Environmental Engineering Reference
In-Depth Information
To achieve desirable anti-knock properties, fuel refiners change the composition of the fuel,
utilizing more volatile components that increase vapor pressure and thereby evaporative emissions
and are more prone to generate ozone. It has been found that the addition of oxygenated fuel com-
ponents, such as methanol or ethanol, improves fuel performance and reduces exhaust emissions,
especially in older vehicles, so incentives to employ these additives have been utilized. Another
fuel additive, MTBE (methyl tertiary butyl ether), has been required by some states with ozone
problems, but has been found to be environmentally harmful in fuel leaks to ground water, in which
it is very soluble.
Natural gas is a clean vehicle fuel, yielding reduced exhaust emissions and no fuel vapor prob-
lem because it is very unreactive in photochemical ozone production. But storing natural gas in a
vehicle, either as a compressed gas in high-pressure tanks or as a refrigerated liquid at
253
◦
C,
is difficult and expensive, and it limits the vehicle range between fuel refills. At the present time,
natural gas vehicles are restricted to fleet vehicles with limited daily range operating out of central
fuel depots.
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8.8
CONCLUSION
Among all transportation vehicles in the United States, light-duty passenger vehicles and trucks, in
aggregate, are the predominant users of fuel and emitters of air pollutants. Transportation accounts
for about a quarter of U.S. energy use, so substantial improvements in fuel efficiency and emissions
of light-duty vehicles could contribute proportionally to reductions in national fuel consumption
and pollutant emissions.
A substantial gain in vehicle fuel efficiency is primarily a matter of improved vehicle design
and only secondarily does it turn on the improvement of engine efficiency. Current U.S. vehicles
differ much more in vehicle than in engine fuel efficiency, with the larger, more massive vehicles
having poorer vehicle fuel efficiency than the smaller, lighter ones.
The vehicle design parameters affecting vehicle fuel efficiency are vehicle mass, aerodynamic
drag, and rolling friction (in order of decreasing importance). For a given vehicle size, vehicle
mass can be reduced below current designs by substitution of lighter materials of equal strength,
particularly in the vehicle frame, without impairing vehicle safety in collisions. As vehicle mass
is reduced, less power and mass is needed for the engine, transmission, wheels, tires, fuel tank,
and so on, compounding the gain in frame mass reduction. By careful attention to vehicle shape,
aerodynamic resistance can be reduced. Efficient tires, in addition to mass reduction, lower the
rolling resistance. Altogether, these technologies can improve vehicle fuel efficiencies independent
of improvements to engine efficiency.
Improvements in engine fuel efficiency are closely constrained by the requirement to limit
exhaust pollutant emissions. In the past, engine fuel efficiency has gradually improved while
exhaust emissions were greatly reduced. There is still room for continued improvement in both
respects for both SI and CI reciprocating engines.
The most fuel-efficient current vehicle, the hybrid electric vehicle, can achieve two times the
vehicle fuel efficiency of current reciprocating engine vehicles of similar size and performance.
Utilizing the same principles of vehicle design, vehicles powered by CIDI engine could achieve
nearly comparable vehicle fuel efficiencies as current hybrids. Additional improvements seem
likely in the future, given the long history of experience with these conventional technologies.
