AFR Calculator (Air-Fuel Ratio)
Calculate the stoichiometric air-fuel ratio from a fuel's chemical formula, or determine lambda (λ) and mixture classification for any combustion process.
🔥 Air-Fuel Ratio Calculator
Enter the number of carbon (C), hydrogen (H), and oxygen (O) atoms in your fuel molecule: CxHyOz
✅ Result
What Is the Air-Fuel Ratio (AFR)?
The air-fuel ratio (AFR) is the mass ratio of air to fuel present during combustion. It is one of the most important parameters in engine design, combustion engineering, and emissions control. The AFR determines whether a mixture burns completely, how much power is produced, and what emissions are generated.
For example, an AFR of 14.7:1 for gasoline means that 14.7 kg of air is needed for every 1 kg of gasoline for complete (stoichiometric) combustion.
Stoichiometric Air-Fuel Ratio
The stoichiometric AFR is the exact ratio at which all the fuel and all the oxygen in the air are completely consumed, leaving no excess of either. At stoichiometry, combustion is theoretically complete, producing only CO2 and H2O.
How to Calculate the Stoichiometric AFR
For a hydrocarbon fuel with the general formula CxHyOz, the balanced combustion equation is:
The stoichiometric AFR is then calculated as:
- Moles of O2 required: nO2 = x + y/4 − z/2
- Mass of O2: mO2 = nO2 × 32 g/mol
- Mass of air: Since air is 23.2% O2 by mass: mair = mO2 / 0.232
- Mass of fuel: mfuel = 12x + y + 16z g/mol
- AFR = mair / mfuel
Combustion: C8H18 + 12.5 O2 → 8 CO2 + 9 H2O
Step 1: nO2 = 8 + 18/4 − 0/2 = 12.5
Step 2: Mass of O2 = 12.5 × 32 = 400 g
Step 3: Mass of air = 400 / 0.232 = 1724.1 g
Step 4: Fuel mass = 12(8) + 18 + 0 = 114 g
Step 5: AFR = 1724.1 / 114 = 15.12:1
Air-Fuel Ratio of Common Fuels
| Fuel | Chemical Formula | Stoichiometric AFR |
|---|---|---|
| Methanol | CH3OH | 6.47:1 |
| Ethanol | C2H5OH | 9.00:1 |
| Butanol | C4H9OH | 11.21:1 |
| Diesel | C12H23 | 14.50:1 |
| Gasoline | C8H18 | 14.70:1 |
| Propane | C3H8 | 15.67:1 |
| Methane | CH4 | 17.19:1 |
| Hydrogen | H2 | 34.30:1 |
Lambda (λ) — The Equivalence Ratio
Lambda (λ) is a dimensionless ratio that compares the actual AFR to the stoichiometric AFR:
- λ = 1.0: Stoichiometric mixture — perfect balance of air and fuel
- λ > 1.0: Lean mixture — excess air, less fuel. Better fuel economy, lower CO/HC emissions, but higher NOx and potentially lower power.
- λ < 1.0: Rich mixture — excess fuel, less air. More power, but higher CO/HC emissions and wasted fuel.
Rich vs. Lean Mixtures
| Property | Rich (λ < 1) | Stoichiometric (λ = 1) | Lean (λ > 1) |
|---|---|---|---|
| Power output | Maximum near λ ≈ 0.85–0.9 | Moderate | Reduced |
| Fuel economy | Poor | Good | Best |
| CO emissions | High | Moderate | Low |
| NOx emissions | Low | Peak | Moderate |
| HC emissions | High | Low | Low to moderate |
| Exhaust temperature | Cooler | Hottest | Moderate |
How to Use the AFR Calculator
- Common Fuels tab: Select a fuel from the dropdown to instantly see its stoichiometric AFR, balanced equation, and mass breakdown.
- Custom Formula tab: Enter the number of C, H, and O atoms in your fuel molecule to calculate the stoichiometric AFR for any fuel.
- Lambda Calculator tab: Enter the stoichiometric AFR and your measured actual AFR to determine lambda and classify the mixture as rich, lean, or stoichiometric.
Frequently Asked Questions
What is the ideal air-fuel ratio for gasoline?
The stoichiometric (ideal) AFR for gasoline is approximately 14.7:1, meaning 14.7 kg of air for every 1 kg of gasoline. In practice, engines run slightly rich (~12:1–13:1) for maximum power, or slightly lean (~15:1–16:1) for fuel economy.
Why does hydrogen have such a high AFR?
Hydrogen (H2) has a very low molecular weight (2 g/mol) compared to hydrocarbon fuels. Since it takes 0.5 mol of O2 to combust 1 mol of H2, the mass of air required per unit mass of fuel is much larger, resulting in an AFR of about 34.3:1.
What does a lambda sensor measure?
A lambda sensor (oxygen sensor) in a vehicle's exhaust system measures the residual oxygen content in exhaust gases. The engine control unit uses this signal to determine whether the mixture is rich or lean and adjusts fuel injection accordingly to maintain near-stoichiometric operation for optimal catalytic converter efficiency.