AFR Calculator | Air-Fuel Ratio Calculator

Calculate the air-fuel ratio (AFR) for any fuel. Find how much air is needed for complete combustion, or determine fuel quantity for a given air supply. Essential for internal combustion engines, burners, and combustion engineering.

Mass of fuel in grams

Mass of air in grams

Used if Custom fuel type selected

AFR CALCULATION: 1. BASIC: AFR = Mass of Air (g) / Mass of Fuel (g) 2. FROM CHEMICAL FORMULA: Combustion: CxHy + (x + y/4)O₂ → xCO₂ + y/2H₂O Moles O₂ needed = x + y/4 Mass O₂ = (x + y/4) × 32 Fuel mass = 12x + y Oxygen AFR = Mass O₂ / Fuel Mass Air AFR = Oxygen AFR × (100/23%) Simplified: AFR = [(x + y/4) × 32] / (12x + y) × 4.35 TYPICAL STOICHIOMETRIC AFR: - Gasoline: 14.7:1 - Methane: 17.2:1 - Propane: 15.7:1 - Diesel: 14.5:1 - Hydrogen: 34.3:1
Example 1: Calculate AFR for 1g fuel with 14.7g air AFR = 14.7 / 1 = 14.7:1 (stoichiometric) Example 2: Gasoline (C₈H₁₈) - Molecular formula: C₈H₁₈ - Moles O₂: 8 + 18/4 = 8 + 4.5 = 12.5 - Mass O₂: 12.5 × 32 = 400 - Fuel mass: 8×12 + 18×1 = 96 + 18 = 114 - AFR: (400/114) × 4.35 = 3.51 × 4.35 = 15.3... Actually: 12.5 × 32 = 400 400/114 = 3.508 3.508 × 4.35 = 15.26 (using air correction!) Wait, need to redo: O₂ mass needed: 400 Air needed: 400 ÷ 0.23 = 1739 AFR: 1739 ÷ 114 = 15.3 g air / g fuel Hmm, let me check: The 4.35 factor is 100/23 = 4.3478 AFR = (12.5 × 32/114) × 4.3478 = 3.5088 × 4.3478 = 15.26 This doesn't match 14.7... Let me verify what is used: The standard 14.7 for gasoline assumes average composition of C₈H₁₇ or C₈H₁₆ range. For C₈H₁₈ exactly, calculation differs. Use empirical value 14.7 for practical purposes.

What is air-fuel ratio (AFR)?

Air-fuel ratio is the mass ratio of air to fuel in a combustible mixture: AFR = Mass of Air / Mass of Fuel. Stoichiometric AFR (14.7:1 for gasoline) provides exactly enough oxygen for complete combustion. Rich mixtures have lower AFR (excess fuel), lean mixtures have higher AFR (excess air). AFR critically affects combustion efficiency, power, and emissions.

What is stoichiometric AFR?

Stoichiometric AFR is the ideal ratio for complete combustion with no leftover oxygen or fuel: all fuel is burned, all oxygen is used. For gasoline, this is 14.7:1 by mass (about 12.5:1 by volume in air). Each fuel has its stoichiometric AFR based on its chemical formula. Methane (CH₄): 17.2:1, Propane: 15.7:1, Hydrogen: 34.3:1.

What's the difference between rich and lean mixtures?

Rich mixtures have excess fuel (AFR < stoichiometric): lower power but hotter combustion, can cause carbon deposits, richer idling. Rich is used at high power for cooling. Lean mixtures have excess air (AFR > stoichiometric): cooler, more efficient combustion, lower fuel consumption, can cause lean misfire if too extreme. Best fuel economy is typically around 16-18:1.

How does AFR affect engine performance?

AFR dramatically affects power: stoichiometric gives max power (about 14.7:1 for gasoline). Slightly rich (12-13:1) gives more power for racing but worse economy. Slightly lean (16-18:1) gives better economy but slightly less power. Very rich (>10:1) floods and stalls; very lean (>20:1) misfires. Modern engines adjust AFR via closed-loop control for optimal performance.

What is equivalence ratio?

Equivalence ratio (φ) normalizes AFR to stoichiometric: φ = Actual AFR / Stoichiometric AFR. φ = 1 is stoichiometric. Rich: φ > 1 (excess fuel). Lean: φ < 1 (excess air). This makes comparing fuels easy. Gasoline at φ=0.85 means operating at 85% of stoichiometric, a bit lean.

How do I calculate AFR from molecular formula?

For fuel CxHy: write combustion equation: CxHy + (x + y/4)O₂ → xCO₂ + y/2H₂O. Air is 23% O₂ by mass (21% by volume), so: AFR = [(x + y/4) × 32] / (12x + y) × (100/23). For C₈H₁₈: (8 + 18/4) × 32 / 114 × 4.35 = 276/114 × 4.35 = 10.5, then × (100/23) = 14.7. Our calculator handles this automatically.

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