Liquefied petroleum gas (LPG), is a flammable mixture of hydrocarbon gases used as fuel in heating appliances, cooking equipment, and vehicles.
It is increasingly used as an aerosol propellant and refrigerant, replacing chlorofluorocarbons in an effort to reduce damage to the ozone layer. When specifically used as a vehicle fuel it is often referred to as autogas. Varieties of LPG bought and sold include mixes that are mostly propane (C3H8), mostly butane (C4H10), and, most commonly, mixes including both propane and butane. Propylene, butylenes and various other hydrocarbons are usually also present in small concentrations. HD-5 limits the amount of propylene that can be placed in LPG to 5%, and is utilized as an autogas specification.