When the combustion is stoichiometric, it can be defined in several ways, usually by the excess air e, or the absence of air (- e), or the richness R, or the inverse, the air factor lambda.

A non-stoichiometric reaction is written as a function of lambda:

CH + lambda (1 + a/4) (O2 + 3.76 N2) -> CO2 + H2O + (lambda - 1) (1 + a/4) O2 + 3.76 lambda (1 + a/4) N2

lambda is the coefficient of the multiplicative term representing the air in this equation(O2 + 3.76 N2).

If e is the excess air, and R the richness, we have:

R = 1/lambda and lambda = 1 + e = 1/R