What is Fuel air ratio or Air fuel ratio?

From the perspective of combustion and engine efficiency, the relative proportions of fuel and air in the engine are crucial. It is stated as either the ratio of the fuel mass to the air or the other way around. With SI engines, the fuel air ratio nearly never changes during a range of operation. With CI engines, the fuel flow varies directly with the load whereas the air flow at a fixed speed does not vary with load. As a result, the phrase fuel-air ratio is frequently used in place of air-fuel ratio.

The chemically correct or stoichiometric fuel air ratio is defined as the amount of air required for full combustion of all the fuel in the mixture.

A mixture containing more fuel than the chemically correct mixture is called a rich mixture and a mixture containing less fuel (or excess air) is called a lean mixture. The ratio of the actual fuel air ratio to the stoichiometric fuel-air ratio is called the equivalence ratio.