Liquefied petroleum gas (LPG), also called LPG, GPL, LP Gas, liquid petroleum gas or simply propane, is a flammable mixture of hydrocarbon gases used as a fuel in heating appliances and a clean-burning fossil fuel that can be used to power internal combustion engines. LPG-fueled vehicles can produce significantly lower amounts of some harmful emissions and the greenhouse gas carbon dioxide (CO2). LPG is usually less expensive than gasoline, it can be used without degrading vehicle performance, and most LPG used in U.S. comes from domestic sources.
The availability of LPG-fueled light-duty passenger vehicles is currently limited. A few light-duty vehicles—mostly larger trucks and vans—can be ordered from a dealer with a prep-ready engine package and converted to use propane. Existing conventional vehicles can also be converted for LPG use. Since propane is stored as a liquid in pressurized fuel tanks rated to 300 psi, LPG conversions consist of installing a separate fuel system if the vehicle will run on both conventional fuel and LPG or a replacement fuel system for LPG-only operation.
Propane Fuel Basics
Also known as liquefied petroleum gas (LPG) or autogas, propane is a clean-burning, high-energy alternative fuel that’s been used for decades to power light-, medium- and heavy-duty propane vehicles.
Propane is a three-carbon alkane gas (C3H8). It is stored under pressure inside a tank and is a colorless, odorless liquid. As pressure is released, the liquid propane vaporizes and turns into gas that is used for combustion. An odorant, ethyl mercaptan, is added for leak detection.
Propane has a high octane rating and excellent properties for spark-ignited internal combustion engines. It is non-toxic and presents no threat to soil, surface water, or groundwater. Propane is produced as a by-product of natural gas processing and crude oil refining. It accounts for about 2% of the energy used in the United States.
Of that, less than 2% is used for transportation fuel. Its main uses include home and water heating, cooking and refrigerating food, clothes drying, powering farm and industrial equipment. Rural areas without natural gas service commonly rely on propane as a residential energy source. The chemical industry uses propane as a raw material for making plastics and other compounds.
Propane as an Alternative Fuel
Interest in propane as an alternative transportation fuel stems mainly from its domestic availability, high-energy density, clean-burning qualities, and its relatively low cost. It is the world’s third most common engine fuel and is considered an alternative fuel under the Energy Policy Act of 1992.
Autogas is a mixture of propane with smaller amounts of other gases. According to the Gas Processors Association’s HD-5 specification for propane, it must consist of at least 90% propane, no more than 5% propylene, and 5% other gases, primarily butane and butylene. (See fuel properties.)
Propane is stored on board a vehicle in a tank pressurized to about 150 pounds per square inch—about twice the pressure of an inflated truck tire. Under this pressure, propane becomes a liquid with an energy density 270 times greater than the gaseous form. Propane has a higher octane rating than gasoline, which can decrease engine knock. However, it has a lower Btu rating than gasoline, so it takes more fuel to drive the same distance. Propane’s clean burning characteristics allow the engine to have increased service life.