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Used in connection with combustion and air/fuel ratios. The stoichiometric, or chemically correct ratio is the mass of air in Kg required to completely burn one Kg of fuel.

For petrol, this ratio is approximately 14.7:1 and for diesel, it is approximately 14.5:1.

The Greek letter λ is used to represent this ratio.

When λ = 1, this is the stoichiometric or chemically correct ratio of air to fuel. In other words there is sufficient air present to fully burn a given quantity of fuel.

When λ < 1, the amount of air present is less than what is required to burn a given quantity of fuel. In other words, the mixture is too rich.

When λ > 1, the amount of air present is more than what is required to burn a given quantity of fuel. In other words, the mixture is too lean.

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