Natural Gas as a Transportation Fuel
Only about one tenth of one percent of all of the natural gas in the United States is currently used for transportation fuel. About one third of the natural gas used in the United States goes to residential and commercial uses, one third to industrial uses, and one third to electric power production.
Natural gas has a high octane rating and excellent properties for spark-ignited internal combustion engines. It is nontoxic, non-corrosive, and non-carcinogenic. It presents no threat to soil, surface water, or groundwater.
Natural gas is a mixture of hydrocarbons, predominantly methane (CH4). As delivered through the nation's pipeline system, it also contains hydrocarbons such as ethane and propane and other gases such as nitrogen, helium, carbon dioxide, hydrogen sulfide, and water vapor.
Most natural gas is extracted from gas and oil wells. Much smaller amounts are derived from other sources such as synthetic gas, landfill gas, and coal-derived gas.
Compressed and Liquefied Natural Gas
Because of the gaseous nature of this fuel, it must be stored onboard a vehicle in either a compressed gaseous (compressed natural gas, CNG) or liquefied (liquefied natural gas, LNG) state.
Compressed Natural Gas
To provide adequate driving range, CNG must be stored onboard a vehicle in tanks at high pressure—up to 3,600 pounds per square inch. A CNG-powered vehicle gets about the same fuel economy as a conventional gasoline vehicle on a gasoline gallon equivalent (GGE) basis. A GGE is the amount of alternative fuel that contains the same amount of energy as a gallon of gasoline. A GGE equals about 5.7 lb (2.6 kg) of CNG.
Liquefied Natural Gas
To store more energy onboard a vehicle in a smaller volume, natural gas can be liquefied. To produce LNG, natural gas is purified and condensed into liquid by cooling to -260°F (-162°C). At atmospheric pressure, LNG occupies only 1/600 the volume of natural gas in vapor form. A GGE equals about 1.5 gallons of LNG. Because it must be kept at such cold temperatures, LNG is stored in double-wall, vacuum-insulated pressure vessels. Typically LNG fuel systems are only used with heavy-duty vehicles.
Visit the Alternative Fuels and Advanced Vehicles Data Center to learn more about natural gas.