The yield of gasoline on distilling crude petroleum is of the order of 10 to 45 per cent., but in order to satisfy the enormous demand for motor fuel this yield may be increased by thermal treatment of heavy fractions such as gas oil at temperatures in the region of 500°C. Under these conditions, the oil is “cracked” or broken down to give gas, gasoline, and heavier fuels. Cracking increases the anti-knock value of gasoline as well as the yield.