The air-fuel ratio (AFR) is the mass ratio of air to a solid, liquid, or gaseous fuel present in a combustion process.
Engineers often use Lambda (λ), which is the ratio of actual AFR to stoichiometric AFR. λ = 1 is stoichiometric, λ < 1 is rich, and λ > 1 is lean. The Equivalence Ratio (φ) is the inverse of Lambda (φ = 1/λ). Modern cars use oxygen sensors (Lambda sensors) to keep λ very close to 1.0 for the catalytic converter to work efficiently.
