Environmental Implications of Alternative-Fueled Automobiles:  Air Quality and Greenhouse Gas Tradeoffs

We analyze alternative fuel-powertrain options for internal combustion engine automobiles. Fuel/engine efficiency, energy use, pollutant discharges, and greenhouse gas emissions are estimated for spark and compression ignited, direct injected (DI), and indirect injected (II) engines fueled by conventional and reformulated gasoline, reformulated diesel, compressed natural gas (CNG), and alcohols. Since comparisons of fuels and technologies in dissimilar vehicles are misleading, we hold emissions level, range (160 and 595 km), vehicle size class, and style (a 1998 Ford Taurus sedan) constant. At present, CNG vehicles have the best exhaust emissions performance while DI diesels have the worst. Compared to a conventional gasoline fueled II automobile, greenhouse gases could be reduced by 40% by a DI CNG automobile and by 25% by a DI diesel. Gasoline- and diesel-fueled automobiles are able to attain long ranges with little weight or fuel economy penalty. CNG vehicles have the highest penalty for increasing range, due to their heavy fuel storage systems, but are the most attractive for a 160-km range. DI engines, particularly diesels, may not be able to meet strict emissions standards, at least not without lowering efficiency.