What is air-fuel ratio ?

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Air–fuel ratio (AFR) can be defined as the ratio of mass of air to the mass of fuel present in an internal combustion engine. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric air-fuel ratio. The ratio (14.7:1) is considered as the ideal ratio for lowest emissions.
(a) Rich Mixture – A lower air-fuel ratio implies that the mixture contains less air than the 14.7:1 ratio. Such a mixture is said to be a rich mixture.
(b) Lean Mixture – Similarly a higher air-fuel ratio implies that the mixture contains more air and therefore it is considered as a lean mixture.