Hybrid-electric:These vehicles are powered by both a gasoline engine and an electric motor. The gasoline engine is generally small and efficient. The electric motor provides additional power when needed, often during acceleration. Hybrid-electric vehicles run on gasoline.

Ethanol: These vehicles run on an alcohol-based fuel produced from fermented corn, barley and wheat. All gasoline vehicles are capable of running on a blend of gasoline and ethanol containing up to 10 percent ethanol. Flexible-fuel vehicles can run on an 85 percent ethanol blend. There are more than 1,000 public filling stations around the country with ethanol blend fuels.

Diesel or biodiesel:Diesels have internal-combustion engines that use air compression to ignite fuel to power a vehicle. Diesels are more fuel efficient than similar gasoline engines but have high emissions of particulates and smog-forming nitrogen oxides. New, "clean" diesel fuels, including ultra-low sulfur and biodiesel, will reduce these pollutants. Biodiesel is made of biodegradable vegetable oil, animal fat and recycled grease. Regular diesel fuel is petroleum-based. There are more than 600 gas stations in the United States that carry biodiesel.

Natural gas:Already commonly used in homes, businesses and industrial complexes, natural gas can power some vehicles. This clean-burning fuel can be stored on board in a compressed gaseous or liquefied state. Honda makes the only dedicated natural-gas vehicle, the Civic GX. It is possible to retrofit vehicles to run on natural gas. More than 700 gas stations in the United States dispense compressed natural gas.

SOURCES: Department of Energy, Environmental Protection Agency