United States Air and Radiation EPA420-R-93-010
Environmental Protection October 1993
Agency
vvEPA Report to Congress:
Emission Control
Technology for Diesel
Trucks
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Emission Control Technology
for Diesel Trucks
Report to Congress
October 1993
U.S. Environmental Protection Agency
Office of Air and Radiation
Office of Mobile Sources
Regulation Development and Support Division
Engine and Vehicle Regulations Branch
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Executive Summary
Technologies for controlling exhaust emissions from diesel engines are
currently m a state of rapid development. The primary motivation for th?effort is
to improve emission control while maintaining good fuel economy. EPA's regulaVo™
initiatives, most recently in response to the Clean Air Act Amendments of 1990 have
set increasingly stringent standards for controlling exhaust emissions froToS
SS? ^^ feCtiV6' *~* C°ntro1 re<^ments will result Tn" ouced
emissions of particulate matter and oxides of nitrogen, the primary emission
components of concern from diesel engines, by more than 90 and 60 percent
respectively, from uncontrolled levels. Some 1993 model year certification engines
are already performing at these levels. The ongoing development of technologies to
contr?! ^abiKt ^ ** ^^^ °apable °f fiirther imP™vements in emission
This report provides an overview of the various technologies being developed
to meet the exhaust emission standards for diesel engines. Each technology is
evaluated for its potential emission control benefits, as well as its cost and effect on
fuel economy and durability. Table 1 provides a summary of the relative advantages
and disadvantages of the major emission control technologies described in this report
This summary is a very simplified characterization and should be used only as a
supplement to the body of the report. The text of the report also includes a
discussion of various possible modifications to diesel fuel and several alternative
tuels. It is important to note that the evaluation of the relative costs and benefits of
the various technologies represents a snapshot of the current stage of development
ot each technology. Further technological advances should expand the ranee of
alternatives available to manufacturers.
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Table 1
Diesel Emission Control Technologies1
Technology
Charge air compression and afbercooling
Closed crankcase
Combustion chamber configuration
Electronic controls
Exhaust gas recirculation
Fuel spray pattern
Increased compression ratio
Induced swirl
Injection pressure increase
Injection rate shaping
Injection timing retard
Reduced crevice volume
Smoke controls
Oxidation catalyst
Reduction catalyst4
Particulate trap
Use of ceramics
Water injection4
PM
Emissions
4
4
4
4
T
4
4
i
4
i
T
4
4
i
<-»
i
4
<-»
NO,
Emissions
4
<-»
4
4
4
4
T
<-»
«-»
4
4
«-»
<-»
<->
4
«-»
<->tot
4
Fuel
Economy
<->tot
<-*
«-»
T
«-»• to 4
<-»
<->tot
«-»
<-»
«-»
4
T
<-»
4-»
f-»
4
T
<-^
System
Durability
<->to T
«->to 4
<->
«->
4
<->
«-» to 4
<-^
<-»
4-*
<-*
«-»
<-*
4->tO 4
«-» to 4
«-» to 4
<-»
4
Cost2
$$
$
$
$$
$$
$
$
$
$$
$$
$
$$
$ -
$$$3
$$$5
$$$$
$$
$$$$
KEY:
Effects
"<-»" = little or no effect
"4" = decrease
"T" = increase
Costs
"$" = less than $50
"$$" = $50 - $500
"$$$" = $500 - $2,000
F = $2,000 and up
lrThe table assumes a technology baseline from the 1993 model year.
2Cost estimates are very approximate, especially for those technologies still under development. Estimates are
based on heavy heavy-duty vehicles; smaller vehicles and engines may involve lower costs.
3Costs may be lower. '
40perator action may be required.
5Costs may be higher.
ll
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Table of Contents
Chapter 1 Introduction 1
A. Emission Constituents 1
1. Comparison of Diesel and Gasoline Engines 1
2. Particulate Matter 3
3. Oxides of Nitrogen 3
4. Other Constituents 4
B. Diesel Truck Classification and Standards 5
C. Current Technology 8
Chapter 2 Engine Technologies 11
A. Charge Air Compression and Cooling 11
B. Induced Swirl in Intake Air 13
C. Exhaust and Intake System Optimization 13
D. Intake Air Dilution 13
E. Fuel Delivery System 14
F. Closed Crankcase 16
G. Smoke Controls 16
H. Combustion Chamber Modification 17
Chapter 3 Aftertreatment Technologies 19-
A. Catalytic Converters 19
1. Principles of Operation 19
2. PM Control 20
3. NO. Control 21
B. Particulate Traps ; 23
1. Principles of Operation . . •. 23
2. Emission Reductions and Fuel Economy 25
3. Durability 25
4. Cost 26
C. Advanced Aftertreatment Technologies 27
Chapter 4 Fuel Technologies 29
A. Diesel Fuel and Lubricants 29
1. Reduced Sulfur 29
2. Increased Cetane Rating 30
3. Lower Aromatic Content , 30
4. Increased Volatility 31
5. Oxygenate Additives 32
6. Biodiesel 32
7. Other Modifications to Diesel Fuel 32
8. Lubricants 33
m
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B. Natural Gas 33
1. Emission Reductions 33
2. Fuel Availability and Cost 34
3. Required Engine and Vehicle Modifications 35
C. Liquefied Petroleum Gas 36
D. Alcohols 37
1. Emission Reductions 37
2. Fuel Availability and Cost 37
3. Required Engine and Vehicle Modifications 38
E. Electric Vehicles 39
Appendix A: Glossary
Appendix B: Definitions of Vehicle Classes
Appendix C: Nomenclature
IV
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Chapter 1 Introduction
A,
en« Protection
that impact actuen,n teo^r f vtSs ^1™ ^1 O*- factors
report. Factors not addressed include tiTSi ^ °," lde *he scope of ths
" U
. cors no aressed include tii , s
quality of aftermarket parj tte teSunt and ^f "g ,qUallty °f 6ngine oils' the
y o aermaret par tte teun and , s' e
report such terms are marked with a »f" wh^ ^referent %n % ' ^°^houi the
appropriate. reference to the glossary may be
A. Emission Constituents
!• Comparison of Diesel and Gasoline F!nnr
'See H.Rep. Report No. 102-902, 102nd Congress, Second Session 44 (1993).
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methods of (1) delivering fuel to the combustion chamber, (2) controlling power
output, and (3) achieving ignition. Additionally, the physical and chemical properties
and combustion characteristics of diesel fuel are much different from those of
gasoline. These differences result in different levels of the various combustion
products and thus differences in the challenges for emission control.
In gasoline engines, fuel and air are thoroughly mixed before entering the
combustion chamber. A throttle controls power output by regulating the mass of air
(or air and fuel) entering the engine; the fuel is metered to maintain the correct air-
fuel ratio. Under most conditions the fuel and air are combined in chemically correct
proportions to achieve complete combustion of the fuel. A spark from the sparkplug
in the combustion chamber ignites the gaseous mixture of fuel and air; the flame then
propagates through the fuel-air mixture.
A gasoline engine generally emits unburned fuel from incomplete combustion
in gaseous form as HC rather than PM. The gasoline forms very little PM because
it evaporates and is thoroughly mixed with the intake air before ignition; also,
combustion occurs under approximately stoichiometrict conditions. Partial burning
of gasoline corresponds also with increased CO emissions. Uncontrolled gasoline and
diesel engines form comparable levels of NOZ. In addition to the exhaust emissions
produced in the combustion process, liquid gasoline vaporizes readily to form
evaporative hydrocarbons. These vapors may be emitted to the atmosphere during
refueling of the vehicle or during periods of driving or parking.
In a diesel engine, the liquid fuel is injected directly into the combustion
chamber after the air has been heated by compression. The fuel is injected in the
form of a mist of fine droplets, which mix with the air. Power output is controlled by
regulating the amount of fuel injected into the combustion chamber, without
throttling (limiting) the amount of air entering the engine. The compressed air heats
the injected fuel droplets, causing the fuel to evaporate and mix with the available
oxygen. At several sites where the fuel mixes with the oxygen, the fuel autoignitesf
and the multiple flame fronts spread through the combustion chamber.
PM and NOX are the emission components of most concern from diesel engines.
Incomplete evaporation and burning of the fine fuel droplets result in emissions of
the very small, carbon-rich particles of PM. The high temperatures and excess
oxygen associated with diesel combustion can cause the nitrogen in the air to combine
with available oxygen to form NOX. Because of the presence of excess oxygen,
hydrocarbons evaporating in the combustion chamber tend to be completely burned
and HC and CO are not emitted at high levels. Evaporative emissions from diesel
engines are insignificant due to the low evaporation rate of diesel fuel. Since PM and
NOX are the primary emissions of concern from diesel engines, they are the focus of
this" report. The following discussion briefly describes the characteristics of PM, NOX,
and other emission components.
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2. Particulate Matter
In addition to the particulate formed from fuel droplets, small amounts of
lubricating oil that escape into the combustion chamber can contribute to PM As
engine improvements have decreased the amount of unburned fuel contributing to
particulate matter, lubricating oil has become an increasingly important source of
PM. Higher temperatures in the combustion chamber or faster burning would lower
rates of PM emissions, either by decreasing the formation of particulates or by
oxidizing those particulates that have formed. However, such adjustments made to
limit PM emissions often lead to increased NOX formation, as discussed below,
resulting in a difficult trade-off between reducing PM emissions or reducing NO'
emissions.
The particulate matter of most concern is less than 10 microns in size, referred
to as PM-10. Particulate matter in diesel exhaust is all much less than 10 microns
in size. These particles remain suspended in air for long periods and, because of
their small size, can be inhaled deeply into lung tissue, increasing the possibility of
respiratory problems. EPA has therefore set a National Ambient Air Qualitv
Standard for PM-10.
Diesel particulate matter consists primarily of a soluble organic fractionf and
carbonaceous material, with small quantities of sulfates and adsorbed water. The
soluble organic fraction contains polycyclic aromatic hydrocarbons, some of which are
potential human carcinogens. Use of low-sulfur fuel reduces the sulfate content of
diesel exhaust emissions. (Fuel sulfur also leads to gaseous emissions of oxides of
sulfur, as described below.)
3. Oxides of Nitrogen
The high temperatures resulting from combustion of diesel fuel create favorable
conditions for the formation of NOX. Nitrogen and oxygen in the intake airf react
together in the combustion chamber at high temperatures to form NOX. Measures to
reduce combustion temperatures lead to lower NOX levels, but can increase PM levels.
NOX emissions are an important precursor to ground-level ozone, or smog,
which reaches its highest concentrations in urban areas. Emissions of HC, the other
main pollutant acting as a precursor to ozone, come from many natural as well as
man-made sources. On the other hand, NOX emissions come almost exclusively from
man-made sources. The National Academy of Sciences recently determined that
reducing NOX emissions has been underemphasized in past efforts to reduce ground-
level ozone concentrations.2 In addition, human exposure to NOX may cause lung
^Rethinking the Ozone Problem in Urban and Regional Air Pollution, National Academy of Sciences
1992.
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irritation and increase the susceptibility to respiratory illness and pulmonary
infection. NO, emissions are also associated with acid deposition.
4. Other Constituents
Smoke is the visible soot emitted from engines. The formation of smoke
particles in a properly operating diesel engine is typically limited to periods of
acceleration or other increases in engine load. During accelerations, the fuel injection
rate may increase faster than the accompanying rate of increase in airflow due to
increased engine speed. If the increased fuel flow leads to an air-fuel ratio less than
about 22 to 1, smoke may form. Some strategies to lower PM emissions also lower
smoke. It appears likely, however, that the new PM standards adopted by EPA will
cause smoke emissions to be largely eliminated from properly operating engines, even
under high load and most acceleration conditions.
The amount of smoke present in the exhaust stream is quantified by measuring
the amount of light that can pass through the exhaust plume. Because it is easily
observed, smoke is a major concern to the public and frequently leads to the
misperception that diesel engines are unregulated and are major contributors to
urban air pollution.
Levels of CO and HC emissions from a diesel engine are inherently very low,
because the fuel burns in the presence of excess oxygen. Emission control strategies
that tend to increase PM levels, however, also may increase CO and HC levels.
Strategies to reduce PM should not lead to an increase—and may lead to a
decrease—in CO and HC emissions.
Sulfur contained in diesel fuel can react with oxygen in the combustion or the
after-treatment process to form oxides of sulfur (SO,). Currently, the only practical
method of reducing SOX emissions in diesel exhaust is to reduce the concentration of
sulfur in the fuel. Starting October 1,1993, EPA's regulations require that diesel fuel
sold for use in motor vehicles have no more than 0.05 weight percent sulfur.
Combustion of diesel fuel can produce toxic emissions. Many of the toxic
combustion products are bound in the particulate matter. Decreasing particulate
formation through engine modification or aftertreatment would reduce exposure to
the toxic compounds associated with the particulate matter. These toxic compounds
would be partially or completely decomposed into less harmful products and emitted
in gaseous form. Many other toxic components, such as benzene^ 1,3 butadiene, and
formaldehyde, are emitted as gases. Measures to reduce HC emissions also reduce
these toxic emissions. The use of alternative fuels may also reduce toxic emissions,
as described in Chapter 4.
The two other main components of diesel exhaust are carbon dioxide and
water, both natural by-products of the combustion process. Neither the carbon
dioxide nor the water present in diesel exhaust poses any known health problems to
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humans. However, because of concerns over global warming, there is an interest in
reducing carbon dioxide emissions through improved fuel efficiency. Diesel engines
require substantially less fuel to operate than comparable gasoline engines and thus
emit less carbon dioxide for an equivalent amount of operation.
B. Diesel Truck Classification and Standards
EPA classifies diesel trucks in various ways for purposes of controlling
emissions; relevant definitions of vehicle classes from the Code of Federal Regulations
are listed in Appendix B. The primary distinction is between light-duty trucks and
heavy-duty vehicles.
1. Light-Duty Trucks
In general, light-duty trucks are vehicles designed for carrying property or
more than 12 people, with a gross vehicle weight rating! up to 8,500 pounds. The
engine and the vehicle of a light-duty truck are typically built by the same
manufacturer. Light-duty trucks are small enough to be easily tested as complete
vehicles on a chassis dynamometer (for simulated driving in the laboratory). Exhaust
emissions are measured in grams per mile (g/mi).
EPA first set HC, CO, and NOX emission standards for light-duty diesel trucks
for the 1975 model year, as detailed in Table 2. EPA adopted more stringent-
standards beginning with the 1979 model year and introduced a particulate standard
for the 1982 model year. Standards for each pollutant were tightened in subsequent
years. The most recently adopted emission standards are 0.25 g/mi for nonmethane
hydrocarbons, 3.4 g/mi for CO, 1.0 g/mi for NOX, and 0.08 g/mi for PM, beginning with
the 1994 model year. Compared to the period before EPA adopted emission
standards, the emission levels represented by the latest standards indicate a 80 to
90 percent reduction for each pollutant.
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Table 2 .
Emission Standards: Light-Duty Diesel Trucks
Model
Year
1975
1979
1982
1984
1987
1988
1994
Pollutant (g/mi)
HC
2.0
1.7
1.7
0.8
0.8
0.8
0.25 (NMHO*
CO
20
18
18
10
10
10
3.4*
NO,
3.1
2.3
2.3
2.3
2.3
1.2*
1.0*
PM
—
—
0.6
0.6
0.26
0.26
0.08*
*Standard varies with light-duty truck subclass. Standard applies for 5 years or 50,000 miles. A relaxed
standard applies beyond that for the statutory useful life of the truck. Indicated value applies to
the lightest subclass. See 40 CFR 86 for details.
2. Heaw-Dutv Diesel Engines
Vehicles with a gross vehicle weight rating over 8,500 are considered heavy-
duty vehicles. Heavy-duty engines are used in a wide range of heavy-duty vehicle
categories, from small utility vans to large trucks. Because the manufacturer of one
type of heavy-duty engine may sell its engines to multiple vehicle manufacturers for
use in different applications, EPA emission standards for heavy-duty vehicles are
based on tests performed on the engine alone (and any associated aftertreatment
devices).
Testing of a heavy-duty engine is conducted on an engine dynamometer. The
engine, separate from the vehicle chassis, is mounted on a test stand for laboratory
operation. Emissions are measured in grams per braket horsepower-hour (g/bhp-hr).
This unit of measure recognizes the large variation in work output inherent in heavy-
duty applications. Because light-duty vehicles and heavy-duty engines are tested
differently, the numerical values for the standards are expressed in different units;
it is therefore difficult to compare values for light-duty vehicle and heavy-duty engine
standards.
Heavy-duty engines are further classified as either light, medium, or heavy.
The same emission standards apply to all heavy-duty diesel engines, regardless of
subclass (though the defined useful life varies considerably).
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«, i oS ^TS "* V^P"**8 category of heavy-duty vehicles. Beginning with
the 1993 model year the standard for PM emissions from heavy-duty efgTnes
designed for use m urban buses is more stringent than for other heavy-duty engines
Therefore strategies used by engine manufacturers to meet the stringent emission
standards for urban buses will likely be useful for meeting emission standards for
other engines.
Heavy-duty diesel engines for nonhighway applications (e.g., locomotives
marine vessels and construction and other equipment) are not currently regulated'
However, EPA is developing regulations for certain classes of nonhighway
applications in accordance with section 213 of the Clean Air Act.
with to™ I*6* a Sm°ike !£"?*£ for heavy-duty diesel engines beginning
with the 1970 model year, and added a CO standard and a combined HC and NO
standard for the 1974 model year, as detailed in Table 3. Beginning in the 1979
model year EPA added a HC standard, while retaining the combined HC and NO
standard. Beginning in the 1985 model year, EPA added a NOX standard, dropped*
the combined HC and NO, standard, and converted from steady-state to transient
5"^?oo H5\C0' and N0* missions. EPA introduced a particulate standard for
the 1988 model year. Since the 1985 model year, only the NOX and PM standards
have been tightened. Emission standards will be 4.0 g/bhp-hr NO for all 1998 and
later model year heavy-duty engines and a 0.10 g/bhp-hr PM standard for 1994 and
later model year heavy-duty engines. The PM standard for urban bus engines will-
be further tightened in the 1996 model year.
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Table 3
Emission Standards: Heavy-Duty Diesel Engines
Model
Year
1970
1974
1979
1985*
1988
1990
1991
1993
1994
1996
1998
Pollutant (g/bhp-hr)
HC
—
—
1.5
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
CO
—
40
25
15.5
15.5
15.5
15.5
15.5
15.5
15.5
15.5
NO,
—
—
—
10.7
10.7
6.0
5.0
5.0
5.0
5.0
4.0
HC+
NOX
—
16
10
—
—
—
—
—
—
—
—
PM
—
—
—
—
0.60
0.60
0.25
0.25 truck
0.10 bus
0.10 truck
0.07 urban bus
0.10 truck
0.05 urban bus
0.10 truck
0.05 urban bus
Smoke*
A:40%; L:20%
A:20%; L:15%; P:50%
A:20%; L:15%; P:50%
A:20%; L:15%; P:50%
A:20%; L:15%; P:50%
A:20%; L:15%; P:50%
A:20%; L:15%; P:50%
A:20%; L:15%; P:50%
A:20%; L:15%; P:50%
A:20%; L:15%; P:50%
A:20%; L:15%; P:50%
'Emissions measured in percent opacity during different operating modes: A=Acceleration; L=Lug; P=Peaks
during either mode.
*Starting in 1985, the test cycle was changed from steady-state to transient operation for emissions of HC,
CO, and NO,.
C. Current Technology
Manufacturers have made very substantial progress over the last two decades
to achieve the current level of emission control performance from diesel engines.
Emission reductions have been accomplished mainly through major improvements in
engine technology rather than aftertreatment technology. Most engine modifications
either decreased temperatures in the combustion chamber (for NOX control) or
increased the mixing of fuel and air in the cylinder (for PM control). To reduce NOX
emissions, manufacturers have relied extensively on charge air coolingt- To reduce
PM emissions, manufacturers have increased pressures for fuel injection!, improved
turbochargingt, improved combustion chamber swirlf, and introduced injection rate
8
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shapingf. Some engines for urban buses utilize exhaust after-treatment to meet more
stringent PM standards. For better control of the combustion process, manufacturers
have begun to use electronic controlsf. In addition, manufacturers have achieved
reductions in particulate emissions by reducing the consumption of lubricating oil,
without sacrificing engine durability.
Several 1993 model year certification engines have achieved the 1994 PM
standard of 0.10 g/bhp-hr, some without the use of after-treatment or alternative fuels.
Additionally, some 1993 model year certification engines have tested at levels below
the 1998 NOX standard of 4.0 g/bhp-hr. One methanol-fueled engine has
simultaneously controlled both PM and NOX to these levels.
Manufacturers are continuing to optimize these technologies to meet future
emission standards. This report describes and evaluates the prospects for further
improvements in emission control, and the associated trade-offs, from these and other
technologies. Chapter 2 focuses on further developments in engine technologies;
Chapter 3 discusses after-treatment devices; and Chapter 4 explores the use of various
alternative fuels for diesel truck applications.
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10
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Chapter 2 Engine Technologies
Many methods of engine-based emission control have been investigated since
the advent of emission standards for diesel engines. These methods range from
minor hardware improvements to redesigns of major engine components (such as the
combustion chamber), to introduction of electronic control of the fuel delivery system.
This chapter provides a discussion of various technologies available to diesel engine
manufacturers to meet new emission standards.
Diesel engines are divided into two basic types: two-strokef and four-strokef
engines. Generally, the technologies discussed in this section apply to either two-
stroke or four-stroke engines. Unique characteristics of each type of engine are
pointed out where applicable.
Another basic choice in designing diesel engines relates to the method of fuel
injection. For direct injection, the fuel is injected directly into the open combustion
chamber. Indirect injection is also possible, in which case the fuel is typically injected
into a prechamber with a glow plug for easier starting with a cold engine. Direct
injection engines have better fuel economy, but may be difficult to start, especially
in cold weather. Direct injection is used in most commercial applications of diesel
engines in the U.S. Indirect injection engines are most often used in vehicles that
experience frequent starts with a cold engine, such as light-duty trucks.
The technologies described in this chapter are evaluated with consideration of
the following factors: emissions, fuel economy, durability, and cost. The effects of a
technology on engine performance are noted as appropriate. Additionally, any
requirements for a vehicle operator to provide materials (such as water) for continued
functioning of an emission control technology are noted. In
A. Charge Air Compression and Cooling
Compressing the charge air (or intake air) before it enters the engine increases
the airflow into the combustion chamber. Until the recent emphasis on improved
emission controls, manufacturers used charge air compression exclusively to increase
power output without increasing engine displacement. Charge air compression
substantially heats the intake air and is therefore increasingly used with charge air
cooling, as described below. Charge air cooling prevents increased NOX emissions and
engine durability problems that could otherwise be caused by the higher
temperatures in the combustion chamber resulting from charge air compression.
If the amount of fuel injected is not increased with the increase in airflow and
the temperature of the air entering the combustion chamber is kept constant, the
additional intake air can reduce NOX emissions by diluting the combustion mixture
and lowering peak temperatures. The lower temperatures may increase PM
emissions by preventing full evaporation and combustion of the fuel, or by preventing
11
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complete oxidation of PM. If the decrease in combustion temperatures is small
enough, however, PM emissions may be unaffected or may even decrease, due to the
additional oxygen available for combustion.
Charge air compression can be accomplished in various ways, but the dominant
method used for diesel engines is turbocharging. A turbocharger uses the waste
energy in the exhaust gas to drive a turbine; the turbine is connected to, and thus
spins, a centrifugal compressor, which compresses the intake air. During acceleration
the turbine takes time to increase in speed (i.e., turbocharger lagt), which can result
in a small delay in acceleration.
Other methods for charge air compression have been used as well. In two-
stroke diesel engines a positive-displacement air pump (or compressor) scavenges! the
engine and increases the airflow through the engine. While these air pumps are very
effective at moving large volumes of air, they must be mechanically driven by the
engine, causing a loss of usable power. Also, this type of air pump results in higher
intake air temperature than from a turbocharger, requiring a greater amount of
cooling to prevent increased NOX emissions and engine durability problems.
Increasing charge air compression should not involve a large cost penalty, since
most diesel engines already have some form of turbocharger or positive-displacement
air pump. The costs associated with increased charge air compression are primarily
for development rather than for hardware. For example, fuel injection systems may
need to be recalibrated or aftercoolers may need to be upgraded, as described below.
Charge air cooling is accomplished with aftercoolersf on diesel truck engines.
There are two types of aftercoolers, each with unique characteristics: air-to-liquid
aftercoolers and air-to-air aftercoolers. The first manufacturers to introduce charge
air cooling used air-to-liquid aftercoolers, with the engine coolant as the cooling
medium. Air-to-liquid aftercoolers using engine coolant can lower the intake air
temperature only to a level near the operating temperature of the engine. However,
the temperature of the intake air, and thus the level of emission control, remains
relatively constant over a wide range of ambient temperatures.
Air-to-air aftercoolers use a stream of outside air flowing through the device
to cool the intake air. By using ambient air, an air-to-air aftercooler can cool the
compressed intake air to a temperature approaching that of the ambient air.
Manufacturers are now extensively using air-to-air aftercoolers, because the more
effective cooling contributes to lower NOX emissions. Intake air temperature from an
air-to-air aftercooler is highly dependent on ambient temperature—NOX control would
therefore be most effective in low ambient temperatures. However, unless
manufacturers limit the effectiveness of cooling in winter conditions, very low intake
air temperatures may lead to increased PM emissions. Converting to an air-to-air
aftercooler introduces a moderate cost penalty.
12
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Another possibility for getting more cooling than the conventional air-to-liquid
aftercooler is an air-to-hquid aftercooler using a separate coolant system. Such a
system would cool intake air temperatures almost as effectively as an air-to-air
aftercooler and could reduce seasonal variations in intake air temperature
Introducing a separate liquid system for aftercooling would be significantly more
complex and costly than either of the other systems.
B. Induced Swirl in Intake Air
Increasing the turbulence of the intake air entering the combustion chamber
a-e inducing swirl) can reduce PM emissions from diesel engines by improving the
mixing of air and fuel in the combustion chamber. Historically, swirl was induced by
routing the intake air to achieve a circular motion in the cylinder. Manufacturers
are however, increasingly using "reentrant" piston designs, in which the top surface
of the piston is cut out to allow fuel injection in a smaller cavity in the piston to
induce additional turbulence.
Excessive swirl may prevent full penetration of the fuel in the swirling air
stream, especially under light load conditions. The resulting incomplete combustion
would cause reduced fuel efficiency and increased emissions. The amount of swirl
should be matched with other engine parameters, such as fuel injection pressure
compression ratio, and the shape of the top surface of the piston.
Induced swirl has several benefits. Induced swirl may prolong engine life by
reducing the amount of dilution of engine oil by the fuel, thereby preventing a loss
of lubrication. The cost of producing a heavy-duty engine with induced swirl would
be primarily for research and development to modify the intake manifold, the intake
valve, or piston bowl. Incorporating induced swirl should have little effect on fuel
economy; the increased turbulence may, however, slightly decrease fuel economy by
causing heat from the combustion chamber to be lost to the engine block and the
cooling system.
C. Exhaust and Intake System Optimization
Aftertreatment devices may increase backpressure, thereby restricting the flow
of intake air and exhaust gases. Manufacturers using after-treatment devices
therefore have a greater incentive to reduce pressure losses in the intake and exhaust
systems and to reduce heat losses in the exhaust system. Reducing heat losses in the
exhaust products increases the energy available to power the turbocharger and
maintains higher exhaust gas temperatures, which increases the effectiveness of most
after-treatment devices.
D. Intake Air Dilution
Intake air dilution in diesel engines to lower combustion temperatures is
currently being researched. Displacing some of the intake air with inert materials
13
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lowers combustion temperatures by diluting the mixture in the cylinder and
absorbing heat from the burning fuel. The reduced temperatures caused by intake
air dilution, with or without aftercooling of the intake air, result in lower levels of
NOX emissions. Two potential methods are exhaust gas recirculation (EGR) and
water injection.
Exhaust gas recirculation uses gases from the exhaust stream to dilute the
combustion mixture. The recirculated exhaust gases absorb a portion of the energy
released during combustion of the fuel, decreasing the peak combustion temperature
and reducing NOX formation. EGR in gasoline-fueled engines is most often
accomplished by routing a portion of the exhaust stream from the exhaust system
into the intake air. A metering valve controls the flow rate of the recirculated
exhaust gases. As an alternative, manufacturers may use "internal" EGR, in which
the timing of the intake and exhaust valves is coordinated, so a portion of the exhaust
gases from the previous combustion event are retained in, or drawn back into, the
combustion chamber.
There are, however, potential drawbacks in the application of EGR to diesel
engines. The abrasiveness of the particulate matter in the exhaust stream may cause
accelerated wear in the engine and turbocharger. Also, the particulate matter would
form deposits on components of the engine intake system, decreasing the heat
transfer capability of the aftercooler and potentially decreasing the effectiveness of
the turbocharger. In addition, by reducing combustion temperatures and decreasing
the amount of air available for combustion of the fuel, EGR may cause incomplete
combustion, especially at high load, resulting in increased smoke, PM, and CO
emissions.
Water injection is a second form of intake air dilution. Water has a very high
capacity to absorb heat as it flashes from liquid to gas (steam). Much like EGR,
vaporizing and heating the water lowers peak combustion temperatures, thus
lessening the formation of NOX. However, water injection imposes significant
problems. First, the water can cause corrosion of engine components. Second, use
of water with dissolved impurities (such as calcium carbonate) would lead to deposits
in the water injection system and in the engine; purified water would be needed to
avoid such deposits. Third, insulation or additives would be necessary to prevent the
water from freezing in winter. Finally, water injection requires the engine operator
to refill the water reservoir periodically—likely without a tangible incentive—in
contrast to EGR, which functions without operator participation.
s
E. Fuel Delivery System
Many aspects of the fuel delivery system may be modified to improve the
emission control of a diesel engine. They include injection pressure, fuel spray
pattern, injection rate and timing, and use of electronic controls. Each modification
used alone typically has some penalty, such as higher cost or lower efficiency, as a
14
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trade-off for the emission benefit. These modifications can be used together to
optimize the combination of individual benefits and penalties.
Increasing injection pressure improves the atomization pf the fuel and
increases the mixing action with the intake air in the combustion chamber This
combination of reduced droplet size and improved mixing leads to more complete
combustion and decreased formation of PM. A drawback of higher injection pressures
is the cost involved in reinforcing the fuel-injection system and the engine to deal
with higher pressures, which might otherwise cause a decrease in durability.
The design of the fuel injector nozzle determines the fuel spray pattern
Nozzles are designed to provide the best spray pattern for each combustion chamber
configuration. The spray pattern from the nozzle needs to be optimized to reduce fuel
dropout on the surfaces of the combustion chamber and to achieve good mixing with
the compressed intake air. Another design consideration for fuel injectors is to limit
sac volume. Sac volume is the small volume of fuel remaining in the tip of the
injector at the end of injection. This fuel may dribble out of the injector at low loads
causing increased PM emissions. At high loads this effect is minimized during the
combustion stroke, because the much higher pressures in the combustion chamber
hold the fuel in the nozzle. However, during the following low-pressure exhaust
stroke, the fuel may dribble from the injector and be emitted as HC. Some fuel
injection nozzles have been designed to close the injector spray orifices quickly and
completely at the end of the injection stroke. Durability problems, however have
been reported with some of these fuel injectors. '
Retarded injection timingf is a very inexpensive means of reducing NO
emissions. Retarded injection timing lowers combustion pressures and temperatures"
thus reducing NOX formation. The lower combustion temperatures result in increased
PM emissions and cause a loss in fuel economy; therefore, substantial timing retard
is no longer commonly used.
Increasing the overall injection rate shortens the duration of fuel injection
Increased injection rate allows a delay in the initiation of fuel injection, similar to
retarded injection timing, causing lower combustion temperatures and reduced NO
formation. Increasing the injection rate avoids the PM and fuel economy penalties"
of retarded injection timing, because the termination of fuel injection is not delayed.
Injection rate shaping is a new strategy in which the rate of fuel injection is
deliberately varied over the duration of the injection period, to reduce emission
formation. Typically a small, early burst of fuel enters the combustion chamber
initially; injection of the majority of the necessary fuel is slightly delayed until the
fuel in the combustion chamber ignites. Injection rate shaping reduces PM formation,
because less time is required to ignite and burn the fuel droplets injected into the
combusting mixture. Similar to retarded injection timing, delaying injection of most
of the fuel lowers peak combustion pressures and temperatures, thus reducing NO
formation. *
15
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Finally, use of electronic controls! enables the designer to implement and
precisely control the modifications described in this section to achieve improved
emission control with minimum penalty to fuel economy. Various systems have been
developed and are in use, but much research remains in applying electronic controls.
For example, optimizing fuel management on a cylinder-to-cylinder basis achieves the
maximum benefits from each of these methods under varying operating conditions.
F. Closed Crankcase
Since an engine's piston rings cannot provide a perfect seal with the cylinder
walls, a small fraction of the products of combustion leaks past the piston rings into
the crankcase. This flow of materials, known as blowby gases, mixes with the mist
of lubricating oil present in the crankcase and must be vented from the crankcase.
Components called coalescers are typically installed at the crankcase vent to collect
and retain in the crankcase most of the entrained oil. In past designs, blowby gases
were vented directly to the atmosphere as complex hydrocarbons, some of which may
be considered toxic air emissions.
Preventing the discharge of the mixture of materials to the atmosphere is
known as closing the crankcase and is achieved by routing the blowby gases to the
engine air intake. EPA regulations require the use of closed crankcase systems on
all gasoline-fueled engines, but only on naturally aspirated diesel-fueled engines (i.e.,
those without charge air compression). EPA exempted diesel-fueled engines with
charge air compression because of the possibility of blowby gases decreasing the
effectiveness of turbochargers and aftercoolers, which could cause a net increase in
emissions.
For diesel engines equipped with charge air compression, closing the crankcase
depends on the development of designs that protect turbochargers and aftercoolers
from damage. Closed crankcase systems are now available for some locomotive and
marine applications. Development of such systems for truck applications should be
feasible at a low cost. There appear to be no negative effects on fuel efficiency or
engine performance.
G. Smoke Controls
Smoke from properly operating diesel engines occurs primarily during periods
of engine acceleration, and to a lesser extent during engine luggingt operations. With
the introduction of stringent standards for controlling emissions of particulate matter,
manufacturers have nearly eliminated visible smoke from the exhaust of properly
operating diesel engines. Manufacturers control smoke emissions during engine
acceleration with a component called a puff limiter, which limits the rate at which
additional fuel is supplied to the engine during accelerations. A puff limiter may
operate with either mechanical or electronic controls.
16
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The primary disadvantage of a puff limiter is that it may cause somewhat
slower or delayed acceleration. Costs associated with the use of a mechanical puff
limiter are relatively low. Electronically controlled puff limiters are more expensive
desigts ^^ ^ and 16SS SUScePtible to tampering than mechaS
H. Combustion Chamber Modification
Manufacturers have achieved significant emission reductions through changes
to the combustion chamber. Additional modifications to the combustion chamber may
provide further improvements in emission control. Combustion chamber parameters
of interest include (1) the shape of the chamber and the location of the fuel injector
U) the location of the top piston ring relative to the top of the piston, and (3) the
compression ratio. The introduction of ceramic coatings to surfaces of the combustion
chamber is another possible modification in the experimental stage.
^ f Efforts to redesign the shape of the combustion chamber and the location of
the fuel injector have been directed primarily at optimizing the relative motion of the
air and the injected fuel. The goal is to limit the formation of NO without an
increase mPM or conversely, to reduce PM without an increase in NO,* Reductions
in both NO, and PM may be possible with some combustion chamber configurations
currently under development. However, significant problems in the areas of
structural durability and emission control durability have been attributed to these
configurations. Additional benefits can be realized in the form of reduced HC and CO
emissions, accompanied by little or no penalty in fuel efficiency. Costs would be
limited primarily to research and development and to tooling changes, which would
not be very large if spread over many engines.
The location of the top piston ring relative to the top of the piston has
undergone significant investigation. The location of piston rings has been modified
to reduce the crevice volumef, while retaining the durability and structural integrity
of the piston and piston ring assembly. Improvements result in reduced HC
emissions and, to a lesser extent, in reduced PM emission. Costs associated with a
relocation of the top ring can be substantial. Raising the top piston ring requires
modified routing of the engine coolant through the engine block to prevent the raised
ring from overheating. Also, the machining needed for the engine block would likely
require more precise tolerances.
Compression ratio is another engine design parameter that impacts emission
control. In general, higher compression ratios cause a reduction in PM emissions and
improved fuel economy, but also cause an increase in NOX emissions. However,
higher compression ratios require a stronger engine structure, which may increase
weight and cost. The increased engine weight and frictional losses somewhat offset
the fuel economy benefit of higher compression ratios, especially at very high
compression ratios. Conversely, lower compression ratios generally cause a reduction
in NO, emissions, while causing an increase in PM emissions and decreased fuel
17
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economy. Also, low compression ratios can lead to problems with starting a cold
engine.
Manufacturers are researching the possibility of adding ceramic materials to
the surfaces of the combustion chamber. Ceramic coatings may provide effective
insulation, allowing the engine to retain more energy in the products of combustion.
Retaining more energy in the combustion chamber increases peak combustion
temperatures, resulting in decreased PM emissions and possibly increased NOX
emissions. Also, a greater portion of the total energy contained in the fuel can be
converted into useful work in the engine, improving the engine's fuel efficiency.
When combined with other modifications such as retarded injection timing and
reduced fueling rate, the use of ceramics can result in reduced NOX emissions without
a loss in fuel efficiency. Expected costs for the use of ceramics would be moderate.
In some applications introduction of a waste heat turbine to extract additional
energy from the exhaust gases may be possible. A waste heat turbine would probably
be used in conjunction with the application of ceramics. Energy extracted by the
turbine would be transmitted to the crankshaft through an appropriate set of
reduction gears. Converting some of the energy in the waste products to useful work
on the crankshaft would increase overall engine efficiency. By reducing fuel
consumption, addition of a waste heat turbine would lead to a corresponding
reduction in overall exhaust emissions. Costs associated with the addition of a waste
heat turbine would likely be high; the durability is unknown.
18
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Chapters After-treatment
signita equipment suppliers have performed
signmcant research in response to the emission standards required by the Clean Air
Act Amendments of 1990, and by similar control programs i^ California Th I
stringency of the PM and NO, standards have led manufacturer^ *t Tp^
aftertreatmentt as a supplement to engine-based emission control tecEo
primary exhaust aftertreatment technologies are currently available
converters and particulate trap oxidizer, This chapter di^'S^h
°f other ad— d aftertreatment techndoS
rP°ratin! 6XhaUSt aftertreatl*ent is typically more expensive than
engine designs, but aftertreatment can result in additional Mission
NOnP^eTtTeatmer$ ^ *° l6SSen the trade-°ffs involved in con* both
^i\f emissions. For example, use of aftertreatment devices to control PM
NO T™a«± ^^ 6ngme ^SignerS Wlth m°re flexibility to focus ™ Deducing
JNUX formation m the engine, and vice versa.
A. Catalytic Converters
Catalytk converters have played a major role in reducing emissions from-
ret^
CC key 6ngine V*****^™ ^n conver
i u CC2 key 6ngine V*****^™' ^n convert most of the HC CO
l a S^olinB engine into carbon dioxide, nitrogen, and water
vapor. The technology base established over the years from the use of iatiyste on
gasoline-fueled passenger cars provides a foundation for development of catalysts for
'
1- Principles of Operation
The gases that exist in a vehicle's exhaust stream, if given enough time, would
naturally react with each other to form less harmful products. However, even in the
high temperatures of a vehicle's exhaust pipe, these reactions proceed far too slowly
to be comp eted before the gases are expelled into the atmosphere. Certain materials
such as platinum, palladium, and rhodium, act as catalysts to speed up these
reactions. Catalyst materials must contact the targeted reactants, but are not
themselves consumed m the ensuing reaction. An automotive catalytic converter
channelsthe exhaust stream past a catalyst material distributed over an extremely
large surface area created by a highly porous matrix. The large surface area greatly
increases the opportunities for catalyst-reactant contact within the small volume of
a catalytic converter shell.
19
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Diesel engine catalysts are of two basic types: oxidation catalystst and
reduction catalysts t. Oxidation catalysts use the free oxygen present in a vehicle's
exhaust stream to oxidize substances such as CO, HC, and some constituents of PM.
In contrast, oxidation catalysts do not affect NOX emissions, because NOX is itself an
oxidant.
Reduction catalysts focus specifically on NOX elimination. Reduction catalysts
therefore require an adequate concentration of substances in the catalytic converter,
called reductants or reducing agents, that react readily with NOX. These catalysts
are described further in the discussion on NO_ control below.
'JC
The three-way catalystt strategy used successfully in automotive gasoline
engine applications does not readily transfer to diesel emission control efforts. Three-
way catalysts enhance both oxidation and reduction reactions, relying on the careful
control of the exhaust gas mixture coming from the engine to mutually eliminate HC,
CO, and NO,. In contrast to gasoline engines, the exhaust from diesel engines has
excess oxygen, so the oxygen molecules entering a three-way catalyst would
overwhelm the far fewer NO, molecules in competition for available CO, HC, and PM
molecules. Without a supply of additional reductants, such catalysts have little effect
on NOZ emissions.
2. PM Control
Oxidation catalysts for diesel engines serve primarily to reduce PM emissions.
PM emissions from diesel engines are composed of carbonaceous particles (the
primary constituent of soot), a soluble organic fractionf, sulfates, and adsorbed water.
As discussed below, particulate traps are effective at filtering out the carbonaceous
component. In contrast, oxidation catalysts operate largely on the soluble organic
fraction and have little effect on the carbonaceous portion of PM in diesel exhaust.
This limits the reduction in PM emissions that an oxidation catalyst can achieve.
Furthermore, an oxidation catalyst converts some portion of the sulfur dioxide
present in the exhaust stream to sulfate PM. Because the increased sulfate PM can
offset the reduction in the soluble organic fraction, an oxidation catalyst may actually
increase total PM emissions. A major goal in the development of oxidation catalysts
for diesel engines is therefore to increase oxidation of the soluble organic fraction
while decreasing the oxidation of sulfur dioxide. Reducing the sulfur content of on-
highway diesel fuel to a maximum of 0.05 percent by weight, required by EPA
nationwide beginning October 1, 1993, provides a major boosf to the viability of
oxidation catalyst technology for diesel engines (55 FR 34120, August 21, 1990 and
57 FR 19535, May 7, 1992). The lower fuel sulfur content produces a corresponding
decrease in sulfur dioxide emissions, making it easier to design a catalyst that forms
less sulfate PM. However, this level of fuel sulfur still presents a serious challenge
to the effectiveness of oxidation catalysts. Further reductions in the sulfur content
of diesel fuel would lead to additional reductions in sulfur dioxide emissions, but
removing more sulfur would significantly increase the cost of diesel fuel.
20
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"" C
the
total PM emisrions from
very dependent on the m .?"^ AC*Ual "***»• •»
carbonaceous portion) from ^ngine-ou e^aut BefaZ *aCtl°n 'C°mPared to th*
is largely unaffected, oxidation catalvsto for Hi. f T . Carbonaceous portion
PM emissions as effectively " o^^L^^l™^™* '—
passenger CSLTS. Oxidation catalysts are attrarti™ t • ~, ° C° enussi°ns in
may allow engines that alread> havTfi?lv ^™ deSlgners because
standards-at a significantly lower cit
manufacturer is conskering
certified at a 0.08 g/bhp-fa even
standard for urban buses" Cots «
range from $300 to $2,000."
to meet stringent
*rapS' One
*° an en«ine
, S 2'°5 ^
catalyst have been estimated to
catalysts with sufficientaby
3. NO. Control
s ™ n- However-
a catalyst is to lower NO
characteristic of diesel en
called a lean NOX catalyst
ust
years" T116 *oal
f OXy^en-rich S^s mixture
°f °atalyst is therefore sometimes
diesel enes ^ »« NOX from
exhaust stream. Two means of arW«L» !2 • PP y, reducing agent in the
s°'&te Adsorpt"m - Perfomance °f D
^- N.
SAE
11,
i.n, to Richard O. Wikon, U.S. EPA, September
21
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For catalysts relying on the injection of a reducing agent into the exhaust
stream, urea, ammonia, and diesel fuel are reducing agents that have been
investigated. Urea and ammonia are effective reducing agents in industrial
applications, achieving very high NOX reduction efficiencies in steady-state
operation.7 However, the operating environment of motor vehicles requires these
systems to be effective under frequently varying load conditions, particularly in urban
areas where engine loads can vary considerably over the course of a trip. This may
require a sophisticated control system to ensure that injection of the reductant closely
tracks the transient conditions. In addition, designs would need to avoid injecting
excessive amounts of reducing agents, which could produce toxic emissions and
offensive odors. The effectiveness projected for reduction catalysts in diesel trucks
should therefore be lower than that achieved in industrial applications. One
investigator believes that NOX reduction efficiencies of 25 percent from such systems
may be expected by the early 2000s.8
Relying on urea or ammonia for effective catalyst operation raises an additional
concern. These chemicals are consumed to achieve NOX reduction, so vehicle
operators would need to maintain an adequate supply. Some operators would likely
not maintain an adequate supply of the required reducing agents, unless running out
of the reducing agent would somehow degrade vehicle performance.
Using diesel fuel as the injected reducing agent would resolve the concern over
operator participation. In such a design, a small amount of diesel fuel is sprayed'
upstream of the catalytic converter in metered amounts, corresponding to engine NOX
output. NOX reduction efficiencies of 30 to 80 percent have been reported in steady-
state experimental systems.9'10 The reported NOX reduction efficiency of 80 percent
corresponded with a 5 percent loss in fuel economy. Fuel efficiency decreases because
fuel is injected for NOX reduction instead of producing power output from the engine.
Performance in the far more demanding operating environment of heavy-duty diesel
engines is not yet established. Also, it may not be possible to design tamper-proof
systems. Operators would have an incentive to disconnect the fuel flow to the
catalyst in order to increase fuel economy, thus potentially negating any expected
NOX reduction.
There is considerable interest in the diesel exhaust-NO, catalyst concept, in
which the hydrocarbons present in the exhaust stream break down the NOX.
Hydrocarbons are trapped in zeolite molecular sieves when they are emitted from the
engine at high levels. The stored hydrocarbon is then available for reaction during
7"Catalytic NO, Reduction in Net Oxidizing Exhaust Gas," W. Held, SAE 900496, 1990.
8Acurex, p. 3-36.
""Catalytic Reduction of NO, and Diesel Exhaust," S. Sumiya, et al, SAE 920853, 1992.
10"Catalytic Reduction of NO, in Actual Diesel Engine Exhaust," M. Konno, et al, SAE 920091, 1992.
22
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of hvHrn >f i pT0^UCtl0^ ^ zeolite molecular sieves allow selective trapping
of hydrocarbon mo ecules and prevent hydrocarbon oxidation by the oxygen in he
exhaust stream. Diesel exhaust-NO, catalysts are attractive because theTfequ^re no
injection of supplemental reducing agents. 7 reqmre no
To be effective, diesel exhaust-NOx catalysts will likely require exhaust
temperatures higher than those from most current diesel engines ffigher
temperatures can be achieved with lower engine air-fuel ratios; however, reducing the
amount of oxygen may introduce a penalty for fuel economy and PM emissions. NO
^ T^nTon8 5° t07° PerC6nt haVe been pr(»ected'but fuel econo^y would*
decrease by 10 to 20 percent due to unavoidable pumping losses. Currently, catalysts
of this type have achieved NOX reduction efficiencies of 20 percent.11
Because much development remains for reduction catalysts for diesel
applications, reliable cost and durability projections are not yet available However
this technology appears promising. '
B. Particulate Traps
1. Principles of Operation
A particulate trap oxidizert ("trap") filters particulate matter from the exhaust
stream and later oxidizes (burns) the filtered particulate, reducing PM emissions -
The basic element of a trap system is a structural shell containing filter material
The trap also may include a system for heating the filter to oxidize the particulate
and regenerate the trap filter, a microprocessor for controlling filter regeneration and
an air supply. A trap system typically dampens the engine noise enough that it
replaces the muffler on a vehicle.
Filters currently under development are either ceramic wall-flow monolith
filters or filter tubes covered with multiple layers of a yarn-like ceramic material
The filter material contains many small holes that allow the exhaust gases to pass
through while collecting the particulate from the raw exhaust. In some applications
the filters are covered with a precious metal catalyst such as platinum to aid in the
oxidation of the particulate matter, as discussed below.
There are two main methods for oxidizing the particulate: heating the filter
with an auxiliary heat source and using a catalyst. In the case of filter heating an
electrical heater or a fuel burner supplies the auxiliary heat to the filter and is
controlled by a microprocessor. As the trap collects particulate, the microprocessor
monitors such parameters as the trap temperature, the exhaust backpressure the
pressure drop across the trap, and the airflow through the engine. Regeneration
begins when the measured parameters indicate that regeneration is necessary and
"Acurex, p. 3-39.
23
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can be done effectively. The exhaust flow is then diverted to bypass the filter during
the regeneration process, either to a second trap or through the exhaust stream to the
atmosphere. The trap is regenerated by supplying heat to the filter until the
collected particulate has been oxidized. The oxidized particulate is converted to
carbon dioxide and water and passes through the trap. The oxygen needed to react
with the particulate is drawn from the outside air by an auxiliary blower.
For traps that use catalysts to oxidize the particulate, the catalyst material is
typically loaded directly on the filter (a catalyzed trap). Also under development are
catalyzed trap systems, in which an additive to diesel fuel supplies the catalyst.
When exhaust temperatures are in the right range, the catalyst promotes particulate
oxidation. For either kind of catalyzed system, the oxygen needed to react with the
particulate is drawn from the exhaust stream. Because catalysts are effective in only
a limited exhaust temperature range, some types of engines also may require the
addition of an electrical heater or fuel burner to assist the oxidation process.
Exhaust filtering may be continuous or intermittent. Trap systems with
catalysts can utilize continuous filtering; the catalyst promotes oxidation
continuously, so the exhaust can be passed continuously through a single trap. Trap
systems regenerated by auxiliary heating can also utilize continuous filtering but
require two traps; when the first trap has been loaded and conditions permit, the
exhaust is diverted to the second trap, allowing the first trap to regenerate. Single-.
trap systems regenerated by auxiliary heating utilize intermittent filtering, in which
case the unfiltered exhaust is diverted through a muffler during trap regeneration,
then redirected to the trap when regeneration is complete.
The main application for trap development has been for urban bus engines.
EPA has set more stringent particulate emission standards for urban bus engines
than for other heavy-duty engines, beginning with the 1993 model year. In addition,
the Clean Air Act specifically requires EPA to implement a retrofit/rebuild program
for 1993 and earlier model year urban buses to reduce PM emission levels from in-use
buses. The tighter emissions standards for new urban bus engines and the possible
need to retrofit a bus to meet standards may cause an increased reliance on traps for
urban buses.
There are approximately 700 urban buses currently equipped with traps; these
buses have accumulated more than 25 million miles of collective in-use operation.12
The New York City Transit Authority (NYCTA) operates most of these urban buses.
Nearly all the urban buses using traps have noncatalyzed, electrically regenerated,
dual-trap systems with ceramic wall-flow monolith filters.
12 Letter from Bruce Bertelsen, Manufacturers of Emission Controls Association, to William Reilly, U.S.
EPA, January 19, 1993.
24
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2. Emission Reductions and Fuel Economy
The leading motivation to develop traps has been to reduce PM emissions
Traps function primarily to reduce the carbonaceous fraction of particulate matter
(soot), but also achieve some reductions in the soluble organic fraction Based on
results from testing heavy-duty engines, a noncatalyzed, electrically regenerating
dual-trap system can reduce total PM emissions by more than 80 percent Such traps
do not significantly affect HC, CO, and NOX emissions. Use of a catalyzed trap filter
employing an auxiliary heat source can provide slightly better particulate reductions
and can reduce HC and CO up to 50 percent and 70 percent, respectively13 Such
traps have little effect on NOX emissions.
Little information exists on the emission-control performance of catalyzed trap
systems not relying on an auxiliary source of heat for oxidation. However, because
the catalyst would be optimized for use without auxiliary heating, a catalyzed trap
system without auxiliary heating should be able to achieve PM, HC, and CO emission
reductions almost as great as those for catalyzed traps with auxiliary heating.
Some trap systems cause a decrease in fuel economy. For catalyzed traps
without auxiliary heating no effect on fuel economy is expected, because the traps
operate continuously, independent of the engine, and therefore have no power
requirements and likely can be designed to induce no backpressure. For a trap
system with auxiliary heating, the increased exhaust backpressure from the collected
particulate and the power required to generate electricity to heat the filter decrease
fuel economy. Based on information from NYCTA, the fuel economy decrease from
such traps has been as great as four percent.14 Further improvements to trap
system designs may reduce the fuel economy impact.
3. Durability
Current information on the durability of traps is limited. The main source of
information on the durability of in-use trap systems is from NYCTA. NYCTA
installed traps on 400 new urban buses put into service in early 1991. As of late
1992, these trap-equipped buses had operated for an average of 45,000 miles. The
low-mileage durability of the NYCTA trap systems has been promising. However,
none of these urban buses has achieved a mileage accumulation near the useful life
requirement for urban bus engines of 290,000 miles.
Although the low-mileage durability of the NYCTA trap-equipped buses is
promising, the long-term durability of particulate traps remains unproven and an
"Particulate Trap Technology Demonstration at New York City Transit Authority," Kong Ha et al
SAE 910331, 1991. .
14Kong Ha, SAE 910331, 1991.
25
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area of concern. A particulate trap involves a complex system that includes a variety
of hardware and software to filter and burn the particulate in the exhaust.
Manufacturers of urban bus engines have certified trap-equipped engines for model
year 1993, emitting less than 0.05 g/bhp-hr PM. Certification requirements include
a demonstration of durability for the useful life of the engine. In contrast to the
controlled laboratory conditions of certification testing, however, in-use operating
conditions vary greatly from one vehicle to another. In addition, certification testing
is performed on a well maintained engine run solely on an engine dynamometer, in
contrast to in-use engines operated with widely varying degrees of maintenance under
conditions potentially more demanding than those for certification. The ongoing
mileage accumulation of trap-equipped buses and further technological development
will establish the level of performance that such systems can achieve over the full
useful life of urban bus engines.
4. Cost
Based on comments from urban bus operators, current dual-trap systems cost
around $15,000. In recent comments on EPA's urban bus retrofit/rebuild program,
a manufacturer of dual-trap systems noted that increased trap production would
likely cause wholesale trap costs to decrease to a level between $5,000 and $6,500 per
system. Distribution costs would be additional. There is some uncertainty in the
expected costs of trap systems due to the uncertainties in the ultimate market share
for traps, coupled with the large investments made in trap development. If oxidation
catalyst technology capable of meeting the PM standards becomes available within
the next few years, as urban bus engine manufacturers have indicated, catalysts may
capture a significant share of the market. In this case, trap costs may need to be
higher to allow recovery of development costs over fewer trap systems, which might
further lessen the demand for traps.
The use of traps may also increase the expense of operating a vehicle through
decreased fuel economy. For those trap systems with a fuel economy penalty,
assuming an average two percent decrease in fuel economy and a lifetime mileage of
500,000 miles, an urban bus would use an additional $2,000 of fuel over its
lifetime.15
Based on information from trap manufacturers, a single-trap, noncatalyzed
system is expected to cost about half as much as a dual-trap system. Systems with
catalyzed traps, not regenerated by auxiliary heating, are expected to cost nearly the
same as the dual-trap systems discussed above (assuming increased production).
Systems with catalyzed traps that are regenerated by auxiliary heating would be
slightly more expensive than noncatalyzed trap systems. A catalyst in the form of
a fuel additive would increase the cost of diesel fuel by a few cents per gallon.
15 Final Regulatory Support Document for the 1994 and Later Model Year Urban Bus Particulate
Emissions Standard, U.S. EPA, February 1993 (Docket A-91-28, item V-B-1).
26
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C. Advanced Aftertreatment Technologies
Chemical aftertreatment with cyanuric acid is being developed as a method to
reduce NOX emissions. As exhaust gas passes over cyanuric acid pellets the
substance gives off a gas (HCNO), which reacts with NOX to form nitrogen caVbon
dioxide, and water. California has identified this as a "best available technology" for
stationary sources. Successful use of cyanuric acid for exhaust aftertreatment in
truck applications requires carefully controlling the delivery of HCNO to the exhaust
stream. Further study is necessary to be able to deliver the right quantity of HCNO
at a point where the surrounding temperatures allow effective reaction with NO and
to reduce system costs. One approach may be instead to add cyanuric acid to diesel
fuel, as described in Chapter 4.
Researchers are also attempting to use electronic aftertreatment methods to
reduce NOX and PM emissions. These technologies have been associated with
controlling emissions from coal-fired power plants, but they may also be viable in
truck applications. Two approaches getting the most attention have shown potential
though substantial additional effort is needed before the technologies are ready for
commercial application. One approach, called pulsed plasma, involves an electrical
discharge in the exhaust stream to produce electrons, creating highly reactive
radicals and ions from the available water and oxygen molecules. These radicals and
ions then oxidize particulate matter and react with NOX to form nitrogen, oxygen and
water. '
The second approach, called oscillating wave, involves an electrode that
produces an oscillating discharge of electrons in the exhaust stream. Similar to
pulsed plasma, the electrons from the oscillating wave produce radicals and ions. The
radicals and ions oxidize particulate matter and react with both NOX and
hydrocarbons in the exhaust to reduce emissions of these compounds.
27
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28
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Chapter 4 Fuel Technologies
Altering the fuel used in diesel engines can contribute significantly to improved
emission control. Current research efforts are focused on either modifying the
properties of diesel fuel or using an alternative fuel for diesel enginesf. Cleaner
diesel fuel and several potential alternative fuels, including natural gas, liquefied
petroleum gas, methanol, ethanol, and electricity are discussed below. There are
many important issues connected with the use of reformulated or alternative fuels
that are beyond the scope of this report, such as energy security and safety.
Published literature from recent years includes extensive examination of these
issues.16
The emission benefits of improved fuels are difficult to compare with the
benefits of engine modifications or aftertreatment. Since cleaner fuels are available
immediately to all vehicles, a seemingly small improvement in emission control can
make a real improvement in air quality. In contrast, emission reductions from engine
modifications, as a percent reduction from some baseline, are limited by fleet
turnover, aging, and tampering.
A. Diesel Fuel and Lubricants
The composition and some of the properties of diesel fuel can be adjusted in an~
attempt to improve emission control performance. Most of the following possible
changes to diesel fuel are interdependent, for example, an increase in a fuel's cetane
number is usually associated with a decrease in aromatic content and an increase in
volatility. It is therefore difficult in some cases to determine separately the effects
of changing individual parameters.
1. Reduced Sulfur
Sulfur occurs naturally in crude oil and, unless removed, also occurs in refined
diesel fuel. Two basic problems are associated with sulfur in diesel fuel. First, the
sulfur in the fuel reacts with the available oxygen to form oxides of sulfur; about 3
percent of the fuel sulfur is directly emitted as particulate in the form of sulfuric
acid.17 Oxidation catalysts worsen this effect by enhancing the conversion of sulfur
oxides to sulfate particulates. Oxides of sulfur can also react in the atmosphere to
form PM. Second, for vehicles equipped with particulate trapsf or catalysts!, fuel
sulfur can cause significant deterioration in the substrate materials, decreasing the
effectiveness and durability of the trap or catalyst.
"Extensive lists of such references were compiled in a series of special reports by EPA's Office of Mobile
Sources, for methanol (September 1989), natural gas (April 1990), and ethanol (April 1990).
""Cost-Effectiveness of Diesel Fuel Modifications for Particulate Control," M.C. Ingham and R.B.
Warden, Chevron, SAE 870556, 1987.
29
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As of October 1, 1993, new federal requirements limit sulfur levels for on-
highway diesel fuel to 0.05 weight percent (see 40 CFR 80.29). The low-sulfur fuel
reduces the soluble organic fraction of PM emissions. Further reductions in sulfur
levels may increase the potential for PM reductions. Industry testing showed that
a sulfur reduction from 0.05 to 0.01 weight percent caused an eight percent decrease
in total PM emissions from an engine emitting at a level of 0.08 g/bhp-hr.18
However, reducing fuel sulfur levels below 0.05 weight percent would significantly
increase costs.
2. Increased Cetane Rating
The cetane rating! is a measure of the tendency of a fuel to autoigm'te.
Increasing the cetane rating of diesel fuel makes autoignition of the fuel in the
combustion cylinder easier. A fuel's cetane rating can be increased either with a fuel
additive that enhances autoignition, or through modified processing of diesel fuel at
the refinery. The federal requirement limiting the sulfur content of on-highway
diesel fuel also established a lower limit on the cetane index for the same fuel. On-
highway diesel fuel must have a minimum cetane index of 40 (or alternatively, a
maximum aromatic content of 35 volume percent).
Currently, the nationwide average cetane number is approximately 44 for on-
highway diesel fuels.19 A recent paper showed a NOX emission decrease of about.
four percent from increasing the fuel cetane number from 44 to 53 for an engine
emitting approximately 5 g/bhp-hr; the paper estimated the cost of increasing the
cetane number from 40 to 50 to be $0.30/gallon.20
3. Lower Aromatic Content
A typical gallon of diesel fuel consists of about 20 to 45 percent aromatic
hydrocarbons by volume. Decreasing the aromatic content of diesel fuel, which is
closely correlated with increased cetane rating, seems to have a greater potential for
decreasing both PM and NOX emissions than changing fuel composition in other ways.
To reduce emissions, the state of California has limited aromatic content in diesel
fuel to a maximum of 10 percent, effective October 1, 1993. A diesel fuel with a
higher aromatic content may be marketed in California only if it is shown to have
emission characteristics as good as, or better than, a standard test fuel with an
aromatic content of 10 percent.
18"Diesel Fuel Property Effects on Exhaust Emissions from a Heavy Duty Diesel Engine that Meets
1994 Emissions Requirements," Christopher I. McCarthy, Amoco, and Warren J. Slodowske, Navistar, et
al, SAE 922267, 1992.
19"National Fuel Survey, Diesel Fuel, Summer 1992," and "National Fuel Survey, Diesel Fuel, Winter
1992," Motor Vehicle Manufacturers Association, 1992.
20McCarthy, et al, SAE 922267, 1992.
30
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NO „ f PM quantlfie* ^e imPact °f reduced aromatic content in diesel fuel on
NO and PM emissions. Reducing the aromatic content from 40 to 20 percent
resulted m a 4 percent reduction in NOX (from 5 g/bhp-hr NOX) and a 7 percent
reduction in PM (from 0.085 g/bhp-hr).21 In another study, a 10 percent reduction
percent i™**™8 ^^ ^ ^^ *" ^omatic ^nt from 40 to lS
i Th\
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5. Oxygenate Additives
Adding a small percentage of certain oxygenated compounds to diesel fuel can
decrease PM emissions. An oxygenated compound adds oxygen to the fuel, making
it more likely that the hydrocarbon molecules will be able to react with oxygen. For
example, adding 5 percent of diethylene glycol dimethyl ether, which increases the
oxygen content of the fuel to 1.8 volume percent, was shown to reduce PM by 20
percent.27 However, adding oxygenates could lead to an increase in NOX and
aldehyde emissions.
6. Biodiesel
Derivatives of vegetable oils or animal fats can be mixed with diesel fuel for
combustion in a diesel engine. The fuel mixture, known as biodiesel, has received
much attention in Europe as a potential source of renewable fuel. Also, Congress
identified biodiesel as an alternative fuel in the National Energy Policy Act of 1992.
The biodiesel fuel mixture includes oxygen and, as with oxygenate additives, would
likely lead to a decrease in PM emissions while risking an increase in NOX emissions.
Costs for biodiesel are estimated to be quite high, but actual costs would be highly
dependent on the level of consumption and other variables.
7. Other Modifications to Diesel Fuel
Various additional modifications to diesel fuel are under consideration for
reducing diesel exhaust emissions. The costs and emission-reducing potential of these
changes may not be well understood or quantified. First, increasing the kinematic
viscosity of diesel fuel has been correlated with reduced PM emissions.28 Second,
addition of detergents or other chemicals may reduce the formation of deposits that
impair precise control of fuel flow; such deposits can lead to increased emissions.
Finally, cyanuric acid as a fuel additive may provide large reductions in
emissions. As described in Chapter 3, cyanuric acid can decompose NOX in the
exhaust stream through chemical aftertreatment. Using cyanuric acid as a fuel
additive, however, may be a more convenient way of introducing it into the
combustion path. However, there are several obstacles to use of cyanuric acid as a
fuel additive. For example, cyanuric acid alone does not mix with diesel fuel.
Additionally, the cyanuric acid would have to be converted to HCNO during the
expansion stroke to reduce NOX to nitrogen and water. Also, it must not create other
health-related problems. '
27"Toward Improved Diesel Fuel," J.E. Bennethum.and R.E. Winsor, SAE 912325, 1991.
^"Description of Diesel Emissions by Individual Fuel Properties," Noboru Miyamoto, Hokkaido
University, et al, SAE 922221, 1992.
32
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8. Lubricants
Strategies to reduce PM emissions have focused primarily on improving fuel
combustion, so an increasing proportion of PM comes from the lubricating oil.
Refiners are conducting research to formulate lubricating oils that form less PM. One
possibility is to replace the metal additives commonly used in lubricating oil with
nonmetallic compounds in order to reduce the noncombustible (ash) portion of the oil.
Using synthetic oils or partial synthetic oils may reduce formation of PM
emissions. Conventional formulations of lubricating oil evaporate over a wide range
of temperatures. The portion that evaporates at lower temperatures may evaporate
in the crankcase and diffuse into the combustion chamber, increasing PM emissions.
Synthetic oils can be formulated to evaporate over a narrow, high-temperature range.
Using such a synthetic lubricating oil would reduce the contribution of oil evaporation
to PM emissions. A partial synthetic oil, made by displacing the most volatile portion
of a conventional oil with the synthetic material, may yield equivalent results.
B. Natural Gas
Natural gas, predominantly composed of methane, is currently used as 'an
alternative fuel for some fleet vehicles. For example, the state of Texas recently
adopted a requirement that school buses be converted to operate on natural gas.
Delivery vehicles and dump trucks are some other common applications for natural-
gas conversions.
Manufacturers are developing two basic designs of natural gas engines. One
design requires air-fuel ratios very close to those needed for stoichiometric
combustionf, similar to gasoline-fueled engines. The other design maintains a lean
air-fuel mixture. Both designs are viable as heavy-duty engines, though a published
comparison against 20 different design criteria showed lean-burn to be superior
overall.29 The following discussion applies to both lean-burn and stoichiometric
designs, with exceptions noted as appropriate.
Consideration of safety impacts is an important part of a full evaluation of
alternative fuels for motor vehicles. Discussion of safety issues is, however, outside
the scope of this report. Previous EPA publications, as noted above, contain a
thorough treatment of the safety implications of using the various alternative fuels.
1. Emission Reductions
Natural gas has the potential for significant reductions in PM emissions.
Because the fuel is burned in stoichiometric or lean conditions, there is enough
29"A Lean Burn Turbocharged, Natural Gas Engine for the U.S. Medium Duty Automotive Market,
D.P. Clarke, Ricardo Consulting, and P.K Das, Navistar, SAE 921552, 1992.
33
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oxygen available to prevent formation of particulate matter. Unburned fuel would
instead typically result in formation of HC and CO, which can be controlled through
optimized engine designs or after-treatment.
Use of natural gas would reduce toxic emissions such as benzene and 1,3
butadiene, because there are only small amounts of complex hydrocarbons in the fuel
to form toxic products. For stoichiometric natural gas engines, EPA has estimated
a decrease in toxic air emissions of 99 percent, compared to gasoline-fueled engines.
For lean-burn engines, however, EPA expects emissions of formaldehyde to increase
from the levels produced by diesel-fueled engines, yielding an expected net decrease
in overall toxic air emissions in the range of 19 to 35 percent.30
Because methane has a higher hydrogen-to-carbon ratio than diesel fuel,
burning natural gas produces less of the global-warming gas carbon dioxide.
However, this benefit is partially or completely offset by the lower efficiency of
engines operating on natural gas. Increased methane emissions from natural gas
vehicles also counter the benefit of reduced carbon dioxide emissions. EPA has
estimated that, compared to diesel fuel, there is a 13 to 15 percent net increase in
global warming potential from using natural gas as a vehicle fuel.31 Utilizing
sources of methane that would otherwise be burned as waste or vented to the
atmosphere would mitigate this increase in global warming potential.
2. Fuel Availability and Cost
Natural gas is plentiful in underground reserves, both in the U.S. and in other
parts of the world. Also, the existing infrastructure for natural gas delivery
facilitates distribution of the fuel without large incremental costs. However,
compressing or liquefying the natural gas into a vehicle's storage tank requires
expensive equipment and significant energy consumption. Centrally fueled fleets are
therefore the most attractive opportunity for introducing natural gas vehicles. A
disadvantage of using the current natural gas distribution network without
modification for engine consumption is the high variability in quality of the natural
gas in the pipelines for use in motor vehicles. Manufacturers have to design engines
for the worst-case gas quality, foregoing some of the efficiency improvement that
would otherwise be available with natural gas.
Natural gas used as a motor fuel is estimated currently to cost $0.54 per
equivalent diesel gallon on an energy basis.32 This estimate does not include any
30"Analysis of the Economic and Environmental Effects of Compressed Natural Gas as a Vehicle Fuel,
Volume II: Heavy-Duty Vehicles," Office of Mobile Sources, U.S. EPA, April 1990, page 5-13.
31U.S. EPA, Natural Gas Report, April 1990, page 5-13.
32Letter from Jeffrey L. Clarke, American Gas Association, to EPA Air Docket, September 8, 1992
(Docket A-91-28, item IV-D-57).
34
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taxes, nor does it take into account the lower thermal efficiencies that natural *as
engines general y have relative to their diesel-fueled counterparts. The currfnt
exemption of natural gas from most transportation taxes clearty provides a bifcos
advantage for natural gas. EPA has projected for the year 2000 that the cfst of
m
a e cst of
almost am±^ni? ^ effef °f ^^a thermal e^encies, could increase to
almost $2.00 per gallon diesel equivalent.33 This EPA estimate aqsiimp« nnrr'
projected cost of natural gas for delivery to a service staUon or^et £S£ a
6 *
to t lowerhe6™ ? M "" gall°n>' «* * 33 " penalty
tor the lower thermal efficiency for current-technology natural gas enrines
Sxl ±"Sffi 6 C°St °f de'iVering natUral gaS' «»tinued exemption to, nlghway
taxes and efficiency improvements to dedicated natural gas engines wouW cause
comparison' DOE
3- Required Engine and Vehicle Modifications
Manufacturers can design new vehicles to run on natural gas, but existing
diese -fueled vehicles also can be converted for use with natural gfs. Addition £?
vehicles either converted or initially designed to use natural gas can operate
exclusively on natural gas (dedicated vehicles), or they can operate fither on n£SS
gas or diesel fuel (dual-fuel or bi-fuel vehicles). Converting a car to run on natura
gas is estimated to cost as little as $3,000.35 Converting an urban bus to run on
natural gas has been estimated to cost up to $24,000.36 Vehicles designed initially
to run only on natural gas should have costs comparable to diesel-fueled vehicles and
may be optimized m some ways to improve fuel efficiency and emission control
compared to diesel fuel.37 Dual-fueled vehicles will not achieve the full emissions
benefit available from natural gas, because their engines cannot be fully optimized
for burning natural gas. The major benefit of dual-fueled vehicles is aiding the
transition to fleets of dedicated natural gas vehicles.
Natural gas is typically stored on a vehicle as a compressed gas. As a gas the
fuel takes up more space than conventional liquid fuels. Furthermore, to maintain
high pressures the fuel tanks are very heavy-increasing the weight of the fuel
storage system by up to five times compared to diesel fuel. The increased fuel storage
33"Esti mated Diesel-Equivalent Pump Price of Compressed Natural Gas," EPA memorandum from
Susan Stefanek to Joanne Goldhand, May 21, 1993.
34Annual Energy Outlook, Department of Energy, 1993.
37
""^ to EPA ** Docket, Sept™b.r 8, !992
U.S. EPA, Natural Gas Report, April 1990, page 4-20.
35
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weight decreases fuel economy and can decrease a vehicle's load-carrying capacity.
The increased weight and low energy density of compressed natural gas forces a
design trade-off with vehicle range. Lighter (but more expensive) materials, such as
composites and fiber-wrapped steel and aluminum, are under development. These
materials can potentially reduce the total weight of fuel storage by 25 to 40 percent
compared to current steel tanks.38
Another option for reducing the volume and mass of fuel storage is the use of
liquefied natural gas. Liquefying greatly reduces the fuel volume and provides an
opportunity to remove the impurities found in pipeline-quality natural gas. For some
heavy-duty applications, reducing storage volume is a critical advantage that
increases the viability of natural gas as a fuel. Instead of being stored in gaseous
form under high pressure, liquefied natural gas must be stored cryogenically, which
is somewhat more expensive. Also, if the vehicle is not driven for long periods, it
must vent small amounts of the fuel over time as the liquefied natural gas vaporizes
in the fuel tank. The energy required to convert the natural gas to a liquid increases
the cost and decreases the overall energy efficiency of using liquefied natural gas.
Refueling would involve more expensive equipment, but could be done more quickly
than with compressed natural gas.
Substantial development opportunities exist for optimizing natural gas engines
for controlling emissions. These include port fuel injection, electronic feedback control
of the air-fuel mixture, combustion chamber shape, exhaust gas recirculationf, pilot
diesel injectionf, and the use of two spark plugs per cylinder to reduce fuel burn time.
Natural gas engines may require use of aftertreatment devices to adequately
control exhaust emission levels. Stoichiometric engines may require three-way
catalystsf to control NOX, HC, and CO. Lean-burn engines also may need to oxidize
formaldehyde emissions with a catalyst.
C. Liquefied Petroleum Gas
Liquefied petroleum gas (LPG) is composed primarily of propane. Automotive
grade LPG (as specified in ASTM D 1835) has a maximum of 5 percent propylene and
no more than 2.5 percent butane and heavier hydrocarbons. Worldwide there are
many vehicles operating on LPG. Although LPG is stored as a liquid under moderate
pressure, it evaporates before entering the combustion chamber. LPG therefore
shares many of the emission advantages of natural gas.
A gallon of diesel fuel contains as much energy as 1.6 gallons of LPG, so LPG
requires more storage space than diesel fuel. However, because of the different
physical densities, the amount of fuel required to supply an equivalent amount of
energy involves approximately the same weight for either fuel.
38U.S. EPA, Natural Gas Report, April 1990, page 3-10.
36
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uPP,y of LPC from refineries
D. Alcohols
similar. The two fuels are therefore treaed to^ S*^™1 3nd methano1
differences noted as appropriate ™£%£i ^°5f]her in the ^wing discussion, with
but are outside the scope of^hTs report ^ 1SSUeS °f Safety are ™P«rtant,
!• Emission Reduction^
PM .
and inced formoofpcaerAS T C°mplete combusti"n of fuel
in formation of HC and TO wn,^ be^rn'7^fiaelruldinSteadresult
designs or aftertreatment AddwT^?on«l^T^ ^Ugk "P^^ engine
various chemical additiveT) m^y "cet^'TTeS ^ "
-provements; these additives ^m^SS H°C,
greater than from diesel fuel. Because £lhof fuJ, »f t ^ n6™33'0118 may be
gasoline, however, there is little information avSlw^ ^"^ """P8™! ""*
alcohol and diesel fuels. uuormatlon available comparing toxic emissions from
2- Fuel Availability and f!naf
"Emissions of Greenho
Argonnne National Laboratory. De^^^f Z^^
"LiNiVbbD/TM-22, November 1992.
37
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reduction in the unit cost. EPA has projected the costs for using methanol to be
comparable to the costs for using diesel fuel.40
Ethanol is derived from grain crops and is primarily a domestic product.
Cellulose and municipal waste also can be used as feedstocks. Fuel costs are
significantly higher than with diesel fuel; however, with the current tax credit of
approximately $0.54 per gallon of ethanol mixed with gasoline (at ten percent
ethanol), "gasohol" is cost-competitive as a highway fuel oxygenate. Current law does
not extend the tax credit to neat ethanol used as fuel.
Ethanol and methanol both currently have only a limited infrastructure for
distribution as a fuel product.
3. Required Engine and Vehicle Modifications
Manufacturers can design new vehicles to run on an alcohol fuel, but existing
diesel-fueled vehicles also can be converted for use with an alcohol fuel. Additionally,
vehicles either converted or initially designed to use an alcohol fuel can operate
exclusively on the alcohol fuel (dedicated vehicles), or they can operate either on the
alcohol fuel or diesel fuel (dual-fuel or bi-fuel vehicles). Vehicles designed initially
to run on an alcohol fuel may have somewhat lower costs than conversion vehicles,
and may be optimized in some ways to improve fuel efficiency and emission control
compared to diesel fuel. The following discussion evaluates some of the hardware*
changes required to build or convert a vehicle for operating on an alcohol fuel.
Starting a cold alcohol-fueled engine is more difficult than starting a cold
diesel-fueled engine. To improve cold-starting, alcohol-fueled engines normally
require an ignition system, typically consisting of a spark plug or a glow plugf.
Alternatively, a fuel additive could improve the ability of the fuel to ignite. For
example, ethanol and methanol are commonly mixed with 10 or 15 percent gasoline
for improved ignition in spark-ignition engines. Modifications to the ignition system
would involve hardware costs; fuel additives may prevent the need for hardware
changes, but could increase operating costs.
Ethanol and, to a greater extent, methanol corrode many metals and cause
deterioration of plastics and some other polymers. These effects can be overcome
with proper material selection, including, for example, use of some stainless steel and
teflon materials in the fuel system. These materials increase the cost of the fuel
system components.
The alcohol fuels also react with some conventional engine oils, so the oil loses
its lubricating properties. Modified engine oil is available for alcohol-fueled engines
^"Analysis of the Economic and Environmental Effects of Methanol as an Automotive Fuel," Office of
Mobile Sources, U.S. EPA, September 1989.
38
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at little extra cost; however, if an operator would use the wrong oil, engine lubrication
could deteriorate to the point of engine failure.
In contrast to diesel fuel, alcohol fuels have very poor lubricating properties.
Pure alcohol fuels would therefore require redesigned pumps, injectors, and plumbing
to avoid excessive wear. A lubricant could instead be added to the fuel; such a fuel
additive would increase operating costs and could decrease emission benefits.
Alcohol-fueled engines may require an oxidation catalystf to adequately control
aldehyde, CO, and HC emissions. As discussed above, oxidation catalysts add a
substantial cost.
Because of lower energy density, alcohol fuels require larger fuel tanks and a
higher rate of fuel flow and fuel injection, resulting in an increased hardware cost.
A gallon of diesel fuel contains as much energy as 2.3 and 1.7 gallons of methanol
and ethanol, respectively. A smaller cooling system in a dedicated, optimized
methanol- or ethanol-fueled vehicle could partially offset the increased weight for fuel
storage.
E. Electric Vehicles
Electric trolleys or streetcars are being used successfully for mass transit in
cities; the low speeds, local operation, and reduced acceleration requirements for'
these applications are well suited to using electric power. For trucks involved in
long-distance travel, however, limited access to quick recharging, high battery costs,
and high power and acceleration requirements currently prevent widespread use of
electric vehicles. Further developments, primarily in battery design, fuel cells, and
quick-charge infrastructure may improve the prospects for electric trucks in the
future.
Electric vehicles approach the zero-emission level. While there is no
combustion occurring on the vehicle, the generation of the electricity produces some
pollution (as with the production of any motor vehicle fuel). However, on an energy
equivalent basis, fossil-fueled power plants have lower emissions than highway
vehicles. Nonfossil-fueled power plants emit at even lower levels. The indirect
emissions attributable to electric vehicles vary with the source of electric power
production prevalent in each part of the country.
The infrastructure for "fueling" electric vehicles is well established. With an
inexpensive apparatus, a standard outlet could charge a battery overnight. High-
voltage charging technologies, with charge times approaching the refueling times of
conventional vehicles, are under development, but are unlikely to be widely available
in the next few years.
39
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Appendix A
Glossary
In addition to the terms used in the report, this glossary contains other terms
commonly found in the literature pertaining to motor Vehicle Engines and controTof
their emisS10ns. This glossary is, however, not intended to include allsuch terms
Aftercooler. An intake air cooler with the heat exchanger operating after
compression in the turbocharger or supercharger. peraung alter
Compare with: Intercooler.
Aftertreatment. Any technology in which polluting compounds released from a
combustion process are converted to less harmful products in the exhaust
stream. Aftertreatment devices for internal-combustion engines Se Wca%
either catalytic converters or participate traps. 'ypicauy
Autoignition. The ignition of fuel without the application of an auxiliary ignition
source, such as an electrical spark or glow plug. *B'"«on
Synonymous with: Self-ignition.
Boost pressure. A measure of the level to which intake air pressure is elevated"
above atmospheric pressure. Commonly expressed in terms of inches of mercury
( Hg), inches of water ( H2O), pounds per square inch (psi), or kilopascals (kPa).
Bottom dead center. Denotes the position of the piston when it is at its lowest
point in the cylinder. At this point, the piston reverses its direction of travel and
there is momentarily no relative motion between the piston and the cylinder
along the axis of the cylinder.
Brake engine power. The power that the engine delivers, or can deliver at its
power output point (e.g., at the flywheel). Usually expressed in terms of
horsepower or kilowatts.
Brake-specific emissions. The mass of a pollutant emitted per brake horsepower-
hour. The most commonly used units of mass are grams and pounds.
Brake-specific fuel consumption (bsfc). The measure of the efficiency with which
the energy contained in the fuel is converted to useful work in the engine
Commonly expressed either in terms of pounds of fuel consumed per brake
horsepower-hour or in terms of grams of fuel consumed per brake kilowatt-hour.
Catalyst. A substance that accelerates a chemical reaction, but itself undergoes no
permanent chemical change. For motor vehicle emission control applications,
A-l
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catalysts are classified as oxidation catalysts, reduction catalysts, or three-
way catalysts.
Synonymous with: Catalytic converter on occasion.
Catalytic converter. An aftertreatment device in which the rates of selected
chemical reactions are accelerated. The catalytic converter is an assembly
forming part of the engine exhaust system, including such major components as
the structural shell, the substrate, and the catalyst material.
Synonymous with: Catalyst on occasion.
Cetane rating. A measure of the tendency of a fuel to autoignite when injected
into air heated by compression. The cetane rating is a characteristic of fuels for
diesel engines. A fuel with a high numeric cetane rating autoignites at a lower
temperature than a fuel with a low numeric cetane rating. The cetane rating is
in the form of either a cetane number, directly measured with a diesel engine, or
a cetane index, calculated from various physical properties of the fuel. The
cetane number and cetane index for a fuel typically correlate very closely.
Compare with: Octane rating.
Charge air.
Synonymous with: Intake air.
Charge air cooler.
Synonymous with: Intake air cooler.
Combustion chamber. The space formed between the cylinder head, the cylinder
wall or cylinder liner, and the top of the piston, including all cavities or pockets
in the cylinder head and the top of the piston. Combustion of the fuel occurs in
the combustion chamber.
Compression-ignition engine.
Synonymous with: Diesel engine.
Compression ratio. The numerical value resulting from dividing the cylinder
volume at bottom dead center by the cylinder volume at top dead center.
Compressor section. The components of a supercharger or turbocharger that
compress the intake air.
Crevice volume. Generally, all volumes in the combustion chamber through
which the flame does not progress. Crevice volume includes valve, fuel injector
and spark plug recesses, and the annular volume between the piston and
cylinder wall, above the topmost piston ring.
A-2
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Diesel engine. An internal-combustion engine in which the air in the cylinders
is heated by compression to the autoignition temperature of the fuel. The fuel
is injected into the combustion chamber near the end of the compression
stroke.
Synonymous with: Compression-ignition engine.
Direct injection (DI) engine. A diesel engine in which the fuel is injected into
an open combustion chamber.
Compare with: Indirect injection (IDI) engine.
Distributor pump. An injection pump in which fuel delivery is metered and
directed to each engine cylinder by a distribution device. One pumping element
serves all of the engine's cylinders.
Divided combustion chamber. A combustion chamber in which the combustion
space is divided into two or more distinct compartments, between which are
restrictions small enough to cause considerable pressure differences to exist
between the compartments during the compression and combustion processes.
Electronic control. An electric or electronic system that directs the operation of
one or more control components.
Exhaust gas recirculation (EGR). A system that returns a portion of the exhaust
gases to the combustion chamber. With EGR the temperature of the gases in
the chamber is lowered, causing a reduction in the formation of oxides of
nitrogen.
Compare with: Internal exhaust gas recirculation
Four-stroke engine. A reciprocating internal-combustion engine in which the
functions of (1) the introduction of new charges of air and fuel, (2) compression
of the air (or air and fuel), (3) combustion, (4) extraction of work, and (5)
expulsion of the products of combustion are performed in each cylinder during
four passes of the piston between its extremes of travel in the cylinder. The four
passes of the piston correspond to two revolutions of the crankshaft.
Fuel injector. The component that introduces pressurized and metered fuel, in the
form of a mist or spray, into the engine. In the case of a diesel engine the fuel
is introduced into the combustion chamber.
Glow plug. An electrically heated component located in the combustion chamber
to provide a hot surface for igniting the fuel. Glow plugs are used either to allow
or enhance starting of diesel engines at low ambient temperatures.
Gross vehicle weight rating. See Appendix B.
A-3
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Heat exchanger. A component used to transfer heat between two or more
physically separated fluid flow streams of dissimilar temperature. A heat
exchanger physically separates two or more fluids, while offering little resistance
to the flow of thermal energy between the fluids.
Indirect injection (IDI) engine. A diesel engine in which the fuel is injected into
a prechamber.
Compare with: Direct injection (DI) engine.
Injection rate shaping. A strategy in which the rate of fuel injection is deliberately
varied over the duration of the injection period.
Injection pump. A device that meters the fuel and delivers it under pressure to the
fuel injectors.
Injection timing. The point in time during a diesel engine operating cycle when
fuel is initially injected into the combustion chamber. Usually expressed as
a number of crankshaft rotational degrees that the piston is away from top
dead center on the compression stroke.
Injector.
Synonymous with: Fuel injector.
In-line pump. An injection pump with two or more pumping elements arranged
in line, each pumping element serving one engine cylinder only. A pump that
has the elements arranged in line and in more than one bank (e.g., in a "V") is
a special case of an in-line pump.
Intake air. The air moved into the engine to serve as the source of oxygen for the
combustion of fuel.
Synonymous with: Charge air.
Intake air cooler. A heat exchanger in which the intake air, heated as a result
of compression in the supercharger or turbocharger, is cooled prior to
entering the cylinders of the engine. An intake air cooler can be either an
aftercooler or an intercooler.
Synonymous with: Charge air cooler.
Intercooler. An intake air cooler with the heat exchanger between two stages
of a multistage compressor.
Compare with: Aftercooler.
Internal-combustion engine. An engine in which useful work is extracted directly
from the heat energy released during the combustion process; the products of
combustion are used as the working fluid (typically to drive a piston or turbine).
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In contrast, powerplants for generating electricity use a secondary fluid such as
steam to extract useful work from a combustion process.
Internal exhaust gas recirculation. A form of exhaust gas recirculation in
which a small portion of the exhaust gases is deliberately retained in, or drawn
back into, the cylinder by appropriate phasing of the points in the cycle at which
the intake valve is opened and the exhaust valve is closed.
Lugging. A mode of engine operation in which the load on the engine is greater
than the available engine power. Engine rotational speed is forced to decrease
during lugging.
Octane rating. A measure of the properties of a fuel, for use in spark-ignition
engines, to prevent autoignition when mixed with air and heated by
compression, both before ignition by the spark and during burning of the mixture
following ignition. A fuel with a higher numeric octane rating than another fuel
can prevent autoignition at higher temperatures and pressures.
Otto-cycle engine.
Synonymous with: Spark-ignition engine
Oxidation catalyst. A catalyst (catalytic converter) that promotes the oxidation
of hydrocarbons, carbon monoxide, and some types of particulate matter to
water and carbon dioxide.
Open combustion chamber. A combustion chamber in which the combustion
space incorporates no restrictions that are sufficiently small to cause large
differences in pressure between different parts of the combustion chamber during
the compression and combustion processes.
Particulate matter. A combination of solid and liquid materials present in exhaust
gas in the form of minute particles. This material is predominantly carbon or
hydrocarbon, derived from the incomplete combustion of the fuel. Engine
lubricating oil and trace constituents in the fuel (e.g., sulfur) are also found in
particulate matter from diesel engine exhaust.
Particulate trap. A filtration assembly for after-treatment of exhaust from diesel
engines. A particulate trap forms part of the engine exhaust system and
removes particulate matter. A particulate trap assembly includes such major
components as a structural shell, the filter medium, a means for oxidizing
collected particulate matter, and a control system. Various methods of
particulate trap regeneration are used to oxidize the filtered material.
Synonymous with: Trap.
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Particulate trap regeneration. The process of oxidizing particulate material
that has been filtered from the exhaust stream in a particulate trap. The
filtered material can be oxidized by heating the trap to the ignition temperature
of the filtered materials, or by lowering the ignition temperature of the filtered
material to the normal trap operating temperature with catalyst materials.
Heating the trap to the ignition temperature of the filtered material can be
achieved through combustion of diesel fuel in the trap or by an electric heater.
Air must be provided to oxidize the filtered material.
Pilot diesel injection (ignition). A method for achieving ignition in an engine,
without the use of an electrical source of ignition, of a fuel which does not readily
autoignite (e.g., natural gas or an alcohol). After air is compressed and heated
to the autoignition temperature of diesel fuel, a small quantity of diesel fuel is
injected into the combustion chamber. The diesel fuel then autoignites and
initiates burning of the rest of the fuel.
Prechamber. That portion of a divided combustion chamber in which combustion
is initiated. The prechamber is usually separated from the remaining portion of
the combustion chamber by a restriction.
Puff limiter. A component that limits fuel input during engine acceleration. Puff
limiters are commonly used on diesel engines to control smoke emissions
during spool-up of the turbocharger.
Pulse-tuning. A design that takes advantage of pressure waves in an engine's
intake system, exhaust system, or both to increase the flow of air through the
engine. The pressure waves, resulting from the opening and closing of intake
and exhaust valves, allow more effective movement of air over a limited range
of engine speeds.
Reduction catalyst. A catalyst (catalytic converter) that promotes the chemical
reduction of oxides of nitrogen to elemental oxygen and nitrogen.
Retarded injection timing. A timing for injection of fuel into the combustion
chamber that is delayed relative to the timing that provides optimum fuel
efficiency. Retarded injection timing is commonly used on diesel engines as a
means to control emissions of oxides of nitrogen. The amount of timing retard
is seldom identified. When identified, timing retard is expressed as a number
of degrees of rotation of the crankshaft.
Roots blower. A positive-displacement supercharger employing two parallel
counter-rotating rotors whose meshing lobes sweep air through the blower.
Scavenge. The process of purging the products of combustion from the cylinder in
preparation for the next cycle.
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Self-ignition.
Synonymous with: Autoignition
Soluble organic fraction. That portion of participate matter that can be
dissolved in an organic solvent.
Spark-ignition engine. An internal-combustion engine in which the fuel is
ignited by a spark.
Synonymous with: Otto-cycle engine
Spool-up. The process of increasing the rotational speed of the rotating components
of a turbocharger. Spool-up delays a desired increase in the air throughput of
the turboeharger. .
Stoichiometric combustion. Combustion of a mass of fuel in the presence of the
chemically ideal mass of air to achieve complete oxidation. The proportion of fuel
to air can be expressed either as the mass ratio of the air to fuel, or as the ratio
of fuel to air.
Supercharger. An air pump used to supply charge air at pressures above
atmospheric, typically mechanically driven by the engine.
Three-way catalyst. A catalyst (catalytic converter) that promotes the oxidation-
of hydrocarbons and carbon monoxide and promotes the reduction of oxides of
nitrogen.
Top dead center. Denotes the position of the piston at its highest point in the
cylinder. At this point, the piston reverses its direction of travel and there is
momentarily no relative motion between the piston and the cylinder along the
axis of the cylinder.
Trap.
Synonymous with: Participate Trap.
Turbocharger. A centrifugal air pump driven by engine exhaust gases and used to
supply intake air at pressures above atmospheric.
Turbocharger lag. The time required for a turbocharger to spool-up. Because
of turbocharger lag, the flow of intake air may be momentarily insufficient to
provide optimum engine performance and emission control during engine
acceleration.
Synonymous with: Turbocharger response time.
Turbocharger response time.
Synonymous with: Turbocharger lag.
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Two-stroke engine. A reciprocating internal-combustion engine in which the
functions of (1) the introduction of new charges of air and fuel, (2) compression
of the air (or air and fuel), (3) combustion, (4) extraction of work, and (5)
expulsion of the products of combustion are performed in each cylinder during
two passes of the piston between its extremes of travel in the cylinder. The two
passes of the piston between its extremes of travel in the cylinder correspond to
one revolution of the crankshaft.
Unit fuel injector. An assembly that receives fuel under supply pressure and is
then actuated by an engine mechanism to pressurize, meter, and inject the
charge of fuel into the combustion chamber at the proper injection timing.
Waste gate. An assembly that allows total exhaust gas flow to reach the
turbocharger when boost pressure is below a specified value. A portion of
the exhaust gas flow is diverted around the turbocharger once the specified boost
pressure has been achieved.
Water injection. A system that introduces water into the intake air and
subsequently into the combustion chamber. With water injection, the peak
temperature of the gases in the combustion chamber is lowered, causing a
reduction in the formation of oxides of nitrogen.
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Appendix B
Definitions of Vehicle Classes
f P ?e ^"f™« definitiems are from various sections of title 40, part 86 of the Code
of Federal Regulations. These definitions are specific to the Federal Motor Chicle
Emission Control Program. California's definitions may be slightly different.
Gross Vehicle Weight Rating (GVWR) means the value specified by the
manufacturer as the maximum design loaded weight of a single vehicle.
Heavy-duty engine means any engine which the engine manufacturer could
reasonably expect to be used for motive power in a heavy-duty vehicle.
Heavy-
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Medium heavy-duty engines may be sleeved or non-sleeved and may be
designed for rebuild. Rated horsepower generally ranges from 170 to 250.
Vehicle body types in this group would typically include school buses,
tandem axle straight trucks, city tractors, and a variety of special purpose
vehicles such as small dump trucks, and trash compactor trucks. Typical
applications would include commercial short haul and intra-city delivery and
pickup. Engines in this group are normally used in vehicles whose GVWR
varies from 19,500 to 33,000 Ibs.
Heavy heavy-duty engines are sleeved and designed for multiple rebuilds.
Their rated horsepower generally exceeds 250. Vehicles in this group are
normally tractors, trucks, and buses used in inter-city, long-haul
applications. These vehicles normally exceed 33,000 Ibs. GVWR.
Urban Bus means a passenger-carrying vehicle powered by a heavy heavy-duty
diesel engine, or of a type normally powered by a heavy heavy-duty diesel engine,
with a load capacity of fifteen or more passengers and intended primarily for
intra-city operation, i.e., within the confines of a city or greater metropolitan
area. Urban bus operation is characterized by short rides and frequent stops.
To facilitate this type of operation, more than one set of quick-operating entrance
and exit doors would normally be installed. Since fares are usually paid in cash
or tokens, rather than purchased in advance in the form of tickets, urban buses
would normally have equipment installed for collection of fares. Urban buses are
also typically characterized by the absence of equipment and facilities for long-
distance travel, e.g., rest rooms, large luggage compartments, and facilities for
stowing carry-on luggage. The useful life for urban buses is the same as the
useful life for other heavy heavy-duty diesel engines.
Vehicle curb weight means the actual or the manufacturer's estimated weight of
the vehicle in operational status with all standard equipment, weight of fuel at
nominal tank capacity, and the weight of optional equipment computed in
accordance with 40 CFR 86.082-24.
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Acronym
ASTM
CO
C02
DI
EGR
EPA
FR
g/bhp-hr
GVWR
HC
IDI
Ibs.
LPG
NG
NMHC
NOX
NYCTA
PM
PM-10
SOX
SOF
Appendix C
Nomenclature
Description
American Society for Testing and
Materials
carbon monoxide
carbon dioxide
direct injection
exhaust gas recirculation
Environmental Protection Agency
Federal Register
grams per brake horsepower-hour
gross vehicle weight rating
hydrocarbon
indirect injection
pounds
liquefied petroleum gas
natural gas
nonmethane hydrocarbon
oxides of nitrogen
New York City Transit Authority
particulate matter
particulate matter less than 10
microns in size
oxides of sulfur
soluble organic fraction ,-
The temperature at which 90
percent of the fuel is evaporated
during the distillation procedure
specified in ASTM D 86.
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