What is biodiesel?

Biodiesel is a diesel fuel refined from vegetable oils or fats. In the United States most commercial biodiesel is made from soybean oil, but it can also be refined from animal fats, recycled restaurant grease and other sources.

Biodiesel producers use a chemical process called transesterification to separate the glycerin in the oil from the methyl esters (the chemical name for biodiesel).

Biodiesel often is sold in a blend with regular petroleum-based diesel. Most diesel engines can use a blend of B20 (20 percent biodiesel and 80 percent petroleum diesel) or less with little to no engine modification, although some manufacturers don't recommend it and will invalidate warranties. Many diesel engines can use 100 percent biodiesel with some modifications.

Source: Department of Energy Alternative Fuel Data Center, National Biodiesel Board

Cost
   $2.53 per gallon

Efficiency relative to gasoline
   109 percent

Fuel sources (U.S.)
   Primary: Vegetable oil (soybeans); crude oil
   Secondary: Animal fat; recycled restaurant grease

Number of biodiesel-fueled cars on the road
   N/A

Number of fueling stations
   475

Number of available models
   16

What is natural gas?

Natural gas, like petroleum, is a fossil fuel found underground. However, it burns more cleanly than petroleum. Natural gas is about 95 percent methane, and may contain small amounts of other gases such as nitrogen, helium, carbon dioxide and water vapor. Most of the natural gas used in the United States is produced domestically.

Most cars, buses and other vehicles than run on natural gas use gas that's been compressed at 3,000 or 3,600 pounds per square inch and stored in cylinders.

Natural gas is becoming a particularly common method of fueling buses -- about 20 percent of new buses run on natural gas. CNG-fueled passenger cars are less common -- only one model was sold in 2007 -- but it is possible to retrofit gasoline-powered cars to run on natural gas for about $2,000 to $4,000.

Source: Department of Energy Alternative Fuel Data Center

Cost
   $2.12 per gasoline-gallon equivalent

Efficiency relative to gasoline
   38 percent

Fuel sources (U.S.)
   Primary: Natural gas
   Secondary: N/A

Number of natural gas-fueled cars on the road
   118,000

Number of fueling stations
   732

Number of available models
   1

What is ethanol?

Ethanol is an alcohol-based fuel that can be distilled from the starch or sugars in crops such as corn, barley and sugar cane. Almost all of the ethanol produced in the United States comes from corn, while most of the ethanol produced in Brazil comes from sugar cane.

Much of the ethanol sold in the United States is sold in a blend called E10 (10 percent ethanol and 90 percent gasoline). Most gasoline engines can run on E10, however, E10 is not considered an "alternative fuel" according to government standards. Flexible fuel vehicles, in contrast, are designed to run on E85, a blend of 85 percent ethanol and 15 percent gasoline, or even higher concentrations of ethanol.

Ethanol also can be distilled from material called cellulose in plants like trees and grasses -- this kind of ethanol is called cellulosic ethanol -- but the method of producing it is less developed and more expensive. However, many researchers believe that if the process were improved, cellulosic ethanol could provide a cleaner, more efficient energy source than corn-based ethanol. (For more information, see Questions of Efficiency, Environmental Impact Loom over Increased Ethanol Use.)

Source: Department of Energy Alternative Fuels Data Center

Cost
   $3.47 per gallon

Efficiency relative to gasoline
   71 percent

Fuel sources (U.S.)
   Primary: Corn; crude oil
   Secondary: Sugar or starch-based plants (sugarcane, wheat, sorghum); cellulosic plants (wood, grasses)

Number of ethanol-fueled cars on the road
   246,000

Number of fueling stations
   1,166

Number of available models
   21

What is electricity?

Electricity, stored in a battery, can be used to power vehicles. Right now, there are no major car manufacturers making standard passenger-car-sized electric vehicles. Most electric vehicles manufactured today are low-speed, light-duty neighborhood vehicles.

However, major manufacturers such as Honda and General Motors did make electric cars for several years in the late 1990s and early 2000s, though they never became a major part of the car market. These vehicles were generally designed to be plugged into the power grid to recharge. So although electric vehicles produce no tailpipe emissions, making the electricity used to power them does generally produce greenhouse gas emissions.

Source: Department of Energy Alternative Fuels Data Center

Cost
   $0.12 per kilowatt-hour

Efficiency relative to gasoline
   3 percent

Fuel sources (U.S.)
   Primary: Coal
   Secondary: Petroleum, natural gas, wood, solar, wind, nuclear

Number of electricity-fueled cars on the road
   51,000

Number of fueling stations
   444

Number of available models
   0

What is hydrogen?

Hydrogen is abundant in nature, in water, hydrocarbons such as coal, and biomass. However, it doesn't exist alone -- it's always combined with other elements to make those substances.

There are many ways to derive pure hydrogen from the substances. Thermal processes use heat and pressure to separate the hydrogen in natural gas, coal or biomass from other elements. The majority of hydrogen produced in the United States comes from a thermal process called natural gas reformation.

Some hydrogen producers use a process called electrolysis that uses electricity to separate water into its constituent hydrogen and oxygen parts.

There are two types of hydrogen-fueled cars. Hydrogen internal combustion engines are similar to the regular combustion engines fueled by gasoline, only modified to burn gaseous hydrogen. Hydrogen fuel cells, on the other hand, electrochemically convert hydrogen to electricity -- and produce only water as an emission. (For more information, see How a Fuel Cell Works.)

Hydrogen fuel cell vehicles are not yet commercially available. However, prototypes are being tested in public and private fleets.

Source: Department of Energy Alternative Fuels Data Center, National Hydrogen Association

Cost
   $5.60 per kilogram

Efficiency relative to gasoline
   99 percent

Fuel sources (U.S.)
   Primary: Natural gas
   Secondary: Solar, nuclear

Number of hydrogen-fueled cars on the road
   119

Number of fueling stations
   62

Number of available models
   1