LPG stands for liquefied petroleum gas, and its is mainly produced as a byproduct from the refining of crude oil.
There tend to be two types of LPG; Butane is used fuel for camping stoves, or in lighters and as a replacement for CFC gases in aerosol cans.
Propane, meanwhile, is the fuel that is used for road vehicles, because it has a much higher calorific value than butane. In short, it burns better.
It’s possible to use the gas as a fuel in cars because when it is stored in a pressurised tank under only slight pressure, it reverts to a liquid form and takes up almost 250 times less volume, which means you can store a great deal of it in a small space. This is also why it can be used in a conventional pressurised fuel system.
LPG is extremely cheap to produce because until relatively recently it was regarded as a waste product.
However, it has since been discovered that its octane rating is actually much higher than that of petrol, so it requires no additives in order to make it burn efficiently. Indeed, the only thing that is added to it during the refining process is a pungent odour, so that it can be detected if it ever manages to leak out of its container.