel economy, thus gaining a devastating
competitive advantage. At the same time, this practice would impair the overall protec-
tion of the environment, since the light-heavy duty engines have shorter lives and lower
power levels, and thus a reduction of 1 g/BHP-hr in such an engine has much less effect
than in a-larger, longer-lived model.
Careful design of any emissions averaging regulation would be required in order to avoid
undesirable results such as those described. One possible approach would be to weight
each engine's brake-specific emissions by its rated horsepower and expected lifetime
mileage. This would be more representative of each engine's effects, but it would still
tend to favor light-heavy duty manufacturers, since the average horsepower in use is a
smaller fraction of the rated horsepower for these engines than for the other heavy-duty
classes. This also fails to account for the differences in contributions to urban pollution
levels between different classes — an additional gram of lifetime NOx emissions from a
transit bus, for instance, is much more harmful than an additional gram from a line-haul
truck. Thus, to achieve real equity an averaging scheme would need to account for
estimated life, average power levels, and use patterns, as well as brake-specific emis-
sions.
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An alternative, and possibly simpler approach, would be to allow averaging of emissions
only within specific subclasses of heavy-duty vehicles — for instance, allowing bus emis-
sions to be offset only against other bus emissions. One workable subclassification for
this purpose would be the scheme of light-heavy, medium-heavy, line-haul, and transit
bus proposed in this report. This approach would eliminate most of the perverse incen-
tives inherent in a simple averaging scheme, while retaining the advantage of sim-
plicity. Reducing the range of engines over which emissions can be averaged would
reduce the manufacturer's flexibility, however, and thus reduce the benefits of averag-
ing. It might also introduce some unnecessary competitive dislocations, since some
manufacturers with only one or two engine models in a subclass would be at a disadvan-
tage.
7 J ACHIEVABLE NOx AND PARTICULATE STANDARDS
In setting an achievable motor-vehicle emissions standard, it is necessary to consider six
questions.
1. What is the lowest emissions level technically achievable in new
engines?
2. How is this technical limit affected by other emissions standards and/or
other possible changes in the environment?
3. How much deterioration in emissions will occur during that portion of
the vehicle's life covered by the regulation?
4. How much allowance for engine-to-engine, test-to-test, and laboratory-
to-laboratory variation in emissions measurements must be allowed in
order to avoid having to remove an engine from production solely due to
statistical fluctuations?
5. How much margin should the manufacturers be allowed? (This might be
rephrased as "How close to the fire shall we hold their feet?")
6. What level of control is necessary and justified, in order to provide
maximum protection to the environment at an acceptable cost?
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The first two questions — the limits of technical possibility and the effects of other
regulations — have been addressed in the previous chapters. Achievable engine-out emis-
sions levels and the NOx-particuIate tradeoff were discussed in Chapter 4. Estimates of
the near-term and intermediate-term limits of technicaJ feasibility for the NOx-
particulate tradeoff — developed in Chapter 4 — will form the basis for the analysis
here. In addition, this discussion will consider the effects of trap-oxidizers, as discussed
in Chapter 5. Possible changes in the environment — such as the changes in diesel fuels
discussed in Chapter 6 — will be considered only by default; it will be assumed that no
major changes in diesel fuel quality or other environmental variables with effects on
emissions will occur. As this report deals only with the technology of emissions control,
the last question posed will not be addressed here.
7.3.1 Translation from Low-Mileage Emissions to Standards
Questions three through five on the list above are very important ones, yet they are fre-
quently neglected in environmental analyses. There is a tendency to regard the tech-
nically feasible low-mileage emissions level and the technically feasible standard as the
same. This tendency is wrong — the low mileage emissions level is only one factor enter-
ing into the determination of a feasible standard. Other, equally important, concerns are
the variability of emissions, the portion of the engine's or vehicle's life that the standard
is to apply to, the degree of deterioration that can be expected in that time, and the
auditing and verification procedures. The effect of each of these concerns must be
accounted for in order to determine a technologically feasible standard. In addition, in
the use of "technology-forcing" standards such as those contemplated for heavy-duty
diesel engines, the speculative nature of the projected technically feasible low-mileage
emission level should also be taken into account, and some slack should be provided to
compensate for the uncertainty in the projection.
The three questions concerning deterioration, variability, and slack can usefully be com-
bined into a single one: "How far above the technological limit for low mileage emissions
should the standard be set?" In this way it is possible to combine in one number all the
necessary allowances. In estimating feasible values for this number, it is instructive to
consider the allowances recommended by the manufacturers themselves. Each of the
five major U.S. engine manufacturers recommended a set of diesel NOx and particulate
standards to EPA in 1982, and each provided an estimate of the low-mileage emission*;
target that would be necessary to meet this standard. These values, and the ratios
between them* are shown in Table 7.2.
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Table 7.2
Manufacturer's Recommendations for Heavy-Duty
Emissions Standards
NOk Particulate
Low-Mileage % Low-Mileage %
Manufacturers Standard Target Margin Standard Target Margin
Mack 8.0 6.5 23 0.79 0.50 58
Cummins 7.5 6.7 12 0.80 0.66 21
International 10.0 7.7 30 0.68 0.44 55
Harvester 8.0 6.1 31 0.77 0.50 54
7.0 5.9 30 0.79 0.51 55
6.0 4.6 30 0.83 0.54 54
Caterpillar 6.0 5.3 13 0.60 0.46 30
General Motors 8.0 6.05 32 0.85 0.54 37
10.7 8.09 32 0.70 0.44 37
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As Table 7.2 indicates, different manufacturers differ strongly in the allowances beyond
the basic low-mileage emissions level that they recommend. These values range from 12
to 32 percent for NOx, and from 21 to 58 percent for particulates. On the basis of the
information shown, and ERC's own analysis of the issue, the authors recommend
minimum margins of 10 percent and 25 percent for NOx and particulates respectively.
Thus the emissions standard for NOx should be at least 10 percent, and the standard for
particulates at least 25 percent, above the estimated technological limit for low mileage
emissions. These factors include allowances for deterioration, random variation, and
some error in the estimate of technical feasibility, either for the industry as a whole or
for some manufacturer.
These margin levels are comparatively strict, and will not be easy to meet. They have
been derived using the following assumptions.
1. The standard is based on averaging within classes or across a
manufacturer's entire heavy-duty product Line, as discussed in Section
7.2.2, reducing the effects of random variation in tests and of
differences in the emissions control abilities of different engines.
2. Average full-life NOx deterioration factors are taken equal to zero,
and average particulate deterioration is 15 percent.
3. Deterioration is measured over the full estimated useful life of the
engine.
4. Manufacturer's recommended maintenance procedures are followed
throughout the durability test.
5. Reasonably representative and statistically valid auditing procedures
are followed.
6. Reasonable non-conformance penalties for non-compliance are in
effect.
Under these assumptions, the technologically feasible standard levels can approach the
feasible low-mileage emissions levels rather closely. Generally, the statistical variation
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in emissions from engine to engine and from test to test is a major concern in setting the
margin of compliance, so that random variations in test results do not result in failing an
audit. However, averaging across the entire product line should reduce this concern,
since the standard deviation of the mean of several measurements on different engines
would be less than the standard deviation of the measurements themselves. In addition,
the use of nonconformance penalties would reduce the adverse consequences of failing an
audit, and thus make a greater probability of failure tolerable.
These qualifying assumptions should be noted carefully, and their validity in any specific
instance should be checked. Use of similarly tight margins in circumstances where the
assumptions above do not apply could well result in regulations which are effectively
beyond the limits of technical feasibility, or which result in an unwarranted level of risk
for manufacturers.
7.3.2 Limits of Feasibility for NOx and Particulate Standards
Estimates of the technological limits on near-term and intermediate term NOx and par-
ticulate emissions have already been presented in Section U.k. Because of the inverse
relationship between NOx and particulate emissions, these estimates were presented
graphically, in the form of a plot of the lowest achievable particulate emissions versus
NOx. These graphs can be adapted to indicate the lowest feasible particulate standard as
a function of the NOx standard. This is done by multiplying the NOx coordinate of every
point on the graph by 1.10 (corresponding to the 10 percent margin for deterioration,
random variation, and slack derived in Section 7.3.1) and multiplying the particulate
coordinate of each point by 1.25 (corresponding to a 25 percent margin).
Figure 7.1 shows both the estimated technological limit for low-mileage emissions and
the standard derived from that limit for heavy-duty diesels in the near term. Figure 7.2
shows the same data for intermediate term engine-out technology. Both of these figures
have been based on the data in Figure 4.12, and thus all of the reservations and cautions
concerning those data apply here as well. In particular, the engine-out emissions frontier
for the intermediate term is somewhat speculative, due to the general unavailability of
data, and should be better defined before being used in setting a final rule.
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PARTICULATE
(g/bhp tw)
1.0
.9 -
B -
.6 -
.5 -
.4 -
.2 -
11 Ł
CM 6 I
( p» iictur t "in
Gil 6 2 1
(pre-product!on)
101 J)J
normally flv»iratcd - A A
r iirI ir>« hri ri|i<<1 —
turlwit h.i rf|cd/ -
atIprtooleri
uiik nnvm ------
I
~
o
it
A-A Estimated low-mileage NO /particulate
levels achievable In the*near term
A'-A1 Feaslhle NO /particulate standards
derived froft A-A
5
NO,
10
I g/bhp hf)
Figure 7.1: Estimates of achievable near-term particulate emissions standards
vs. NO standards for heavy-duty diesel engines.
x
-------
I
Ul
PARTICULATE
(g/bhphr)
\.o
.9 -
.8 -
.6 -
.5 -
.4 -
.3 -
.2 -
.1 -
ID)
01
normally aspirated -
A
A
lurbochdryed -------
¦
D
ttirbocharyed/
•
O
aftercooled
unknown
A
> i
""1
2
-r
4
C-C Hldiiulnt of ranye ol eitiiintes of
NO /|Mrtuulati' levels achievable In
th& Intermediate term (see Fl9 ' 12)
C-C' Feasible NO /particulate standards
derived froft C-C
5 6
NOx (g/bhp hr)
1-
9
IO
Figure 7.2: Estimates of achievable intermediate-term particulate emissions
standards vs. NO^ standards for heavy-duty diesel engines.
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Figures 7.1 and 7.2 show the levels of standards estimated to be achievable with engine-
out control technology. In the near term, engine-out technology is all that will be avail-
able. In the intermediate term, however, trap-oxidizers will also be available for parti-
culate control. The effect of trap-oxidizers on the achievable particulate standard can
be estimated by multiplying the particulate coordinates of the curve in Figure 7.2 by
0.25, reflecting a trap-oxidizer efficiency level of 75 percent. This level has been
selected in order to avoid ruling out catalytic wire-mesh trap-oxidizers, which cannot
achieve as great an efficiency as the monolith and silica-fiber traps. Monolith trap effi-
ciency is generally about 90 percent, and the silica-fiber trap should be in the 80-90 per-
cent range. However, the wire-mesh trap has other beneficial characteristics (odor, HC,
and CO reduction, as well as reducing the soluble organic fraction of the particulate)
which makes it undesireable to rule it out of consideration. For purposes of computing
the low-mileage target emissions, however, a trap efficiency of 85 percent or more
should be assumed, reflecting the greater efficiency possible with the monolith and
candle systems.
7.3.3 Alternatives for Emissions Standards
This section offers several possible scenarios for future NOx and particulate emissions
standards. All of the five are technically feasible by the criteria discussed above; they
differ in areas such as the relative stringency of control (i.e., the willingness to impose
economic penalties on the industry in order to reduce emissions) and the degree of con-
centration on reducing NOx as opposed to particulate emissions. Because of the limited
technological capabilities in the near term, all five scenarios are identical until the
intermediate term — 1990 for the medium-heavy, line-haul, and transit bus classes, and
1988 for the light-heavy duty class. Beyond this point, the scenarios differ.
Table 7.3 describes and names each of the emissions scenarios considered, and indicates
the numerical standards and the estimated low-mileage emission levels associated with
each. These scenarios are further discussed below.
1. Moderate Control Scenario - This scenario includes only a single, emis-
sions standard to go into effect in 1987, with no tightening of the
standard in 1990. It includes an engine-out NOx standard of 6.0
g/BHP-hr (chosen because this number is one being considered by
EPA), combined with the feasible particulate standard for that NOx
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Table 7.3
Numerical Standards and Low-Mileage Emissions
Levels for Five Feasible Heavy-Duty Regulatory Scenarios
NO„ Particulate
1" —1 rfc — ¦ i i. - — ¦ ¦ . ¦ . . i M . i
Scenario Date Standard LMT Standard LMT
1. Moderate Control
Light-Heavy 1986 6.0 5.5 0.62 0.50
Ail Others 1987 6.0 5.5 0.62 0.50
2. Moderate NO^/Best Engine-Out Particulate
Light-Heavy 1986 6.0 5.5 0.62 0.50
1988 5.0 <*.5 0.56 0.45
All Others 1987 6.0 5.5 0.62 0.50
1990 5.0 4.5 0.56 0.45
3. Moderate NO^/Trap Oxidizers
Light-Heavy 1986 6.0 5.5 0.62 0.50
1988 5.0 4.5 0.14 0.08
All Others 1987 6.0 5.5 0.62 0.50
1990 5.0 4.5 0.14 0.08
4. Strict NO„/No Trap-Oxidizers
¦ ' W 1 ¦ 1
Light-heavy 1986 6.0 5.5 0.62 0.50
1988 4.0 3.6 0.72 0.58
All Others 1987 6.0 5.5 0.62 0.50
1990 4.0 3.6 0.72 0.58
5. Strict NOx/Trap-Oxidizers
Light-Heavy 1986 6.0 5.5 0.62 0.50
1988 4.0 3.6 0.18 0.09
All Others 1987 6.0 5.5 0.62 0.50
1990 4.0 3.6 OAS 0.09
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standard. Meeting these standards would require only near-term tech-
nology such as improved turbocharging, charge-air cooling, and fuel
injection systems. Thus the economic impact of the regulations wouJd
be less than for the other scenarios. Some such impact would still
occur, however — the NOx limitations would increase fuel consumption
by several percent in the near term for instance.
2. Moderate NO^, Best Engine-Out Particulate Control - This scenario
includes the same regulations as the moderate control scenario in
1987, followed in 1990 by a moderate NOx standard and the strictest
feasible engine-out particulate standard given that •NOx standard.
Compliance with this standard would require the use of electronic
governor and fuel injection timing controls, high-precision fuel injec-
tion, and extensive engine optimization, but probably not EGR. Most
of these would be added by the manufacturers anyhow, in order to
improve fuel consumption. Thus the economic impact of this scenario
would be fairly low — the major effect would be due to an increase in
fuel consumption of a few percent due to the NOx controls.
3. Moderate NO^, Strict Particulate Control with Trap-Oxidizers - This
scenario is identical to Scenario 2, except that a tighter particulate
standard — requiring the use of trap-oxidizers — is adopted. The
economic impact of this scenario would be greater, due to the added
cost and fuel consumption of the trap-oxidizers.
Strict NO^ Scenario, No Trap-Oxidizers - This scenario assumes that
strict control of NOx emissions is necessary, and that higher costs in
fuel economy and particulate emissions are an acceptable price to pay
for this. It consists of an NOx standard of 4.0 grams per BHP-hr,
together with the most stringent engine-out particulate standard
achievable at the NOy level. The initial cost of this scenario would be
moderate, but the cost in fuel economy would be very high.
5. Strict NOx Scenario with Trap-Oxidizers - This scenario assumes that
strict control of both NOx and particulate emissions is necessary, and
that society is willing to absorb substantial costs in order to achieve
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this. It consists of the same NOx regulations as the fourth scenario,
combined with a particulate standard which is strict enough to require
trap-oxidizers. The economic impact of this scenario would be signifi-
cant — to the moderate initial cost and high fuel-economy costs of the
fourth scenario would be added the high initial costs and moderate
fuel-economy costs of trap-oxidizers.
None of these scenarios would need to be applied across the board — different groups
within the heavy-duty class could be regulated according to different philosophies.
Indeed, it is strongly recommended that this approach be considered. In particular, it is
recommended that a very strict approach (corresponding to Scenario 5) be considered for
transit buses, while a moderate trap-oxidizer standard such as in Scenario 3 should be
considered for light-heavy and medium-heavy trucks. For line-haul trucks, a moderate
engine-out particulate standard such as Scenario 2 should be considered. These standards
appear likely, on the basis of ERC's qualitative analysis, to produce the best tradeoff
between emissions, fuel-economy, and initial cost. However, qualitative analysis is not
enough to justify a quantitative standard — thus it is only recommended that these levels
be considered, and not necessarily adopted. This consideration should include a firm
quantitative analysis of the costs and benefits of each approach, and this quantitative
analysis should be used as the basis for regulation.
7A RECOMMENDATIONS
The author's recommendations for emissions standards and related issues can be sum-
marized as follows:
1. In establishing regulations for heavy-duty engines, EPA should consider
separately the four major subclasses discussed in Chapter 2. This does
not necessarily mean that different regulations should be adopted for
each subclass. Rather, the costs and benefits of regulation should be
considered separately for each subclass, and the best regulation for
that subclass (which might or might not be the same as for some other
subclass) shoiild be adopted.
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2. Special consideration should be given to accelerating the imposition of
strict emissions standards on light-heavy engines (since they are capa-
ble of meeting these standards more quickly) and to imposing very
strict standards on transit buses. Special consideration should be given
to imposing less strict standards on line-haul trucks,
3. Any scheme for emissions averaging must be very carefully designed in
order to prevent it from being used in ways which would result in
greater emissions overall. One simple approach would be to permit
averaging only within subclasses of heavy-duty engines. If averaging
across subclasses is permitted, it should properly account for the
expected lifetime, expected average power level, and expected frac-
tion of urban operation of each engine.
4. Figure 7.1 is a plot of estimated feasible particulate standards versus
the NOx standard in the near term (19S7 or 1988). Because the
technology involved is not radically different from what is now in use,
this frontier is considered to be fairly well defined. Any emissions
standards applying to that period should be chosen to fall on or above
this frontier.
5. Figure 7.2 shows the estimated feasible engine-out particulate
standards as a function of the NOx standard for intermediate-term
(1990 or 1991) application. The information in this figure is much
more uncertain than that in Figure 7.1, and should be clarified by addi-
tional research before being used as a basis for regulation. Feasible
trap-oxidizer based standards for the intermediate term can be
obtained by multiplying the feasible engine-out standard by 0.25,
reflecting an average trap efficiency of 75 percent.
6. Light-heavy duty engines could comply with standards similar to those
in Figures 7.1 and 7.2 more quickly than the other heavy-duty classes.
Implementation dates of 1986 for the near-term standard and 1988 for
the intermediate-term standard appear to be feasible.
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8.0 SUMMARY AND CONCLUSIONS
Chapters Two through Seven of this report have presented largely separate, although
interelated, analyses of a number of issues related to diesel particulate standards and
control. These issues range from definition of representative classes of heavy-duty
vehicles (Chapter Two) and defining the requirements for commercial feasibility (Chap-
ter Three) to detailed technical analyses of engine-out and aftertreatment control tech-
nologies (Chapters Four and Five). Chapter Six deals with the effects of fuels on
emissions, with special attention to the effects of changes in fuel quality. Chapter
Seven, the final technical chapter, draws together the analysis from the preceding chap-
ters to derive estimates of feasible near and intermediate-term hfeavy-duty particulate
standards. This chapter also discusses a number of other issues related to particulate
control regulations.
Except for Chapter Seven, the analyses presented are largely independent and
separable. Because of this, as well as the very large amount of information presented
and the large number of conclusions reached, the summary and conclusions have been
divided into sections, with a separate section for each chapter. These are given below.
CHAPTER 2: CLASSIFICATION OF HEAVY-DUTY ENGINES AND VEHICLES
Summary - .Chapter Two examines the heavy-duty vehicle and heavy-duty engine indus-
tries in the United States, and presented ERC's classification scheme for heavy-duty
vehicles. The structure of the heavy-duty vehicle industry is quite different from that
for light-duty vehicles. Heavy-duty trucks are not standardized - rather, they are sold
with a wide variety of options as to engine model, drivetrain, body type, and auxiliary
equipment. Thus, not all heavy-duty vehicle manufacturers produce engines, and not all
engine manufacturers produce vehicles. Since engines, rather than vehicles, are the
regulated items in the heavy-duty class, this report deals mainly with engine manufac-
turers. The major heavy-duty diesel engine manufacturers in the U.S. are Cummins
Engine, Caterpillar Tractor, Mack Trucks, Detroit Diesel-Allison Division of General
Motors, and International Harvester. A number of foreign firms, notably Daimler-Benz
(Mercedes), Fiat (IVECO) and Volvo also import heavy-duty diesel engines installed Ln
their own trucks.
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ERC has proposed a classification ol heavy-duty vehicles along functional lines, rather
than strictly by weight class. This classification is used throughout the report, and
serves as the basis for a number of important conclusions. Four classes of heavy-duty
vehicles were defined: light-heavy duty (primarily pickup trucks and vans, with some
specialized types); medium-heavy duty (all unitary trucks, and all other large heavy-duty
vehicles except those in the next two classes); line-haul trucks (large, heavy, extremely
powerful tractor-trailer combinations used for Jong-haul trucking); and transit buses.
Although these classes still exhibit considerable heterogeneity, they are much more
homogeneous in their essential characteristics than is the class of heavy-duty vehicles as
a whole.
Conclusions - The major conclusions of Chapter Two are given below.
1. Heavy-duty vehicles are a very heterogeneous group, far more so than light-duty
vehicles.
2. This heterogeneity can be reduced to a manageable level by subdividing the heavy-
duty class into four subclasses: light-heavy, medium-heavy, line-haul, and transit
bus. These should be considered separately for regulatory purporses.
CHAPTER 3: COMMERCIAL FEASIBILITY AND HEAVY-DUTY ENGINES
REQUIREMENTS FOR EMISSIONS CONTROL TECHNOLOGIES
Summary - Chapter Three examines the requirements which an emissions control device
must meet in order to be considered commercially feasible in heavy-duty service. These
requirements are both technical and economic. The major criteria for feasibility are the
following: effectiveness, durability, reliability, cost, and the effects of the technology
on the fuel economy, performance, durability, and reliability of the engine. In addition,
it is necessary to consider the technology's effects on safety and maintenance require-
ments, its resistance to tampering, weight and bulk, ease or difficulty of manufacturing
and system integration, and possible environmental effects.
The criteria for feasibility in heavy-duty vehicles are generally more stringent than those
for emissions controls in light-duty service. This is due to the more rigorous usage of
heavy-duty engines and equipment,, as well as to the greater technical sophistication of
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the users and the much larger amounts of money involved. Both of these latter traits
would tend to encourage tampering with emissions-control devices which degrades fuel
economy, reliability, or performance. Since heavy-duty engines are designed to be re-
buildable indefinitely, a sufficiently strict emissions standard could even prove counter
productive (as well as very destructive to the industry), since older engines not subject to
the standard could be rebuilt and substituted for new ones if that were economically
favorable. This would be most likely to occur as the result of a strict NOx standard,
since such a standard would have a highly adverse effect on performance and fuel
economy.
Conclusions - The following conclusions resuit from the discussion in Chapter Three.
1. The criteria for commercial feasibility of emissions controls in heavy-duty service
are different from, and generally more stringent than, those for light-duty
applications. This is especially true of economic criteria such as fuel economy
effects.
2. The importance of individual criteria is different for different subclasses of heavy-
duty vehicles. In light-heavy vehicles, for instance, first cost is the dominant
concern. For line-haul trucks, in contrast, the initial cost of the technology is
negligible compared to the potential cost of losses in fuel economy and per-
formance.
3. Heatfy-duty engine purchasers, especially those buying heavy trucks, have sub-
stantially more freedom of choice than purchasers of light-duty vehicles. Thus
they can switch engines to avoid a poor performer, and could even substitute un-
controlled rebuilt engines for new ones if that were economically favorable. They
may also be more likely to tamper with emissions control devices which have
serious deleterious effects in costs of performance.
4. As a result, there are limits on environmental agencies1 power to reduce heavy-duty
emissions by means of new engine standards. Too severe a standard would be
counterproductive. This possibility should be borne in mind during standards
development.
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CHAPTER 4: ENGINE-OUT EMISSIONS CONTROL FOR HEAVY-DUTY ENGINES
Summary - Chapter Four deals with the potential for control of heavy-duty particulate
emissions by means of "engine-out" methods, which are techniques to reduce the amount
of particulate in the exhaust as it leaves the engine. Engine-out technologies are dis-
tinguished from aftertreatment technologies, which rely on separate processing to purify
the exhaust.
Chapter Four begins with an examination of the fundamental physics and chemistry of
combustion and pollutant formation in the diesel engine. This discussion concluded that
the commonly observed tradeoff between reduced NOx and increased particulate
emissions in diesel engines is a fundamental characteristic of thet combustion process,
and thus is subject only to very limited control. The same is true for the tradeoffs
between NOx and fuel economy, and NOx and hydrocarbon emissions. The best prospects
for improvement in these tradeoffs appear to lie in improved control of injection timing
and other engine variables, and in technologies such as charge-air cooling which increase
the oxygen content of the charge tn the cylinder while reducing the peak combustion
temperature.
Chapter Four also examines present and future engine-out emissions control technologies
for heavy-duty diesels. Because of the NOx/particulate tradeoff, both NOx and par-
ticulate control technologies were examined (in practice, of course, a given technology
may often provide either NOx or particulate control, depending on the settings of engine
parameters). Many technologies show present or future promise for improving on the
NOx/particJlate and NOx/fuel-economy tradeoffs. Promising technologies which could
be implemented by 1987 or 1988 include improved engine/turbocharger matching,
improved charge-air cooling, and high-pressure/high-precision fuel-injection systems.
In the intermediate term (1990 or 1991), the technology with the greatest promise is
optimal electronic control of fuel injection timing, coupled with an electronic governor
and (for a low NOx standard) electronically-modulated exhaust-gas recirculation (EGR).
EGR would be necessary to meet a low NOx standard without gross degradation in fuel
economy, but would not be used otherwise, due to its deleterious effects on engine
durability. In the longer term (after 1991), uncooled or "adiabatic" engine technology
offers the promise of substantial decreases in particulate emissions, at little or no cost in
increased NOx. However, this technology is still in the early stages of development, and
any predictions as to when or if it will become available would be premature.
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The last part ol Chapter Four deals with the average engine-out emissions Jeveis which
are estimated to be attainable by 1987-S8, and by 1990-91 (near-term and intermediate-
term periods, respectively). Since NOx and particulate emissions are interrelated, they
were considered together. A plot of measured particulate emissions levels vs. NOx levels
for heavy-duty engines reveals three fairly distinct groups: Group I, consisting of stan-
dard technology engines with parameters set to meet present Federal regulations; Group
I[, containing standard-technology engines set to meet California's more restrictive NOx
standard; and Group III, which contains more advanced-technology engines (many of
which are still in the prototype state). These groups are shown in Figure <>.11.
Group I engines generally exhibit low particulates but high NOx, while
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3. Injection timing retard and exhaust gas recirculation do not improve on the
NOx/particulate tradeoff, but are likely to be used to adjust NOx and particulate
levels along the tradeoff curve.
Turbocharging, charge-air cooling, improved fuel-injection, and electronic engine
controls all have beneficial effects on fuel consumption and performance, thus they
are likely to be introduced independently of any emissions standards.
5. When used to reach low NOx levels, injection timing retard significantly degrades
fuel economy, and is thus likely to be supplemented or supplanted by EGR, which
has a lesser effect on fuel consumption. Otherwise, EGR would not be used, since
it is regarded as detrimental to engine durability.
6. Indirect injection results in a major loss in fuel economy, thus it is only eco-
nomically feasible in light-duty and light-heavy duty engines, where it is already
universal (although small DI engines are being introduced into the light-heavy
class). Because of the advantages of IDI and the technological sophistication of
new DI engines in this class, light-heavy duty engine manufacturers are presently
able to achieve emissions levels which the larger direct-injected (DI) heavy-duty
engines will not be able to attain until about 1989, and could probably attain sig-
nificantly lower levels (corresponding to an intermediate-term standard) by 1988.
7. Technologies likely to be available for common use in DI engines in the near term
are, turbocharging/charge-air cooling, improved fuel injection, engine efficiency
improvements, and injection timing retard. Aggressive application of all of these
techniques, combined with optimization, could produce an average low-mileage
NOx/particulate tradeoff curve similar to line A-A in Figure V.12.
8. Technologies Likely to be in common use in the intermediate term include those
listed for the near term, plus electronic engine controls and still further improve-
ments in charge-air cooling and engine optimization. By aggressive application of
these techniques, combined with EGR for very low NOx levels, engine manufactur-
ers could probably attain an average low-mileage NOx/particuIate tradeoff curve
lying between lines B-B and B'-B' in Figure 4.12.
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CHAPTER 5: TRAP-OXIDIZER SYSTEMS FOR HEAVY-DUTY VEHICLES
Summary - The trap-oxidizer is a particulate control system consisting of a durable filter
(the "trap") which removes particulate material from a vehicle's exhaust system, com-
bined with a system for regenerating the trap by burning off ("oxidizing") the collected
materiai. Chapter Five discusses the present state of trap-oxidizer technology, with
special attention to the relatively small amount of work that has been done on trap-
oxidizers for heavy-duty vehicles.
Trap-oxidizer technology for light-duty vehicles is quite advanced. One trap-oxidizer
equipped car is now in production, and many models are expected to be available by
1987. However, heavy-duty vehicles have a number of characteristics which will make
the application of trap-oxidizers more difficult. The most important of these is the
much greater lifetime mileage typical of heavy-duty vehicles — as much as 500,000 to
1,000,000 miles in line-haul trucks. Other problems include the fragmentation of the
industry, and the fact that many different models of engine may be offered in a single
truck chassis, implying that the chassis would need to accommodate many different
models of trap-oxidizers as well. These difficulties, as well as the lesser amount of de-
velopment work in the heavy-duty area, will increase the lead-time required for the
introduction of trap-oxidizers in heavy-duty vehicles. Even making favorable assump-
tions, heavy-duty trap-oxidizers are unlikely to be available before 1990 or 1991. The
most advanced manufacturer in trap-oxidizer development appears to be Daimler-Benz
which has developed a highly successful heavy-duty trap-oxidizer system that it states it
is "confident" will be ready for production in 1990.
Tampering and institutional resistance would be significant problem with trap-oxidizers
in heavy-duty use. Trap-oxidizers would almost certainly degrade fuel consumption
slightly, which could lead to their removal by truck owners or users. These problems
would be most severe in the line-haul and medium-heavy truck classes, which are also the
groups in which the durability and reliability issues are of greatest concern. In contrast,
application of trap-oxidizers to light-heavy vehicles would be much simpler — for these
much smaller engines, a straightforward adaptation of light-duty technology would be
possible. Because of this, light-heavy trap-oxidizers could be available well before the
other classes — probably as early as 1988.
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Four generic types of trap-oxidizer systems now appear promising for heavy-duty use.
These are: (1) a ceramic monolith trap, regenerated by a diesel fuel burner; (2) a ceramic
monolith trap with continuous regeneration by means of catalytic fuel additives; (3) a
catalytic wire-mesh trap, using one of several inexpensive regeneration systems; and
(<0 a trap using woven silica yarn on a perforated metal substrate, with regeneration by
the injection of catalysts into the exhaust. This last is the system developed by Daimler-
Benz. It is too early to predict which of these may be adopted for heavy-duty use,
although the monolith/additive system appears to be the current leader in light-duty
applications. System descriptions and cost estimates were developed for trap-oxidizer
systems using each of these approaches in each of the four classes of heavy-duty
vehicles.
The first cost estimates for these systems ranged from $
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2. Trap-oxidizer technology for heavy-duty vehicles is much less advanced, due to a
slower start and the greater difficulty of the development task. Except for light-
heavy vehicles (which could use an adaptation of the light-duty technology) heavy-
duty trap-oxidizer systems are not likely to be available before 1990 or 1991.
Trap-oxidizer systems for light-heavy vehicles could be available by 1988 if — as
expected — successful trap-oxidizer systems for light-duty vehicles appear by 1987.
3. At least four generic types of trap-oxidizer systems now appear promising for
heavy-duty use. However, there are unanswered questions and unresolved technical
difficulties associated with each, and it is premature to predict which, if any, of
them will eventually be adopted.
V. Generic cost estimates for each of the four promising trap-oxidizer systems in each
class of vehicles are given in Tables 5.1 to 5A. In general, these costs are quite
high. Discounted life-cycle costs range from $566-$715 for light-heavy vehicles, up
to $3^62-5^0^7 for line-haul trucks, with costs for medium-heavy trucks generally
around $1500 and those for transit buses around $3,000.
5. A large fraction of the total lifecycle cost of the trap-oxidizer system in the
heavier trucks is due to increased fuel consumption, maintenance expenses, and
trap replacement. These costs could be avoided by removing the trap shortly after
purchase. Without an effective enforcement program, this practice could be
expected to become widespread.
6. The cost effectiveness of trap-oxidizers in light-heavy and medium-heavy vehicles
would be greater than that in light-duty cars and trucks, while that for transit
buses would be many times greater. The cost effectiveness of trap-oxidizer control
on line-haul trucks is doubtful, since they spend little time in urban areas. This
suggests that transit buses should receive the highest priority for regulation, while
consideration should be given to exempting Line-haul trucks.
CHAPTER 6: EFFECTS OF FUELS ON DIESEL EMISSIONS
Summary - Chapter Six reviews the effects of fuel variables on heavy-duty emissions.
and discusses the possible effects of the current degradation of fuel quality on emis-
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sions. The major quality indices for diesel fuel are cetane number, aromatic content
(this is closely correlated with cetane number), volatility, and sulfur content. The recent
trend toward heavier and lower-quality crude oils has led to degradation in most of these
indices, with the effects on cetane number, aromatic content, and sulfur content being
the most significant. Cetane number and aromatics content have a strong effect on
particulate emissions, with aromatic content seeming to be the more important of the
two. Thus, a continuation of the trend toward lower cetane can be expected to lead to
higher emissions in use, and cetane-improving additives (since they do not affect the
aromatic content) are unlikely to improve this greatly.
The effects of volatility on emissions do not seem to be very significant, at ieast within
the range of present day diesel fuels. Thus, volatility changes are not of major concern.
Sulfur content, on the other hand, is significant, both alone and in conjunction with cata-
lytic trap-oxidizers. Sulfate formed from fuel sulfur contributes significantly to particu-
late emissions, and high sulfur fuels may also increase the organic fraction of the par-
ticulates. The precious-metal catalysts in catalytic trap-oxidizers can oxidize SO2 to
additional sulfate, which then combines with water to form sulfuric acid. This material
is then emitted to the atmosphere. Although the sulfate conversion problem can prob-
ably be controlled, the steps required to do so will also make regeneration more diffi-
cult. Reducing the sulfur content of diesel fuel would eliminate this problem, and would
also eliminate a small but significant contribution to urban SO2 levels (including
secondary particulate formed by 502 oxidation in the atmosphere) and acid deposition.
There would be synergetic benefits as well — the de-aromatization process used to
reduce aromatic content and upgrade cetane ratings can also be used to remove sulfur
from diesel fuel. Thus, the cost of both operations might not be much more than that of
either one alone, and this cost would probably be rather small — of the order of a few
cents per gallon.
Conclusions — The major conclusions of Chapter Six were the following.
1. There has been a substantial degradation in the average quality of diesel fuel,
especially in cetane number and aromatic content. This can be expected to lead to
greater in-use emissions than would otherwise be experienced.
2. The aromatic content of diesel fuel has the greatest effect on particulate emis-
sions, and this effect seems to be partly independent of the aromatics effect on
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cetane number. Thus, cetane improving additives may not reduce particulate emis-
sions, even though they restore the cetane number to an acceptable level. Thus,
from an emissions standpoint, a regulatory limit on aromatic content, as well as or
instead of cetane number, might be in order.
3. The volatility levels of diesel fuel have only minor effects on emissions, at least
within the range of experiments. Thus, fuel volatility does not seem to merit any
special concern at this point.
Poor-quality diesel fuel results in higher fuel consumption, and poor performance,
as well as increased emissions. In addition, the need to tolerate a broad range of
cetane numbers makes it more difficult to achieve low NOx levels and may harm
fuel economy. Thus, regulations to fix a narrower range of cetane and/or aromatic
content might provide economic benefits which would partially or wholly offset the
greater cost of producing the fuel. This possibility should be investigated further.
5. There is a potential for a highly beneficial synergism between fuel desulfurization,
improvements in Cetane, and reduction in aromatic content, all of which can be
accomplished in essentially the same process. The benefits of desulfurization
would include reduced human exposure to S02-reduced secondary particulate
formation a small reduction in acid precipitation, and increased ability to use cata-
lytic traps. Catalytic traps greatly reduce diesel odor, HC, and CO emissions, as
well as reducing the soluble organic portion of the diesel particulate. The feasi-
bility of this approach should be investigated carefully.
CHAPTER 7: EMISSIONS STANDARDS FOR HEAVY-DUTY DIESEL ENGINES
Summary — Chapter Seven, the final technical chapter, draws together the results of all
of the preceding analysis to arrive at conclusions regarding the feasible levels of particu-
late standards. Due to the interrelationship between NOx and particulates, feasible NOx
standards are considered as well. In addition, the chapter deals with several other,
related issues, such as the effects of emissions averaging and the possible subdivision of
the heavy-duty clas for regulatory purposes.
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Chapter Seven begins by discussing the present and proposed heavy-duty NOx and par-
ticulate regulations, then moves on to a discussion of the issues invoived in emissions
averaging and subdivision of the heavy-duty class for regulatory purposes. A major con-
cern with emissions averaging relates to the comparability of different classes of heavy-
duty engines. A reduction of 1 gram/BHP-hr from a light-heavy engine results in a much
smaller effect on environmental quality than an equivalent reduction in a larger engine,
due to the much smaller number of BHP-hr generated over the engine's life. This could
be avoided by weighting each engine's emissions by its rated power and/or estimated life,
but this would greatly complicate the regulation.
Another solution to the averaging problem would be to permit averaging only within sub-
classes of engines, e.g., within the light-heavy class, the line-haul class, etc. This would
retain many of the benefits of a more general averaging standard (reduced cost of com-
pliance and reduced uncertainty), while ensuring a rough comparability of the emissions
being averaged. This would also facilitate the establishment of separate numerical
standards and/or standard effective dates for the different classes, which is itself a good
idea. There would be a cost, however, in reduced flexibility, and this approach might
unfairly penalize some manufacturers who have only one or two engine lines in a given
subclass.
Light-heavy duty diesels are able to adopt light-duty emissions control technology, and
thus could meet a stringent emissions standard more quickly than the engines in the other
classes. Since this group seems likely to increase rapidly in size, it would be worthwhile
to consider an earlier implementation date for standards in this group. Transit buses
should, also be singled out for special regulatory concern, due to the fact that bus emis-
sions result in much moce. human exposure than those from most other vehicles. On the
other hand, consideration should be given to exempting line-haul trucks from any
extremely stringent standard, due to the fact that they operate mostly outside urban
areas, while they would suffer disproportionately from any adverse effects on fuel
economy or durability.
The last part of Chapter Seven deals with feasible emissions levels and emissions stand-
ards. First, the relationship between low-mileage emissions and a feasible standard is
developed. Next, this relationship is combined with the estimated low mileage
NOx/particulate tradeoff curves developed in Chapter Four to obtain plots of the feasi-
ble particulate standard as a function of the NOx standard for both the 1987-88 and
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1990-9' time frames. Finally, these plots are used to derive feasible particulate
standards corresponding to a number of different regulatory scenarios.
All the scenarios assume an initial (1987-88) NOx standard of 6.0 g/BHP-hr, for which the
feasible particulate standard is 0.62 g/BHP-hr for medium-heavy, line-haul, and transit-
bus vehicles. The scenarios differ in their assumptions concerning the 1990 standard —
two NOx levels (6.0 and 4.0 grams) and three levels of particulate control (no change
from 1987-88, strictest feasible engine-out control, and strictest feasible control with
trap-oxidizers) are considered. Feasible particulate standards for these scenarios range
from 0.72 g/BHP-hr (for strict NOx with no trap-oxidizers) to 0.16 (Moderate NOx with
trap-oxidizers). These estimates assume that emissions averaging is in use, a 4>0 percent
average quality level (AQL), the use of full electronic engine* controls (which are
important in limiting deterioration), and the use of non-conformance penalties rather
than more draconian measures to deal with non-compliance.
Conclusions — The major conclusions and recommendations developed in Chapter Seven
are the following.
1. In establishing regulations for heavy-duty engines, EPA should consider the four
major subclasses of heavy-duty vehicles separately. This does not necessarily mean
that different regulations should be adopted for each subclass. Rather, the costs
and benefits of regulation should be considered separately for each subclass, and
the best regulation for that subclass should be adopted.
2. Special consideration should be given to imposing earlier emissions standards on
light-heavy engines, and to imposing strict standards on transit buses. Special con-
sideration should be given to imposing less strict standards on line-haul trucks.
3. Emissions averaging should be permitted in order to minimize the effects of ran-
dom variation and to permit a wider variety of engine models to be offered. Any
emissions regulation must be carefully designed in order to allow maximum flexi-
bility without introducing unfair competitive advantages or jeopardizing air quality
goals.
Figure 7.1 is a plot of estimated feasible particulate standards versus the NOx
standard in the near term (1987). Because the technology involved is not radically
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different from what is now is use, this frontier is considered to be fairly well
defined. Any emissions standards applying to that period should be chosen to fall
on or above the frontier.
Figure 7.2 shows the estimated feasible engine-out particulate standard as a func-
tion of the NOx standard for intermediate-term (1990 or 1991) application. The
information in this figure is much more uncertain, and should be clarified by addi-
tional research before being used as a basis for regulation. A feasible trap-oxidizer
based standard can be derived from the feasible engine-out standard by multiplying
it by 0.25, reflecting a trap-oxidizer efficiency of 75 percent.
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APPEfCIX A: SUMMARY CF THE REVIEWERS' COMMENTS
The review draft of the report was conpleted in March, 1984, and dispatched to
interested parties for review in May. Six organizations (all of which were
manufacturers either of heavy-duty engines, heavy-duty vehicles, or both)
returned comprehensive written contents; a further three organizations and
individuals supplied shorter ccnments by telephone. Several of the reviewers
also supplied additional data in areas of interest along with their coranents.
In their corments, the reviewers pointed out a number of minor errors and a
few major errors in the draft report, as well as identifying a number of
sections in which the wording of the report was apparently unclear. The
errors so identified have been corrected in the final report, and the obscure
sections revised to clarify their intent. These modifications require no
further ccmment. Every reviewer who returned comments also took exception to
scare of the major conclusions and recommendations of the report, and presented
counter-arguments or data to support his views. These arguments have been
carefully considered, and in some cases have led to modification of the
conclusions and recommendations. This Appendix describes some of the major
issues raised by the reviewers presents the authors' response to them, along
with a description of the changes — if any — made in the final report as a
result.
Useful Life and Nonconformance Penalties
The sections of the draft report dealing with useful life were written before
EPA's November, 1983 rulemaking on the subject. The draft thus erroneously
discussed useful life and deterioration factors in terms of half-life rather
than the full-life based rules. In addition, the draft report took no notice
of EPA's proposal to make use of monetary non-conf or nance penalties for
non-corrpliance with the emissions regulations, rather than prohibiting the
sale of non-complying engines. Both of these errors have been corrected in
the final report.
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These corrections have had offsetting effects on the amount of slack required
between the feasible low-mileage emissions level and the feasible standard.
Increasing the useful life increases the amount of slack required, while the
switch to non-conformance penalties decreases it. Given the low deterioration
factors typical of heavy-duty diesels, the latter effect is the more
important. The estimated slack requirements for NOx and particulate emissions
have thus been reduced from 15 percent and 30 percent to 10 percent and 25
percent, respectively.
Classification Scheme For Heavy^-IXity Vehicles
Several reviewers took issue with the classification scheme proposed for
heavy-duty engines and vehicles. Most felt that since EPA regulations apply
to heavy-duty engines, any classification scheme used should reflect only
engine characteristics — the argument being that the same engine might be
used in several different classes of trucks. This objection arose mostly in
connection with the suggested exemption of line-haul engines from stringent
controls — with several reviewers suggesting that the development of two
versions of the same engine for line-haul and medium-heavy use would increase
development costs and administrative difficulties.
This argument ignores the fact that it is the vehicle, not the engine, which
determines usage patterns, and thus determines the feasibility and
cost-effectiveness of emissions control. Exenption of line-haul engines
rather than line-haul vehicles from stringent emissions control standards
would result in many line-haul engines being placed in medium-heavy trucks,
with a consequent increase in urban emissions.
The argument also overstates the degree to which engines are actually used
across different classes. True line-haul engines are specialized for that
purpose (through the shape of the torque curve, engine/turbocharger natching,
and rated RPM), and are not generally used in medium-heavy trucks. Similarly,
engines specialized for stop-and-go driving in medium-heavy trucks are not
well suited to line-haul use. Transit-buses also have special engines, such
as the Cmrmins NHHTC and Detroit Diesel-Allison's bus engines, as do
light-heavy duty vehicles. Although seme manufacturers would have to develop
two engine versions for line-haul and medium-heavy use, this is cornnonly done
in any case due to the differences in torque curves and driveability
requirements. The proposed inplementation of the line-haul exenption on a
case-by-case basis would also reduce administrative and inventory problems
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with the two engine versions, since engines built to the more lenient standard
would presumably need to be special-ordered by the holder of the exenption.
lingering
The report suggests that tampering with trap-oxidizers and engine-out NDx
controls is likely to be especially prevalent in the line-haul class, and less
of a problem in the medium-heavy, transit-bus, and light-heavy classes.
Several reviewers took issue with this, arguing that the desire for an
economical and reliable vehicle is not limited to the line-haul class, and
that taupering would thus probably be conrnon in the other classes as well.
This argument ignores the special sociology and economics of line-haul
trucking. Host medium-heavy and light-heavy trucks are owned by craimercial
operations whose major business is not trucking — e.g. utilities, stores,
tradesmen, etc. Trucking costs are generally a minor factor in their overall
costs, and thus a small increase in these costs is not of major concern. Such
organizations, since their trucking operations are generally local to their
area of business, are also more concerned about public relations and public
image, both of which could be expected to suffer if their illegal tanpering
became known.
Line-haul trucking, on the other hand, is dominated by individuals and fleets
for which trucking is the najor or the only business, and the cost of
operating the trucks their major operating expense. Under these
circumstances, even a small increase in cost per mile nay become significant
enough to prompt illegal action. The sociology of the industry, which is
characterized by extreme independence and distrust of government regulation
(as well as active defiance of troublesome laws such as speed limits) would
also tend to promote such actions. For this reason, the authors consider
tanpering and similar activities to be a much more severe problem in the
line-haul class than in others.
Electronic Controls
The draft report contained a rather optimistic assessment of the potential of
electronic control systems for improvements in emissions, fuel-economy, and
driveability. Host of the reviewers felt that the assessment overstated the
potential benefits of electronics, and several expressed doubts that
electronic controls would offer anv significant emissions benefits *<- all- Qq_
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the basis of additional research in the area and new information, we consider
the first point to be well taken, and have revised the discussion of
electronics accordingly. However, we still expect significant and important
benefits from their use. These benefits would be especially marked at low NQx
levels, since precise timing and optimization become increasing important as
timing is retarded. In the light of the confidential data available to us,
and of the startling results in light-duty emissions control via electronics
that have been reported in the literature, we do not believe that the
doubter's position (that electronics will offer no significant benefits) can
be supported.
Trap-Qxidizers
Several reviewers objected to the report's characterization of light-duty
trap-oxidizer technology as "well-developed", and expressed doubt that
trap-oxidizers for either light or heavy-duty vehicles could be feasible any
time in the foreseeable future. Seme reviewers also comnented that the
assessment of the Daimler-Benz "candle" trap was overly optimistic, and that
the cost estinetes for ceramic monolith traps were iruch lower than prices
indicated by the trap's suppliers.
With regard to the first two points, the authors feel that developments since
the completion of the draft report have only tended to confirm their
assessments. As of this writing, one light-duty manufacturer (Mercedes) is
presently, selling trap-oxidizer equipped vehicles in California, and several
other manufacturers are expected to introduce such systems in 1986 or 1987.
We believe it is thus fair to characterize the technology as "well-developed".
Our assessment of the Daimler-Benz trap-oxidizer system for heavy-duty
vehicles has also been confirmed by Daimler-Benz itself. In testimony before
the EPA, a Daimler-Benz spokesman stated that Daimler is "confident" that the
system described could be in production by 1990.
Hie authors' estimates of trap-oxidizer prices are apparently much lower than
those being quoted now by trap manufacturers (the reviewers cited figures of
up to $1,000 for traps for a line-haul truck). However, except in light-duty
vehicles, trap-oxidizers are not yet in mass production, and thus costs could
be expected to be higher. There is probably also a _arge premium being
charged for the supplier's FQ and engineering support. The report's estimates
are for traps in large-volume production, for which the economics would be
very different. nnns-irieHng the ^comparatively low pricps of—similar
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components such as catalytic converter substrates, the authors do not feel
that prices as high as those indicated by the reviewers can long exist in a
carpetitive market under nass production.
Special TrAaHnpnf fnr Light-Heavy tXity Engines The draft report had
recormended not only earlier but also more stringent emissions standards for
light-heavy duty engines, on the grounds that the IDI engines used in this
ciass are inherently cleaner than the larger DI engines. Several reviewers
objected to this reconmendation. The objections were based on two premises:
first, that the establishment of the suggested standards would eliminate the
small DI engines new being introduced into the light-heavy duty class; and
secona, that IDI engines are not, in fact, inherently cleaner than DI engines,
and thus could not conply with a more stringent standard.
Both of these points have merit — the standards proposed in the draft report
would effectively have eliminated DI engines from the light-heavy class, with
a consequent loss in fuel economy. Additional data on the emissions
capabilities of IDI engines also indicate that their advantage over DI engines
may be less than we had estimated, and that very advanced DI engines might be
able to attain the same emissions levels as the best IDI's. We have modified
the relevant sections of the report accordingly, and are no longer
rec amending a more stringent numerical standard in the light-heavy class.
Because of, the technology similarity between light-heavy and light-duty
engines and vehicles, however, we still believe that light-duty emissions
control technology could be adapted to this class, and thus that these
vehicles would be capable of complying with a given emissions standard several
years before any of the heavier classes.
One caimenter also expressed the opinion that operation patterns in the
light-heavy class are not in fact very similar to those of light-duty trucks,
and thus that adaptation of light-duty technology would not be
straightforward. This opinion was not supported by any data, however, nor are
the authors aware of any but impressionistic data bearing on the question of
light-heavy duty use patterns. Examination of the operating characteristics
for light-duty trucks, however, indicates that a substantial fraction of them
operate at least occasionally in the kind of fully-loaded, cargo-hauling mode
which is said to be typical of light-heavy operation, and thus that any
feasible emissions control systems for these trucks would need to be able to
cope with such operation. For this reason, we consider that any adaptations
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required to convert feasible light-duty technology to light-heavy use would be
minor ones of degree, rather than kind, and that the required lead-time would
thus be short.
Definition nf Frets iblp Standards
Several camenters objected to the report's estimates of the low-mileage
emissions levels attainable, and to the estimates of achievable standards
derived from these. These ccmnenters generally made two points: that the
emissions levels shewn were over-optimistic, and that they were "speculative"
{i.e. not based on demonstrated technologies), and should thus be discounted.
We consider the second point to be without merit — for any emissions standard
to be technology-forcing requires that it be set lewer than the level that can
be achieved with demonstrated technology. Estimating the emissions levels
attainable with such a standard will thus always require 'speculation". The
issue of over-optimism is more debatable. However, this is basically a matter
of engineering judgement, and such judgements car. be verified only by the test
of time. We believe that the estimates shown fairly reflect the potential for
emissions control in heavy-duty engines, and We are encouraged in our position
by the fact that General Motors (1984b) has apparently arrived at very similar
estimates of achievable engine-out particulate levels at moderate NDx. We
acknowledge, hewever, the speculative nature of the estimates given,
especially for the intermediate term, and have emphasized the need for
additional study in the report.
Sulfur Effects in Diesel Fuel One commenter suggested making extensive
revisions to the sections dealing with fuel quality to reflect our
newly-developed understanding of the effect of sulfur in fuels on
particulates. On the basis of our review of the issues and of the new data
available Inotably Chevron# 1984 and CARB, 1984) we agree with this
suggestion, and we have modified the relevant sections extensively as a
result.
Cnfit and Supply Effect of Diesel-Fiipl Standards
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ftie draft report suggested that EPA further investigate setting standards for
mininum cetane and/or maxinum aromatic content in diesel fuelf as well as
possible limits on its sulfur content. Several reviewers apparently
interpreted this as reconmending such standards, and stated that the report
should have addressed the effects of such standards on the costs and supply of
diesel fuel. The draft report had stated that such standards would reduce the
supply and increase the cost of the fuel, but no quantitative estimates of
these effects were available. Hie subsequent appearance of the CARB study
(CARB, 1984) provided some useful order-of-magnitude data, which are cited in
the final report.
The draft report had suggested a minimum cetane number of 44*, and a maximum
aromatic content of 30 percent as a suitable fuel-quality standard. One
reviewer objected to these limits, suggesting values of 40 cetane and 40
percent aromatics instead. Subsequent data on the distribution of quality
indices in current diesel fuels (Pless, 1984) led to a compromise suggestion
of 42 cetane and 35 percent aronatics in the final report.
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Energy and Resource Consultants, Inc.
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Energy and Resource Consultants, Inc.
APPENDIX C: ORGANIZATIONS CONTACTED
California Air Resources Board
Caterpillar Tractor Company
Chevron Research Corporation
Corning Glassworks
Cunriirvs fogine Coirpany
Detroit Diesel Allison Division of General Motors
Ford Motor Con^any
Freightliner Corporation
International Harvester
IVEGO
Johnson-Mat they, Inc.
Lubrizol Corporation
Mack Trucks
Mercedes-Benz of America
Ontario Research Institute
Renault
Southwest Research institute
U.S. EPA Office of Mobile Source Emissions
Volkswagen -
Volvo-White Trucks
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