Air-Fuel Ratio (AFR) Calculator

Air-fuel ratio (AFR) is the mass ratio of air to fuel in a combustion mixture. It indicates how much air is needed for complete combustion of a given amount of fuel. The stoichiometric AFR represents the ideal ratio for complete combustion with no excess air or unburned fuel.

Lambda (λ) is the ratio of actual AFR to stoichiometric AFR. λ = 1 means perfect stoichiometric mixture, λ > 1 indicates lean mixture (excess air), and λ < 1 indicates rich mixture (excess fuel). Lambda is crucial for engine tuning and emissions control.

For a hydrocarbon fuel CₓHᵧ, the stoichiometric AFR is calculated using the balanced combustion equation. The oxygen demand is (x + y/4) moles O₂ per mole fuel. Since air is ~21% O₂, the AFR = (oxygen demand × 4.76 × 32) / molecular weight of fuel.

Gasoline (C₈H₁₈) has a stoichiometric AFR of approximately 14.7:1, meaning 14.7 kg of air is needed to completely burn 1 kg of gasoline. Most gasoline engines operate near this ratio for optimal efficiency and emissions control.

Rich mixtures (λ < 1) have excess fuel, producing more power but higher CO and HC emissions. Lean mixtures (λ > 1) have excess air, improving fuel economy but potentially causing higher NOₓ emissions and engine knock. Stoichiometric mixtures provide balanced performance.

AFR significantly impacts power, fuel economy, and emissions. Rich mixtures provide maximum power but poor fuel economy. Lean mixtures improve efficiency but reduce power and can cause engine damage. Modern engines use oxygen sensors to maintain optimal AFR.

Equivalence ratio (φ) is the inverse of lambda: φ = 1/λ. It represents the actual fuel-to-air ratio divided by the stoichiometric fuel-to-air ratio. φ = 1 is stoichiometric, φ > 1 is rich, and φ < 1 is lean.