The catch is that the path to the pump might not be so easy as the overall picture for pollution and energy, which the engineers call "well to wheels", might have drawbacks to equal those of gasoline. The supply chains for diesel, ethanol and hydrogen are immature but that should change in a few years, as the most important choice for consumers in car showrooms may be what kind of fuel they want to use.
"Buying a car is not going to be about color choices or automatic versus manual transmission," said Allen Schaeffer, the executive director for the Diesel Technology Forum, a trade association. "It is going to be about getting into a powertrain. Current choices are diesel and ethanol."
Carmakers are selling diesel models in Europe that are clean, odor-free and peppy. Computer control over fuel injection has reduced diesel clattering noise, and ultraslow-sulfur diesel, now widely available in the United States, has made it possible for carmakers to install filters and other devices to clean up the exhaust.
Chrysler and Mercedes-Benz are offering diesels in 45 states, and Mercedes is planning to sell one that meets the stricter requirements of California, which have also been adopted by New York, Massachusetts, Maine and Vermont. Although diesel engines cost more to make and buy, they can make sense for a car owner.
For one, they use fewer gallons per trip than gasoline engines.
In addition to regular diesel fuel, which is refined from petroleum, there is biodiesel. Chemicals extracted from soy or other vegetables, or from beef tallow or other animal fats, burn well in a diesel engine. These substances become waxy at low temperatures, so they are usually blended in small quantities with petroleum diesel. But like ethanol, producing biodiesel requires farmland, which could otherwise be used to raise food. Yet making biodiesel takes less natural gas and other fossil fuels than making ethanol. A gallon of diesel will power a car 20 to 40 percent more miles than a gallon of gasoline, though the energy gain and the reduction in carbon dioxide emissions are not that large.
The reason is that a gallon of diesel fuel contains more carbon than a gallon of gasoline. It also has more energy, about 138,000 British Thermal Units (B.T.U.) versus about 118,000 for gasoline. That distinction may be lost on consumers, because motor fuel is sold by a unit of volume, the gallon, not a unit of energy.
Despite these issues, there is a real advantage to driving a diesel engine because it burns fuel at a higher temperature than a spark-ignited gasoline engine does, thus squeezing more work from the fuel. Skeptics still abound. Lee Schipper, a former oil industry executive who leads a transportation and environmental study program at the World Resources Institute, said that what pushed European drivers to diesel was a tax policy that made the fuel cheaper, but buyers there tend to drive more, so they don't save on total consumption. "There are limits to diesel," Dr. Schipper said. "Unless a diesel car is driven the same as a gasoline car, on 35 percent less fuel per kilometer, the CO2 benefit is marginal and may be negative. Hybridization might be a better option," he said.
The United States consumes about 140 billion gallons of liquid transportation fuel a year, about 6 billion gallons come from ethanol. Half of all gasoline contains some ethanol, which ordinary cars can burn at a concentration of up to 10 percent. About six million cars can now use any mixture of ethanol and gasoline, up to 85 percent ethanol, known as E85. Domestic carmakers view ethanol as a way to cut gasoline consumption and to avoid making major changes in their production.
Ethanol has strong political support. "I'd rather be paying farmers than the people overseas for the energy that fuels this country," President Bush told auto workers at a speech at a Ford plant in Claycomo, Mo., earlier this year.
The downside on ethanol is that it contains fewer BTUs than gasoline, so motorists would have to purchase more to travel the same distance as gasoline and the corn used for fuel could also be used to feed animals.