Propane and butane, used separately or in mixtures, are the principal LP-gases. They are either extracted from natural gas or produced as a result of processing crude oil at refineries. At normal temperatures and atmospheric pressure, it exists as a vapor.
Producers convert propane to a liquid by a combination of moderate pressure and cooling.
This greatly reduces space requirements, since approximately 270 gallons of propane vapor can be converted to one gallon of liquid. Moderate pressure holds the propane in liquid form until needed. A specially designed pressure vessel is necessary to maintain this pressure. LP-gas has been a common fuel in homes and businesses since the early 1920's. Engine fuel use of propane dates back to the 1930's.