Liquefied petroleum gas or propane is a type of fuel that can be used in the transportation sector to help improve our nation's air quality and ease our dependence on imported petroleum. About two-thirds of the propane available today is a by-product of natural gas processing; the remainder comes from crude oil refining.
Propane, composed primarily of propane and butanes with smaller amounts of propylene and butylenes, is supplied in four grades of different composition. HD-5, the only grade appropriate for automotive applications, is 95 percent propane and five percent butanes. The terms liquefied petroleum gas and propane are often used interchangeably.
Engines have been powered with propane gas since 1912. Interest in using propane as a transportation fuel has continued to increase in recent years. Today, propane is the most widely used alternative motor fuel and the third most widely-used motor fuel overall, ranking right behind gasoline and diesel. The Department of Energy estimates that there are approximately 350,000 propane-fueled vehicles are on our nation's roads and more than 3.5 million in service throughout the world.
The United States is one of the world's largest producers of propane, generating about 230 million gallons in 1992. Almost 92 percent of the U.S. demand for propane gas is sourced domestically, therefore the supply is reliable and free from political upheaval. In addition, propane is readily available in regions with natural gas production or petroleum refining. Propane's dual source supply65 percent is extracted from natural gas production, 35 percent is a by-product of crude oil refiningprotects against shortage.
There are currently more than 10,000 public propane refueling stations in the United States and refueling a propane vehicle takes about the same amount of time as refueling a gasoline vehicle. Most gasoline-powered cars and trucks can be converted to operate on propane in either a single-fuel propane only or a dual-fuel either gasoline or propane configuration. Modifications include changes to the fuel delivery and carburetion system.
Propane's infrastructure has the benefit of being well established. Storage and transportation of propane gas in sealed, extra strength, pressurized tanks eliminates evaporative emissions or spillage. Propane does not pollute wells and underground water sources because it vaporizes when released and is not water soluble.
Propane can be burned directly in modified automotive engines. The gas is stored on-board in liquefied form in tanks pressurized to 160 pounds per square inch. Propane has about 80 percent of the energy of gasoline on a volumetric basis, however, so it requires more storage volume to provide the same driving range as a gasoline-powered vehicle.
Although the driving range of propane is still somewhat shorter than gasoline, it provides the longest driving range of any alternative fuel more than 250 percent further than compressed natural gas, about 60 percent further than methanol and 25 percent further than ethanol. Propane has 100+ octane and leaves no lead varnish or carbon deposits; the engine is cleaner and lasts longer than gasoline engines, which helps explain its popularity as fuel for fleet vehicles and industrial engines.
The chemical and physical properties of propane give it good driveability compared to gasoline. In addition, because it is introduced to the engine as a gas rather than as a liquid that must first be vaporized propane can provide quicker cold-starting with fewer emissions in a properly designed system.
Propane gas has the potential to produce less carbon monoxide and smog-causing reactive hydrocarbons than gasoline does. It is clean-burning, producing virtually no particulates or sulfur emissions. California Air Resources Board and U.S. EPA tests have shown that emissions of hydrocarbons and carbon monoxide are consistently reduced 50 to 80 percent and nitrous oxide emissions are reduced five to 30 percent after installing a propane conversion system.