United States Air and Radiation EPA420-S-03-001
Environmental Protection February 2003
Agency
&EPA Independent Peer Review of
EPA Technical Report
EPA420-S-02-012:
"The Effect of Cetane Number
Increase Due to Additives on
NOx Emissions from
Heavy-Duty Highway Engines"
> Printed on Recycled Paper
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EPA420-S-03-001
February 2003
Of
EPA
"The Effect of Number Due to
Additives on NOx Emissions from
Heavy-Duty Highway Engines"
Peer Reviewers:
Dr. Larry Caretto Dr. Nigel Clark
California State University Northridge West Virginia University
18111 Nordhoff Street P.O. Box 6106
Northridge, CA 91330-8348 Morgantown, WV 26506
Ph# 818-677-6448 Ph# 304-293-2514
Review Period: September 4, 2002 through November 22, 2002
Assessment and Standards Division
Office of Transportation and Air Quality
U.S. Environmental Protection Agency
NOTICE
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.
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Background and Purpose Of This Document
In June of 2002 we released a technical report in draft form for public comment entitled,
"The Effect Of Cetane Number Increase Due To Additives on NOx Emissions From Heavy-Duty
Highway Engines" (EPA420-S-02-012). Stakeholders were given until July 15, 2002 to provide
comments to EPA. After comments were received and reviewed, a number of modifications
were made to the technical report. The resulting Revised Draft Technical Report was then given
to Drs. Larry Caretto and Nigel Clark to perform independent peer reviews according to the
guidelines given in EPA's Science Policy Council Handbook on Peer Review (EPA100-B-00-
001). Both of these peer reviews were completed by November 22, 2002, with reports submitted
to the EPA.
This document provides a summary of EPA responses to each of the peer reviewers'
recommended changes or additions to the technical report or the analyses contained therein. In
cases where the technical report has been modified in response to a peer review recommendation,
a short description of the modification is given here. Modifications to the technical report itself,
however, do not include references to the specific recommendation made by a peer reviewer.
Apart from comparing the responses from the two peer reviews to the four specific
questions raised in the original charge, this document does not present or respond to any peer
reviewer comments supporting or otherwise agreeing with analyses presented or statements made
in the Revised Draft Technical Report. The reader is referred to the peer review reports
themselves for these statements of agreement or support. The peer review reports are included at
the end of this document.
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Table of Contents
Original Charge To Peer Reviewers 1
Comparison of Answers From Peer Reviewers To The Four Specific
Questions Raised In The Charge 4
Peer Reviewer Recommendations and EPA Responses 6
Peer review report from Dr. Larry Caretto 13
Peer review report from Dr. Nigel Clark 26
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Original Charge To Peer Reviewers
Dear Dr. XXXX
This letter serves to establish your intent to be a peer reviewer for the draft EPA technical
report entitled, "The Effect of Cetane Number Increase Due To Additives on NOx Emissions
from Heavy-Duty Highway Engines." A copy of this report is enclosed. If you agree to perform
a review of this document as described in this letter, please indicate so by responding to David
Korotney via regular mail or e-mail. Once we receive this response, we will submit the purchase
order for your services to our purchasing department. You can begin work after we have
informed you that the purchase order for your services has been authorized.
The draft technical report at issue describes an analysis conducted by the Environmental
Protection Agency in which existing emissions data from heavy-duty diesel engines was used to
determine the degree of correlation between NOx emissions and increases in cetane number
brought about through the use of cetane improver additives. The report estimates the percent
change in the heavy-duty engine NOx inventory that would be expected to occur if cetane
improver additives are used in diesel fuel. These estimates might then be used, for instance, in
the context of State Implementation Plans or credit trading programs.
We are submitting this information to you so that we may obtain peer review of the
methodology, assumptions, and conclusions we have drawn in this technical report. Your review
should make use of your general understanding of the technology used in heavy-duty diesel
engines, your statistical expertise, and your knowledge of the relationship between fuel
properties, combustion mechanisms, and emissions of regulated pollutants. In your review, you
should clearly distinguish between recommendations that are necessary to insure the adequacy of
the predictions, and those that you believe should be considered but are not critical. Your
comments should be sufficiently detailed to allow thorough understanding by EPA or other
parties familiar with the work.
In addition to any issues, comments, or recommendations you include in your review of
the draft technical report, you should also address the following specific questions/topics:
1. Is the range of fuels and engines in the database sufficient to form the basis of
estimated effects of additized cetane on NOx emissions for the in-use fleet?
2. Is it reasonable to conclude that additized cetane effects on NOx are independent
of engine model year?
3. Is it reasonable to assume that all engines sold after 2002 will make use of exhaust
gas recirculation?
4. Is there reasonable agreement between the data and the model?
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We would appreciate receiving the results of your review within six weeks of start of
work. During your review, questions about what is required in order to complete the review or
about the draft technical report itself can be directed to the Peer Review Leader via phone, e-
mail, or regular mail:
David Korotney
U.S. EPA
2000 Traverwood Dr.
Ann Arbor, Mi 48105
Ph# 734-214-4507
Fax 734-214-4051
korotney.david@epa.gov
Your final review should be provided in written form via regular mail. Your comments
should include a cover letter stating your name, the name and address of your organization, what
material was reviewed, the number of hours you spent on the review, a summary of your
expertise and qualifications, and a statement that you have no real or perceived conflicts of
interest. Please also provide an electronic copy of your comments and cover letter, either via e-
mail or on a floppy disk included in your submittal via regular mail. The electronic version
should be in WordPerfect, MS Word, or some format that can be converted to WordPerfect or
MS Word. Please do not provide the draft technical report or your comments to anyone else.
As we have discussed, you will be paid a flat fee of $XXX for this peer review. This fee
was calculated based on an estimated XX hours of review time at a rate of $XX per hour.
However, spending fewer or more hours than our estimate of XX hours will not affect the fee
paid for this work. You may expect to receive payment in full within forty-five (45) days of
submitting your invoice and final comments to David Korotney.
Thank you for your time and consideration.
Sincerely,
Chester L. France
Director
Assessment and Standards Division
Office or Transportation and Air Quality
Enclosures
cc: D. Korotney, ASD/NC
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Comparison of Answers From Peer Reviewers To The Four Specific
Questions Raised In The Charge
Only summaries of the answers from each peer reviewer to the four questions posed in the charge
are given here. The reader is referred to the peer review reports for the full answers. EPA
responses to any recommendations made by the peer reviewers are addressed in the next section.
1. Is the range of fuels and engines in the database sufficient to form the basis of estimated
effects of additized cetane onNOx emissions for the in-use fleet?
Caretto: The range of values for natural cetane and cetane difference are sufficiently broad
to form the basis for the resulting regressions. When the database fuels are
compared to in-use surveys, the range of the fuels is the same and there should be
no significant differences in the additized cetane response for the two sets of fuels.
The data on engines built during the 1990's should be representative of the onroad
national fleet. The EPA should, however, compare the technology groups of the
engines in the database with the engines in the fleet.
Clark: The experimental fuels identified in the SAE papers cover a wider [total] cetane
range than is commonly purchased at the pump. This does not diminish the
statistical validity of the conclusions. It is probably wise to exclude 2-stroke
diesel engines.
2. Is it reasonable to conclude that additized cetane effects on NOx are independent of
engine model year?
Caretto: The analysis supports the conclusion that the additized cetane effects are
independent of model year.
Clark: I think that the incidence of advanced injection management (such as split
injection and rate shaping) may be lower than the report suggests. However, in
Figure II.A.2-2 [of the Revised Draft], the range of the NOx response to cetane
does shrink for later model years, suggesting that later model year engines exhibit
a more precise sensitivity to cetane rating.
3. Is it reasonable to assume that all engines sold after 2002 will make use of exhaust gas
recirculation?
Caretto: The conclusion that cetane has no effect on EGR-equipped engines is not
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supported by tests on different engines. However, engines designed to meet the
emission requirements for 2003-and-later will likely be designed so that cetane
has no effect on NOx. Therefore, it is still valid to assume that no NOx reduction
credit should be given for cetane use in highway engines designed to meet the
"2004" emission standards.
Clark: Fearing new EGR tchnology, some line haul companies purchased 2002
technology tractors at a higher rate than predicted, resulting in a lower rate of
purchase for 2003+ engines. Thus the rate that EGR-equipped engines enter the
fleet may be lower than estimated in the draft report. In addition, it is perhaps not
EGR alone, but the complex flow and thermal management schemes, coupled
with advanced electronic controls, that change the engine's reaction to fuel
properties.
4. Is there reasonable agreement between the data and the model?
Caretto: The small p-values for the regression show a good fit of the model to the data.
The wide scatter of individual data points around the 45° line in Figure in.B-2 is
typical of such plots when examining the effects of fuels on emissions.
Clark: Even test-to-test variability in transient engine testing is substantial, as evidenced
by "round robin" engines tests organized by the EMA.
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Peer Reviewer Recommendations and EPA Responses
Recommendation 1: EPA should compare the distribution of engine technology groups in the
database with those in the in-use fleet to determine how well the range of
engines tested matches those in-use (Caretto)
EPA response to Recommendation 1
Although Figure II.A.2-1 from the Revised Draft Technical Report did present the distribution of
engine model years from the database, there was no comparison to the distribution of in-use
model years. We have therefore added this comparison to this Figure.
Recommendation 2: Include confidence limits for the final regression equation (Caretto)
EPA response to Recommendation 2
For multi-parameter regression equations of the sort generated by our maximum likelihood
curve-fitting approach, there is no straightforward mechanism for combining term-specific
standard error estimates into confidence limits. The SAS procedure "proc mix" employed in our
analysis does have the capability of generating confidence intervals around every observation in
the database, but this falls short of confidence limits around the regression equation itself. In
order to generate a quantitative measure of uncertainty, we compared predicted to observed
values to estimate the standard error associated with the residuals. A description of this analysis
has been added to Section IV.B of the final report, along with a graph illustrating the results.
Recommendation 3: Review evidence for the cause of low cetane sensitivity in engines
equipped with EGR. If the lack of a cetane effect is due not just to EGR,
revise the report accordingly. (Caretto)
EPA response to Recommendation 3
The evidence identifying a specific cause for lower cetane sensitivity in engines with advanced
technology is not conclusive. The primary test program on which our conclusions regarding
EGR-equipped engines were based did not include an evaluation of the impacts of injection rate-
shaping or other combustion management techniques. In addition, this test program used a
prototype, not production, engine with EGR. However, as this peer reviewer pointed out, our
assumption of zero-cetane sensitivity for 2003+ model year engines is appropriate regardless of
whether that cetane insensitivity is caused by the presence of EGR or some other technology.
We have modified the text to highlight this fact. The fact that lower cetane sensitivity in future
engines cannot be unambiguously associated with a specific technology does suggest that a future
test program is warranted to investigate this issue, but it does not appear to be necessary in order
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to draw conclusions about additized cetane effects on the in-use fleet.
Recommendation 4: Describe any alternatives to equation (5) that were considered to model the
effects of natural cetane changes on NOx (Caretto)
EPA response to Recommendation 4
As described in Section n.B of the final technical report, the high degree of colinearity between
natural cetane and other fuel properties presents problems when trying to correlate natural cetane
with emissions. In work completed in 2001 and described in the Staff Discussion Document1,
attempts to include natural cetane in correlations between fuel properties and NOx emissions
failed - the natural cetane term was not significant, its impact on NOx being surrogated to
aromatics and density. The use of additized cetane instead of natural cetane avoids the problems
associated with colinearities between fuel properties, and thus results in a less ambiguous and
more straightforward correlation with NOx. Given that natural and additized cetane describe
overlapping combustion effects, we continue to believe that it is appropriate to use the
correlation based on additized cetane to represent the effects of natural cetane on NOx. In
addition, we believe that this approach will be environmentally conservative, i.e. result in fewer
NOx benefits being ascribed to increases in natural cetane than could actually happen in-use;
since changes in natural cetane are usually accompanied by (and in fact usually brought about
through) changes in aromatics, NOx emissions from existing engines may be reduced through
both the higher cetane and the lower aromatics, resulting in a larger emissions effect than if
cetane alone changed. The text in the final technical report has been modified to more clearly
state these conclusions.
Recommendation 5: Clarify the policy guidance on the application of this analysis to nonroad
engines (Caretto)
EPA response to Recommendation 5
Section LA was added to clarify that the final technical report is primarily intended to provide the
EPA's current understanding of a technical issue. Although the technical analysis contained in
the report may be used in EPA policy decisions, those decisions are generally made and applied
outside of the technical report proper. Thus the draft technical report's conclusion that "it might
be appropriate to apply" the regression equation to nonroad engines was intended to indicate
EPA support for this approach on a technical basis while leaving a definitive policy decision to
be made at another time or in a more specific context. A sentence has been added to the text
indicating that the application of equation (2) to nonroad in any specific context may be
1 "Strategies and Issues in Correlating Diesel Fuel Properties with Emissions," Staff
Discussion Document, EPA report number EPA420-P-01-001, July 2001
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dependent on the availability of supporting data or other relevant factors.
Recommendation 6: Provide the slope and p-value for the comparison between cetane effects
on NOx and model year (Caretto)
EPA response to Recommendation 6
Figure in.A.2-2 in the final report has been modified to include the regression lines, and Table
IU.A.2-1 has been added to show the coefficients and p-values for the least-squares regressions.
Recommendation 7: Provide the actual slope for the regression done on the data shown in
Figure in.B-2 [of the revised draft report] as well as the confidence limits.
(Caretto)
EPA response to Recommendation 7
Figure IV.B-3 was added to the final report, showing the regression line and the associated
confidence interval. The actual slope of the regression line and its p-value have been included in
the text.
Recommendation 8: Insert an abstract stating the purpose of the report and the overall
conclusions. (Caretto)
EPA response to Recommendation 8
An abstract has been included.
Recommendation 9: The numbering of tables and figures and their associated references must
be corrected. (Caretto)
EPA response to Recommendation 9
All the table and figure numbers have been corrected for the final report, along with their
associated references.
Recommendation 10: Insert the equation defining the variance inflation factor into the paragraph
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that introduces this concept. (Caretto)
EPA response to Recommendation 10
The equation defining the variance inflation factor was inserted as suggested.
Recommendation 11: Clarify in the text associated with Figure I.B-1 [of the Revised Draft
report] that the data comes from an in-use survey. Clarify that the values in
Table I.B-1 are derived from the same in-use survey data shown in Figure
I.B-1. Clarify whether the in-use survey data shown in Figure II. A. 1-1 is
the same as that used in Figure I.B-1. (Caretto)
EPA response to Recommendation 11
Language was added to Section IB to clarify that the analyses were based on an in-use survey
conducted by the Alliance of Automobile Manufacturers in 1999. The in-use surey data shown
in Figure HA. 1-1 was collected in 2000, but a review of the 1999 and 2000 surveys indicate that
the differences are very minor and would not materially affect the analysis or conclusions. Also,
the table and figure numbers were changed in accordance with Recommendation 9.
Recommendation 12: Insert the number of engines at the top of each bar in Figure II. A.2-1.
(Caretto)
EPA response to Recommendation 12
The suggested change has been made to Figure II. A.2-1. Also, the figure number was changed in
accordance with Recommendation 9. Per Recommendation 1, the same Figure has been used to
show the distribution of model years for the in-use 2003 fleet.
Recommendation 13: Indicate somewhere in the report the location of the Web site containing
all the background for this report. (Caretto)
EPA response to Recommendation 13
The Web site has been inserted into Section IB.
Recommendation 14: Correct the spelling of "calendar". (Caretto)
EPA resyonse to Recommendation 14
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The suggested change has been made to all occurances in the report.
Recommendation 15: Clarify that "additives" used to increase the cetane number of diesel fuel
are limited to peroxides, nitrates, and similar compounds added in very
small quantities to specifically and solely affect cetane number. (Clark)
EPA response to Recommendation 15
The text in Section n of the final report has been amended to clarify that other potential bulk
blending components such as biodiesel, Fischer-Tropsch diesel, etc. which may increase cetane
number were not included in the analysis and thus are not directly represented by the final
estimates of additized cetane effects on NOx. The text associate with equation (5) in the final
report was also amended to specifically prohibit the application of the correlation to
circumstances wherein biodiesel or Fischer-Tropsch fuels are used.
Recommendation 16: Point out that the anticorrelation between additized and natural cetane, as
highlighted by the value of-0.35 in Table I.B-2, is due to product
requirements rather than properties inherent in the fuel makeup. (Clark)
EPA response to Recommendation 16
The text in Section n of the final report has been amended to highlight this point.
Recommendation 17: Clarify that the following terms are synonomous: additized cetane,
additized cetane number, cetane difference, and CETANE_DIF. (Clark)
EPA response to Recommendation 17
These terms are indeed synonomous. The text throughout the final report has been modified to
be more consistent in this respect, with "additized cetane" being the primary descriptor. Where
necessary, explicit clarifications have been added.
Recommendation 18: Clarify that base fuels in the database had a CETANE_DIF value of zero.
(Clark)
EPA response to Recommendation 18
Section HI of the final report was amended to clarify this fact.
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Recommendation 19: Expound on the impact that repeat measurements have on the database and
the subsequent analyses. (Clark)
EPA response to Recommendation 19
Section HI. A of the final report states that repeat emission measurements were not averaged nor
were they limited in any fashion. The SAS curve-fitting procedure "proc_mix" that was
employed in our analysis is able to account for large numbers of repeat observations without
allowing them to overly influence the results. This fact has been added to the description of the
analysis in Section in.B.
Recommendation 20: Though I understand the argument for log, would a linear fit have changed
the conclusions? (Clark)
EPA response to Recommendation 20
The peer reviewer did not advocate a separate analysis to answer this question, and was referring
specifically to the use of the natural log of NOx as the dependent variable instead of just NOx.
We continue to believe that the primary reason for using the natural logarithm of NOx emissions,
i.e. to mitigate the heteroscedastic nature of the NOx measurements, is sufficient to support its
use. Besides being a common approach to treating dependent variables in these types of
analyses, the use of the natural log also provides the advantage that the conversion of the
regression equation to provide the % change in NOx is more straightforward, as described in
Section IV. A of the final report. We do not believe that a linear fit would have changed the
conclusions. The nonlinearity introduced through the use of the natural log was very minor in
the analyses presented in the Staff Discussion Document, and it is unlikely that the very small p-
values shown in Table IV.A-2 of the final report would have increased to values at or above 0.10
if a linear fit had been employed. In addition, the use of a log transform for NOx emissions
introduces some upward curvature to the correlation which is offset by the existence of the
squared cetane difference term in equation (2) which results in an overall downward curvature.
If we had used NOx as the dependent variable instead of log NOx, it is likely that the effect of
the squared cetane difference term would have been stronger, resulting in effectively the same
overall downward curvatuve shown in Figure ni.A-1 of the final report.
Recommendation 21: State whether the final estimates of additized cetane effects on NOx apply
to locomotives. (Clark)
EPA response to Recommendation 21
The discussion of applicability to nonroad engines in Section IV. C of the final report was
amended to specifically exclude locomotives.
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Recommendation 22: The draft report indicates that there is also a possibility that natural cetane
numbers nationwide will increase with the introduction of ultra-low sulfur
highway diesel fuel in 2006. Please indicate whether this result would be
due to hydrogenation of the fuel while removing sulfur. (Clark)
EPA response to Recommendation 22
The peer reviewer's understanding is correct, and the text has been modified accordingly.
Recommendation 23: The base level of NOx was not considered in the analysis. It is therefore
unclear whether absolute or percent reduction of NOx is more appropriate.
(Clark)
EPA response to Recommendation 23
In fact the base level of NOx was included in the analysis. Dummy terms for each engine were
included in the regression analysis, and these terms permit each engine to have its own base
emissions level. As a result, the percent change estimates provided by equation (4) are
apropriately scaled for the base emission level of the engines in the database. In addition, a
review of the database indicates that there is no correlation between the cetane impacts on
percent change in NOx and the base NOx level of the engine. Thus the percent change values
estimated in our analysis should be applicable to engines with any base NOx level.
Recommendation 24: Off-cycle (often steady-state) effects may not have been properly
considered. (Clark)
EPA response to Recommendation 24
The peer reviewer cites the exclusion of data collected on the Japanese steady-state cycle as one
reason for this concern. In fact that database retained data collected on other steady-state cycles,
namely the European R49 cycle. The text in the final report has been amended to highlight this
fact. Even so, experience with diesel engine emissions testing strongly suggests that impacts of
changes in cetane on NOx emissions are essentially the same for transient and steady-state
cycles.
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REPORT REVIEW
EPA420-S-02-012 - THE EFFECT OF CETANE NUMBER
INCREASE DUE TO ADDITIVES ON NOx EMISSIONS FROM
HEAVY-DUTY HIGHWAY ENGINES
Review Prepared for U. S. Environmental Protection Agency
Office of Transportation and Air Quality
Assessment and Standards Division
October14, 2002
L. S. CARETTO
7805 Cowper Avenue
West Hills, CA91304
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TABLE OF CONTENTS
Table of Contents i
Introduction 1
Discussion 2
Scope of the report 2
General Comments 2
Is the range of fuels and engines in the database sufficient to form the
basis of estimated effects of additized cetane on NOx emissions
for the in-use fleet? 3
Fuels 3
Engines 4
Is it reasonable to conclude that additized cetane effects on NOx are
independent of engine model year? 5
Is it reasonable to assume that all engines sold after 2002 will make use of
exhaust gas recirculation? 5
Is there reasonable agreement between the data and the model? 8
Other comments 9
Minor editorial comments 10
Recommendations 12
Recommendations necessary to ensure the adequacy of the predictions .... 12
Recommendations that should be considered but are not critical 12
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INTRODUCTION
The U. S. Environmental Protection Agency (EPA) requested an independent peer
review of the report entitled "The Effect of Cetane Number Increase Due to Additives on
NOx Emissions from Heavy-Duty Highway Engines." This report, numbered EPA420-S-
02-012, is dated June 2002 and is labeled as a revised draft technical report.
The letter requesting the review1 asked that the following questions be addressed.
1. Is the range of fuels and engines in the database sufficient to form the basis of
estimated effects of additized cetane on NOx emissions for the in-use fleet?
2. Is it reasonable to conclude that additized cetane effects on NOx are
independent of engine model year?
3. Is it reasonable to assume that all engines sold after 2002 will make use of
exhaust gas recirculation?
4. Is there reasonable agreement between the data and the model?
The overall directions for review also call for a clear distinction "between
recommendations that are necessary to ensure the adequacy of the predictions and
those that [. . . ] should be considered but are not critical."
The body of this review is in the next section. That discussion section presents a brief
overview of the report and some general comments. It then addresses the four
questions listed above. Finally, additional comments, some simply editorial, are made
regarding the report. The recommendations section summarizes the comments and
distinguishes between essential recommendations and those that are not critical.
The report draft provided for review was an intermediate draft in which a blank page
with the heading "I. EPA Technical Reports" was inserted as a placeholder for material
to be added later. Although the remaining sections of the report were renumbered to
be consistent with this new section, the tables and figures were not. The discussion in
this review of specific report sections, figures, and tables uses the numbers in the draft
even though they are not consistent.
Chester L. France letter to Larry Caretto, August 22, 2002.
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DISCUSSION
Scope of the report
The EPA report provides a statistical analysis of existing data on the emission effects of
two variables in fuel used in on-highway heavy-duty diesel engines: natural cetane and
additized cetane. The latter is defined as the increase in cetane produced by diesel
fuel additives designed to increase cetane number.
This report is the result of a process that started in fall 2000 when "EPA voiced concern
about the NOx emission reduction credits claimed by Texas [for its low emission diesel
(LED) program]." Prior to this report there were meetings with "numerous
stakeholders," a July 2001 contractor report by Southwest Research Institute (SwRI),2
and a July 2001 EPA staff discussion document by EPA.3 There was also a public
workshop held on August 28, 2001. EPA established a web site4 for this issue that has
various reports and memoranda related to this study and stakeholder comments from
the public meeting.
The introduction to the report notes that EPA's conclusions on the Texas program were
transmitted in a separate memo. It further states that "there is no widely-applicable,
EPA-approved model for estimating the emission impacts of more general changes in
diesel fuel properties. At this time, EPA has no plans to pursue such a model."
General Comments
In a study like this one, it would be useful to have some quantitative measure of the
uncertainty in the results, such as the final regression equation plotted in Figure III.B.1
on page 21. Regression equations by other researchers are shown on the same plot
indicating a wide spread in the work of various groups. On this graph it appears that an
increase of five in the cetane number due to additives will reduce NOx by about 2%
using the regression equation proposed here. However, the other regression equations
shown in the figure predict a NOx reduction in the range of 1 % to 3% for the same
increase in cetane number. Showing the confidence limits on this regression would
Robert L. Mason and Janet P. Buckingham, "Diesel Fuel Impact Model Data Analysis Plan
Review, Draft Final Report, Work Assignment 2-7, EPA Contract 68-C-98-169, Southwest Research
Institute, July, 2001.
3U. S. Environmental Protection Agency, Strategies and Issues in Correlating Diesel Fuel
Properties with Emissions, Staff Discussion Document EPA420-P-01-001, July 2001.
4http://www.epa.gov/otaq/models/analysis.htm
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give the reader some indication of the reliability of the equation for making predictions.
Other areas where confidence limits would add to the report are noted below.
Is the range of fuels and engines in the database sufficient to form
the basis of estimated effects of additized cetane on NOx emissions
for the in-use fleet?
Fuels
There are two separate questions one can ask regarding the fuels. The question asked
here regards the range of fuels. This is separate from the representativeness of the
fuels that is addressed in section III.A-1 of the report.
The data to respond to both issues - range and representativeness - are shown in
Figures II.A-1 and II.A. 1-1 to II.A. 1-6 of the report. Figure II.A. 1-1 shows a wide range
for the natural cetane and cetane differences in the database fuels. This figure shows
that the range of values for these properties, which are the ones used in the
regressions derived in the report ,are sufficiently broad to form the basis for the
resulting regressions.
The issue of representativeness is less certain. Because of the small effect of fuels on
emissions (compared to engine effects) and the correlations among various fuel
properties, studies of fuel effects on emissions use specially chosen fuels. Such fuels
have a wide range in properties to enable effects to be determined with a reasonable
number of experimental measurements. In addition, these fuels are often custom
blended to avoid collinear effects. This results in experimental fuels that are somewhat
different from in-use fuels. This is shown in Figures II.A. 1-1 to II.A. 1-6 of the report.
The distribution of fuel properties, especially sulfur, aromatics, specific gravity and T90,
is much narrower in the in-use survey data than it is in the fuels used in the database.
Only natural cetane has a similar distribution in both cases.
The report compares the fuel property distributions from the survey data and the report
database in terms of the ranges that encompass 90% of the data. This comparison
leads the report authors to the conclusion on page 12 that the "database is fully
representative of in-use fuels." This conclusion is correct for the range of properties,
but not for the distribution. A more appropriate conclusion is that the fuels in the
database have been selected for experimental studies of fuel effects on emissions.
Although there are differences in the distribution of properties when the database fuels
are compared to in-use surveys, the range of the fuels is the same and there should be
no significant differences in the additized cetane response for the two sets of fuels.
Engines
17
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The representativeness of the engines used in the report is analyzed in section III.A.2 in
terms of the model year distribution. Twenty-two engines were present in the database
with the following model-year distribution: 1991 (5), 1993 (4), 1994 (7), 1996 (5), and
1998 (1). Although the report notes that two-stroke engines and engines with exhaust
gas recirculation (EGR) were excluded from the database, there is no analysis of the
technology groups to which the remaining engines belong.
Table II.C-2 in the staff discussion document3 lists of the number of engines and the
relative amount of emission data in the various technology groups in the original
database. The table shows 73 engines in 16 technology groups in the original
database.5 The technology groups in the original database represent a wide range of
in-use engines. Technology groups F and T6 account for 60% of the emissions data in
the original database. When the data for two-stroke engines and EGR-equipped
engines are eliminated, there are 65 engines in the original database and groups F and
T account for 64% of the emissions data there.
The database-preparation discussion in section III.A of the report refers to the staff
discussion document for information about the basic database. The database-
preparation procedures were presumably responsible for reducing the number of
engines from 73 in the original database to 22 used in this study.
The data on engines built during the 1990s should be representative of the onroad
national fleet. In addition, since the proposed final equation allows no credit for engines
produced in 2003 and later, the model year mix used for the engines that receive
cetane credit will not change significantly as new engines are introduced into the fleet.
On this basis, the engines used in the database should be representative.
Because there is no analysis in the report that compares the technology groups of the
engines in the database with the engines in the fleet, it is not possible to answer the
question of whether or not the range of engines in the database is sufficient. Such an
analysis would give a better picture of the similarities and differences between the
onroad fleet and the fleet in the database.
On page 23, the report notes that one might use local inventory data on fleet
distribution. Urban areas are expected to have a larger proportion of older engines than
the national fleet. The regression equation in this report should also be applicable to
these areas with the different fleet distributions.
5The text that references this table states that there were 75 engines (some of which were the
same engine with different injection timings) in 17 technology groups. The discussion here is based on
the data actually in the table.
6Both of these technology groups represent turbocharged, direct-injection engines. Group F uses
mechanical (inline or rotary) fuel injection and has displacements less than or equal to 9.4 L. Group T
uses electronic unit injection and has displacements greater than 9.4 L.
18
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Is it reasonable to conclude that additized cetane effects on NOx are
independent of engine model year?
The report contains an analysis in which the effect of a unit increase in cetane on NOx
reduction was analyzed as a function of model year. The data for this analysis are
plotted in Figure II.A.2.2 on page 14 of the report. According to the discussion on that
page, two separate analyses - one accounting for an engine effect and one without
such an effect - showed no statistically significant slope at the p = 0.1 level.
This analysis supports the conclusion that the additized cetane effects are independent
of model year. However it would be interesting to learn the slope of the regression
slope and its p-value.
Is it reasonable to assume that all engines sold after 2002 will make
use of exhaust gas recirculation?
The significance of this question is related to the application of the proposed model to
the future in-use fleet. The report proposes, in the discussion that begins at the bottom
of page 22, that there is no NOx reduction credit for cetane increase in 2003-and-later
engines. The absence of cetane effect on 2003-and-later engines is attributed to the
presence of EGR on these engines. Because it is not intuitively obvious why EGR
should negate the effect of cetane, this review examined the basis for the conclusion
about EGR effects.
New emission control technologies are likely to be required to meet the diesel emission
standards for 2004. Those standards lower the allowed non-methane hydrocarbons
(NMHC) from 1.2 to 0.5 g/bhp-hr, and the current NOx standard of 4 g/bhp-hr will be cut
approximately in half.7 These emission reductions on the diesel engines that have
already been subject to significant control will require new control technologies of which
exhaust gas recirculation (EGR) is the most likely candidate. The even more stringent
diesel emission standards for 2007 will almost certainly require add-on controls for NOx
and particulate matter (PM) standards; however, the 2007 standards will continue to
require control in the engine itself in addition to the add-on controls.
Under the terms of a 1998 settlement to a civil suit, seven engine manufacturers, who
produce nearly all the heavy-duty diesel engines sold in the U. S., agreed to implement
the 2004 standards for all engines sold on or after October 1, 2002. Thus, the emission
7There is no actual NOx standard for 2004. Instead, the engine manufacturers must meet a
combined NMHC + NOx standard of 2.4 g/bhp-hr; manufacturers can choose to meet an optional NMHC
+ NOx standard of 2.5 g/bhp-hr combined with a NMHC standard of 0.5 g/bhp-hr.
19
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control technology anticipated for the 2004 standards will be on virtually all heavy-duty
diesel engines starting from this October 2002 date. The report uses 2003 and later as
the starting point for the installation of technology that meets the new standards.
EPA's relationship with manufacturers regarding future certification plans should give
them more data about specific manufacturer's plans regarding future emission control
technology than is available to this reviewer. On page 13, the report notes that "there
are indications that some manufacturers will not use EGR in their 2002+ engines."
Although no quantitative data are presented, there is an implication that the majority of
new engines will be using EGR.
Page 15 of the report cites a report by the Heavy-Duty Engines Workgroup (HDEWG)
as the "primary source for [EPA's] data on the effects of additized cetane on an EGR-
equipped engine." The results from the HDEWG were published in a series of SAE
papers in 2000. One paper, which presented the statistical analysis of the study,8 had
the following observations about the effect of cetane:
1. The overall conclusion was that the engine had a "very low sensitivity to cetane
number. . . [that] differs from the results of similar experiments in engines that
are not equipped with EGR."
2. Table 5 of the paper showed that the regression coefficient for NOx with cetane
had a small positive value whose p-value (level of significance) was 0.024.
3. Figure 2 showed that an increase in cetane number from 42 to 52 would
increase NOx by 1.3%.
The paper reported data with and without EGR, but did not explicitly compare the
effects of cetane on NOx, with and without EGR. Table 1, below, is taken from Table 8
of the statistical results paper.8 The NOx results in this table are the averages of the
entries for two fuels with the same cetane number in the original paper. The fuels that
have an N in their designation are natural cetane fuels that do not have any additized
cetane. E.g., fuels 16 and 16N are both reported as having a "target" cetane level of
52. (The paper contained an analysis that showed the effect of cetane was the same
whether the cetane number was produced by an additive or by natural cetane.)
.^W;-Ff (OTper^^ Withpif E©R8> •*#*
Fuels
7N+14N
8+8N
Cetane
42
47
NOx data with EGR
g/bhp-hr
2.368
2.415
Percent change
Per step
2.0%
Initial to
final
NOx without EGR
g/bhp-hr
3.740
3.803
Percent change
Per step
1.7%
Initial to
final
Robert L. Mason et al. "EPA HDEWG Program - Statistical Analysis," SAE Paper 2000-01-
1859, presented at International Spring Fuel and Lubricants Meeting, Paris, France, June 19-22, 2000.
20
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|l6+16ISl| 52 | 2.346 | -2.9% | -0.9% | 3.684 | -3.1% | -1.5%
The results of Table 1 show that the effect of cetane on NOx is similar with or without
EGR, for the engine used in the study. In both cases there is an increase of about 2%
as the cetane number is increased from 42 to 47 and a decrease of about 3% as the
cetane number is increased from 47 to 52. The overall effect of increasing the cetane
number from 42 to 52 is a NOx decrease of 0.9% with EGR and a NOx decrease of
1.5% without EGR.
This table shows that the effects of cetane on NOx for this engine are similar with and
without EGR. Although the engine has a low sensitivity to cetane, it is likely that this
effect is not due to EGR. Instead, it is due to some other differences between this
engine and engines tested previously.
One possible explanation may be found in the paper that presents the HDEWG engine
test results.9 Figures 9 to 12 of that paper, and the discussion of those figures, notes
that "there is very little premixed burning." Since the premixed burning phase of diesel
combustion is the one that is affected by changes in cetane number, an engine that is
designed to nearly eliminate this premixed-burning phase should have a low sensitivity
to cetane. The report notes (on page 13) that EPA has "limited information" suggesting
that this hypothesis is correct.
According to the full database that EPA used for this study,10 all the EGR engine data
(technology group L) are taken with a single engine, a Caterpillar 3176. Thus the
conclusion that cetane has no effect on EGR-equipped engines is not supported by
tests on different engines.
This section is supposed to address the following question: is it reasonable to assume
that all engines sold after 2002 will make use of exhaust gas recirculation? This
question is better restated: Is it reasonable to assume that engines calibrated to meet
emission requirements for engines manufactured in 2003-and-later will be designed so
that cetane has no effect on NOx emissions?
The answer to the revised question is a guarded "yes." The only data available for fuel
effects on such an engine indicate that cetane will not have a sizeable effect on NOx.
(Furthermore the effect of cetane in the one engine tested is a slight - but statistically
significant - increase in NOx.) A review of the SAE papers discussing the tests on this
engine suggests that the explanation for this effect may be the control of the
9Andrew C. Matheaus, etal. "EPA HDEWG Program - Engine Tests Results," SAE Paper 2000-
01-1858 presented at International Spring Fuel and Lubricants Meeting, Paris, France, June 19-22, 2000.
10This was taken from the Excel file hdd-db7.xls, available at the EPA diesel analysis web site,
http://www.epa.gov/otaq/models/analysis.htm.
21
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combustion process to reduce premixed burning rather than EGR. However, this
proposed explanation does not change the conclusion of the report that no NOx
reduction credit should be given for cetane use in highway engines designed to meet
the "2004" emission standards.
The conclusion could be less guarded if data were available in more than one engine
designed to meet these standards. However, there are no data that conflict with the
observations that one engine calibrated to meet the 2004 emission standards shows a
small effect of cetane on NOx emissions and that effect is an increase NOx.
Is there reasonable agreement between the data and the model?
The p-values for the significance of the regression coefficients, shown in Table III.A-2
are all less than 0.0001 except for the natural cetane coefficient which is 0.0057; these
small p values show a good fit of the model to the data.
Figure II.B-2 on page 22 shows a comparison of predicted versus observed NOx
changes. This plot shows a significant scatter around the 45° line on which all data
should ideally fall. Most of the individual data points lay in a band that is ±2.5
percentage points on either side of the 45° line. The wide scatter of individual data
points around the 45° line is typical of such plots when examining the effects of fuels on
emissions. It would be interesting to know the confidence limits for the mean observed
change. The report notes that a regression line through these data had "a slope that is
close to 1.0." The confidence limits on the mean for this regression line would give a
quantitative estimate of the agreement between the data and the model.
Although it would be helpful to have the quantitative confidence limit recommended in
the previous paragraph, the low p-values for the regression coefficients and the visual
comparison of the observed and predicted data in Figure III.B-2 show reasonable
agreement between the data and the model.
Other comments
The suggestion on page 25 that a modified version of equation (4) could be applied to
changes in natural cetane is not consistent with the equations previously derived.
Equation (4) is derived from the percent reduction equation (2). Equation (2) is derived
from the basic regression equation (1) by assuming that the percent reduction is due
only to additized cetane. The natural cetane of the base fuel and the additized fuel are
assumed to be the same. Accordingly, the natural cetane number term drops out of
equation (2). If equation (2) were written to consider changes in both additized cetane
and natural cetane it would be written as follows.
22
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fractional change in NOx = exp{ -0.015151 [ cetDifff - cetDiff, ]
+ 0.000169 [ cetDifff - cetDiffj ]2
- 0.006014 [ natCetf - natCet; ]
+ 0.000223 [ (natCetf)(cetDiff) - (natCetj)(cetDifj) ] - 1 }
In this equation natCet, and natCetf represent the initial natural cetane number of the
initial (base) fuel and natCetf represents the natural cetane number of the final
(modified) fuel. Similarly cetDiff, and cdtDifff represent the cetane difference due to
additives in the base fuel and the modified fuel, respectively. Equation (5) substitutes
the natural cetane terms for the additized cetane terms and does not consider any
possible changes in cetane additives between the base and modified fuel.
The discussion of equation (5) implies that the additized cetane equation may be used
to estimate, for regulatory purposes, the effects of changes in natural cetane. No
reason is given in that discussion for not providing an equation that specifically
considers the effects of natural cetane on NOx. Presumably, this was due to the
problems associated with modeling natural cetane because of its correlations with other
fuel properties. The report should discuss any alternatives to equation (5) that were
considered and rejected.
The report could benefit from a brief abstract stating the purpose of the report and the
overall conclusions. At present one gets to the bottom of page six before learning that
EPA has "determined that correlating of additized cetane with NOx is an appropriate
means for providing inventory impact information to anyone considering the use of
higher cetane diesel fuel."
The conclusion near the bottom of page 24 that it "might be appropriate to apply" the
regression equation to nonroad engines is not very definitive. Should an area planning
a diesel fuel control program claim credit for NOx reductions from nonroad engines? If
no policy decision on this issue has been made, the report should say so explicitly. Of
course, there are no data on cetane effects in nonroad engines, but one would expect
that the NOx effects in nonroad and onroad engines to be similar. A given percent
reduction in NOx emissions could become a more significant emission reduction in
nonroad engines due to the higher emission rates and longer lifetimes for these
engines as compared to highway engines.
Minor editorial comments
The numbering of tables and figures (and the references to these) needs to be redone
to reflect the addition of a new section I, EPA Technical Reports. (This section has only
its title and a note that the section will be included in the final report.) There are also
references to sections that will have to be corrected; e.g., the reference in the next-to-
last line on page 12 should be "Section III.B" instead of II.B.
23
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The equation defining the variance inflation factor should be placed in the final
paragraph of page 3 where this term is introduced.
The reference for the data in Figure 1 .B-1 on page 3 is a 1999 AAM fuel survey. It
would be helpful to the reader to indicate briefly in the text, at the point where the
reference is made, that the data in the figure come from an in-use survey. This would
make it clear that the Figure 1 .B-1 and Table 1 .B-1 are based on the same data set. At
present the mention of in-use survey data, at the top of page 4, is not clear. In addition
there is presently a reference to AAM data collected in 2000 on page 9 of the report. Is
this a different survey or the same one as cited earlier?
It would be helpful to modify the bar chart in Figure II.A.2-1 on page 13 so that the
number of engines for each model year is shown at the top of the bars.
No mention is made of the EPA diesel fuel analysis web site in the report. This is a
useful site for anyone who wants to review the background for the present report. This
site could be mentioned in the introduction or as a preface to the references section.
The word calendar is misspelled11 at various places in the report: page 23, last line in
both second and third paragraph; page 25, last paragraph, second and sixth lines.
nThe spelling used in the report, calender, is allowed by the spell checker in Word Perfect.
According to www.dictionary.com, a calender is "a machine in which paper or cloth is made smooth and
glossy by being pressed through rollers." This misspelling may be in other places in the report not listed
above.
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RECOMMENDATIONS
Recommendations necessary to ensure the adequacy of the
predictions
The comments in the previous section deal mainly with editorial changes and
recommendations for inclusion of more statistical information about existing results.
Based on this review there appears no problem with the immediate use of the
equations in the report to make useful estimates of the NOx reductions due to the
addition of cetane improvers to diesel fuel.
If there is significant use of this approach in future years, the database that justifies the
assumption that engines built in 2003-and-later will have no NOx benefit from cetane
addition should be based on a larger set of engine data.
Recommendations that should be considered but are not critical
The various recommendations that were made in the previous section are not critical to
ensure the adequacy of the predictions. However, these recommendations,
summarized below, could improve the usefulness of the report.
° Compare the distribution of engine technology groups in the database with those
in the in-use fleet to determine how well the range of engines tested matches
those in use.
° Include confidence limits for the final regression equation and the prediction of
measured and predicted data.
° Review evidence for cause of low cetane sensitivity in engines equipped with
EGR. If you agree that the lack of a cetane effect is not just due to EGR, but to
other advanced emission control systems, revise the report accordingly.
° Consider a future test program to verify cetane effects on advanced emission
control systems, if cetane increases become a significant NOx control strategy.
° Describe any alternatives to equation (5) that were considered to model the
effects of natural cetane changes on NOx.
° Clarify the policy guidance on the application of this analysis to nonroad engines.
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PEER REVIEW OF EPA DRAFT TECHNICAL REPORT
'The Effect of Cetane Number Increase Due To Additives on NOx Emissions from Heavy-
Duty Highway Engines" (EPA420-S-02-012 / June 2002)
Reviewer: Nigel N. Clark
The South Lockhouse
362 Old Lock 13 Rd.
Morgantown, WV 26501
November 17, 2002; revised December 6, 2002.
26
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PEER REVIEW
The EPA document (EPA420-S-02-012 / June 2002) provides a careful and credible account of
the methodology used to arrive at the relationship between diesel fuel cetane number and NOx
production for the case of engines that most closely resemble the present day on-highway heavy-
duty fleet. The results of this study carefully delineate the different influences of "natural" cetane
rating, arising from the gross fuel composition, and of the cetane enhancement arising from the
use of fuel additives. The methodology is sound, and the conclusions appear to be reasonable
and do not contradict current wisdom on the topic.
On page 3 of the document, the way in which diesel fuel cetane number is attained is categorized
into the "natural" and "additized" methods. The report subsequently points out that fuel
enhancing additives are used in small quantities in the fuel. However, this report does not allow
in its language for the possibility of blending modest quantities of high grade products (such as
Fischer-Tropsch paraffinic distillate, or oxygenates) into the fuel. Presumably it is thought that
these products would always represent a substantial fraction of the fuel, and therefore be part of
the "natural" fuel, but it may prove worth defining the line between additive and blend stock to
avoid ambiguity.
The beginning of the report is structured around the relationship between aromatics and cetane
number. This relationship is well known, and is usually associated (simplistically) with an
aromatic versus straight chain paraffin tradeoff. Presumably the scatter in Figure I.B-1 is due to
two factors, namely (i) inaccurate measurement of properties and (ii) the presence of varying
quantities of other compounds including olefins and branched species that do not lie directly on
the aromatic-straight chain paraffin axis. It is not necessary for clarity of the report to raise this
issue, but it does explain the fact that natural cetane and aromatic content enjoy only a 0.57 anti-
correlation value in Table I.B-2. I am also told by a refinery expert that many tests and
reportings of aromatic content include other compounds with the aromatics. In particular,
napthenes may be grouped with aromatics. This will tend to confuse correlations.
An observation, perhaps not central to the document, concerns the strong relationship between
T10, T50 and T90 in Table I.B-2. The strength of this relationship may change between the
cases of examining only an ensemble of Diesel #2 products, versus an ensemble of a mixture of
#1 and #2 products.
The argument on page 5, that the aromatics and specific gravity terms were quite capable of
accounting for natural cetane in the NOx model, is clear and credible.
On page 6 it is stated that "Additized cetane is largely uncorrelated with other diesel fuel
properties, as shown by the low variance inflation factor in Table I.B-1." However, in Table I.B-
2, the value of-0.35 correlating additized cetane and natural cetane is not negligible. This anti-
27
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correlation is due to human intervention and the need to enhance fuels with poor natural cetane
by using additives to meet cetane minimum standards. As the authors observe later in the
document, there is no need to employ additives in a fuel that already has a high natural cetane
rating. Perhaps the document could point out that this anticorrelation does exist, but is due to
product requirements rather than properties inherent in the fuel makeup. I find it hard to judge
how many readers will find this nuance in the statistics obvious, hence my recommendation.
The document needs to be consistent in its meaning of "additized cetane" and "additized cetane
number", and careful in the usage. The footnote to Table I.B-1 provides the definition as a
"difference": are all usages in the text intended to relect difference, or do some reflect the total
value? On p.7, in the "Analytical approach" section, the variable CETANE_DIF is mentioned.
This is the same as the "additized cetane" defined in the footnote. It would just help the reader to
use the same terms, or reference them as the same in the text.
On p.7, presumably a base fuel had a value of CETANE_DIF of zero, and was included. If I am
mistaken, the then the text may easily be misread.
Table II.A-1 is very important in showing how the process of eliminating "extraneous" natural
fuels separated the natural cetane and additized cetane variables further. However, this
separation is merely due to the fact that the experimental fuels identified in the SAE papers cover
a wider cetane range than is commonly purchased at the pump. So the database has a range of
natural cetane that reflects pump fuel, a range of additized cetane that reflects pump fuels, but a
range of total cetane (natural plus additized) that exceeds that of pump fuel. The database range
is evidently about 38 to 64 from Figure HA-1. This does not diminish the value of this review,
or diminish the statistical validity of the conclusions: simply put, the motivation for the original
research studies differed from the motivation of a refiner blending a product for the market.
On p.8, there is mention that only engines that were tested on both unadditized and additized fuel
were considered. When an engine is used to determine emissions using the transient Federal Test
Procedure, the engine is first mapped for full torque. Two approaches are possible. Either (i) the
engine may be mapped (to determine the torque curve) on a baseline fuel and this one map may
be used for generating the test from CFR dimensionless variables for testing and comparing a
range of fuels, or (ii) the engine may be mapped on each individual fuel. Essentially, the
difference accounts for the de-rating of the engine by some fuels, which may be noticeable with
fuels such as diesel emulsions and biodiesel. The influence will usually be greater on PM than
NOX. In this case the mapping issue is not likely to matter much, because fuel density will play
the greatest role, and the additives do not change density measurably.
Presumably no test was repeated so many times (see p.8) that the database would be substantially
warped by the repeats.
On p. 12 the "clean diesel fuel" of California is mentioned. At least one refinery in California
still produces a genuine 10% aromatic fuel, but most fuels are "test out" blends, with far higher
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aromatics. In fact, the issue of using additized test-out fuels in California is precisely the issue
being addressed in the present EPA report.
It is probably wise to exclude 2-stroke diesel engines. A few are left in garbage trucks and local
dray tractors, but most were used in transit buses and are now being replaced with 4-stroke units.
The authors may be interested to know that Caterpillar used EGR engines to meet California
NOX standards decades ago, but then abandoned the technology. There are very few of these old
trucks left in the national fleet, and they differ substantially from advanced electronic EGR
engines.
On p. 13 the report states "engines built after 1999 will not constitute a majority of the highway
engines NOX inventory for several more years." Strictly speaking, it is true, but new trucks see
the highest VMT, and 2000-2002 trucks already comprise a substantial fraction of the fleet.
Also, fearing new EGR technology, some line haul companies purchased 2002 technology
tractors at a higher rate than the depressed economy might have predicted (but I have no record
of the quantity of these purchases.)
On p. 13 the absence of effect on EGR engines is discussed. This is a very complex issue. It is
not perhaps the EGR alone, but the complex flow and thermal management schemes, coupled
with advanced electronic controls, that change the engine's reaction to fuel specifications. Mann
et al. (SAE 982486) presented some detail on the interaction of an engine's fuel injection and
control system with fuel parameters. The truth is that the present EPA report is geared to heavy
heavy-duty engines with similar unit injector systems and similar injection timing, with
turbochargers and intercoolers (that were mimicked in a test cell). Perhaps the largest variation
was whether the turbochargers were wastegated or not. It is good that the report examines the
influence of the model year on cetane effects. I think that the incidence of advanced injection
management (split injection and rate shaping) may be lower in practice than the report suggests,
since a 4 g/bhp-hr standard can be met with injection retard, particularly if there is relief through
off-cycle operation (see below). What is interesting in Figure H.A.2-2, though, is that the range
(and standard deviation) of the response shrinks for later model years, which does suggest a more
precise "sensitivity" to the cetane rating.
On p. 14: was a linear fit tried? Engineers tend to be linear people, though I understand the
argument for log, and log is often used in emissions work when some higher emitters are
encountered. Would a linear fit have changed conclusions? (I am not advocating substantial
additional work if this is not known.)
On p. 15, it is true that 2-stroke diesels will see little highway use, but they will remain in use in
locomotives for some time to come.
On p. 16 "cetane difference" is used: once again, terms should be constant throughout.
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The inclusion of the squared terms and interaction term (see p. 16) is important. The interaction
term is particularly important, because it will reveal the fact that cetane increases above a certain
level are not beneficial. Most NOxis produced at high load, and hence at high boost, where in-
cylinder temperatures are high and promote short ignition delays. These delays become so short
in some circumstances, that an increase in cetane number has little effect.
On p. 19, presumably the small increase in natural cetane in 2006 would be due to hydrogenation
of the fuel while removing sulfur.
The argument that the turnover is artificial is completely plausible. The "straight line correction"
may be needed for "legal defensibility", but in truth it is not really necessary. No-one is likely to
explore fuels above 60 cetane and the turnover is small even up to 65 cetane. The text
recognizes this issue.
On p.22 measurement variability is mentioned. Even run-to-run variability in transient engine
testing is substantial, as evidenced by "round robin" engine tests organized by the EMA. With
small cetane changes, one is fortunate to be able to draw any concrete conclusion.
On p.23, EGR engines are mentioned again. As mentioned previously in this review, EGR is
accompanied by other control changes. Pilot (split) injection, with a small pilot quantity, has the
potential to eliminate cetane effects almost entirely, except in extreme cold operation cases with
poor quality fuel.
In table ni.B-1, presumably the high VMT of newer trucks in the fleet has been adequately
modeled.
On p. 24, it is argued that off-road engines may benefit according to a similar formula. The
exceptions (since 2-stroke technology was excluded) might be GM locomotives, older generator
sets (with DDC 71, 92 and 149 series engines) and some marine engines (both GM/DDC and
larger bore units). If equation (2) is used for on-road it will at least be conservative (that is,
underestimate benefits) because engines with older technology and mechanical injection are
likely to be more cetane sensitive. Comments on advanced injection timing presented
immediately below are also relevant to the off-road engine issue.
This report specifically excluded engine tests employing the Japanese steady-state cycle. Also, a
variable that was not considered in this study was the base level of NOx from the engine. These
facts lead to two concerns, namely (i) one can reach different conclusions by considering percent
reduction and absolute (brake specific) reduction of NOX, and (ii) "off-cycle" (often steady-state)
effects may not be properly considered. Off-cycle effects are now well understood, and typically
lead to an increase in brake-specific NOX of 2.5 times in the NTE zone for a 5g/bhp-hr. certified
engine. Many on-road engines may spend the bulk of their lives emitting at a 12g/bhp-hr level
rather than at a transient certification level. It is not clear that cetane enhancement will have the
same effect for an advanced injection timing engine as opposed to an engine that has timing set
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close to TDC. The ignition delay is related to the "integral of the inverse of the exponential
function of temperature", and the in-cylinder temperature history will differ for the cases of
advanced and TDC timing. The report is predicated on the fact that the FTP represents engine
operation and controller (injection timing) behavior, but this might not be so. All of the engines
in this study for MY 1990 to MY 1998 will have 4 g/bhp-hr to 6 g/bhp-hr (less safety margin)
emissions on the FTP, and the combustion will have a very similar location of peak pressure (as
crank angle). Both the off-road engines (which could be 8 to 12 g/bhp-hr engines) and the off-
cycle engine operation will not have this constancy. In the case of a 12 g/bhp-hr engine, should
one consider absolute or percent reduction of NOX, and is either truly applicable? This issue
goes to the heart of the report, but there is probably little that can be done about it, because few
tests will have been performed on US engines with varying cetane rating with non-FTP or steady-
state (eg. EURO/OICA) cycles.
Clearly this report cannot deal with all eventualities, but engines under cold start conditions or
cold/hot climate operation may deviate from the 68 to 86 degree FTP window of operation and
yield different NOX. In all these cases, correcting using brake specific NOX rather than percent
NOX would prove conservative.
The last issue that I feel obliged to raise is broader in philosophy than this report addresses.
Problems arise in relating emissions and cetane rating because the original definition of cetane
number is based on an antiquated engine, with low injection pressure, low in-cylinder charge
motion and operating conditions atypical of a present day on-road engine. Fuel providers tailor
their product in this regard not to optimize modern engine operation but to cater to this old cetane
specification. Cetane may therefore not be correlated unequivocally to ignition delay in modern
engines. If cetane rating (by comparison with standards) were measured using a modern engine,
with a more meaningful measure of premixed burn, then the difference in effects between natural
and additized cetane would be much reduced, and correlations would be superior.
The request for review of this report specifically sought an opinion on whether all post-2002
engines would employ EGR. Most manufacturers have favored the use of EGR to achieve 2003
standards, although there is the option of not meeting standards and absorbing penalties. It is
true that EGR technology with present fuel sulfur levels presents materials concerns, and that
cooling EGR adds substantial burdens to ultimate efficiency of the engine in vehicle applications.
In other words, EGR adds to engine manufacturing cost and complexity, and will detract from
fuel economy. It is my opinion that EGR cannot enable 2007-2010 standards without substantial
additional engine and controls alterations. It is possible that suggested aftertreatment devices,
including SCR catalysts and NOX adsorbers, will reach a level of success where it is more
economical to run a non-EGR engine with aftertreatment rather than an EGR engine with
aftertreatment to reach levels below about 0.5g/bhp-hr. of NOX. In this way, engines in the next
decade may not employ EGR, but EGR will predominate over this decade. Also, once engines
are fitted with aftertreatment, it is unlikely that the conclusions of this EPA report will be
applicable.
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In summary, this EPA report has presents a thorough and credible analysis of the effects of
additized cetane on NOX emissions. The assumptions and statistics are reasonable and
defensible. The only concerns are that some non-road engines may differ in injection timing and
NOX production from on-road engines, so that the application of this method would be
approximate. Also, the reality of off-cycle, high NOX, operation may not be reflected by FTP
tests where timing remains closer to TDC.
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