As cracking methods have evolved more and more LPG has been produced by refineries. Gasoline is a light fraction of petroleum which is quite volatile and burns rapidly . Straight run gasoline refers to gasoline produced by distillation instead of cracking, although it really doesn't make a difference. Today, the average American uses 450 gallons of gasoline a year . Octane Number rates a fuel's ability to avoid premature ignition called knock .
Premature ignition reduces an engine's power and quickly wares it out .
Isooctane is then added to heptane until the mixture has the same knock characteristics as the fuel being tested, and the percent isooctane is taken as the unknown fuels octane number. Tetraethyl lead used to be a common anti-knock additive which would raise a fuels octane number. High octane fuel can be used in engines with high compression ratios which in turn produce much more power . However, the additive is no longer used because of concerns over lead pollution . Naphtha is a light fraction of petroleum used to make gasoline .