Calculators · Fuel
Air–fuel ratio, lambda and fuel pump calculator
Every fuel burns completely at its own stoichiometric air–fuel ratio: about 14.7 parts air to one part petrol, but only 9.8 for E85. Lambda expresses any mixture relative to that ideal, which is why wideband sensors and tuners work in lambda. These calculators convert between air–fuel ratio and lambda for common fuels and estimate the fuel pump flow an engine needs.
Air–fuel ratio to lambda
Converts an air–fuel ratio reading into lambda and equivalence ratio for the selected fuel. Lambda below 1 is rich (extra fuel), above 1 is lean (extra air).
λ = AFR / AFR_stoich φ = 1 / λ
Lambda to air–fuel ratio
Turns a target lambda into the air–fuel ratio for a particular fuel. Useful when a tune specifies lambda but your gauge shows air–fuel ratio, or when switching fuels.
AFR = λ × AFR_stoich
Fuel pump flow required
Estimates the fuel flow a pump must deliver to support a given power, including a safety margin for pressure drop and pump ageing. Pump ratings fall as fuel pressure rises, so compare the result with the pump’s flow at your operating pressure.
fuel [kg/h] = hp × BSFC × 0.4536 L/h = fuel / density × (1 + margin)
Worked example: a 400 hp petrol engine
- A wideband reads 12.5 : 1 on petrol: λ = 12.5 / 14.7 = 0.85.
- The same λ 0.85 on E85 is 0.85 × 9.8 = 8.3 : 1.
- 400 hp at a BSFC of 0.50 uses 200 lb/hr = 90.7 kg/h of fuel.
- At 0.74 g/cc that is 122.6 L/h; with a 20% margin, choose a pump that flows at least 147 L/h at operating pressure.
Stoichiometric air–fuel ratios
| Fuel | Stoichiometric AFR | Full-load AFR (λ ≈ 0.85) |
|---|---|---|
| Petrol (gasoline) | 14.7 : 1 | about 12.5 : 1 |
| E10 | 14.1 : 1 | about 12.0 : 1 |
| E85 | 9.8 : 1 | about 8.3 : 1 |
| Ethanol (E100) | 9.0 : 1 | about 7.7 : 1 |
| Methanol | 6.4 : 1 | about 5.4 : 1 |
| Diesel | 14.5 : 1 | runs lean, λ above 1.2 |
| LPG / propane | 15.5 : 1 | about 13.2 : 1 |
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About these results
Results are theoretical estimates from standard engineering formulas. Real-world figures depend on conditions these formulas do not capture. Always follow the manufacturer's specifications for maintenance, loading and safety decisions.
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