Alternative fuels for internal combustion engines have been the
subject of numerous studies. The new U.S. Renewable Fuel Standard
has made it a requirement to increase the production of ethanol and
advanced biofuels to 36 billion gallons by 2022. Because corn-based
ethanol will be capped at 15 billion gallons, 21 billion gallons
must come from the advanced biofuels category. A potential source
to fill the gap may be butanol and its isomers as they possess fuel
properties superior to ethanol. Recently, concerns have been raised
about emission of currently non-regulated constituents, aldehydes
in particular, from alcohol-based fuels.
In an effort to assess the relative impact of the U.S. Renewable
Fuel Standards on emissions from a modern gasoline engine, both
regulated and non-regulated gas constituents were measured from the
combustion of three different alcohol isomers in a modern
direct-injected (DI) spark ignition (SI) gasoline engine. Exhaust
gas recirculation (EGR) was disabled to avoid changes in emissions
due to slight changes in EGR ratio, thereby allowing direct
comparison of the emissions results. A standard emissions bench in
combination with a Fourier Transform Infrared (FTIR) analyzer was
used to characterize the exhaust stream before catalyst in gasoline
operation as well as operation using several gasoline/alcohol
blends. Ethanol, n-butanol and iso-butanol were used as blending
agents. Relevant exhaust constituents were measured as a function
of blend ratio for several engine load and speed conditions.
Oxides of nitrogen emissions decreased with increased alcohol
content while formaldehyde and acetaldehyde show a clear, positive
correlation with blend ratio. Although an apparent reduction in
aromatic hydrocarbon emissions was observed with increased alcohol
content, this was likely caused in large part by dilution of
aromatics (high volume percent in gasoline) in the fuel blend
rather than changes in the fuel chemistry due to combustion.
Examination of the major precursors (propene, 1,3-butadiene, and
acetylene) to benzene, and therefore particulate matter (PM),
revealed significant increases consistent with iso-butanol
emissions poised to produce benzene via the C₃/C₃ route, whereas
n-butanol emissions appeared primed to proceed through the C₄/C₂
route.