Regulation of Fuels and Fuel Additives: Renewable Fuel Standard ...

content. Table VIII.A.1.c-1 shows the effect of adding 11 volume percent MTBE or 10
volume percent ethanol to non-oxygenated gasoline on these emissions.
Base Fuel
Table VIII.A.1.c-1
Effect MTBE and Ethanol on Nonroad Exhaust Emissions
4-Stroke Engines 2-Stroke Engines
11 Volume
Percent MTBE
10 Volume
Percent Ethanol
11 Volume
Percent MTBE
10 Volume
Percent Ethanol
Exhaust VOC -9% -15% -1% -1%
Non-Exhaust VOC 0% 26% 0% 26%
CO -13% -21% -8% -12%
NOx +24% +37% +12% +18%
As can be seen, higher oxygen content reduces exhaust VOC and CO emissions
significantly, but also increases NOx emissions. However, NOx emissions from these
engines tend to be fairly low to start with, given the fact that these engines run much
richer than stoichiometric. Thus, a large percentage increase of a relative low base value
can be a relatively small increase in absolute terms.
Evaporative emissions from nonroad equipment are impacted by only RVP, and
permeation by ethanol content. Both the RVP increase due to blending of ethanol and its
permeation effect cause non-exhaust VOC emissions to increase with the use of ethanol
in nonroad equipment. The 26 percent effect represents the average impact across the
U.S. in July for both 2-stroke and 4-stroke equipment. We updated the NONROAD2005
hose permeation emission factors for small spark-ignition engines and recreational
marine watercraft to reflect the use of ethanol.
For nonroad toxics emissions, we base our estimates of the impact of fuel quality
on the fraction of exhaust VOC emissions represented by each toxic on MOBILE6.2 (i.e.,
the same effects predicted for onroad vehicles). The National Mobile Inventory Model
(NMIM) contains estimates of the fraction of VOC emissions represented by the various
air toxics based on oxygenate type (none, MTBE or ethanol). However, estimates for
nonroad gasoline engines running on different fuel types are limited, making it difficult to
accurately model the impacts of changes in fuel quality. In the recent NPRM addressing
mobile air toxic emissions, EPA replaced the toxic-related fuel effects contained in
NMIM with those from MOBILE6.2 for onroad vehicles. 85 We follow the same
methodology here. Future testing could significantly alter these emission impact
estimates.
2. Diesel Fuel Quality: Biodiesel
EPA assessed the impact of biodiesel fuel on emissions in 2002 and published a
draft report summarizing the results. 86 At that time, most of the data available was for
85 71, Federal Register, 15804, March 29, 2006.
86 “A Comprehensive Analysis of Biodiesel Impacts on Exhaust Emissions,” Draft Technical Report, U.S.
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