How this calculator works
AFR is the mass of air divided by fuel mass. The formula mode balances complete CHO combustion and divides oxygen demand by the oxygen mass fraction of air, defaulting to 0.232. Real fuels and blends need their actual composition. An engine operating AFR can differ from stoichiometric AFR because of mixture control and combustion conditions.
Formula
AFR = m_air/m_fuel. For C_aH_bO_c, stoichiometric O2 = a + b/4 - c/2 mol per mol fuel.
Worked example
34.4 kg air and 2 kg fuel give AFR 17.2. Methane needs 2 mol O2 per mole CH4; at oxygen mass fraction 0.232 the stoichiometric AFR is about 17.19.
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Frequently asked questions
Is AFR a volume ratio?
No. This calculator uses mass ratios.
Can I calculate a blend?
Use a measured blend AFR or calculate oxygen demand from a verified composition. A single formula does not describe arbitrary gasoline or diesel blends.
What is excess-air ratio?
It is actual AFR divided by stoichiometric AFR for the same fuel and air basis.