A clear liquid derived from petroleum that is used mainly as a fuel in internal combustion engines. One barrel (159 l) of crude oil yields an average 72 l of petrol. Combustion is the oxidation of the fuel: 2C8H18 + 25O2 → 16CO2 + 18H2O, so 1 kg of petrol with 3.51 kg of oxygen yields 3.09 kg of CO2 and 1.42 kg of H2O. This is an exothermic reaction, 1 kg of petrol containing about 42.4 MJ of energy.