Air fuel mixture

It is essential to supply proper air-fuel mixture to the engine cylinder in order to operate the engine at various loads and speeds. There are three types of air-fuel mixtures.
Stoichiometric mixture: It is a mixture in which the proportion of fuel and air is so accurate that proper combustion of fuel takes place. For a particular fuel, the amount of air required for complete combustion is calculated from the chemical equation. For the combustion of I kg of hydrocarbon fuel like octane (C₈H₁₈), 15 kg of air is required. Air fuel ratio of 15:1 is considered a correct ratio for hydrocarbon fuels.
Lean mixture: This mixture contains less fuel and more air. Air fuel ratio of more than 20:1 is considered as lean mixture.
Rich mixture: This mixture contains more fuel and less air. Air fuel ratio of upto 7:1 is considered as rich mixture.