The ratio 14:1 used to burn 14 parts of air by one part of fuel (by weight) to form a more efficient mixture to achieve maximum power rather than the stoichiometrically correct ratio of 14.7 : 1, which is maintained in a three – way catalyst engine, is called Air Fuel ratio. The ordinary engines run on weak mixtures with air/ fuel ratios in the region of 15/16: 1whereas lean – burn engines run on still weaker engines under the air/ fuel ratio of 18 – 20: 1, during part – throttling.