Air-Fuel Ratio to Lambda Converter (Petrol, Diesel, E85, Ethanol, LPG)
Convert an air-fuel ratio to lambda (λ) and back for the fuel you burn, with the equivalence ratio and how far the mixture is from ideal.
Results
What this tool does
Lambda is the air-fuel ratio divided by the ratio at which the fuel burns completely with nothing left over — the stoichiometric ratio. λ = 1 is that ideal mix, below 1 is rich (extra fuel) and above 1 lean (extra air). Wideband sensors and many tuning tools speak lambda because it means the same for every fuel, while air-fuel ratios do not: petrol is about 14.7:1 but ethanol only about 9:1, so '12.5:1' says very different things. The stoichiometric values here are commonly used figures; real fuels vary a little (ethanol in petrol, propane and butane in LPG, the ethanol share of E85).
Formula
λ = AFR ÷ AFR_stoich · AFR = λ × AFR_stoich · φ = 1 ÷ λ
Variables
| Symbol | Meaning | Unit |
|---|---|---|
fu | Fuel (and its stoichiometric air-fuel ratio) | — |
afr | Air-fuel ratio (kg of air per kg of fuel) | — |
lm | Lambda (λ) | — |
LM | Lambda (λ) | — |
AF | Air-fuel ratio (kg of air per kg of fuel) | — |
PH | Equivalence ratio φ (fuel-air, 1 ÷ λ) | — |
EX | Air in excess (+) or short (−) of the ideal amount | % |
Worked example
- Fuel (and its stoichiometric air-fuel ratio)14.7
- Air-fuel ratio (kg of air per kg of fuel)12.5
- Lambda (λ)0.850
- Equivalence ratio φ (fuel-air, 1 ÷ λ)1.176
- Air in excess (+) or short (−) of the ideal amount-15.0 %
Limitations
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.