MEMORANDUM 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY 

2565 PLYMOUTH ROAD 

ANN ARBOR, MICHIGAN 48105-2498 

OFFICE OF 

AIR AND RADIATION 

SUBJECT: Consideration of Confidential Business Information on the Relationship between 

Engine-Out NOx Emissions and Fuel Consumption 

FROM: 

TO: 

DATE: 

Charles Moulis, Senior Environmental Engineer 

Assessment and Standards Division 

Matt Spears, Director 

Heavy-Duty Onroad and Nonroad Center 

August 3, 2012 

This memorandum summarizes our analysis of the relationship between engine-out NOx 

emissions and fuel consumption for the 2012 Nonconformance Penalty (NCP) Final Rule, which 

is also summarized in Figure 3-2 of the Final Technical Support Document for that rule. 

Engine-Out NOx and Fuel Consumption Data 

Before and during the rulemaking to establish NCPs for model year 2012 and later heavy heavyduty diesel engines, engine manufacturers provided us with information about the relationship 

between engine-out NOx emissions and fuel consumption for their heavy heavy-duty engines. 

Most of this information was identified as confidential business information (CBI) by the 

manufacturers so we are not releasing it to other manufacturers or anyone else. The Clean Air 

Task Force also provided public data collected by M.J. Bradley. These data are shown together 

in Figure A, which plots normalized fuel consumption versus engine-out NOx emissions. 

Manufacturer data is presented with fuel consumption normalized relative to each 

manufacturer's own compliant 0.20 glhp-hr (tailpipe) engines. 

These data do not fall on a single line, which is not unexpected since absolute performance can 

vary significantly from one engine model to another. This can be corrected to some extent by 

replotting the data so that fuel consumption is normalized to equal 1.00 at 3 glhp-hr engine-out 

N consistent with the ·ant · · . This ach is shown in F. B. 

second set of data from M.J. Bradley is added to the plot. This second set was excluded from the 

first figure because the data are not from engines compliant with the 2010 NOx standard, but 

rather represented earlier engines. 
EPA Analysis of Engine-Out NOx and Fuel Consumption Data 

To complete the NCP rule, EPA needed to generate a single estimate of the difference in fuel 

consumption between an engine with engine-out NOx emissions at 3.0 glhp-hr (the estimated 

engine-out emissions of a compliant engine) and an engine with engine-out NOx emissions at 4.8 

g/hp-hr (the estimated engine-out emissions of the baseline engine). To evaluate other 

alternative cost scenarios, similar estimates were also needed relative to engines with engine-out 

NOx emissions at 0.50 and 1.2 g/hp-hr. Therefore, we developed a single curve of relative fuel 

consumption versus engine-out NOx emissions in the 0.50 to 4.8 g/hp-hr range. This curve is 

show along with the data in Figures A and B. 

Given the limited amount of data available and the inconsistencies between the various sources, 

it was not possible to develop an appropriate curve by a purely statistical analysis of the data 

such as averaging the data. Instead, we subjectively developed the curve based on the following 

principles: 

• Manufacturer data should be roughly weighted based on each manufacturer's 

approximate share of the heavy heavy-duty engine market. 

• Data provided in the 3.0 to 4.8 glhp-hr range should be given a greater weight because 

this is the range that represents the expected change in engine-out emissions and resulting 

fuel consumption from the baseline engine to a compliant engine. Thus, this is in the 

range used to define the final penalty amount. 

• Data provided in more detail should be weighted more highly than data provided in lesser 

detail. 

• The line should be continuous, but curve slightly to reflect the known trend of flattening 

at higher NOx levels. 

• The projections for NOx levels below 3.0 g/hp-hr should represent fuel consumption 

changes observed and reported for 2010 engines relative to 2009 engines. 

• The curve should be somewhat conservative to protect complying manufacturers, but not 

unreasonably conservative. It should be conservative with respect to both the analysis 

used to set the penalty and the alternative scenarios. 

The result was the line shown in the figures which follows the following equation: 

Relative Fuel Consumption= 0.0007 x (NOx) 2 - 0.0157 x (NOx) + 1.0408 

This curve projects that there would be a 1.9 percent decrease in fuel consumption associated 

with increasing engine-out NOx emission from 3.0 to 4.8 glhp-hr, equivalent to a negative slope 

of 1.056 percent per g/hp-hr. This slope is compared to the slopes of the raw data sets in Figure 

C. In this Figure, lines lying above the EPA line (i.e., lines with shallower slopes) represent 

projections of smaller fuel consumption impacts, while lines lying below the EPA line (i.e., lines 

with steeper slopes) represent projections of larger fuel consumption impacts 
Although the derivation of this curve was ultimately subjective, it can be evaluated to some 

degree by the following observations: 

• The slope of the curve falls within the range of data for engine-out NOx levels both 

above and below 3.0 shown in 

• The slope of the curve (as shown in Figures Band C) is conservative (i.e., it projects a 

slightly higher fuel consumption penalty for relative to the baseline) 

with to the all data for 

• The curve is relatively straight. However, it curves very slightly to be steeper for engineout NOx levels below 3.0 glhp-hr and less steep for engine-out NOx levels above 3.0 

glhp-hr. A more curved line would have been less conservative by projecting a smaller 

fuel consumption advantage for the optimized 0.50 glhp-hr SCR engine used as the 

baseline for the penalty (which has engine-out NOx emissions at 4.8 glhp-hr). A more 

curved line would have also projected a larger fuel consumption disadvantage for a nonSCR 0.50 and 1.2 glhp-hr baseline engines considered in alternative scenarios. 

• The 2.25 percent difference in fuel consumption between our ' 3.0 n /,....,._,.. 

- .. t~· .. ·~~ and an with NOx emissions at 1.2 is within 

This value is significantly lower than the values 

reported by SCR engine manufacturers in their marketing materials, but those marketing 

values are generally assumed to reflect best-case engine operating scenarios. 

• The curve results in a reasonable 1.1 percent change for reducing emissions from 1.2 to 

0.50 glhp-hr. 

Thus, we concluded that this curve was most consistent with the available data and the guiding 

principles described above. 

Comparison to Tailpipe NOx Estimates 

As is described in the Final Technical Support Document, combining this curve with our 

estimates of catalyst efficiency, we project that the fuel consumption for an SCR engine 

optimized for 0.50 g/hp-hr NOx would be 1.9 percent better than an SCR engine optimized for 

0.20 g/hp-hr NOx. This compares very well with the public comments of the Clean Air Task 

Force, Cummins, and Mack, each of whom estimated that there would be a two percent 

difference in fuel consumption between the 0.20 and 0.50 g!hp-hr SCR engines. 
On the other hand, our estimate is much lower than the four percent value recommended by 

P ACCAR. There are several reasons why we believe this is appropriate. First, the relevant 

compliance costs are those representing the 90th percentile costs. Since PACCAR currently 

represents less than ten percent of the heavy heavy-duty engine market and it was the only 

manufacturer to report a fuel consumption impact above two percent, we can conclude that it is 

that the 90th would be less than the value P ACCAR. 

Conclusion 

EPA analyzed the relevant data to estimate the relationship between engine-out NOx emissions 

and fuel consumption for heavy heavy-duty engines. The resulting curve is consistent with the 

available data and the guiding principles listed above. Thus, it is appropriate for the NCP Final 

Rule. This curve was used in that Final Rule primarily to estimate the difference in fuel 

consumption between an engine with engine-out NOx emissions at 3.0 glhp-hr and an engine 

with engine-out NOx emissions at 4.8 glhp-hr (which was used as an input to the cost analysis), 

but also to evaluate other alternative cost scenarios. The projection that the fuel consumption for 

an SCR engine optimized for 0.50 glhp-hr NOx would be 1.9 percent better than an SCR engine 

optimized for 0.20 glhp-hr NOx compares very well with the public comments of the Clean Air 

Task Force, Cummins, and Mack, each of whom estimated that there would be a two percent 

difference in fuel consumption between the 0.20 and 0.50 g/hp-hr SCR engines. 
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Fuel Consumption vs. Engine Out NOx Emissions 

Contains Confidential Business Information 

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Figure B- Data Renormalized Relative to 3 g/hp-hr 

Fuel Consumption vs. Engine Out NOx Emissions 

Contains Confidential Business Information 

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Fuel Consumption vs. Delta Engine Out NOx Emissions 

Contains Confidential Business Information 

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