🔥 Air-Fuel Ratio (AFR) Calculator
Determine mixture strength, lambda, and equivalence ratio for combustion analysis and engine tuning.
Use consistent units (kg, g, or lb). Any mass unit works if both inputs match.
Enter fuel mass in the same unit used for air.
How to Use This Calculator
Measure air and fuel mass
Record the amount of air and fuel entering the engine or combustion chamber over the same interval.
Enter the values
Input the air mass and fuel mass using consistent units (kg, g, lb, etc.).
Calculate mixture data
Click "Calculate AFR" to see the actual AFR, lambda, equivalence ratio, and mixture classification.
Interpret the mixture type
Use the lambda and equivalence ratio to determine whether the mixture is lean, stoichiometric, or rich.
Formula
AFR = mair / mfuel
λ = AFR / AFRstoich
φ = 1 / λ
AFRstoich for gasoline ≈ 14.7. Adjust the stoichiometric value for alternate fuels (e.g., E85 ≈ 9.8).
Full Description
The air-fuel ratio (AFR) expresses how much air mixes with a given mass of fuel during combustion. A stoichiometric mixture contains just enough oxygen to burn all the fuel completely. Mixtures with more air than stoichiometric are lean, while mixtures with less air are rich.
Lambda (λ) normalizes AFR to the stoichiometric value, making it easy to compare different fuels. A lambda greater than 1 indicates a lean mixture, while values below 1 indicate excess fuel. The equivalence ratio (φ) is the reciprocal of lambda and is widely used in combustion research and emissions testing.
Understanding mixture strength helps tune engines for power, efficiency, and emissions compliance. Adjusting AFR impacts exhaust gas temperature, combustion stability, knock resistance, and catalytic converter performance.
Frequently Asked Questions
What is a stoichiometric AFR?
It is the ideal ratio where all fuel burns completely with the available oxygen. For gasoline this is approximately 14.7:1, while diesel and alternative fuels have different values.
Can I use different units for air and fuel?
No. Both masses must use the same unit because AFR is a ratio. Mixing units (kg and lb) will produce incorrect results.
What lambda range is safe for engines?
For gasoline spark-ignition engines, lambda near 1.0 is used for cruising and emissions. Lean mixtures (λ > 1.1) improve efficiency but raise NOx, while rich mixtures (λ < 0.9) are used for power enrichment and knock suppression.
How do I adjust AFR for other fuels?
Replace the stoichiometric reference value with the fuel-specific number when interpreting lambda. The calculator assumes 14.7 unless you adjust the inputs accordingly.