Regulatory Support Document
An Updated Assessment of
the Feasibility of Trap-Oxidizers
June 1983
Prepared By
Jeff Alson
Richard Wilcox
Standards Development and Support Branch
Emission Control Technology Division
Office of Mobile Source Air Pollution Control
Office of Air, Noise and Radiation
U. S. Environmental Protection Agency
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Table of Contents
Page
I. Introduction ......... 1
II. Highlights of the Feasibility Study ... 2
A. Status of Development 2
B. Leadtime 3
III. Reconsideration of the Feasibility
Study's Conclusions 5
A. Efficiency, Backpressure, and
Durability Performance 5
B. Regeneration Mechanisms 7
C. Regeneration Control Systems 14
1. Positive Regeneration 14
2. Self Regeneration 17
D. Overall Evaluation of Status of
Trap-Oxidizer Development 17
E. Prototype Development Leadtime 18
F. Production Development Leadtime '20
IV. Conclusion 21
A. Applicable Model Year 21
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I. Introduction
On March 5, 1980, EPA published a Final Rule which, for
the first time, established particulate emission standards for
diesel-powered light-duty vehicles and light-duty trucks (45 FR
14496). That rulemaking established standards of 0.6 g/mi for
all 1982 through 1984 diesel passenger cars and light trucks.
The 1985 and later model year standards were set at 0.2 g/mi
for passenger cars and 0.26 g/mi for light-duty trucks. These
latter standards were clearly "technology-forcing," (i.e., EPA
could not show at the time of promulgation that the standards
were feasible but projected that the standards could be met by
1985). The Agency's technical projection was based on the
expectation that trap-oxidizers would be reasonably perfected
by the 1985 model year.
The feasibility of trap-oxidizers has been a controversial
issue. Since compliance by the entire fleet with the 1985
standards is dependent upon the successful application of
trap-oxidizers, the Agency has constantly monitored the
progress of trap-oxidizer development. EPA published a Request
for Information on June 17, 1981 which announced that the
Agency would be preparing a study of the status of
trap-oxidizer development (45 FR 31677). In March 1982, EPA
published the "Trap-Oxidizer Feasibility Study" (hereafter
referred to as the Feasibility Study) which was based on the
Agency's own work with trap-oxidizers as well as written
comments from interested parties. Comments generally included
information available as of the end of 1981. This study formed
the basis of EPA's Notice of Proposed Rulemaking (NPRM) on
December 1, 1982 which proposed to delay implementation of the
1985 particulate standards for two years (47 FR 54250).
Many comments on the current status of trap-oxidizer
development were received February 17, 1983, which was . the
closing date for comments on the NPRM.* These comments
generally included . information on manufacturer development
programs through the end of 1982. In addition, a study
entitled "Trap-Oxidizer Technology for Light-Duty Diesel
Vehicles: Feasibility, Costs, and Present Status," (hereafter
A few vehicle manufacturers claimed that part or all of
their technical comments to this rulemaking contained
proprietary information and requested that EPA treat such
sections as confidential material. The Agency has
reviewed the comments claimed to be confidential, arid as a
general rule the information presented therein is
representative of material already in the public domain as
well as of comments to this rulemaking which were not
claimed to be confidential.
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referred to as the Technology Study) which was completed on
March 20, 1983 under contract to EPA, involves a recent
independent analysis of trap-oxidizer status.* These sources
of information make it possible for EPA to assess the state of
trap-oxidizer development as of the end of 1982, a full year
later than the assessment possible in the Feasibility Study.
This study is the Agency's summary of the current status of
trap-oxidizers and its projections of future developments. As
such, this study serves as the technical support document for
EPA's decision to delay the particulate standards for two years.
II. Highlights of the Feasibility Study
A. Status of Development
The Feasibility Study found that research into
trap-oxidizers had advanced significantly since the 1985
particulate standards were promulgated. Manufacturers had
identified two basic trap-oxidizer designs and were
concentrating their research efforts on perfecting these
devices. One design utilized a porous, ceramic honeycomb
monolith (hereafter the ceramic monolith trap) similar to the
ceramic substrate used in catalytic converters on many
gasoline-fueled vehicles. The other design utilized an
alumina-coated wire mesh (hereafter the wire mesh trap) . This
filtering medium was often coated with a catalytic material.
The presence of a catalyst can aid regeneration by lowering the
pre-trap exhaust temperature which is necessary to initiate the
particulate burning process. The catalyst functions . to
"light-off" or initiate oxidation of the hydrocarbon (HC)
portion of the trapped particulate or HC and carbon monoxide
(CO) in the exhaust, thereby generating additional heat and
higher temperatures within the trap-oxidizer. The importance
of this will become clearer later in this section.
The Feasibility Study found that these trap designs were
very satisfactory. Trapping efficiency and backpressure were
judged to be acceptable for both new traps and traps with up to
50,000 miles of use that had been regenerated 50-100 times.
Although many cases of trap failure had been reported,
durability problems were almost always due to excessively high
temperatures inside the trap during regeneration. As stated
above, traps had been tested up to 50,000 miles without
problems when regeneration was properly controlled. Therefore,
while trap materials and designs appeared to be satisfactory,
the key to durability was properly controlling regeneration.
This study was prepared under EPA Contract No. 68-01-6543
by Energy and Resource Consultants, Inc., and is available
for inspection in EPA Docket No. A-82-32.
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The first step in properly controlling this process is an
understanding of the conditions which are necessary to initiate
and maintain regeneration. In these areas, the Feasibiltiy
Study found that manufacturers had made significant progress in
identifying the relevant parameters. For non-catalyzed traps,
temperatures in the range of 500°-650°C and oxygen levels of
3-4 percent are required for 2-10 minutes (low temperatures
being associated with the longer times and vice versa) . The
presence of a catalyst in the trap may be able to reduce the
requisite temperature to as low as 310°-350°C.
Because the temperatures required for regeneration do not
always occur during normal vehicle operation, manufacturers
were developing several different systems to initiate and
maintain regeneration. To provide the higher requisite
temperatures associated with non-catalyzed traps, auxiliary
fuel burners or electrical heating appeared to be quite
workable, but would be relatively expensive and, in the latter
case, would require extensive vehicle modifications. For
catalyzed traps, simpler and less expensive systems could
potentially be used because the extent to which the trap
temperature must be raised is significantly less. Two
regeneration systems being developed for this application were
intake air throttling and exhaust stroke fuel injection.
The principal task that remained to complete the
development of a completely functional trap-oxidizer was to
integrate the trap designs and regeneration devices, identified
above, into an automated system. Because of the complexities
involved in determining trap loading, initiating particulate
combustion under the proper oxygen and temperature conditions,
and maintaining combustion without overheating the trap
materials, some form of electronic, control seemed necessary.
These controls would be somewhat similar to those used on most
gasoline-fueled automobiles. Automating the regeneration
process was considered to be technically feasible given more
development time.
B. Leadtime
The Feasibility Study divided the remaining development
work on these systems into two basic categories: prototype and
production. Prototype development culminates in a working
model of the complete trap-oxidizer package, including a fully
automated system for regeneration of the trap under a range of
vehicle operating conditions. As discussed above, nearly all
af the critical parts and design parameters of the system were
either known or readily available. The remaining task in this
area was to integrate the trap designs and regeneration devices
into an automated package. Based on the technical difficulty
of the remaining work, the Feasibility Study estimated that
prototype development should be completed by about January
1983, but noted the inherent uncertainties in such projections.
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Production development follows the completion of prototype
development. This final phase of development includes
assurance testing to refine the system further under a variety
of real-life conditions, modifying the vehicle or engines to
accept the trap system, acquiring production facilities and
tooling, and completing emission certification testing.
Development of leadtime estimates for these activities is
subject to much less uncertainty than that for prototype
development because no technological advances must be made.
The Feasibility Study found that many of the
production-oriented activities could be accomplished
concurrently. The critical path elements were identified as
assurance testing and the acquisition of production facilities
and tooling.
Two phases of assurance testing were believed necessary.
Each phase would involve 50,000 miles of driving and last 3.5-5
months. Between these driving tests would be a 3-4 month
period of data analysis. However, only the first driving phase
and subsequent data analysis (7-9 months) were considered
critical path elements, because manufacturers could begin to
make initial production commitments after these tasks were
complete.
The leadtime necessary for production facilities and
tooling was estimated to be 18-24 months. This estimate was
corroborated by projections that were previously acquired from
heavy-duty engine manufacturers regarding production tooling
and leadtime for a similar technology (i.e., catalysts for
gasoline-fueled vehicles).
The total production development leadtime, then, consisted
of 25-33 months from the completion of prototype development,
which was projected to occur about January 1983. This schedule
would result in trap-oxidizers being ready to be placed on
production vehicles sometime between February 1985 and October
1985. Since vehicle production for a given model year
historically begins around August of the previous calendar
year, this led to two important conclusions. First, a delay of
the new particulate standards beyond the scheduled 1985 model
year was obviously necessary. Second, it was possible traps
would be available for the 1986 model year (development
completed before August 1985), but is was also possible that
they would not be available on a nationwide basis until the
1987 model year (development completed after August 1985). The
serious consequences of selecting an infeasible date led EPA to
follow a conservative approach and, therefore, propose a delay
of the particulate standards until the 1987 model year.
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III. Reconsideration of the Conclusions of the Feasibility Study
A. Efficiency, Backpressure, and Durability Performance
The ceramic monolith honeycomb substrate appears to be the
leading trap-oxidizer filter design. Every major light-duty
diesel manufacturer marketing in the U.S. has conducted
extensive test development programs with ceramic monoliths,
supplied by. either Corning Glass Works or NGK Insulators, Ltd.,
and the majority of recent test data supplied to EPA involved
ceramic monolith traps. Recent submissions lend strong support
to the conclusions of the Feasibility Study regarding
efficiency, backpressure, and durability characteristics of
ceramic monolith traps.
The results of two durability programs demonstrate the
basic integrity of ceramic monolith trap designs under
controlled regenerations. Daimler-Benz has just recently
reported on a major trap-oxidizer development program performed
in Denver in 1981.[1]* Daimler-Benz outfitted twelve of its
Mercedes-Benz vehicles, six naturally aspirated and six
turbocharged, with ceramic monolith traps of varying porosities
and wall thicknesses. All of the naturally aspirated vehicles
successfully completed 50,000 miles of testing with particulate
levels generally around 0.05 g/mi. Daimler-Benz did not
provide the collection efficiencies of the traps, but based on
expected non-trap levels EPA estimates the efficiencies to have
been on the order of 80 percent. None of the turbocharged
vehicles completed 50,000 miles, apparently due to difficulties
in regenerating the trapped particulate. The three traps with
the lower wall thickness failed, while testing of the three
vehicles with the higher wall thickness was halted at between
35,000 and 40,000 miles because of driveability problems.
Although Daimler-Benz did report difficulties with
driveability, and trap failures occurred on the turbocharged
vehicles with the lower wall thickness, this program did
indicate the ability of ceramic monolith traps to survive the
vehicle environment for 50,000 miles if regeneration occurred
properly. These results are particularly impressive in view of
the fact that Daimler-Benz did not control regeneration
conditions nearly as well as they would be under most
regeneration systems, since no positive regeneration technique
was used at all.
A second major ceramic monolith trap durability program
was sponsored by the Agency in late 1981 and early 1982. The
* Numbers In brackets indicate references which are listed
at the end of the report.
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Southwest Research Institute installed a ceramic monolith trap
onto a 1980 Mercedes-Benz turbocharged diesel vehicle. Mileage
accumulation was performed on the road, while regenerations
were conducted under laboratory conditions. Emission tests
were performed every 5,000 miles, with and without the trap.
Particulate reduction was very near 90 percent for the entire
50,000-mile durability run. Backpressure levels were only
moderately higher with the trap, and driveability was
acceptable. (Very high backpressure adversely affects fuel
economy and engine performance.) A more complete description
of the test program and results is given elsewhere.[2] In
general, the trap exhibited stability and high efficiency over
the entire test. No unscheduled maintenance was performed on
either the engine or the trap-oxidizer at any time after the
testing began.
The catalyzed alumina-coated wire mesh trap design has
also been extensively evaluated by nearly every major
light-duty diesel manufacturer marketing in the U.S. Johnson
Matthey, Inc. has been the primary supplier of catalyzed wire
mesh traps. Johnson Matthey proved the basic durability of its
trap design with a successful 50,000-mile demonstration on a
1981 Volkswagen Rabbit at Southwest Research Institute which
was completed in January 1982. The results of the program were
summarized in the Feasibility Study. Particulate reductions
varied between 40 and 60 percent, though the trap was only
designed to be 50 percent efficient due to the relatively low
engine-out emissions of the Rabbit. The trap was also
effective in reducing HC and CO emissions, which would be
expected with a catalyzed trap. Only moderate backpressure
levels were measured. This testing proved the general
durability of the wire mesh filter and catalytic coating under
proper regeneration conditions. In fact, due to the somewhat
crude regeneration system used in this program, regeneration
conditions (e.g., temperature, oxygen level) probably varied
much more than they would under a more advanced regeneration
system.
Few additional results were submitted to EPA on the
catalyzed wire mesh design as a part of this rulemaking.
Several manufacturers still have active programs involving wire
mesh trap designs but did not submit new data on efficiency,
backpressure, or durability. Other manufacturers seem to be
concentrating their development efforts on ceramic monolith
traps. However, EPA, in cooperation with Johnson Matthey, has
recently undertaken a test program to investigate the
possibility of a high-efficiency wire mesh design. The program
is not yet complete, but preliminary results indicate that the
wire mesh trap is capable of particulate reductions as high as
70 to 80 percent over the Federal Test Procedure (urban driving
schedule) and 40 to 70 percent over the Highway Fuel Economy
Test. [3] HC and CO emissions have been reduced by 80 percent
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over the Federal Test Procedure and by more than 90 percent
over the Highway Fuel Economy Test. Based on these results, it
appears that the catalyzed wire mesh design can provide
relatively high collection efficiencies if so designed.
In conclusion, the Agency's analysis of recent information
submitted to EPA and that which has been published in the open
literature supports and strengthens the conclusions in the
Feasibility Study concerning basic filter design and
performance. In particular, the ceramic monolith trap is
capable of maintaining particulate collection efficiencies of
70 to 90 percent with moderate increases in backpressure for up
to 50,000 miles when regeneration conditions are properly
controlled. Also, the catalyzed wire mesh trap can provide
particulate reductions of 50 to 80 percent, along with
significant HC and CO emission reductions, for 50,000 miles
under controlled regeneration. The Agency is not aware of any
attempt to accumulate more than 50,000 miles with either a
ceramic monolith or wire mesh trap, but evidence suggests that
successful operation beyond 50,000 miles is very probable if
proper regeneration conditions are maintained.
B. Regeneration Mechanisms
Much of the industry's research in the last year has
focused on regeneration. The following discussion will
summarize the progress and status of trap-oxidizer regeneration
development.
There are two general approaches to regeneration. Because
of the variable nature of passenger car operation and the
necessity of reliable and periodic regeneration, until recently
most research in this area had concentrated on positive
regeneration systems (those which actively increase exhaust
temperature or otherwise promote regeneration). Positive
regeneration systems involve a decision to regenerate the trap
at a specific point in time with some special mechanism
activated to ensure that regeneration occurs. A second
approach, self-regeneration, relies on attaining regeneration
conditions during normal vehicle operation without the
activation of any special mechanism.
Several positive regeneration systems have been
suggested. All of them share the basic function of raising the
temperature of the particulate in the trap-oxidizer. For
non-catalyzed traps, this typically involves either the
addition of an auxiliary heat source or else the modification
of engine parameters to raise exhaust gas temperature
directly. Examples are fuel burners, electrical heaters, or
air intake throttling. For catalyzed traps, positive
regeneration systems can also include mechanisms which raise
the HC and CO levels in the exhaust. The HC and CO are
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oxidized by the catalyst with the additional heat utilized to
ignite the particulate. Proposed systems for raising the HC
and CO levels for catalyzed trap systems include exhaust stroke
fuel injection, temporary modification of engine parameters,
and individual cylinder throttling. All of these possible
regeneration systems have been described and analyzed in either
the Feasibility Study, the Technology Study, or both. Only the
most advanced positive regeneration systems will be discussed:
the fuel burner for non-catalyzed ceramic monolith traps and
exhaust stroke fuel injection for catalyzed wire mesh traps.
The fuel burner provides the additional heat necessary for
regeneration by igniting an air/fuel mixture immediately in
front of the trap inlet. The burner requires a fuel supply and
fuel injector, a glow plug or spark plug to initiate ignition,
and an oxygen supply. The oxygen may be supplied by a
low-pressure air pump (air-fed burner) or by residual oxygen
present in diesel exhaust (exhaust-fed burner). In addition,
the burner can heat the entire exhaust gas flow (in-line
burner), or can heat just the trap itself while the exhaust gas
is routed away from the trap (bypass burner) . All of these
options have been investigated by various manufacturers.
Ford has published the most extensive results of testing
of burner systems, both air-fed and exhaust-fed in-line
systems.[4] Its most recent paper delineates Ford's extensive
development and evaluation of an air-fed in-line burner.[5]
Ford's burner utilized a burner flame stabilization housing
containing a fuel atomizing nozzle, a spark plug, and
combustion air provided by an external air pump. A short
mixing cone was installed at the burner outlet to distribute
the burner exhaust across the entire front face of the trap.
The burner system was installed just upstream of the ceramic
monolith trap, which was mounted at the outlet of the exhaust
manifold. Ford found that a single burner fuel flow and air
flow setting produced trap inlet temperatures adequate to
initiate regeneration over a wide range of engine operating
conditions (i.e., from idle to 50 miles per hour cruise
conditions). Regeneration effectiveness, defined by Ford to be
a measure of the ability to restore the trap to the zero-mile
backpressure level, was more than 90 percent at trap inlet
temperatures of 650°C with the burner functioning for two
minutes.
Ford installed the trap/burner system onto a diesel
vehicle and, after several attempts, succeeded in operating the
system for a 1,000-mile durability test. During this test, 15
regenerations were successfully performed over a wide range of
operating conditions. With accurate control of trap inlet
temperature and flow rates, no damage to the trap was
observed. Like all manufacturers, Ford also reported general
problems with thermal shock cracking and melting of the ceramic
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monolith trap when regeneration was not properly controlled.
However, it has found that limiting the maximum particulate
loading and trap inlet temperature was effective in avoiding
trap failure. Ford considers the primary remaining problems to
be improving burner operation during transient engine operation
and automating the burner control system. One simple solution
to the transient operation problem is to go to a bypass burner
system, which Ford is researching. The issue of controls will
be discussed later in this section. Based on the results
reported by Ford and other manufacturers, it is clear that
manufacturers have made considerable progress during 1982 in
defining satisfactory operation of burner regeneration systems.
The basic concept of exhaust stroke fuel injection is to
create a diesel analogue of a misfire, by injecting additional
fuel into one cylinder at the beginning of the exhaust stroke.
This fuel is cracked into lighter HC and CO by the heat and
pressure which still exist in the cylinder. The HC and CO are
then exhausted on the piston upstroke and oxidized by the
precious metal catalyst on the wire mesh trap. Johnson Matthey
has stated that the exothermic reaction of the HC and CO
produces a temperature rise of approximately 150°C to 200°C
within the trap. The catalyzed wire mesh trap oxidizes
particulate at approximately 350°C, so exhaust stroke fuel
injection can initiate regeneration for catalyzed wire mesh
traps at exhaust temperatures as low as 200°C. This system
would not work for non-catalyzed traps.
Johnson Matthey utilized an early prototype version of
exhaust stroke fuel injection on its 50,000-mile durability
demonstration test completed in early 1982. When exhaust
backpressure reached a certain level, the driver operated a
manual lever which diverted fuel from one cylinder (which was
in the compression stroke) to another (in the exhaust stroke) ,
which initiated regeneration. Of the 46 regenerations
attempted during the durability test, 37 were successful. The
nine failures simply resulted in additional regenerations.[6]
The diversion of fuel from one cylinder to another results
in the former cylinder being "starved," which can lead to loss
of power and driveability. It will be necessary to modify the
fuel injection system so that extra fuel can be provided to one
cylinder without starving any other cylinder. This might be
difficult for mechanical fuel injection pumps used on current
diesel vehicles, but would likely involve fairly minor
modifications (addition of solenoid valves) to electronically
controlled injection pumps. Several reports have been
published in the last year which indicate that microprocessor-
controlled electronic fuel injection pumps are near production
status, and it is known that a number of prototype diesel cars
with such systems are currently being evaluated. The National
Academy of Sciences has projected that electronically
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controlled pumps will become available by 1985. [7] The
emergence of electronic injection pumps on production vehicles
in this timeframe should ensure the feasibility of exhaust
stroke fuel injection as a satisfactory regeneration technique
for catalyzed wire mesh traps. The new pumps would also
provide the opportunity for alternative means of providing HC
and CO to the catalyzed trap, such as temporary modification of
exhaust gas recirculation and/or injection timing.
Positive regeneration systems, such as the fuel burner for
ceramic monolith traps and exhaust stroke fuel injection for
catalyzed wire mesh traps, appear to be at advanced stages of
development. They are the most understood regeneration systems
and the most likely to be adopted for initial trap-oxidizer
systems. Positive regeneration systems also tend to be
relatively complex. Recently an important breakthrough in
trap-oxidizer development has occurred with self-regeneration
systems which would ensure particulate oxidation during normal
engine operating conditions and thus be simpler than the
positive regeneration systems. Self-regeneration was
considered very unlikely at the time of the Feasibility Study.
Most of this recent work has centered on the use of diesel fuel
additives.
It is well known that catalysts can reduce the ignition
temperature of carbon. Early research, however, had indicated
that catalysts impregnated in the trap were ineffective in
reducing the ignition temperature of particulate stored in the
trap. More recently, it has been found that organometallic
compounds added to diesel fuel can effectively lower the
particulate ignition temperatures. When the fuel is burned,
the metal is liberated from its carrier molecule and forms
either a sulfate or an oxide which is distributed throughout
the particulate in the trap-oxidizer. This is advantageous,
because all the trapped particulate, not just the initial
layer, is in contact with the catalyst.
The use of fuel additives is now the leading avenue of
research for many manufacturers. Three manufacturers have
published results from their additive programs: Ford,
Volkswagen (VW), and General Motors. [8,9,10] These reports
form the foundation for the following analysis.
Several organometallic fuel additives have been evaluated,
including copper, lead, manganese, calcium, and various
combinations of these. Table 1, from the Technology Study,
summarizes the results of screening tests by Ford. Table 1
shows that all of the organometallic additives were effective
in lowering the particulate ignition temperature. Copper was
the most effective individual metal on a concentration basis,
and the combination of copper and lead resulted in the greatest
overall ignition temperature reduction. Volkswagen stated that
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Table 1
Effects on Particulate Ignition
Temperature of Various Organometallic Additives
Ignition
Additive Temperature (°C)
None (baseline fuel)
426
0.5 g/gal calcium
268
0.5 g/gal lead
246
0.25 g/gal copper
232
1.0 g/gal lead
232
0.25 g/gal copper +
232
0.25 g/gal manganese
0.25 g/gal copper +
224
0.25 g/gal lead
0.25 g/gal copper +
191
0.5 g/gal lead
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in order to be able to rely on self-regeneration, an additive
must be able to induce regeneration at approximately 250°C.
Table 1 indicates that all of the additive formulations were
below 250°C except for calcium, which was just slightly above
that level.
Manufacturers have reported promising vehicle tests with
various fuel additives. Ford completed a 10,000-mile
durability test with a ceramic monolith trap using an additive
formulation of 0.25 gram per gallon (g/gal) copper (as copper
naphthanate) and 0.50 g/gal lead (as tetraethyl lead) for the
first 3,500 miles, and doubling the lead content for the
remaining 6,500 miles. The mileage was accumulated with
"normal" on-the-road driving. Ford reported that temperatures
above 191°C were reached often during the testing, resulting in
frequent regenerations. As a result, the oxidation process
generally occurred with relatively low particulate mass
loadings, which led to relatively lower peak trap temperatures
as well. Ford stated that this would assist in avoiding trap
thermal failure. The trap maintained a collection efficiency
of more than 70 percent and exhaust backpressure seldom
exceeded the clean trap backpressure by more than 50 percent,
which also indicated that the trap was being regenerated
frequently and effectively.
Volkswagen operated a diesel Rabbit with a ceramic
monolith trap and a calcium additive of unknown concentration
for more than 25,000 miles of "real world" mileage
accumulation. At about 15,000 miles, particulate and fuel
consumption levels rose, indicating that the trap was possibly
filling with calcium deposits. In a second test, VW used a
manganese additive with a Quantum turbocharged vehicle equipped
with a ceramic monolith trap. The vehicle accumulated more
than 12,500 miles on a chassis dynamometer at a constant speed
of 31 miles per hour. This is probably .close to a "worst case"
scenario, as the power requirements and thus the exhaust
temperatures are very low at such low-speed cruise conditions.
Normal driving, even at low average speeds, typically includes
frequent accelerations, which produce higher exhaust
temperatures which would ease regeneration. That the trap
successfully survived 12,500 miles of worst case operation
indicates that additives should be able to induce
self-regeneration under almost any possible usage pattern. VW
also reported successful regeneration using additives in cold
room testing at -20°C.
Although General Motors has an extensive program underway
to evaluate fuel additives, it has not released many results.
It is known that General Motors has one trap-equipped test
vehicle which has accumulated nearly 30,000 miles with an
unknown fuel additive. GM has stated that backpressure levels
are rising on this vehicle due to additive deposits.
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There are several unresolved issues with respect to the
applicability of fuel additives to induce self-regeneration.
One is the question of how to introduce the additive to the
fuel: should it be premixed into diesel fuel at the. refinery,
or added on-board the vehicle? Blending the additive at the
refinery would obviate the need to design an on-board system,
but would result in either the addition of the organometallic
compound to all diesel fuel, which would result in unnecessary
metallic emissions from non-trapped diesel vehicles, or else a
special grade of diesel fuel for trapped diesels, which would
result in a more expensive and complex distribution system.
The on-board approach, utilizing a reservoir of additive and a
metering system, may be more practical. The amount of additive
required is fairly small and a lifetime supply should be
storable with no serious size or weight problems. On-board
systems would allow optimization of engine, trap, and additive,
and would require no coordination between the auto and fuel
industries. Of course, the need to store the additive in a
highly concentrated form may preclude those additives from
consideration which are toxic in high concentrations (e.g.,
tetra-ethyl lead).
A second issue concerns the compatibility of the fuel
additive and its combustion products (oxides or sulfates) with
the fuel injection system and engine hardware. Fuel filter
plugging and fuel nozzle and engine deposits have been
reported. Manufacturers are currently investigating these
problems.
The two most serious, and interrelated, concerns of fuel
additives are the environmental consequences of their usage and
the possibility of trap plugging with metal deposits.
Extensive analysis by Ford has shown that nearly 100 percent of
the metallic sulfates and oxides are collected in the trap, and
thus, only negligible amounts are emitted in the exhaust. This
is very beneficial from an environmental perspective, since
some of the metals which have been used so far, such as lead
and copper, are quite toxic. Many manganese compounds are
considered hazardous as well. There still remain concerns
about the storage and ultimate disposal of on-board systems
utilizing hazardous substances. As already mentioned, the
environmental ramifications of different fuel additives must be
a basic criterion for additive selection.
The fact that the metallic compounds are collected and
stored in the trap-oxidizer raises the possibility of the trap
plugging with metallic deposits. Both Ford and VW reported
such problems in their initial vehicle tests. Optimization of
trap size and design is one possible solution to trap plugging,
but it has not yet been proved that a trap can function
satisfactorily with fuel additives for 50,000 miles.
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In summary, the utilization of fuel additives to allow
self-regeneration is a recent development which was not
considered a plausible option at the time of the Feasibility
Study. Now it is considered to be one of the leading
alternatives for many manufacturers. Results to date have been
very promising. Additive systems have been studied less than
positive regeneration systems, but they are conceptually
simpler, and would likely require less vehicle/engine
modifications for production. Should the problems discussed
above be resolved, fuel additives would be the regeneration
system of choice for most vehicle manufacturers.
C. Regeneration Control Systems
1. Positive Regeneration
Positive regeneration requires the application of an
on-board system to initiate and control the regeneration
process. This was the most undefined area of trap-oxidizer
development at the time of the Feasibility Study. The
Technology Study examined regeneration control systems in
greater detail and that analysis will be summarized below.
There are three basic functions of any regeneration
control system: to determine when regeneration is required, to
initiate the regeneration process, and to confirm that
regeneration is complete. Other functions, such as determining
whether regeneration is possible or allowable under current
engine operating conditions, and monitoring the regeneration
process for dangerous conditions such as excessive peak
temperatures, will also be necessary on many regeneration
control systems. For example, exhaust stroke fuel injection
produces regeneration with catalyzed wire mesh traps only when
exhaust gas temperatures are above a minimum level. Thus, a
regeneration system utilizing exhaust stroke fuel injection
would have to include control logic to ensure that the minimum
temperature was exceeded before initiating regeneration.
Regeneration control systems can be divided into two
general parts: 1) the identification and design of a suitable
control logic which will ensure effective regeneration, and 2)
the application of system components which will implement that
control process. The components of a regeneration control
system will consist of sensors, actuators, and a mechanism for
control logic.
The first step, the identification of the conditions
permitting regeneration and design of a suitable control logic,
is a fairly straightforward yet time-consuming function. In
order to define the operational requirements of the control
system, it is necessary to understand the characteristics and
limits of the trap regeneration process. Doing so requires
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extensive testing of specific engine/trap/regeneration systems,
which manufacturers have only been able to do relatively
recently. Manufacturers now have considerable data concerning
the minimum particulate, oxygen and temperature levels, and
time necessary for effective regeneration, as well as the
maximum particulate loadings and inlet temperatures which can
be permitted without risk of overheating the trap.
Manufacturers have had time to determine these characteristics
for different engines and regeneration systems. Once these
limits are known, prototype control logic can be designed to
maintain conditions within acceptable limits.
Much progress has also been made in the area of control
systems. Several sophisticated control systems have been
designed, fabricated, and installed on experimental vehicles,
though details have often not been publicly available. For
example, Ford has developed a fuel burner regeneration control
system which carries out the entire process with the push of a
button, and includes the ability to abort if certain conditions
necessitate it. Johnson Matthey has also developed the
necessary logic to control its HC and CO enrichment system
under in-use conditions.
To date, the regeneration systems have still required
human initiation. Production systems will have to be
completely automated, of course. The final step in perfecting
the control process will involve extensive testing to confirm
that the control system successfully maintains regeneration
conditions within acceptable limits under all possible
circumstances. As a result of such testing, refinements to the
prototype design are likely.
The regeneration control system must also include hardware
which can successfully and reliably implement the control
logic. Likely components include sensors, actuators, and a
central processing unit. These items are either generally
available today or should be available very soon.
Depending on the regeneration mechanism used, a wide
variety of sensors may be required for parameters such as
exhaust and trap temperature, engine revolutions and load,
burner ignition detection, etc. With the exception of the trap
temperature and burner ignition sensors, all of these now exist
and are in use on production vehicles. High-temperature
thermocouples would probably suffice for the trap temperature
and burner ignition sensors. Every regeneration control system
would also require some indicator of particulate loading in the
trap. Given the importance of backpressure on vehicle fuel
economy and performance, and its direct relationship with
particulate loading for a given trap, a backpressure sensor
would be the ideal solution. At the time of the Feasibility
Study, the Agency was doubtful that backpressure sensors of
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sufficient durability and reliability could be developed.
Other alternatives, such as using mileage or engine revolution
counters as alternatives to direct measurement of backpressure,
were discussed in the Feasibility Study instead. However, the
situation now appears to be quite different. Ford stated at
the public hearing on this rulemaking that it was already
testing a backpressure sensor developed for this purpose and VW
stated that it did not consider such sensors to be a major
problem. The Technology Study concludes that it is "extremely
probable" that a suitable backpressure sensor will be available
for production regeneration systems in 1987.
Actuators which would be required for regeneration control
systems include valves, solenoids, relays, and possibly
remotely operated accessory clutches for air pumps. Except for
some actuators which might be used on fuel burner systems,
these are all available and have been used on production
vehicles. Ford has investigated the types of actuators which
would be necessary on burner systems and has not reported any
serious concerns in this area.
As discussed earlier, it is expected that by the mid- to
late-1980's many diesel manufacturers will turn to
electronically controlled fuel injection systems. Such systems
will use microprocessors to determine the optimum fuel delivery
rate and injection timing based on information provided by a
variety of sensors. Control logic for trap-oxidizer
regeneration control systems would be simply one additional
function for vehicles already equipped with microprocessors.
Those vehicles which maintain mechanical fuel injection would
have to have a separate logic system. Still, this should not
be a difficult technical task. Most of the control functions
for trap-oxidizer regeneration are on/off functions, rather
than continuous variables. Adding control logic of this nature
will be relatively straightforward compared to the
sophisticated electronics systems which have been developed to
handle multiple continuous variables in three-way catalyst,
closed-loop emission control systems on many current
gasoline-fueled vehicles.
In conclusion, none of the tasks associated with positive
regeneration control systems will be technically infeasible.
Some aspects of the problem, such as defining the operational
limits of the engine/trap/regeneration process, have been time
consuming. Other tasks, such as the development of a suitable
backpressure sensor, will take more time, but indications are
that these problems will soon be successfully resolved.
Significant progress has occurred in regeneration control
system design in the past year. The primary work remaining
will be additional testing of prototype systems resulting in
refinements and improvements.
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2. Self Regeneration
Self-regeneration trap systems would, by definition, not
need any control system. Self-regeneration systems might
incorporate a fail-safe system to prevent excessive particulate
loading and resultant trap overheating, such as a simple bypass
valve venting exhaust around the trap. Positive regeneration
systems might need such a mechanism as well. The fact that
self-regeneration traps would not require control systems is a
major advantage in terms of simplicity and cost.
D. Overall Evaluation of Status of Trap-Oxidizer
Development
On the basis of the information analyzed in the
Feasibility Study and the information which the Agency has
obtained in the past year, the Agency has concluded that the
automotive industry is at a very advanced stage of
trap-oxidizer development. Collection efficiency,
backpressure, and durability characteristics are acceptable
when regeneration conditions are adequately controlled.
Considerable progress has been achieved in positive
regeneration system design and several prototypes have been
tested. Only fairly routine engineering tasks remain to
integrate and automate regeneration control systems which could
be installed in fleet vehicles for additional testing. The
development of self-regeneration systems is a very significant
advancement in trap-oxidizer development. Although the
development of these systems is not quite as advanced as
positive regeneration systems, the technical problems are
generally conceptually simpler and will likely require less
time for design and assurance testing. In addition, the high
level of manufacturer interest in fuel additives indicates the
promise of self-regeneration.
The Technology Study confirms the Agency's position that
trap technology is at a very advanced stage. The Technology
Study, based on information from automotive emission control
researchers in addition to written submissions to EPA,
identified three trap-oxidizer systems (trap design and
regeneration mechanism, plus control system if necessary) which
it considered to be at an advanced stage of development: 1)
ceramic monolith trap/regeneration with fuel burner, 2)
catalyzed wire mesh trap/regeneration by HC and CO enrichment
(such as exhaust stroke fuel injection), and 3) ceramic
monolith trap/self-regeneration using fuel additives. The
Technology Study concluded that there are no technical
uncertainties regarding the feasibility of the ceramic
monolith/fuel burner system. It considered successful
development of the catalyzed wire mesh/HC and CO enrichment
system to be "highly probable" and development of the ceramic
monolith/fuel additive system to be "probable" in the near
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term, and that both of these systems would be preferred if
feasible, due to their greater simplicity and lower costs. If
these systems did not prove to be feasible, then the fuel
burner would be adopted industry-wide.
E. Prototype Development Leadtime
This section will address the issue of how much leadtime
is required for vehicle manufacturers to optimize trap-oxidizer
systems to the stage where workable prototype trap/regeneration
systems can be installed in vehicles for assurance testing.
The Feasibility Study had projected that manufacturers would
reach this point by January 1983. Based on the comments
submitted to EPA in January and February 1983 as a part of this
rulemaking, it appears that manufacturers had not reached the
assurance testing stage by January. Nevertheless, most
manufacturers are very close to having workable prototype
designs.
As summarized in the previous section, trap-oxidizers are
at a very advanced stage of development. The only area where
further development is necessary is in regeneration, whether
positive regeneration or self-regeneration. No major technical
uncertainties exist for positive regeneration systems, such as
fuel burners. Remaining design tasks are relatively
straightforward engineering problems such as development of
high-temperature and backpressure sensors, the automation of
the regeneration control system through the use of fairly
simple electronics, and preliminary testing of the control
systems in order to refine the system designs before the more
extensive fleet assurance testing begins. Most of the
components for regeneration control systems are already used on
production vehicles today, and the experience of automotive
designers with sophisticated electronic controls on
gasoline-fueled vehicles should be of particular assistance in
adapting control systems for diesel vehicles.
Greater uncertainties exist with respect to
self-regeneration systems based on fuel additives. There are
technical uncertainties with respect to on-board additive
storage and metering, although this appears to be a fairly
straightforward design problem. Concerns about the impacts of
metallic deposits on fuel system and engine parts and the trap
itself are more serious, and can be resolved only by additional
development and optimization. Given the high rate of progress
in the last year with respect to fuel additive development,
however, and the understanding which already exists as to the
problems and possible solutions, the Agency expects
manufacturers to resolve these concerns very soon. Quite
possibly the most important questions about self-regeneration
systems revolve around the environmental consequences of their
usage, with respect to any metallic emissions which could be
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toxic, and the safety and ultimate disposal of on-boacd storage
systems. EPA stands ready to work with the manufacturers in
analyzing such problems.
Of course, it is not necessary that both positive and
self-regeneration systems be developed; only one such system is
necessary. There are clearly more technical uncertainties
remaining with respect to fuel additives than with respect to
fuel burners. Many of the uncertainties regarding fuel
additives exist because the concept has only recently been
under serious investigation. The Agency believes that such
questions will be resolved relatively quickly, given the
straightforward nature of the problems and the emphasis many
manufacturers are putting on the development of additives.
Nevertheless, should the additive concept prove unworkable,
positive regeneration systems will undoubtedly be available.
Once complete regeneration systems are developed,
involving positive or self-regeneration, the vehicle
manufacturers can proceed with assurance testing in the field
in order to evaluate comprehensively the entire
trap/regeneration system design in-use. It is not necessary
that a manufacturer have production-ready trap systems
available for the assurance testing fleets, since assurance
testing is itself a part of the final evaluation process.
Rather, the manufacturer must select the most promising trap
design/regeneration system combination(s) so that the assurance
testing process may begin. The manufacturer may need to
include more than one concept in its assurance testing if it is
unable to identify one most promising concept, or if it wants
to be "safe" in case of failure of any particular design.
EPA's analysis of the time necessary for assurance testing
leaves the manufacturer enough latitude to perform the testing
in two separate phases, so improvements may be made in the
midst of the program.
It must be noted that there is evidence that many
manufacturers have already undertaken preliminary fleet
assurance testing, though typically not with completely
automated regeneration control systems. Earlier, the
Daimler-Benz testing program in Denver involving high-mileage
accumulation with 12 vehicles was discussed. Daimler-Benz has
announced that "DBAG will shortly begin a new durability
program on a new generation of trap-oxidizers. This program
incorporates all results and improvements gained up to this
time."[11] Volkswagen has reached the point where it can rank
its candidate systems, and has stated that "Volkswagen is
cautiously optimistic of meeting a 0.2 g/mi particulate
standard in model year 1987."[12] From this statement, EPA
infers that VW is not far from commencing assurance testing.
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The relatively straightforward nature of the engineering
problems which remain, and the pr.ogress which manufacturers
have achieved in the past year with respect to both positive
regeneration control systems and self-regeneration, indicates
that manufacturers are very near to workable trap/regeneration
systems for assurance testing. The EPA technical staff
projects, based on the information available to the Agency as
of February 1983, that another 6-9 months of optimization will
permit all manufacturers to have workable prototype
trap/regeneration systems. This will permit industry-wide
assurance testing to begin in the fall of 1983.
F. Production Development Leadtime
After the successful demonstration of prototype
trap/regeneration systems, additional development will be
required to prepare trap-oxidizers for mass production. In
considering this aspect of the development continuum for the
two positive regeneration systems (i.e., the ceramic
monolith/fuel burner and wire mesh/HC and CO enrichment
systems), the Feasibility Study found that a total of 25-33
months would likely be necessary. The critical path elements
within this schedule of events consisted of a 7-9 month period
for assurance testing and an 18-24 month period for acquiring
production facilities and tooling. These production
development projections are considered to be far more
straightforward than the estimates for prototype development
because no technological advances are necessary. Therefore,
the Agency finds that these projections continue to be
reasonably accurate and that they need not be revised.*
The information EPA acquired during the rulemaking
revealed a new control technology that was not explicitly
considered in the above leadtime schedules. This technology,
the ceramic monolith/fuel additive system, was not clearly
identified at the time the Feasiblity Study was completed. The
available data on this self-regenerating system is limited and
does not allow an independent quantitative assessment of the
requisite production development leadtime. However, the
leadtime requirements can be qualitatively assessed by
comparing this system to the positive regeneration systems.
As discussed previously, the ceramic monolith/fuel
additive system is inherently simpler than the other systems
because of its self-regenerating characteristics. This- aspect
of the system should result in reducing the necessary
It should be noted, however, that these conservative
leadtime projections consider only the time necessary to
enable manufacturers to introduce trap-oxidizers on most
or all models requiring them in order to meet the 0.2 g/mi
particulate standard on a nationwide basis.
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production tooling leadtime in comparison to that required for
the positive regeneration systems, because there are likely to
be significantly fewer and less complex parts. Therefore, at
the very least, this suggests that the leadtime for the ceramic
monolith/fuel additive system would be best represented by the
lower end of the 25-33 month projection. Until additional
information becomes available with which to make this judgment
more conclusively, however, EPA will assume conservatively that
the same amount of production leadtime is required by both
positive and self-regeneration systems (i.e., 25-33 months).
IV. Conclusion
A. Applicable Model Year
Now that the total leadtime requirements have been
assessed, this information can be used to project the model
year in which trap-oxidizer equipped vehicles can be mass
produced. Assuming that successful prototype trap/regeneration
systems are demonstrated in the fall of 1983, and that an
additional 25-33 months are necessary to develop the systems
fully, EPA projects that trap-oxidizers should be available for
production sometime between the fall of 1985 and the summer of
1986. Because vehicle production for each model year generally
begins at the end of the summer of the previous calendar year
{i.e., August) the above completion dates show that
trap-oxidizers will be feasible on a nationwide basis for the
1987 model year. Therefore, based on an assessment of the
information submitted during the rulemaking, EPA concludes that
delaying the 1985 particulate standards for two years is
sufficient to allow for the complete development of
trap-oxidizer technology.
The 2-year delay is supported by the Technology Study
which, as previously described, was an independent assessment
of trap-oxidizer feasibility. This study concluded that the
successful development of trap-oxidizer technology for
introduction in the 1987 model year was highly probable for
both the ceramic monolith/fuel burner and wire mesh/HC and CO
enrichment systems, . and was probable for the ceramic
monolith/fuel additive system.
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References
1. "Comments of Daimler-Benz A.G. to EPA's Proposed
Rule," submitted to EPA by Mercedes-Benz of North America,
Inc., on March 8, 1983.
2. "Diesel Car Particulate Control Methods," Charles M.
Urban, Southwest Research Institute, Larry C. Landman and
Robert D. Wagner, EPA, Society of Automotive Engineers Paper
No. 830084.
3. "Catalytic Diesel Particulate Control System Design
and Operation," Miles F. Buchman and Bernard E. Enga, Johnson
Matthey, Inc., Society of Automotive Engineers Paper No. 830080.
4. "Diesel Particulate Trap Regeneration Techniques,"
W. R. Wade, J. E. White, and J. J. Florek, Ford Motor Company,
Society of Automotive Engineers Paper No. 810118.
5. "Thermal and Catalytic Regeneration of Diesel
Particulate Traps," W. R. Wade, J. E. White, J. J. Florek, and
H. A. Cikanek, Ford Motor Company, Society of Automotive
Engineers Paper No. 830083.
6. "Final. Report on 50,000-Mile Volkswagen Rabbit
Durability Test," B. E. Enga, Johnson Matthey, Inc., and B. B.
Bykowski, Southwest Research Institute, submitted to EPA on
February 12, 1982.
7. Diesel Technology, Report of the Technology Panel of
the Diesel Impacts Study Committee, National Research Council,
1982.
8. "Thermal and Catalytic Regeneration of Diesel
Particulate Traps," W. R. Wade, J. E. White, J. J. Florek, and
H. A. Cikanek, Ford Motor Company, Society of Automotive
Engineers Paper No. 830083.
9. "Regeneration of
Temperatures," B. Wiedemann,
Poettner, Volkswagenwerk AG,
Paper No. 830086.
Particulate Filters at Low
U. Doerges, W. Engeler, and B.
Society of Automotive Engineers
10. General Motors Comments in Response to EPA NPRM,
submitted to EPA by General Motors Corporation on February 22,
1983.
11. Comments of Daimler-Benz A.G. to EPA's Proposed
Rule, submitted to EPA by Mercedes-Benz of North America, Inc.,
on March 8, 1983.
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References (cont'd)
12. Statement by Volkswagenwerk AG, Volkswagen of
America, Inc., and Audi NSU Auto Union AG, before EPA on
J anuary 18, 1983.
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