Liquefied Petroleum Gas: Mainly a byproduct of petroleum refining. It consists of hydrocarbons that are vapors, rather than liquids, at normal temperatures and pressures; its main constituent is propane, and it is often referred to by that name. A relatively small amount of LPG is produced from natural gas.

Formula: C₃H₈ (Propane)

Advantages

Very widely available.

In order to liquefy the fuel, it is stored in sturdy tanks at about 20 times atmospheric pressure; since these are much tougher than typical sheet-metal or plastic gasoline tanks, and since they have a built-in shutoff valve to seal the tank if the fuel lines start leaking, LPG is safer than gasoline.

Less expensive than gasoline.

Because LPG enters the engine as a vapor, it doesn't wash oil off cylinder walls, thus reducing friction between engine parts, which makes the engine life longer.

Doesn't put carbon particles and sulfuric acid into the oil

Disadvantages

Because its source is petroleum, it doesn't do much to help relieve the petroleum dependency problem, and as such it is not even considered an alternative fuel by some definitions.

Its somewhat lower energy content compared to gasoline means a bigger tank is needed to get the same driving range.

The tank will also be heavier because it has to be strong enough to withstand the LPG storage pressure.

A vehicle that runs on LPG will typically be somewhat more expensive (around $1000-2000) than an equivalent gasoline-powered vehicle

The refueling procedure typically involves the release of some raw fuel vapors (unburned hydrocarbons) into the air