United States         Air and Radiation        EPA420-R-01-052
            Environmental Protection                   October 2001
<&EPA     Nonroad Diesel
            Emission Standards
            Staff Technical Paper
                                    y&o Printed on Recycled

                                                                        October 2001

                         Assessment and Standards Division
                       Office of Transportation and Air Quality
                       U.S. Environmental Protection Agency
                                 October 3 0,2001

   This technical report does not necessarily represent final EPA decisions or positions,
It is intended to present technical analysis of issues using data that are currently available.
        The purpose in the release of such reports is to facilitate the exchange of
     technical information and to inform the public of technical developments which
       may form the basis for a final EPA decision, position, or regulatory action.

                                  Executive Summary

       In 1998, EPA adopted more stringent emissions standards for nonroad diesel engines. In
that rulemaking, EPA indicated that in 2001 it would review the upcoming Tier 3 portion of
those standards (and the Tier 2 emission standards for engines under 50 horsepower) to assess
whether or not the new standards were technologically feasible. We are soliciting public
comments on this preliminary technical assessment.  After reviewing the comments, EPA plans
to issue its final assessment early next year on these issues.

       When we set the Tier 3 emission standards in 1998, available information indicated that
the cooled exhaust gas recirculation (EGR) technology developed for highway diesel engines
would be the primary means of compliance with these standards. In conducting our technology
review, we have surveyed the recent engineering and scientific literature on advances in diesel
emissions control. We have also reviewed information provided by engine manufacturers in
support of our 2004 highway standards program, showing the considerable progress they have
made in the design of robust EGR systems for use in highway engines.  In addition, we have
gathered information from engine manufacturers on their design plans for Tier 3 and their testing
and development experience with control technologies they are likely to employ.  This
information shows that cooled EGR is but one of several technologies available to diesel engine
manufacturers to meet the Tier 3 emission standards. This widening of technology  options
comes from the progress of technology development since 1998. In addition,  as we
acknowledged in the  1998 final rule, we envisioned a Tier 3 program more closely aligned with
future highway standards, in particular, achieving comparable control of particulate matter (PM)
for nonroad engines.  Based on the information we have gathered to date,  we reaffirm that the
Tier 3 standards in Title 40 of the Code of Federal Regulations (CFR), Part 89, are feasible in the
timeframe established in the rule. Based on the information to date, we also reaffirm that the
Tier 2 standards for engine under 50 horsepower are likewise feasible. This preliminary
assessment is reinforced by certification test data from  Tier 1 engines in this power  range
showing that many of these engines are already  meeting the Tier 2 standards.

       The 1998 rule did not establish a new Tier 3 program for PM emissions reductions
because of critical unresolved issues connected with the appropriate test procedure for
characterizing transient operating conditions. Instead, the Agency made a commitment in that
rule to  establish an effective program for controlling PM emissions beyond the limited control
achieved under the Tier 2 standards, and to consider adopting measures to better ensure
emissions control in-use.  These actions were, at least in part, planned to occur in the context of
this technology review. Since the 1998 final rule, the belief that further action is warranted has
been reinforced by growing evidence that diesel engine exhaust emissions can cause serious
health problems. EPA has recently issued regulations that will dramatically reduce  emissions
from highway diesel vehicles. As a result, nonroad diesel engines, already a major  source of
harmful particulate matter and ozone-forming compounds, will become a  dominant mobile
source  of these emissions in the future.

       The Agency has already taken some steps toward dealing with nonroad diesel PM and in-

use emissions concerns (such as developing a transient test cycle to better characterize in-use PM
emissions). However, we believe a separate rulemaking is the best approach because it is
increasingly clear that the most effective means of further reducing emissions of PM (and oxides
of nitrogen (NOx), if warranted) is through a "systems" approach that regulates nonroad diesel
engines and fuel in a single coordinated program,  similar to the approach recently taken to
controlling highway vehicle emissions.  This approach would continue the pattern followed
successfully in the past, in which nonroad emissions reduction programs are modeled after
highway programs, with some additional leadtime provided for adaptation of highway
technologies to nonroad diesel applications. EPA plans to initiate such a rulemaking with a
proposal next year.

1.     Introduction

1.1    Purpose

       This EPA staff technical paper presents a preliminary staff assessment of the feasibility of
upcoming emissions standards for nonroad diesel engines.a These standards, referred to as "Tier
3" for engines above 50 horsepower (hp) and  "Tier 2" for engines below 50 hp, were adopted in
a 1998 rulemaking, but do not begin to take effect until the middle of this decade. EPA
announced in the 1998 rule that we planned to perform this assessment to determine the need for
any adjustments to the program. We are soliciting public comments on this preliminary
assessment.  After reviewing the comments, EPA plans to issue a final assessment early next
year on these issues.

       This paper also discusses continuing air quality concerns caused by nonroad diesel
emissions after implementation of the upcoming Tier 3 program. We intend to follow up with a
rulemaking to address these concerns next year. Furthermore, that rulemaking will fulfill
Agency commitments, made in the 1998 final rule, to establish test procedures and standards
levels for controlling PM emissions beyond the limited control achieved under the Tier 2
standards, and to adopt measures to better ensure emissions control in-use.  It is important to note
that we have not made any final decisions regarding the upcoming rulemaking, and we welcome
any and all comments that can help us to develop the best possible program.

1.2    Past EPA Actions

       The EPA has taken measures to reduce harmful emissions from nonroad diesel engines in
two past regulatory actions.  A  1994 final rule, developed under provisions of Section 213 of the
Clean Air Act, set initial emissions standards for new nonroad diesel engines greater than 50 hp
(59 FR 31306, June 17, 1994).  These standards gained modest reductions in NOx emissions and
are referred to as EPA's "Tier 1" standards for large nonroad engines. A subsequent final rule
published in 1998 set more stringent Tier 2 and Tier 3 standards for these engines, as well as Tier
1 and Tier 2 standards for the nonroad diesel engines under 50 hp (63 FR 56968, October 23,
1998). Nonroad diesel fuel quality is not presently regulated by the EPA.

       We also expressed our intent in the 1998 rule to continue evaluating the rapidly changing
state of diesel emissions control technology, and to perform  a review in the 2001 timeframe of
the technological feasibility of the Tier 3 standards, and of the Tier 2 standards for engines rated
under 50  hp.  This review could then result in  additional EPA action to revise the standards
upward or downward, as appropriate.  The 1998 rule did not establish a new Tier 3 program for
PM emissions reductions because of critical unresolved issues connected with the appropriate
       a  Throughout this staff paper, the term "nonroad diesel engine" refers to compression-
ignition engines used in mobile off-highway applications, other than locomotives, underground
mining equipment, and large marine engines, which are regulated separately.

test procedure for characterizing transient operating conditions. Instead we made a commitment
to pursue resolution of these issues and establishment of a PM control program as part of the
2001 feasibility review process.  Therefore the Tier 2 PM standards continue on during the Tier 3
timeframe until changed by the Agency. The 1998 rule also discussed concerns that the advent
of electronic engine controls increase the risk that in-use emissions may not be controlled
adequately, and indicated that the Agency  expected to take action in the future to adopt
supplemental measures to better address this issue.

1.3     The Current Situation

       We have been engaged in the assessment mentioned above for some time now. A key
part of this effort has been meeting with engine manufacturers, equipment manufacturers, and
other stakeholders. The meetings held thus far have been extremely helpful, and have provided
much of the basis for the technical discussion in the following sections. EPA Docket A-2001-28
includes documentation from these meetings. Some engine manufacturers have also provided
written discussions of their views, data, and analyses on this topic, and these too have been put in
the  docket.

       In chapters 3 and 4 of the Regulatory Impact Analysis (RIA) for the 1998 final rule, we
described several technologies that we expected would be used to varying degrees in helping to
meet the Tier 3  standards (and the Tier 2 standards for engines rated under 50 hp). These
technologies included full authority electronic systems and new fuel injection components such
as high-pressure unit injectors and common rail fuel systems.  Key among the forecast Tier 3
technologies was cooled exhaust gas recirculation, which was believed to have the potential to
cut NOx emissions in half. This technology was, at that time,  expected to be an essential piece
of every engine manufacturer's design strategy for meeting the heavy-duty highway diesel engine
standards set in 1997, that take effect in 2004.  The 1998 nonroad rule projected the transfer of
this technology to most nonroad diesel engines in  Tier 3, because the Tier 3 NOx plus
nonmethane hydrocarbon (NMHC) standards were intentionally set at comparable levels (and
with the anticipation that Tier 3 PM standards would be set at a later point), to take effect a
couple of years later.b

       The development of technology for controlling diesel engine emissions has progressed
rapidly in the three years since the 1998  final rule  was published.  As often happens when a
diverse, highly competitive industry tackles challenging new regulatory requirements, the
number of potential control options has expanded  to include new technologies and more effective
application of existing technologies.  At this point, we do not believe that cooled EGR will be the
only means available to comply with the Tier 3 standards, nor  do we believe that any nonroad
       b Engines in the 50-100 hp category were projected to employ non-cooled, or "hot", EGR
systems for reasons of cost, space constraints, less-straightforward technology transfer from the
larger highway engines, and our adoption of a somewhat higher Tier 3 NOx+NMHC
 standard level compared to that of larger engines.

engine designs employing cooled EGR systems will need to rely on them as heavily as was
contemplated in the 1998 final rule.  The next section presents our technology feasibility
assessment of Tier 3 standards in detail.  In evaluating Tier 2 standards for engines under 50 hp,
we now have substantial test and design data resulting from the Tier 1 program that informs our
assessment of the feasibility of these Tier 2 standards. Section 3 presents this assessment in

       EPA has taken several steps toward better controlling PM and in-use emissions from
nonroad engines.  EPA issued guidance in October 1998 on implementation of the defeat device
prohibition for electronically-controlled highway and nonroad diesel engines.1 EPA has also met
with engine manufacturers to discuss supplemental regulations to better ensure that excess in-use
emissions do not occur.  EPA has been working with manufacturers, and has made significant
progress, in developing a transient test procedure to better characterize nonroad engine operation
in the field and thereby to ensure better control of PM.  As discussed earlier, EPA plans to take
up these issues in a future rulemaking. The evaluation of Tier 3 and Tier 2 standards in this
paper is based on the current regulatory requirements, and does not attempt to evaluate the
impact these future actions might have on the already promulgated Tier 3 and Tier 2 standards.

2.     Tier 3 Technology Assessment

       In 1998 we set new Tier 3 NOx+NMHC emission standards for nonroad diesel engines
between 50 and 750 horsepower.0 The schedule we set for introduction of these new standards
included  a gradual phase-in that limited impacts on engine and equipment manufacturers. The
emission levels of these standards were set based largely on our belief that manufacturers of
nonroad engines would be able to apply emission control technologies developed for highway
engines.  The NOx+NMHC standards set for Tier 3 did include provisions that reflect unique
nonroad concerns.  Specifically, the NOx+NMHC standard was set at a less stringent level when
compared to the comparable highway standard, in order to account for differences in aftercooler
performance. Similarly, the standards are phased-in over a number of years according to
horsepower rating. This will allow for a gradual transfer of highway technologies such as cooled
EGR systems to nonroad engines. These highway technologies are already in limited production
and are expected to be in widespread use by 2003, a full three to six years before they will be
needed for nonroad engines.  The resulting emission standards for Tier 3, in grams per
horsepower-hour (g/hp-hr), are shown in  Table 2-1.
                                       Table 2-1
                      Nonroad Tier 3 Emission Standards (g/hp-hr)
Engine Power
belief that engine manufacturers will be able to comply with the Tier 3 standards through the
continued advancement of existing highway diesel engine technologies.

2.1    Combustion Fundamentals and Pollution Formation

       Diesel combustion consists of a complicated series of events, both physical and chemical,
which can be altered through engine design and operation. The sequence and rate at which these
events occur are strongly related to the kinds and quantities of pollutants formed during
combustion. The following sections will describe the combustion and pollutant formation
processes so that the means  of controlling emissions discussed in this paper can be better

2.1.1   Diesel Combustion  Background

       The typical diesel  engine used in nonroad applications operates on a four-stroke cycle
consisting of the intake stroke, the compression stroke, the power (or combustion) stroke, and the
exhaust stroke.  The combustion event provides the energy for engine operation.  It starts at the
end of the compression stroke and continues through the first half of the power stroke. Near the
end of the piston compression stroke, fuel is injected into the cylinder at high pressure and mixes
with the contents of the cylinder (air + residual combustion gases + EGR gases if EGR-
equipped).  This period of premixing is referred to as ignition delay.  Ignition delay ends when
the premixed cylinder contents self-ignite due to the high temperature and pressure produced by
the compression stroke in a  relatively short, homogenous, premixed combustion event.
Immediately following premixed combustion, diesel combustion becomes primarily
nonhomogeneous and diffusion-controlled (the rate of combustion is limited by the rate of fuel
and oxygen mixing).  During this phase of combustion, fuel injection continues creating a region
that consists of fuel only.  The fuel diffuses out of this region and air is entrained into this region
creating an area where the fuel to air ratio is balanced (i.e., near stoichiometric conditions) to
support combustion.d The fuel burns primarily in this region. One way to visualize this
phenomenon is to roughly divide the cylinder contents into fuel-rich and fuel-lean sides of the
reaction-zone where combustion is taking place as shown in Figure 2-1.  As discussed in the
following subsections, the pollutant rate of formation in a diesel engine is largely defined by
these combustion regions and how they evolve during the combustion process.2
       d  Stoichiometric conditions: the amount of air and fuel is balanced at the theoretical, or
chemically correct, level to give complete combustion without any unburned fuel or oxygen
remaining. The ratio of the actual air-to-fuel (A/F) ratio to the ideal stoichiometric ratio is often
referred to as lambda (A,).  In this nomenclature, A = 1 represents an ideal balance, A > 1
indicates an excess of air (oxygen) and A < 1 indicates an excess of fuel.


   Fuel Lean
                                  Piston Bowl
                                     Piston Crown
                                       Fuel Rich
                                    Fuel Spray "Cone"
 Combustion and
Partial -Combustion
                   Top View
                                                                          Fuel Spray
                                                                          Fuel Injector Tip
                             Profile, Partial Cut-away
                                      Figure 2-1
                              Diesel Combustion Schematic
2.1.2   NOx Formation Background

       NOx is formed in diesel engines from molecular nitrogen (N2) and oxygen (O2) in the
stoichiometric combustion region of the diffusion-controlled diesel combustion process
described in the previous section. At the high temperatures present in the combustion zone, a
fraction of the nitrogen and oxygen can dissociate, forming radicals which can then combine
through a series of reactions to form nitric oxide (NO), the primary NOx constituent.  Nitrogen
dioxide (NO2), the other NOx constituent, is formed from NO in the flame region. The NOx
formation rate has a strong exponential relationship to temperature. Therefore high temperatures
result in high NOx formation rates.3 Any changes to engine design that can lower the peak
temperature realized during combustion, the partial pressures of dissociated nitrogen and oxygen,
or the duration of time at these peak temperatures can lower NOx emissions. Most of the NOx
emission control technologies discussed in the following sections reduce NOx emissions by
reducing the peak combustion temperature.

       Researchers  have investigated the limits of diffusion-controlled stoichiometric
combustion in order to determine the minimum NOx emission rate possible from conventional
diesel engine combustion (this includes the use  of EGR, which is discussed later in this paper).4'5
The researchers found that there is a minimum peak combustion temperature below which the
conventional  diesel  diffusion-controlled combustion process can not be maintained. Based on
this observation, experimental data, and theoretical NOx formation rates, a minimum "practical"
NOx emission rate for conventional diesel combustion was estimated at approximately one-half
of the Tier 3 NOx+NMHC standard. A theoretical limit of conventional  diesel combustion was
estimated at approximately one-fourth of the Tier 3 standard.  The researchers  suggest that
reductions in NOx emissions below these levels will require NOx removal aftertreatment systems
or a fundamentally different form of diesel engine combustion. While new technologies are

rapidly being developed for highway engines to reduce NOx emissions below the levels
discussed here (e.g., NOx adsorbers and Homogenous Charge Compression Ignition (HCCI)) it is
unclear to what degree these advanced technologies will be available for application to nonroad
diesel engines. The NOx adsorber technology requires low sulfur diesel fuel (< 15 parts per
million (ppm) sulfur) in order to ensure durability.6 The various forms of HCCI being
investigated for highway applications do not currently provide NOx control beyond 50% engine
load, making its use for nonroad applications uncertain.7

2.1.3   PM Formation Background

       PM emitted from diesel engines is a multi-component mixture composed chiefly of
elemental carbon (or soot), semi-volatile organic compounds, sulfate compounds (primarily
sulfuric acid) and associated water.

       During diffusion-controlled combustion, fuel diffuses into a reaction zone  and burns.
Products of combustion and partial products of combustion diffuse away from the reaction zone
where combustion occurs.  At temperatures above 1,300 K, fuel compounds on the fuel-rich side
of the reaction zone can be pyrolized6 to form elemental  carbon particles.8 Most of the elemental
carbon formed (80% to 98%) is oxidized during later stages of combustion, most likely by
hydroxyl radicals formed during combustion.9'10  The remaining elemental carbon  agglomerates
into complex aggregate chain soot particles and leaves the engine as a component of PM

       From this description, the formation of soot during combustion and emission of soot as
PM following the combustion event can be summarized  as being dependent upon three primary
       1.      Temperature
       2.      Residence time
       3.      Availability of oxidants

Thus, in-cylinder control of PM is accomplished by varying engine parameters that affect these
variables while balancing the resultant effects on NOx emissions and fuel consumption.

       The combination of organic compounds (volatile and semi-volatile) that contribute to PM
are often referred to as the volatile organic fraction (VOF), or the soluble organic fraction (SOF),
depending upon the test procedure used to measure the compounds. The test procedure used to
demonstrate compliance  with the PM emission standard does not differentiate between the
various components of PM. Sulfate PM and VOF are formed after cooling and air-dilution of the
exhaust. Formation of sulfate PM is a function of fuel sulfur content.  In the absence of post-
combustion treatment of the exhaust (i.e., aftertreatment), approximately 1 to 3 % of fuel sulfur
       e Pyrolysis is a high-temperature decomposition that strips hydrogen from the
hydrocarbon fuel molecules.

is converted to sulfate, while the remainder is emitted as gaseous sulfur dioxide(SO2). Post-
combustion treatment of the exhaust using platinum catalysts can oxidize the VOF thereby
lowering PM emissions. VOF emissions are also reduced by the same in-cylinder emission
control strategies used to reduce hydrocarbon (HC) emissions.  But, this can also oxidize up to
50% or more of the SO2 to sulfate PM, depending on the exhaust temperature and the platinum
content of the catalyst.  VOF emissions are also reduced by the same in-cylinder emission control
strategies used to reduce hydrocarbon (HC) emissions.

2.1.4   The NOx vs.  PM Trade-Off

       Diesel engine emission control technology performance is often characterized by its
"NOx vs. PM trade-off. This trade-off refers to the fact that, under many conditions, control
technology designed to reduce one pollutant (e.g., NOx) will do so  while increasing production
of another pollutant (e.g., PM).  For example lower oxygen content (lowering the air-to-fuel
ratio) lowers NOx formation but increases PM formation.  Diesel engine designers must balance
this trade-off in order to accomplish compliance with both the NOx and PM standards. In the
case of the Tier 3 emission standards, since the NOx+NMHC standard is reduced substantially
while the PM standard remains at the Tier 2 level, we would expect there to be a shift in how
design engineers trade off NOx and PM emissions.  The lack of restrictive Tier 3 PM standards
makes it  directionally easier for manufacturers to meet the relatively more restrictive
NOx+NMHC standard by changing the balance of the NOx vs. PM trade-off from the Tier 2
engine designs.

2.1.5   Hydrocarbon Formation Background

       Hydrocarbon  (HC) emissions from diesel  engines primarily occur due to fuel and
lubricant trapped in crevices (e.g., at the top ring land and the injector sac) which prevents
sufficient mixing with air for complete combustion. Fuel-related HC can also be emitted due to
"overmixing" during  ignition delay, a condition where fuel in the induced swirl-flow has mixed
beyond the lean flammability limit.11 Higher molecular weight HC compounds (primarily
lubricant related compounds) adsorb to soot particles or nucleate and thus contribute to
semi-volatile organic PM.  Lower molecular weight HC compounds (chiefly fuel related
compounds) are primarily emitted in the gas phase. Under some cold-start conditions, fuel-
related HC is emitted as a concentrated, condensed aerosol ("white  smoke").  Hydrocarbons can
be controlled in-cylinder by reducing the size and number of crevices, and by reducing ignition
delay. Post-combustion treatment of hydrocarbons can be accomplished via oxidation over
precious-metal and base-metal catalysts.

2.2    Emission Control Technologies

       The following sections describe some of the emission control technologies available to
diesel engine manufacturers. The technologies described here have all been applied in one form
or another to highway diesel engines.  Most of the technologies described in these sections can be

applied directly to similar nonroad diesel engines. There are a few technologies (e.g., cooled
EGR) that have unique issues when applied to nonroad diesel engines.  We discuss those issues
in a separate section following this discussion.

       The RIA documents for the recent heavy-duty highway 2004 standards technology review
rule and for the Tier 3 nonroad emission standards contain additional information regarding the
effectiveness of several of the technologies discussed here, primarily cooled EGR systems.12'13
The reader should refer to these previous rulemaking documents for additional information on
diesel emission control technologies relevant to the Tier 3 emission standards.

2.2.1   Charge Air Cooling

       Lowering the intake manifold temperature (and therefore the initial temperature of the
gases entering the combustion chamber— air and possibly recirculated exhaust), lowers the peak
temperature of combustion and thus NOx emissions.  The NOx reduction realized from lowering
the intake manifold temperature can vary depending upon the engine design but one estimate
suggests NOx emissions can be reduced by five to seven percent with every 10°C decrease in
intake manifold temperature.14  Typically the intake manifold temperature is lowered by cooling
the intake gases through a heat exchanger located between the turbocharger compressor outlet
and the intake manifold. This type of heat exchanger is commonly called an aftercooler since it
cools the gases after they are heated by the compression work done by the turbocharger

       While aftercooling reduces NOx emissions, it was initially developed to improve the
specific power output of an engine by increasing the density of air entering the combustion
chamber. There are two kinds of aftercooling strategies- air-to-water or air-to-air.  Air-to-water
aftercoolers use engine coolant to lower the intake air temperature. This method, however, can
only reduce the temperature of the compressed intake air to the operating temperature of the
engine and significantly adds to the heat load on the cooling system.  The temperature of the
intake air after compression by the turbocharger is approximately 150°C. An air-to-water
aftercooler can only cool the intake charge air to approximately 90°C.

       Air-to-air aftercoolers use a stream of outside air flowing through a separate heat
exchanger to cool the intake air. An air-to-air aftercooler can cool the compressed intake air to a
temperature approaching that of the ambient air. Air-to-air aftercoolers are widely used with
highway  engines.  However, nonroad engines historically have not incorporated air-to-air
aftercooling. Over time, equipment manufacturers are expected to modify their designs to make
space for air-to-air aftercooling technology.  While introducing air-to-air aftercooling requires a
greater degree of engine and equipment modification, the benefits for improved fuel efficiency,
greater engine durability, higher power density,  and better control of NOx emissions make a
compelling case for their widespread use in the  long term.15

       Air-to-air aftercoolers can be somewhat less effective when used on nonroad applications

because of the lack of ram-air coolingf in most nonroad applications. The lack of ram-air cooling
can result in lower aftercooler performance in nonroad applications than for highway vehicles.
The resulting increase in intake charge temperature will reduce the NOx reduction effectiveness
of diesel engine used in nonroad applications as compared to highway applications. This fact is
one of the reasons why we set a less stringent Tier 3 NOx+NMHC standard for nonroad engines
compared to the "equivalent" heavy-duty 2004 highway NOx+NMHC standard.

2.2.2   Fuel Injection Rate Shaping and Multiple Injections

       Historically the relationship between combustion system design and fuel system operation
were fixed functions of the basic engine design and were thus optimized at a single operating
point (or designed as a compromise between several important operating points). At all other
operating points, emission performance was compromised when compared to ideal operation.
These older systems, still in wide-spread use on nonroad  engines,  have limited emission control

       The most recent advances in fuel injection technology are  the systems that use rate
shaping or multiple injections to vary the delivery of fuel over the course of a single combustion
event.  These systems are beginning to be used extensively on light and medium heavy-duty
diesel trucks, a class of engines commonly carried over to nonroad engine applications. Igniting
a small quantity of fuel initially limits the characteristic rapid increase in pressure and
temperature that leads to high levels of NOx formation. Injecting most of the fuel into an
established flame then allows for a steady burn that limits NOx emissions.  Rate shaping may be
done either mechanically or electronically.  Rate shaping has been shown to reduce NOx
emissions by up to 20 percent.16

       For electronically controlled engines, multiple injections may be used to shape the rate of
fuel injection into the combustion chamber.  Recent advances in fuel system technology allow
high-pressure multiple injections to be used to reduce NOx by 50  percent with no significant
penalty in PM. Two or three bursts of fuel can come from a single injector during the injection
event.  The most important variables for achieving maximum emission reductions with optimal
fuel economy using multiple injections are the delay preceding the final pulse and the duration of
the  final pulse.17

       Advanced common rail fuel systems are  being developed and introduced for both
       f "Ram-air" cooling refers to ambient air pushed through the air-to-air aftercooler due to
the speed of the vehicle. For a highway truck traveling at 60 miles per hour, the cooling realized
by the air flowing over the aftercooler can be substantial. Since nonroad equipment is often
stationary (e.g., a generator set) or travels at a slow speed (e.g., a tractor working in a field) this
effect is not very strong for nonroad engines.  Nonroad equipment commonly uses an engine
mounted fan to impart additional air flow through the aftercooler and radiator in order to
compensate for the lack of ram air.


highway and nonroad diesel engines. These fuel systems allow fuel injection rates (as
determined by fuel pressure) and multiple fuel injection events to be changed based upon any
engine operating condition.  This flexibility allows engine designers to tailor the fuel injection
system operation across the entire range of engine operation.  The ability to tailor fuel system
control parameters (injection timing, injection pressure, injection rate and injection
duration/quantity) across the range of diesel engine operation removes these historic
compromises allowing for substantial improvements in engine performance and emissions.18'19

       The dramatic improvements in engine performance and emission controls promised by
these new technologies has led to rapid introductions of common rail fuel systems to nonroad
engines, well in advance of the rate we would have predicted only a few years ago. John Deere
announced that many of its new Tier 2 compliant engines will utilize Denso common rail fuel
systems.20 Similarly, Caterpillar is expanding the use of its Hydraulic Electronic Unit Injector
(HEUI) fuel system in nonroad engines in order to meet the more restrictive Tier 2 standards
while reducing fuel consumption by five to 10 percent.21

2.2.3  Injection Timing Retard

       Delaying the start of fuel injection and thus the start of combustion can significantly
reduce NOx emissions from a diesel engine.  The effect of injection timing on emissions and
performance is well  established.22'23'24'25 Delaying the start of combustion by retarding injection
timing aligns the heat release from the fuel combustion with the portion of the power (or
combustion) stroke of the engine cycle after the piston has begun to move down. This means
that the cylinder volume is increasing and that work (and therefore heat) is being extracted from
the hot gases. The removal of this heat through expansion lowers the temperature in the
combustion gases. NOx is reduced because the premixed burning phase is shortened and because
cylinder temperature and pressure are lowered. Timing retard increases HC, CO, PM, and fuel
consumption, however, because the end of injection comes later in the combustion stroke, where
the time for extracting energy  from fuel combustion is shortened, and the cylinder temperature
and pressure are too low for more complete oxidation of PM. Many of these tradeoffs with
injection timing retard can be changed through the application of new technologies such as
common rail fuel systems and EGR as discussed elsewhere in this section.

2.2.4  Exhaust Gas Recirculation

       EGR reintroduces or retains a fraction of the exhaust gases into the cylinder.  The use of
EGR decreases NOx formation in three different ways:

1.      Thermal Effects Due to Mass: EGR can thermally reduce peak combustion temperature:
Increasing the mass of the cylinder contents by increasing carbon dioxide (CO2) and water vapor
concentrations reduces peak cylinder temperatures during combustion.26 At higher engine loads
(with increased  fuel  injection quantities) with EGR, increased turbocharger boost pressures are
necessary to prevent increased soot formation due to low air-to-fuel ratios.  The increased boost

pressure further increases the mass of cylinder contents.

2.      Dilution Effect: A fraction of the air within the cylinder is replaced with inert exhaust,
primarily CO2 and water vapor.  This reduces the amount of molecular oxygen available for
dissociation into atomic oxygen, an important step in NOx formation via what is know as the
Zeldovich mechanism.27

3.      Chemical Dissociation Effect: Although perhaps of less significance than thermal and
dilution effects, the high temperature dissociation of CO2 and water vapor is highly endothermic,
and thus can reduce temperatures via absorption of thermal energy from the combustion

EGR can be implemented in a variety of ways:

1.      External high pressure loop EGR.  In this case exhaust from the exhaust manifold
(upstream of, or integral with, the turbocharger exhaust turbine) is routed into the intake
manifold (downstream of the turbocharger compressor and the aftercooler).  Under many
conditions the intake manifold is at a higher pressure than the exhaust manifold. Therefore some
means of driving EGR flow is necessary, such as a venturi, intake throttle, pump,  or some
combination of these components. One disadvantage of this approach is that it reduces exhaust
energy available to drive the turbocharger.

2.      External low pressure loop EGR: In this case, exhaust is routed from downstream of the
turbocharger exhaust turbine to the inlet of the turborcharger compressor. This approach is
generally not favored for diesel engines since the turbocharger compressor and aftercooler are
subject to fouling from PM and other exhaust constituents.

3.      Internal EGR:  This can be accomplished via adjustments to valve timing events that
result in increased retention of residual exhaust within the cylinder.  It can also be accomplished
by introducing a separate exhaust valve opening event that allows exhaust from adjacent
cylinders to be drawn into the cylinder  from the exhaust manifold.

       There are both cooled (engine-coolant heat exchanger) and uncooled varieties of external
EGR, and  at least one source has investigated expansion cooling of internal EGR through use of
early intake valve closing (a variant of the Miller-Atkinson cycle).29

       One of the drawbacks of hot EGR is that the high temperature of the EGR increases the
intake manifold temperature substantially. This has two negative effects.  The first is reduced
intake charge density which lowers the fresh air/fuel ratio for a given level of turbocharger boost.
The lower air/fuel ratio can result in higher PM  emissions if the boost pressure is  not increased to
compensate for the lower intake charge density. Even if the boost pressure is increased, the
additional  pumping work done by the turbocharger can cause an increase  in fuel consumption.
The second negative effect of the high intake manifold temperature is that it decreases the

effectiveness of the EGR. NOx is a strong function of temperature, and any increases in the
intake charge temperature will result in higher NOx production during the combustion process.
The impact of the temperature is smaller than the overall impact of the EGR so the net effect is
still a reduction in NOx, but lowering the intake charge temperature would lead to an even
greater NOx reduction. For these reasons, a number of highway manufacturers are using cooled
EGR to meet the 2004 highway emission standards and we anticipate this technology may be
applied to some nonroad applications.30

       Exhaust gas recirculation reduces the air-fuel ratio at a given engine load by two
mechanisms: dilution of the fresh air charge and increased charge temperature. These
mechanisms can be countered by higher intake manifold pressures. Higher intake manifold
pressures would also be necessary to maintain power density and air-fuel ratios sufficient to
prevent excessive PM increases. The additional pressure would increase the charge density and
maintain the desired air-fuel ratio. To accomplish this EGR equipped engines are likely to use
turbomachinery capable of operating at higher pressure ratios. The higher pressure ratios will
also increase the compressor discharge temperature and require additional aftercooler heat
rejection to reduce the fresh air charge temperature back to previous non-EGR levels.

2.2.5  Induced Mixing/Charge Motion

       Inducing turbulent mixing is one means of increasing the likelihood of soot particles
interacting with oxidants within the cylinder. Turbulent mixing can be induced or increased by a
number of means including increased (high) injection pressure, multiple/split injections, intake
port/valve design and piston bowl  design.

       When diesel fuel is injected into the cylinder during combustion the high pressure fuel
spray causes increased motion of the air and fuel within the cylinder.  This  increased motion
leads to great air and fuel interaction and reduced paniculate matter emissions. Increasing fuel
injection pressure increases the velocity of the fuel spray and therefore increases the mixing
introduced by the fuel spray.

       Multiple injection/split injection has been shown to significantly reduce particulate
emissions, most notably in cases that use retarded injection timing or a combination of injection
timing retard and EGR to control NQx.31'32'33'34  The typical diffusion-burn combustion event is
broken up into two events.  A main injection is terminated, then followed by a short dwell period
with no injection, which is in turn  followed by another short injection event, see Figure 2-2. The
second pulse of injected fuel induces late-combustion turbulent mixing. The splitting of the
injection event into two events aids in breaking up and entraining the "soot cloud" formed from
the first injection event into the bulk cylinder contents. Sufficient control of the post injection
event would likely require the use  of either a high-pressure common rail or HEUI fuel injection


                            < I DC)
                          10      20      30
                       Figure 2-2
An example of using multiple fuel injection events to induce late-
combustion mixing and increase soot oxidation for PM control
(Adapted form Pierpont, Montgomery andReitz, 1995).
       Increasing the turbulence of the intake air entering the combustion chamber (i.e., inducing
swirl) can reduce PM 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 and air motion in a smaller cavity in the
piston to induce additional turbulence. Manufacturers are also changing to three or four valves
per cylinder, which reduces pumping losses and can also allow for intake air charge motion. The
effect of swirl is often engine-specific, but some general effects may be discussed.

       At low loads, increased swirl reduces HC, PM, and smoke emissions and lowers fuel
consumption due to enhanced mixing of air and fuel. NOx emissions might increase slightly at
low loads as swirl increases. At high loads, swirl causes slight decreases in PM emissions and
fuel consumption, but NOx may increase because of the higher temperatures associated with
enhanced mixing and reduced wall impingement.35 A higher pressure fuel system can be used to
offset some of the negative effects of swirl, such as increased NOx, while enhancing the positive
effects such as a reduction in PM.36 Intake air turbulence such as "swirl" can be induced using
shrouded intake valves or by use of a helical-shaped air intake port.37 Swirl is important in
promoting turbulent mixing of fuel and soot with oxidants, but can reduce volumetric efficiency.

       Piston bowl design can be used to increase turbulent mixing.  One example of this is the
reentrant piston bowl shown in Figure 2-3. Reentrant bowl designs induce separation of the flow
over the reentrant "ledge" of the piston and help to maintain swirl through the compression
stroke and into the expansion stroke.38

                (A)                        (B)                         (C)

                                       Figure 2-3
                 Schematic examples of a straight-sided piston-bowl (A),
         a reentrant piston bowl (B), and a deep, square reentrant piston bowl (C).

2.2.6   Control of Air-to-Fuel Ratio

       Availability of oxidants (primarily oxygen) is highly dependent on the overall air-to-fuel
ratio at which the engine is operated.  Thus soot formation/oxidation and PM emissions are a
strong function of the overall A/F ratio of the engine. Temporary conditions of fuel-rich air-to-
fuel ratio can occur on turbocharged engines when the engine power demand increases sharply.
This is because the fuel system can increase fueling immediately but the turbocharger takes some
time to increase air flow.  A well known example of this is emission of black smoke when an
older diesel vehicle (without A/F control) accelerates from a stoplight. Electronic control of fuel
injection can limit the injected quantity of fuel until sufficient air supply (i.e., boost pressure) can
be obtained. Electronic wastegate control and electronically controlled, variable geometry
turbochargers (VGT) can also be used to improve turbocharger response and improve PM
emissions under such transient conditions.  Electronic control of EGR is also necessary to
prevent temporary conditions of overly fuel-rich air-to-fuel ratios.

2.2.7  Diesel Oxidation Catalyst

       The flow-through oxidation catalyst reduces HC and PM emissions by oxidizing both
gaseous (volatile) hydrocarbons and the semi-volatile portion of PM known as the volatile
organic fraction (VOF) forming carbon dioxide and water. The VOF consists of hydrocarbons
adsorbed to the carbonaceous solid particles and heavy hydrocarbons that under atmospheric
conditions will condense to form liquid aerosols. The soot or elemental carbon portion of the
PM remains largely unaffected by the catalyst. Although engine design improvements have
reduced VOF emissions, VOF can still comprise a  significant fraction of the total PM mass.

       Diesel oxidation catalysts (DOCs) can also oxidize sulfur species in the exhaust to form
sulfate PM. At higher exhaust temperatures, catalysts have a greater tendency to oxidize sulfur
dioxide to form sulfates, which contribute to total PM emissions. Catalyst manufacturers have
been successful in developing catalyst formulations that minimize sulfate formation.39 Catalyst
manufacturers have also adjusted the placement of the catalyst to a position where the needed
VOF reduction is achieved, but sulfate formation is minimized.40 Nonroad fuel with sulfur
concentrations higher than 0.05 weight percent will limit the effectiveness of diesel oxidation


catalysts but as shown by at least one manufacturer, DOCs can be used to reduce PM emissions
as part of a Tier 3 compliant emission strategy (see Figure 2-4).41
                       380 ppm
                                            1180 ppm

                                          Fuel Sulfur Level
                                                                2000 ppm
                                 Figure 2-4
          Nonroad Tier 3 Particulates as a Function of Fuel Sulfur 8 Mode Cl
            Cycle, Oxidation Catalyst Equipped Diesel Engine

2.2.8   Control of Oil Consumption

       Reducing oil consumption not only decreases maintenance costs, but also VOF and PM
emissions. Reducing oil consumption has been one of the primary ways that highway diesel
engines have complied with the PM standard since 1994.  Oil consumption through the
combustion chamber can be reduced through improvements in piston ring design and through the
use of valve  stem seals. Piston rings can be designed to "scrape" oil from the cylinder liner
surface back into the crankcase reducing the amount of oil consumed during combustion from
the cylinder. Valve stem seals can be used to reduce oil leakage from the lubricated regions of
the engines valvetrain into the intake and exhaust ports of the engine. Engine designs that
incorporate these technologies have reduced VOF and PM emissions.

2.3    Diesel Emission Control Systems  Capable of Tier 3 Compliance

       The preceding sections describe the fundamentals of emission formation in diesel engines
and the control technologies used to reduce these harmful emissions. The control technologies
identified here are all well known and represent existing control technologies applied either to
highway or nonroad diesel engines.  The application of these technologies to nonroad engines
does not require new invention or substantial changes to the technologies. Many of the
technologies discussed here are strongly interrelated, with changes in one area potentially leading
to synergistic improvements in others (e.g., advanced common  rail fuel systems can allow for
reduced NOx and PM emission while improving fuel economy). The integration of these

technologies into a single emission control system design can allow for substantial reductions in
diesel engine emissions. We do not believe that a single emission control technology alone will
be applied in order to meet the Tier 3 emission standards. Rather we believe, based on the
substantial test data provided in the following section, that a number of different systems
approaches incorporating many of the technologies discussed here can be used to comply with
the Tier 3 standards.  In this section, we have included examples of engine designs which
incorporate the previously discussed emission control technologies into total emission control
systems in order to demonstrate the emission control capability needed to meet the Tier 3
standards. These examples serve to show that a systems approach incorporating existing
technologies can achieve compliance with the Tier 3 standards.

2.3.1   Optimized Engine Systems Using Cooled EGR

       The use of EGR for NOx control is a well established technology for gasoline engines
and is experiencing growing acceptance for highway diesel engines.  Cooled EGR systems for
diesel engines are in current production for light-duty vehicles in the U.S. and Europe as well as
for some heavy-duty transit bus applications in the U.S.42 Several diesel engine manufacturers
including Cummins, Detroit Diesel, and Mack have indicated that cooled EGR will be the
primary technology used to comply with the heavy-duty highway 2004 engine standards.  These
same manufacturers are expected to have cooled EGR engines in production starting in 2002,
more than four years earlier than the start of the Tier 3 phase-in.43

       One example of the emission control systems these companies will use is the system
Cummins has announced it will introduce in late 2002 compliant with the heavy-duty highway
emission standards set for 2004.  This engine uses a high pressure loop (exhaust manifold to
intake manifold) cooled EGR system, an advanced high pressure common rail fuel system,
improved turbocharger system, a centrally located fuel injector, and combustion system
enhancements. This system incorporates a jacket water cooled EGR cooler and a variable
geometry turbocharger to drive and control the EGR rate.44  Lower power ratings (similar to
typical nonroad engine ratings) use a conventional turbocharger with a wastegate.  With this
emission control system, the engine is able to meet the heavy-duty highway transient
certification cycle and the Supplemental Emissions Test standards of 2.5 g/hp-hr NOx+NMHC,
0.1 g/hp-hr PM.45  This engine also has  to meet a Not-To-Exceed (NTE) zone cap  of 3.12 g/hp-hr
NOx+NMHC and 0.125 g/hp-hr PM. The NTE zone includes six of the eight nonroad
certification modes (see Figure 2-5).  The six that are included in the NTE region are the most
heavily weighted test modes in terms of the total emission and work used to calculate the
nonroad composite emission rate. Therefore, if an engine is compliant at those six modes it is
highly  likely to be compliant with the eight mode composite level. Since this engine is
compliant with the NTE limit, it is likely that this highway engine would be compliant with the
nonroad Tier 3 emission standards. In fact, the emissions level would likely be close to 2.5 g/hp-
hr NOx+NMHC (well under the Tier 3  standard) over the non-road certification composite.  This
is because compliance with the Supplemental Emissions Test standards includes testing at 13
steady-state test modes which are similar or in some cases coincident  with the nonroad emission
test modes. If this same engine were sold in a nonroad application for Tier 3, it may even be


possible to calibrate the engine with less reliance on cooled EGR while still meeting the Tier 3
               — —Onroad NOx NTE
                - - Onroad PM NTE
                • Nonroad Certification Modes
                                          Speed (rpm)
                                      Figure 2-5
                   Map of Engine Operating Range (Torque vs Speed)

       The emission control system described here represents the kind of technology that we
anticipated would be used to meet the Tier 3 emission standards when we set the standards in
1998. We continue to believe that this approach can be used to meet the Tier 3 emissions
standards, but we no longer are assuming that this will be the only way that emission control
technologies will be combined in order to meet the Tier 3 standards. The following sections
detail two more approaches that we believe some manufacturers could use to comply with the
emission standards.

2.3.2   Caterpillar's ACERT

       Caterpillar has announced that it will produce engines compliant with the Tier 3 emission
standards using existing highway diesel engine technologies. The new engines being developed
by Caterpillar will be marketed under the trade name "Advanced Combustion Emissions
Reduction Technology" or "ACERT." Caterpillar indicated in a letter to the EPA that it is
"prepared to license the ACERT technology, including related patents, to Caterpillar's engine
competitors."46 Caterpillar has  provided data for a mid-range industrial engine (Caterpillar 3126)
that meets the Tier 3 standards (Figure 2-6 shows three compliant calibrations of the ACERT
system).47 Caterpillar describes ACERT as a combination of proven hardware components
integrated in a systems approach to meet emissions  and performance goals. The engines use
open-loop electronic engine controls, the HEUI fuel system, a variable geometry turbocharger,

valve event control, and a diesel oxidation catalyst (DOC). While Caterpillar did not identify a
specific NOx or PM reduction associated with each of the technologies in the ACERT system,
the following discussion describes how the identified technologies may work in the ACERT
                                      Tier III 8-Mode Emissions
                                  Caterpillar 3126 Engine with ACERT
                                     Industrial C Rating (D5, 962)

1 "•'








                                       2.4       2.6       2.8
                                        NOx+HC Emissions (g/hp-hr)
                                         Figure 2-6
                    Tier 3 Emissions Caterpillar 3126 ACERT Engine

       Caterpillar has stated that the HEUI injection system used with ACERT has the ability to
accurately and independently control the number of fuel injection events, fuel injection pressure
and the injected quantity. Multiple injections allow the use of a late "post-injection" event for
PM control, which can allow further injection timing retard for NOx control. Caterpillar has
stated that the VGT used within ACERT is used to allow the electronic control system to regulate
intake air pressure. The use of VGT in concert with electronic control of injection timing and
injection quantity can extend A/F  and PM control over a broader range of engine operating
conditions, thus reducing PM emissions while maintaining NOx control. Control of valve events
can be used to reduce NOx emissions by allowing a degree of internal EGR either by retaining
more residual gases in cylinder or by allowing exhaust flow-reversal.  Control  of valve events
can also be used to provide a degree of expansion cooling of the cylinder contents. Caterpillar
has not commented on the potential use of valve event control for reducing NOx emissions.

       The diesel oxidation catalyst used by Caterpillar provides additional HC control, which
will likely improve the ability to meet the Tier 3 NMHC+NOx  standard.  Using a diesel
oxidation catalyst may also provide a reduction in the semi-volatile organic compounds that
contribute to PM. Caterpillar provided emissions data showing that over the range of fuel  sulfur
levels required for in-use testing under Tier 3, sulfate-make with the DOC selected was


sufficiently low to allow compliance with the Tier 3 PM emission standard (see Figure 2-4).

2.3.3   Hot EGR and Combustion System Optimization

       Under contract with EPA, Southwest Research Institute (SwRI) has been working to
evaluate a variety of means to reduce emissions from a typical nonroad engine, with engineering
support from a number of engine/component manufacturers. For this work a Tier 1 compliant
John Deere 4045H Powertech diesel engine was used as a baseline for emissions improvement
and development. The engine, as delivered by John Deere, has a displacement of 4.5 liters, and
is equipped with a turbocharger, an aftercooler, two valves per cylinder and a mechanical direct
injection fuel system.48 John Deere  has recently announced improvements to this engine that
include the use of four valves per cylinder, an electronic fuel system, and higher capacity
electronics.49 This new improved engine was not available for use at the time the test program
was started.

       One step in the work at SwRI included the addition of an external hot EGR system
controlled by an EGR valve and variable nozzle turbocharger (a particular type of VGT) with an
electronically controlled fuel  system to the engine. This testing demonstrated NOx + HC
emission levels of 4.0 g/hp-hr with  a PM level of 0.12 g/hp-hr over the nonroad test cycle.50 The
NOx reduction performance realized in this step of the development work was limited in part by
limitations on the amount of EGR that could be achieved with the test configuration. Additional
EGR flow could be achieved through the use of an intake-side venturi, an integral EGR turbo
pump,  or other intake flow control strategies. From the testing at SwRI, additional EGR flow to
reduce NOx at rated and peak torque operating conditions could be expected to significantly
reduce NOx emissions.

       An additional design iteration of the engine by SwRI included the use of cooled EGR and
air-to-air aftercooling. This engine demonstrated NOx+HC emissions of 3.3 g/bhp-hr and PM
emissions  of 0.10 g/bhp-hr over the  nonroad test cycle, using fuel sulfur and aftercooling levels
appropriate to nonroad engine applications. The significant NOx and PM emission reductions
demonstrated in this test program were accomplished without the use of a common rail  fuel
system which would be expected to  allow for further reductions in NOx and PM  control due to
the added flexibility it would provide in controlling injection pressure (rate) and multiple
injection events.

       The various design iteration  steps of the development program at SwRI are documented
in Figure 2-7.

" £0-8178 C1 cycle, Tier I (1998)
• Standards are for ..Q Qniv
Rfl-1flfl hp HI Fnninp^ x -;^.
Tier II (2004)
Cooled /
' EGR x ^
Design \
- Target x
Cooled EGR

"* A

/ (no EGR)
I ,

I , I
                                       NOx + HC (g/hp-hr)
                                     Figure 2-7
                          Technology Sequence for SwRI Engine
       John Deere has been investigating a number of system solutions to develop nonroad
engines compliant with the Tier 3 emission standards.  One approach that John Deere has
focused on is a system incorporating a high pressure fuel injection system with injection rate
shaping capabilities, a diesel oxidation catalyst (to lower HCs and PM), and hot EGR (either
internal or external).51 This technology development approach is  consistent with the types of
systems solutions considered in this paper.

2.4    Special Considerations

       The Tier 3 emission standards were developed based upon our assessment that emission
control technologies developed for highway diesel trucks could be applied to nonroad engines
and equipment.  We see considerable evidence that this transfer of technology is already
occurring with the introduction of Tier 2 compliant engines (e.g.,  the use of common rail fuel
systems, air-to-air aftercooling and advanced electronics), and we continue to believe that
highway technology transfer will enable Tier 3 emission  compliant engines. However, we have
always recognized that there are differences between highway vehicles and nonroad equipment
that must be considered in evaluating engine technologies.  These differences include acceptable
levels of heat rejection, limited aftercooler performance without ram-air for nonroad, and

equipment packaging constraints. In large part due to these differences, the Tier 3 standards were
set at a less stringent level when compared to the corresponding highway engine standards. The
standards are both numerically less restrictive and also defined under less restrictive test
conditions (i.e., nonroad has a simple eight mode steady-state test cycle,  highway engines must
meet a demanding transient emission test including both cold and hot cycles). We continue to
believe that these less stringent standards appropriately account for the unique nonroad engine
and equipment issues as discussed in the following sections.

2.4.1  Nonroad Fuel Quality (Sulfur)

       Diesel fuel sold for use in nonroad equipment is not currently regulated by EPA. Typical
nonroad diesel fuel meets ASTM specification D975 which sets a maximum sulfur level of 5,000
ppm. Fuel meeting this standard commonly has a sulfur level of up to 3,000 ppm. For this
reason, as well as for reasons of convenience and availability, many nonroad equipment users
choose to operate their equipment on fuel sold for highway vehicle use.  Highway diesel fuel is
regulated and has a maximum sulfur content of 500 ppm with a typical average sulfur level of
300 ppm. Nonroad engines and equipment are designed to operate on the maximum fuel sulfur
level that may be encountered during the vehicle's life (e.g.,  5,000 ppm). Sulfur in diesel fuel
can cause increased corrosion of engine components, accelerated deterioration of engine
lubricating oil, accelerated engine wear and increases in PM emissions.  Consideration of these
issues is given in nonroad engine designs and maintenance schedules.

       The use of cooled EGR to comply with the Tier 3 emission standards may be more
difficult when compared to highway applications due to the higher fuel sulfur level typical of
nonroad diesel fuel.  Sulfur is an issue since it forms corrosive sulfuric acid (H2SO4) in diesel
exhaust. During combustion sulfur is oxidized 97-99% to  sulfur dioxide  (SO2) and trace amounts
of sulfate (SO3).52 SO3 also forms in the exhaust manifold as equilibrium thermodynamics begin
to favor its formation below ~730°C. However, reaction kinetics limit the SO3 formation rate.53
In diesel exhaust SO3 immediately reacts with water vapor to form aqueous sulfuric acid (-73%
H2SO4 by wt), and this acid begins to condense from about 80 to 145°C  , depending upon engine
operating conditions and fuel sulfur content.54 55 56 Although the acid's concentration is strong,
the acid at this point only accounts for -0.5% of the fuel sulfur. However, once the exhaust cools
below the water vapor dew point (-30 to 80°C), SO2, which  accounts for nearly all of the fuel
sulfur, will begin to react significantly with condensed water to form H2SO4.57  Since nonroad
fuel has six times the sulfur content, the rate of acid condensation will increase by a similar
amount relative to highway.58

       The increase in condensation rate is significant since the increased fuel sulfur content
increases a nonroad engine's exposure to sulfuric acid.  The acid impacts the engine in at least
three ways: direct corrosion of engine components, secondary wear from corrosion byproducts,
and acidification of the engine oil. Condensation can occur in the EGR cooler, after the EGR is
mixed with the fresh air in the intake system, and on the cylinder walls.  Studies have  shown that
increased fuel sulfur levels (350 ppm to 13,300 ppm) increase engine wear significantly,
particularly in conjunction with EGR.59 Material selection will be key to making cooled EGR


work with nonroad fuel.

       In the EGR cooler, condensation can occur over a wide range of operating conditions.60
Condensation in the cooler is a function of the acid/water dewpoint temperature which is
influenced by sulfur concentration, pressure, and the air-fuel ratio of the engine. Increasing
sulfur concentration, increasing pressure, and decreasing air-fuel ratio all increase the dewpoint
temperature and the likelihood of condensation.61 The fuel sulfur level is not an engine design
parameter, but pressure and air-fuel ratio can be manipulated by the engine designers.
Unfortunately, these factors tend to work against each other such that higher air-fuel ratios
require higher pressures and vice versa, so that this trade-off, while it will certainly play into the
engine optimization, will not be able to completely eliminate condensation in the EGR cooler.
Lower coolant temperatures  also increase the condensation rate.62 A simple solution to this could
be to turn EGR off until the coolant temperature has reached its normal operating temperature.
In any case, condensation will still  occur in the EGR cooler. Work has already been done for
EGR coolers that shows materials are available that provide suitable corrosion resistance.63
Higher grade stainless steels and fabrication methods (e.g., laser welding rather than brazing) will
allow EGR coolers to live in this corrosive environment.

       Direct corrosion of the engine components also occurs in the cylinder. Acid condensation
can corrode the piston, rings, and liner. The most damaging corrosion occurs at the top ring
reversal area of the liner. Due to the slow piston speed in this area, lubrication is poor, making
the liner surface finish critical for oil control. The liner surface finish is carefully selected and
controlled to hold oil and provide lubrication for the rings.  The top ring reversal area is the liner
area most exposed to high pressure combustion products, including acids.  Corrosion in this area
compromises the surface finish causing loss of lubrication and further accelerated wear.
Therefore it is very important to select the ring and liner materials to be corrosion resistant.  In
addition to careful selection of the basic materials, plasma sprays  can be used to provide
corrosion resistance.  Tests have shown these sprayed on coatings (like FFS, a material
containing 434 stainless steel and Ni-BN) to be effective in reducing formic acid corrosion of
iron liners.64

       Condensation can also occur when the EGR is mixed with the lower temperature fresh air
in the engine intake system.  The same factors influence the dewpoint temperature and
condensation rate as in the EGR cooler, with the addition of the fresh air temperature.  The fresh
air temperature is a function of the  ambient temperature, engine load (boost level and compressor
efficiency), and aftercooler efficiency. As discussed previously, the aftercooler efficiency of
nonroad equipment is not likely to be as high as found in highway equipment due to the lack of
ram  air. Consequently, the fresh air temperature will be higher than on highway. This will
reduce the condensation rate somewhat compared to highway engines at similar conditions.
What this means is that the condensation rate will not quite scale directly with the fuel sulfur
content  but will be somewhat less depending on the particular engine and equipment.

       Since condensation will be  occurring in the intake system  downstream of the EGR
introduction point, the materials that it comes in contact with will need to be corrosion resistant.65


Corrosion in the intake system is particularly troublesome because it can accelerate cylinder kit
wear. As the intake system corrodes, the corroded materials can flake off and find their way into
the engine. Some of these corrosion products, like aluminum oxide, are very good abrasives that
can rapidly abrade the cylinder liner and rings.  To prevent this, coatings or materials can be used
to reduce or eliminate the exposure of corrosion susceptible materials. Stainless steels or
improved corrosion resistant aluminum materials might be used in place of aluminum castings in
the intake  system, for instance.

       Should nonroad engine and equipment manufacturers choose to use cooled EGR as part
of a Tier 3 engine emission strategy, we believe that they may have to make some changes in
material selection and maintenance intervals compared to similar highway engines due to the
high sulfur content of nonroad diesel fuel.  However, we understand that the identification of
these material changes will come as a natural extension of the extensive work already done by
highway engine manufacturers and their suppliers to develop cooled EGR systems.66 It may be
possible given the rapid progress of highway EGR system development that no changes will be
necessary due to the very high quality and durability levels expected of highway diesel engines.
Additionally, the use of low sulfur highway diesel fuel is  a common practice for many users of
nonroad equipment and would for those users reduce any increased maintenance levels expected
when compared to highway engines.

2.4.2  Nonroad Equipment Design Impacts

       Although assessing the feasibility of new emission standards primarily centers on the
emission control technologies  available to engine designers,  there is an additional concern as
well, stemming from how these emission controls might alter equipment designs. New engine or
aftertreatment characteristics that could compromise the functionality or safety of nonroad
equipment, or that may result in the need for sweeping redesigns of equipment models in too
short a leadtime period, impact the feasibility of emission standards.

       These impacts on equipment manufacturers were thoroughly considered in the RIA for
the 1998 rulemaking that set the Tier 3 standards. As a result, a number  of special flexibility
measures were adopted in that final rule specifically to mitigate these impacts. For example,
equipment manufacturers may exempt a certain portion of their production from the need to use
engines meeting the new standards.  This exemption allowance spans seven years to cover both
Tier 2 and Tier 3 product introductions.

       The physical impacts that the use of new emission controls may have on equipment
designs essentially fall into two broad categories- (1) increased heat rejection and fuel
consumption (resulting in larger heat exchangers, cooling fans, and fuel tanks), and (2) packaging
changes to accommodate engine and heat exchanger profile changes or the addition of exhaust
emission control devices.

       Some manufacturers have expressed a belief that the  potential Tier 3 technologies
discussed in Section 2.2 will result in added heat rejection and a resulting need for larger,


possibly relocated, heat exchangers and cooling fans.67 Cooling of the exhaust gas flow in a
cooled EGR system, for example, requires the use of an additional cooling unit.  It is not evident
that this increased heat rejection would be associated with an overall increase in fuel
consumption and need for larger fuel tanks, however, because the addition of EGR to accomplish
NOx control can allow engine designers to advance injection timing from the retarded timing
strategies used for NOx control in Tier 2 engines, thereby recovering some of the efficiency loss
associated with timing retardation. This, in fact, has been projected for highway diesel engine
designs that use cooled EGR to meet the 2004 model year standards.68 g

       Even though cooled EGR systems have the potential for an increase in heat rejection,
Caterpillar expects that its ACERT technology for Tier 3 will result in at most a slight increase in
heat rejection compared to Tier 2 engines.69 Caterpillar is also projecting that the use of ACERT
technology in nonroad diesels will not reduce fuel economy, reliability, or performance.70

       It seems clear that some packaging impacts will result from Tier 3 technologies.
Although highway engine design experience indicates that EGR plumbing in itself may not
greatly alter the engine profile, adding an EGR cooler could result in additional equipment
redesign in applications with engine compartments tightly constrained by functional or safety
objectives. ACERT technology, too, may involve additional equipment design engineering to
accommodate aftertreatment devices and other changes.

       Although these potential equipment impacts may result in added engineering effort to
match equipment to Tier 3 engines, they are not more severe than what was envisioned in the
feasibility assessment undertaken in the 1998 rule. In fact, the advent of ACERT technology,
with the expectation of little to no increase in heat rejection, provides an additional path to
minimizing equipment impacts not envisioned  in 1998.  The fact that Caterpillar makes not only
engines, but also equipment using those engines in a large variety of applications, reinforces the
expectation that this technology is being developed with the needs of equipment designers in

       The 1998 rule set a multi-tier schedule of emission standards extending through most of
this decade, thus helping engine and equipment manufacturers to plan product redesigns.  The
RIA expressed the expectation, based on manufacturers' comments, that equipment redesigns
for Tier 2 would be made with Tier 3 needs in mind, so that further changes for Tier 3 would be
minimized.71  Nevertheless, we developed the flexibility provisions in the final rule to extend to
both tiers in order not to constrain manufacturers redesign strategies. For example, we provided
a 7 year period, spanning Tier 2 and Tier 3 phase-in dates, in which the equipment manufacturer
exemption allowance provisions could be used. In fact, some Tier 2 engine redesigns have been
accomplished with very little change to parameters that affect equipment designs, highlighting
       g  The reason that fuel efficiency can be maintained even though more engine heat is
rejected to a heat exchanger is that less heat would leave the engine through the exhaust gases, a
likely phenomenon with cooled-EGR systems.


the value of this flexible approach.72 The fact that the final rule had envisioned an added PM
control requirement for Tier 3, a requirement not included in this paper's feasibility assessment,
reinforces our belief that the rule's conclusion of Tier 3 feasibility with respect to equipment
impacts is still appropriate.

2.5    Tier 3 Technology Assessment Conclusions

       In 1998, when we set the Tier 3 emission standards, we did so with the belief that the
technologies being developed for highway diesel engines  (especially cooled EGR) could be
carried over and applied to nonroad diesel engines. Since that time we have followed
developments in both the highway and nonroad diesel engine markets and have observed
continued advancements in emission control technologies. Moreover we have observed an
increasing rate of highway engine technologies (especially electronic fuel system technologies)
being applied to nonroad engines.

       While we did predict that highway engine technologies could be applied to nonroad
engines, the effectiveness of such an approach and the preferred  technology paths developing
from it are different from what we had assumed. In 1998  we assumed that cooled EGR would be
the primary technology used by all engine manufacturers to comply with the Tier 3 standards.
We no longer believe that cooled EGR will be the preferred technology for all engine
manufacturers. It now appears that there are several different system approaches incorporating
existing technologies (as detailed in the preceding sections) available to diesel engine
manufacturers which will allow for compliance with the Tier 3 emission standards. The fact that
diesel engine manufacturers have identified multiple system solutions to Tier 3 compliance
reinforces our assessment that the Tier 3 emission standards are feasible.

       As described in the sections above, the application of highway engine technologies to
nonroad engines can enable substantial reductions in NOx+NMHC emissions to the levels
required by the Tier 3 emission standards. While some changes  to the technologies may be
required in order to address unique nonroad issues, the fact that the Tier 3 emission standards do
not begin to phase-in until 2006 (fully phased-in in 2008), leads us to conclude that there is
ample lead time for these design enhancements to occur.  In addition, the fact that a nonroad
engine and equipment manufacturer (Caterpillar) has provided evidence that it can meet the Tier
3 standards four years in advance of 2006 provides us with additional assurance that the
standards are feasible. For all of these reasons, we continue to believe these standards are
technologically feasible.

3.     Tier 2 Under 50 hp Technology Assessment

       Nonroad diesel engines with power ratings under 50 hp were not regulated in our 1994
rulemaking that set the first Tier 1 standards. Instead, Tier 1 standards for these engines were set
in the 1998 rulemaking and took effect in 1999 and 2000 (phased in by horsepower). Tier 2
standards were also set for these small engines in the 1998 rule, to phase in over 2004-2005
(Table 3-1). These Tier 2 standards were set at stringency levels comparable to the Tier 2
standards for larger nonroad engines, which in turn were based on conventional in-cylinder
control technologies already in-use in highway engines, but with allowance made in the
standards levels for aspects of design and operation unique to small engines, such as the priority
put in this market on simple, low-cost, non-electronic designs. Shorter useful life requirements
were also adopted.  Even with these allowances made, we felt that it was appropriate to include
the small Tier 2 engines in the technology review because at the time of the 1998  rulemaking
these engines had never been regulated by EPA, and we expected that a review undertaken after
Tier 1 designs and certification test results became available would be beneficial.

                                       Table 3-1
                    Small Nonroad Engine Tier 2 Standards (g/hp-hr)
Engine Power
using conventional technologies of proven durability, combined with the 3 to 4 years of leadtime
remaining before Tier 2 implementation, shows conclusively that the Tier 2 small engine
standards are feasible.

                                            Table 3-2
                        Tier 1 Engine Compliance With Tier 2 Standards

Tier 2
Engine Families
Percent Tier 2

4.     Dealing With Future Air Quality Impacts

       Nonroad diesel emissions contribute to air pollution known to have a wide range of
adverse health and welfare impacts. Emissions from nonroad diesels contribute a substantial
percentage of the precursors or direct components of ambient concentrations of ozone, PM,
sulfur and nitrogen compounds, aldehydes, and substances known or considered likely to be
carcinogens. EPA has concluded that diesel exhaust is likely to be carcinogenic to humans by
inhalation at occupational and environmental levels of exposure.74 Nonroad diesel engine
emissions also contribute to adverse environmental effects including visibility impairment, acid
rain, nitrification and eutrophication of water bodies.

       Today, ground-level ozone and paniculate matter remain a pervasive pollution problem in
the United States. In 1999, 90.8 million people (1990 census) lived in the 31  areas designated as
nonattainment areas under the 1-hour ozone national ambient air quality standards (NAAQS).75
Studies of 6 to 8 hour exposures showed health effects from prolonged and repeated exposures,
at moderate levels of exertion, to ozone concentrations as low as 0.08 ppm.76  Prolonged and
repeated ozone concentrations at these levels are common in areas throughout the country, and
are found in areas that are exceeding, and areas that are not exceeding, the 1-hour ozone
standard.77 For example, 153 million people, or 87 percent of the total population in counties
evaluated  (176 million), lived in areas with 2 or more days with concentrations of 0.09 ppm or
higher in 1998, including areas currently meeting the 1-hour NAAQS.78

       The most recent PM10 monitoring data indicate that 14 of the designated PM10
nonattainment areas with a 1999 population of 23 million violated the PM10 NAAQS between
1997 and 1999.1 In addition, there are 25 unclassifiable areas that have recently recorded ambient
concentrations of PM10 above the PM10 NAAQS. Current 1999 PM25 monitored values, which
cover about a third of the nation's counties, indicate that at least 40 million people live in areas
where long-term ambient fine particulate matter levels are at or above 16 //g/m3.79 This 16 //g/m3
threshold is the low end  of the range of long term average PM25 concentrations in cities where
statistically significant associations were found with serious health effects, including premature

       Future inventory projections suggest that adverse health effects caused by air pollution
will continue in the future unless additional emission reductions are achieved. Emissions of
NOx, VOC, PM, and SOx  from all source categories (i.e., mobile, area, stationary) that
contribute to ambient concentrations of ozone, diesel PM, and fine particulate matter are
projected to begin increasing between 2010 and 2025 as economic growth overtakes expected
emission reductions from current control programs. Since the 1998 nonroad diesel engine
rulemaking, the belief that further action is warranted has been reinforced by growing evidence
that diesel engine exhaust causes serious health problems. EPA has recently put in place
       1 PM10 is PM with a diameter of less than 10 microns. PM25 is PM with a diameter of less
than 2.5 microns.

programs to dramatically reduce emissions from highway diesel vehicles.  As a result, nonroad
diesel engines, already a major source of harmful particulate matter and ozone-forming
compounds, will become a dominant mobile source of these emissions in the future.

       Section 213 (a)(3) of the Clean Air Act requires EPA to establish nonroad engine
standards that provide for the "greatest degree of emission reduction achievable through the
application of technology which the Administrator determines will be available for the engines or
vehicles to which such standards apply, giving appropriate consideration to the cost of applying
such technology within the period of time available to manufacturers and to noise, energy, and
safety factors associated with the application of such technology".  In light of the progress being
made in the development of technologies to reduce emissions from highway diesel  engines and
the continuing concerns about nonroad diesel impacts on air quality discussed above, we believe
it may be appropriate to move the control of nonroad emissions beyond the Tier 3 program.
Considering the information gathered in conducting the recent highway diesel engine and fuel
rulemaking, we also believe that a similar "systems" approach is likely to be the most cost-
effective way to pursue this goal. The systems approach recognizes that significant further
reductions in nonroad emissions will require fuel quality improvements and so it entails adopting
nonroad fuel and engine changes in a single coordinated program.

       There are many possible ways of pursuing future standards for nonroad engines and fuels.
For example, one manufacturer has suggested that aftertreatment-based PM standards be
introduced in 2009, along with 15 ppm sulfur fuel, with more stringent NOx standards beginning
to take effect in 2012.81 This implementation schedule may be later than appropriate under the
provisions of the Clean Air Act, but a standards-setting approach along these lines may merit
consideration.  In any approach taken, consideration would need to be given to appropriate
leadtime periods, phase-in and other flexibility provisions, test cycles, in-use emissions control
measures, coordination with highway fuel and engine regulations, State Implementation Plan
emission reduction needs, and international harmonization goals.  We would also expect the
rulemaking to consider the interaction between the existing and new standards, including any
ways in which a coordination of requirements under these sets of standards might be appropriate.

5.     References

1.     U.S. EPA, "Heavy-Duty Diesel Engines Controlled By Onboard Computers Guidance
      On Reporting and Evaluating Auxiliary Emission Control Devices and the Defeat Device
      Prohibition of the Clean Air Act", Manufacturer Guidance Document VPCD-98-13,
      October 15, 1998.

2.     Heywood, John B., Internal Combustion Engine Fundamentals, McGraw Hill 1988,
      pages 567-667.

3.     Heywood, John B., Internal Combustion Engine Fundamentals, McGraw Hill 1988,
      pages 572-578.

4.     Flynn, P., et al, "Minimum Engine Flame Temperature Impacts on Diesel and Spark-
      Ignition Engine NOx Production", SAE 2000-01-1177, 2000.

5.     Dickey, D., et al, "NOx Control in Heavy-Duty Diesel Engines - What is the Limit?",
      SAE 980174, 1998.

6.     U. S. EPA, Regulatory Impact Analysis: Heavy-Duty Engine and Vehicle Standards and
      Highway Diesel Fuel Sulfur Control Requirements, EPA420-R-00-026, December 2000.

7.     Kimura, S., et al, "Ultra-Clean Combustion Technology Combining a Low-Temperature
      and Premixed Combustion Concept for Meeting Future Emission Standards", SAE 2001-

8.     Dec, J.E. and C. Espey, "Ignition and early soot formation in a diesel engine using
      multiple 2-D imaging diagnostics", SAE 950456, 1995.

9.     Kittelson, et al, "Particle concentrations in a diesel cylinder: comparison of theory and
      experiment", SAE 861569, 1986.

10.    Foster, D.E. and D.R. Tree, "Optical measurements of soot particle size, number density
      and temperature in a direct injection diesel engine as a function of speed and load", SAE
      940270, 1994.

11.    Greeves, G.,  I.M. Khan, C. Wang,  I. Fenne,  "Origins of Hydrocarbon Emissions from
      Diesel Engines",  SAE 770259, 1977.

12.    U. S. EPA, Regulatory Impact Analysis: Heavy-Duty Engine and Vehicle Standards and
      Highway Diesel Fuel Sulfur Control Requirements, EPA420-R-00-026, December 2000.

13.    U.S. EPA, Final Regulatory Impact Analysis: Control of Emissions from Nonroad Diesel
      Engines, August  1998.

14.    Dickey, D., et al, "NOx Control in Heavy-Duty Diesel Engines - What is the Limit?",
      SAE 980174, 1998.

15.    U.S. EPA, Final Regulatory Impact Analysis: Control of Emissions from Nonroad Diesel
      Engines., August 1998.

16.    Ghaffarpour, M. and R. Baranescu, "NOx Reduction Using Injection Rate Shaping and
      Intercooling in Diesel Engines," SAE 960845, 1996.

17.    Pierpont, D.A., D.T. Montgomery, and R.D. Reitz, "Reducing Particulate and NOx
      Emissions Using Multiple Injections and EGR in a D.I. Diesel Engine", SAE 950217,

18.    Letter from J.K. Amdall, Caterpillar, to J.R. Holmstead and M.T. Oge, U.S. EPA,
      October 1, 2001, letter body and attachment Appendix A.

19.    Wickman, D., et al, "Diesel Engine Combustion Chamber Geometry Optimization Using
      Genetic Algorithms and Multi-Dimensional Spray and Combustion Modeling", SAE

20.    Osenga, M., "Common Rail, Electronics Headline Deere Tier 2 Diesels", Diesel
      Progress, July 2001, p.44-46.

21.    Letter from J.K. Amdall, Caterpillar, to J.R. Holmstead and M.T. Oge, U.S. EPA,
      October 1, 2001, attachment Appendix E.

22.    Herzog, P., et al, "NOx Reduction Strategies for DI Diesel Engines," SAE 920470, 1992.

23.    Uyehara, O., "Factors that Affect NOx and Particulates in Diesel Engine Exhaust," SAE
      920695, 1992.

24.    Durnholz, M., G. Eifler, and H. Endres, "Exhaust-Gas Recirculation - A Measure to
      Reduce Exhaust Emission of DI Diesel Engines," SAE 920715, 1992.

25.    Bazari, Z. and B. French, "Performance and Emissions Trade-Offs for a HSDI Diesel
      Engine - An Optimization Study," SAE 930592, 1993.

26.    Ropke, S., G.W. Schweimer, and T.S. Strauss, "NOx Formation in Diesel Engines for
      Various Fuels and Intake Gases"  SAE 950213, 1995.

27.    Heywood, John B., Internal Combustion Engine Fundamentals., McGraw Hill  1988,
      pages 572-573.

28.    Kreso, A.M., et al, "A Study of the Effects of Exhaust Gas Recirculation on Heavy-Duty
      Diesel Engine Emissions" SAE 981422, 1998.

29.    Edwards, S.P., et al, "The Potential of a Combined Miller Cycle and Internal EGR Engine
      for Future Heavy Duty Truck Applications" SAE 980180, 1998.

30.    Memo to EPA Air Docket A-2001-28 from William Charmley, U.S. EPA,
      "Documentation of Industry Press Releases Regarding Compliance with Highway HD
      2004 Standards in 2002", October 25, 2001.

31.    Tow, T.C., D.A. Pierpont, and R.D. Reitz, "Reducing Particulate and NOx Emissions by
      Using Multiple Injections in a Heavy Duty D.I. Diesel Engine", SAE 940897, 1994.

32.    Pierpont, D. A., D.T. Montgomery, and R.D. Reitz, "Reducing Particulate and NOx
      Emissions Using Multiple Injections and EGR in a D.I. Diesel Engine", SAE 950217,

33.    Ricart, L.M. and R.D. Reitz, "Visualization and Modeling of Pilot Injection and
      Combustion in Diesel Engines", SAE 960833, 1996.

34.    Mather, D.K. and R.D. Reitz, "Modeling the Influence of Fuel Injection Parameters on
      Diesel Engine Emissions",  SAE 980789, 1998.

35.    Bazari, Z. and B. French, "Performance and Emissions Trade-Offs for a HSDI Diesel
      Engine - An Optimization Study", SAE 930592, 1993.

36.    Herzog, P., et al, "NOx Reduction Strategies for DI Diesel Engines,"  SAE 920470, 1992.

37.    Beard, C.A., "Inlet and Exhaust Systems", in The Diesel Engine Reference Book, Lilly,
      LRC (ed.), Butterworth and Co. (Publishers) Ltd., 1984.

38.    Heywood, John B., Internal Combustion Engine Fundamentals, McGraw Hill 1988,
      pages 866-869.

39.    Voss, K., Y. Bulent, C. Hirt, and R. Farrauto, "Performance Characteristics of a Novel
      Diesel Oxidation Catalyst", SAE 940239, 1994.

40.    Johnson, J., S.  Bagley, L. Gratz, and D. Leddy, "A Review of Diesel Particulate Control
      Technology and Emissions Effects - 1992 Horning Memorial Award Lecture", SAE
      940233, 1994.

41.    Letter from J.K. Amdall, Caterpillar, to J.R. Holmstead and M.T.  Oge, U.S. EPA,
      October 1, 2001, chart A-9.

42.    Detroit Diesel Corporation Press Release, "Detroit Diesel Unveils Year 2000 Series 50
      Bus & Coach Engine", March 17, 2000.

43.    Memo to EPA Air Docket A-2001-28 from William Charmley, U.S. EPA,
      "Documentation of Industry Press Releases Regarding Compliance with Highway HD


      2004 Standards in 2002", October 25, 2001.

44.    Cummins Press Release, "Cummins Takes the Lead in Diesel Emissions Technology:
      New ISB Engine is First to Meet EPA's 2.5-gram NOx Plus NMHC Standard",
      September 10, 2001.

45.    Brezonick, M., "Playing it Cool on Emissions", Diesel Progress, August 2001, pp.28-33.

46.    Letter from J.K. Amdall, Caterpillar, to J.R. Holmstead and M.T. Oge, US EPA, October

47.    Letter from J.K. Amdall, Caterpillar, to J.R. Holmstead and M.T. Oge, U.S. EPA,
      October 1, 2001, letter body and attachment Appendix A.

48.    Memo to EPA Air Docket A-2001-28 from Cleophas Jackson, U.S. EPA, "Summary of
      EPA nonroad test program at Southwest Research Institute", October 2001.

49.    Osenga, M., "Common Rail, Electronics Headline Deere Tier 2 Diesels", Diesel
      Progress, July 2001, p.44-46.

50.    Memo to EPA Air Docket A-2001-28 from Cleophas Jackson, U.S. EPA, "Summary of
      EPA nonroad test program at Southwest Research Institute", October 2001.

51.    Letter from Rick Bishop, John Deere Power Systems,  to Chet France, U. S. EPA,
      October 9, 2001.

52.    Kreso, A.M., J.H. Johnson, L.D. Gratz, S.T. Bagley, and D.G. Leddy, "A Study of the
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      SAE 981423, pp. 1-2,7, 1998.

53.    Kreso, A.M., J.H. Johnson, L.D. Gratz, S.T. Bagley, and D.G. Leddy, "A Study of the
      Vapor- and Particle-Phase Sulfur Species in the Heavy-Duty Diesel Engine EGR Cooler,"
      SAE 981423, pp. 1-2,7, 1998.

54.    McKinley, T.L., "Modeling Sulfuric Acid Condensation in Diesel Engine EGR Coolers,"
      SAE 970636, p.211, 1997.

55.    McKinley, T.L., "Modeling Sulfuric Acid Condensation in Diesel Engine EGR Coolers,"
      SAE 970636, p.211, 1997.

56.    Kreso, A.M., J.H. Johnson, L.D. Gratz, S.T. Bagley, and D.G. Leddy: "A Study of the
      Vapor- and Particle-Phase Sulfur Species in the Heavy-Duty Diesel Engine EGR Cooler,"
      SAE 981423, p.7, 1998.

57.    Kreso, A.M., J.H. Johnson, L.D. Gratz, S.T. Bagley, and D.G. Leddy: "A Study of the
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       SAE 981423, p.7, 1998.

58.     Memo to EPA Air Docket A-2001-28 from William Charmley, U.S. EPA, "Submission
       from Cummins Inc. Regarding Cooled Exhaust Gas Recirculation Technology titled '"05
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59.     Dennis, A.J., C.P. Garner, and H.C. Taylor, "The Effect of EGR on Diesel Engine Wear,"
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60.     Memo to EPA Air Docket A-2001-28 from William Charmley, U.S. EPA, "Submission
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61.     McKinley, T.L., "Modeling Sulfuric Acid Condensation in Diesel Engine EGR Coolers,"
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62.     McKinley, T.L., "Modeling Sulfuric Acid Condensation in Diesel Engine EGR Coolers,"
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63.     Banzhaf, M. and R. Lutz, "Heat Exchanger for Cooled Exhaust Gas Recirculation," SAE
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64.     Rao,V.D.N., et al, "Material Systems for Cylinder Bore Applications - Plasma Spray
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65.     Memo to EPA Air Docket A-2001-28 from William Charmley, U.S. EPA, "Submission
       from Cummins Inc. Regarding Cooled Exhaust Gas Recirculation Technology titled '"05
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66.     Letter from J.K. Amdall, Caterpillar, to J.R. Holmstead and M.T. Oge, U.S. EPA,
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67.     For example, Letter from Rick Bishop, John Deere Power Systems,  to Chet France, U.S.
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       October 19, 2001.

68.     U.S. EPA, Regulatory Impact Analysis: Control of Emissions of Air Pollution from
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69.     Letter from J.K. Amdall, Caterpillar, to J.R. Holmstead and M.T. Oge, U.S. EPA,
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70.     Caterpillar News Release, "Caterpillar ready to meet stringent 2006 Clean Air standards
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71.    U.S. EPA, Final Regulatory Impact Analysis: Control of Emissions from Nonroad Diesel
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72.    Osenga, M., "Common Rail, Electronics Headline Deere Tier 2 Diesels", Diesel
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73.    Memorandum from William Charmley, U.S. EPA, to EPA Air Docket A-2001-28,
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74.    U.S. EPA, Health Assessment Document for Diesel Exhaust: SAB Review Draft.,
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75.    U.S. EPA, National Air Quality and Emissions Trends Report 1999, 2001, Table A-19.

76.    U.S. EPA, Review of National Ambient Air Quality Standards for Ozone, Policy
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77.    U.S. EPA, National Air Quality and Emissions Trends Report 1999, 2001.

78.    U. S. EPA, Regulatory Impact Analysis: Heavy-Duty Engine and Vehicle Standards and
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79.    U.S. EPA Memorandum to Docket A-99-06 from Eric O. Ginsburg, "Summary of 1999
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80.    U.S. EPA, Review of the National Ambient Air Quality Standards for P articulate Matter:
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81.    Letter from W. Passie, Caterpillar, to W. Charmley, U.S.EPA, August 9, 2001.