λ the air-fuel equivalence ratio

The air fuel equivalence ratio is the ratio of the air-fuel ratio to the stoichiometric air-fuel ratio for a given fuel.   It is denoted by the Greek letter λ.

Using the air equivalence ratio, λ is convenient in situation where engines use different fuels.

When the engine is running in rich air-fuel mixture i.e when there is excess fuel, λ is less than one.  When the engine is running with lean air fuel mixture, i.e. when air is more than required, λ is greater than one.