The ignition of propane occurs upon contact with oxygen. Propane is a hydrocarbon called an alkane, which typically produces water and carbon dioxide upon burning. Combustion occurs when propane becomes flammable; a spark is usually needed for this to occur unless the air is hot enough. The temperature has to be at least 920°F (about 433°C) for it to spontaneously burn, and once ignited, propane can burn when it is as cold as -156°F (about -104°C). When exposed to the air, propane typically needs to account for between 2% and 9.6% of the mixture to combust.
If oxygen levels are low, propane can ignite but undergo incomplete combustion. Water and carbon dioxide will form, and so will carbon monoxide which can be poisonous to humans. While a lot of heat is generally produced during propane combustion, the process can create less waste material than gasoline combustion. Propylene and butylene molecular bonds in the propane enable the flame to be visible.
Propane combustion can occur without a spark if the air is hot enough. If the temperature reaches 3,595°F (about 1,979°C), however, propane combustion is unlikely to occur and a propane flame usually stops burning. Proper storage is important because propane is typically heavier than air. It will sink to a low area and propane combustion can be triggered if the gas comes in contact with a pilot light or other ignition source.
Often stored in small pressurized containers, propane usually vaporizes when it hits the air. Barbeque grills may use propane to keep the flame going, so the combustion is controlled. The alkane is sometimes used in combustion engines of buses, trains, as well as industrial equipment such as forklifts. Recreational vehicles, furnaces, and various kinds of heaters can also use propane. Passenger cars in the 21st century sometimes use propane as well, which can be cleaner than gasoline, and may leave fewer deposits in the engine.
Despite its use with engines, propane for powering air conditioning systems built to work with refrigerants can be hazardous. Propane combustion, however, is often essential for petrochemical production, manufacturing semiconductors, powering theme park attractions, and fueling hot air balloons. In addition to being useful in many applications, it is also relatively inexpensive and safe if used properly. Propane is also non-toxic, but can cause suffocation if it is inhaled on purpose.