Petrol (gasoline): Major hydrocarbon fuel, a mixture consisting mainly of hexane, octane and heptanes. 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 vapor supplied to it can explode too quickly, pushing against a rising instead of a descending piston. This effect is known as engine knocks, 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.
