petrol (gasoline) Major hydrocarbon fuel, a mixture consisting mainly of hexane, octane and heptane. One of the products of oil refining, petrol is extracted from crude oil (petroleum). Most modern cars have high-compression internal combustion engines, and the mixture of air and petrol vapour supplied to it can explode too quickly, pushing against a rising instead of a descending piston. This effect is known as engine knock, and to eliminate it many petrol manufacturers add lead(IV) tetraethyl to slow the rate of combustion. However, this additive has been found to be implicated in atmospheric pollution and causes brain damage, and so many manufacturers omit the additive and instead improve the octane number of the petrol by altering the mixture of hydrocarbons.

pet·rol / ˈpetrəl/ • n. British term for gasoline.

petrol †petroleum XVI; refined petroleum used in internal combustion engines XIX. — F. pétrole — medL. petroleum; see next.