Interest in propane as an alternative transportation fuel stems from its domestic availability, high-energy density, clean-burning qualities, and relatively low cost.
Propane, also known as liquefied petroleum gas (LPG) or “autogas,” is a three-carbon alkane hydrocarbon (C3H8). It is a nontoxic, odorless gas at ambient temperature and pressure. It can be stored as a liquid at moderate pressures (300 PSI) or at temperatures below -44oF. In its liquid form, the energy density of propane is 270 times greater than in its gaseous form. However, a gallon of liquid propane has approximately 25% less energy content (BTUs) than a gallon of gasoline.
Propane is a byproduct of natural gas processing and crude oil refinement, and therefore it is not a renewable energy source. Propane fuel used for vehicles must be 90% propane. Butane (C4H10) is a common minor component, as well as propylene and butylene. Commercial propane also contains the odorant Ethyl mercaptan, which aids in leak detection.