Most people know propane as the fuel in a white container attached to a barbecue grill. But propane has long proven its versatility for heating homes, heating water, cooking, drying clothes, fueling gas fireplaces, and as an alternative fuel for vehicles. Propane naturally occurs as a gas at atmospheric pressure but can be liquefied if subjected to moderately increased pressure. It is stored and transported in its compressed liquid form, but by opening a valve to release propane from a pressurized storage container, it is vaporized into a gas for use. Simply stated, propane is always a liquid until it is used. Although propane is non-toxic and odorless, an identifying odor is added so the gas can be readily detected.Liquefied Petroleum Gas (LPG) or Propane
When speaking of alternative fuels, the terms LPG and propane are often used interchangeably. LPG for vehicular use is a mixture containing at least 90 percent propane, 2.5 percent butane and higher hydrocarbons, and a balance of ethane and propylene. Henceforth in this article, LPG used as a vehicle fuel will be referred to as propane.
Propane is a by-product of natural gas processing or petroleum refining. It is a gas at room temperature returns to liquid when compressed. Liquid propane is stored in special tanks that keep it under pressure (about 200 psi). Although stored onboard vehicles as a liquid, propane is returned to a gaseous form before being burned in the engine.
Experiments using propane as a motor fuel were first conducted around 1910. During the 1950’s, the conversion of conventional vehicles to AFVs became popular. A taxi fleet in Milwaukee boasted a fleet of nearly 300 taxis running on propane at that time, and the Chicago Transit system operated more than 500 propane-fueled buses. For on road use, propane is currently used in both light- and medium-duty vehicles as well as heavy-duty trucks and buses. Propane is also a popular choice for non-road vehicles, such as forklifts and agricultural and construction vehicles.
Source: U.S. Energy Information Administration