Alternative fuels: Generally nonpetroleum fuels, including ethanol, synthetic diesel, compressed natural gas, and hydrogen.

Clean diesel: New technology—often using the additive urea—to sharply reduce emissions. New diesels also are smoother, faster, and quieter than previous generations.

Electric vehicle: A car, truck, or crossover running solely on electric power. EVs are the only true zero-emissions vehicles, but they suffer from limited range, long charging times, and high costs.

Flexible-fuel vehicles: Vehicles designed to run on more than one type of fuel, most commonly gasoline, ethanol, or a blend of the two.

Fuel cell: A device that combines hydrogen and oxygen to form water vapor and electric current. That energy can be used to run an electric vehicle’s motor; hence fuel cells are sometimes called “refillable batteries.”

Hybrids: Hybrid-electric vehicles, or HEVs, combine two forms of power to improve fuel economy and reduce emissions. The most common approach combines gasoline and electric motors. The batteries are charged when the vehicle brakes or coasts.

Plug-in hybrid: Similar to a conventional hybrid, but uses advanced batteries that also can be charged from an electric outlet. The goal is to provide enough energy to handle daily commutes on batteries alone but still offer unlimited range on gasoline power.

PZEV: Partial-zero-emission vehicles are some of the cleanest vehicles on the road. They produce less hydrocarbon emissions driving cross-country than a car of the 1970s would emit while parked.