AFR and lambda (λ) for combustion analysis.
The stoichiometric air-fuel ratio is the exact proportion of air and fuel needed for complete combustion with no excess of either. For gasoline (octane, C8H18), the balanced equation 2C8H18 + 25O2 → 16CO2 + 18H2O requires 14.7 kg of air per kg of fuel. Lambda (λ) = actual AFR / stoichiometric AFR. λ = 1 means perfect stoichiometric combustion. λ < 1 is rich (excess fuel, more power but incomplete combustion, produces CO). λ > 1 is lean (excess air, better fuel economy but higher NOx). Modern engines use oxygen sensors (lambda probes) to maintain λ near 1.0 for catalytic converter efficiency. Diesel engines typically run lean (λ = 1.3-4.0). Turbocharged engines may run rich (λ = 0.8-0.9) under boost for cooling and detonation prevention.