Combustion Stoichiometry Calculator

Complete combustion of any hydrocarbon CnHm follows a predictable stoichiometry:

CnHm + (n + m/4) O2 -> n CO2 + (m/2) H2O

Carbon always becomes CO2; hydrogen always becomes water. The oxygen requirement changes with the fuel.

Methane (CH4, n=1, m=4): 1 + 1 = 2 moles O2. Produces 1 mole CO2 and 2 moles H2O. Octane (C8H18, n=8, m=18): 8 + 4.5 = 12.5 moles O2. Chemists multiply through by 2 to avoid fractions: 2 C8H18 + 25 O2 -> 16 CO2 + 18 H2O. Propane (C3H8): 3 + 2 = 5 moles O2. C3H8 + 5 O2 -> 3 CO2 + 4 H2O.

Air is approximately 21% O2 by volume (the rest is mostly N2). So burning one mole of O2 requires 1/0.21 = 4.762 moles of air. This stoichiometric air-fuel ratio is critical in engine design: running rich (too little air) produces carbon monoxide; running lean (too much air) can cause knock and higher NOx emissions.

Excess air percentage = (actual air / stoichiometric air - 1) x 100%. Industrial combustion typically runs 10-20% excess air to ensure complete combustion while minimizing fuel waste. Power stations often target 3-5% excess O2 in flue gas as the control parameter.

Molecular weight of CnHm = 12n + m. The mass-based air-fuel ratio (AFR) = stoichiometric air moles x 28.97 / MW_fuel.

Why anyone runs excess air

The stoichiometric figure is the theoretical minimum, and no real burner runs there. Mixing is never perfect, so some fuel would meet no oxygen at all and leave as carbon monoxide or soot. Feeding in 10 to 20% more air than the equation demands buys enough margin to burn everything, at the cost of heating that surplus nitrogen and sending it up the stack.

Push the excess too far and efficiency falls away, because every extra cubic metre of air arrives cold and leaves hot. Combustion engineers tune this by measuring oxygen in the flue gas: a few percent is healthy, near zero means you are risking carbon monoxide, and high single figures means you are heating the sky.

Reading the air-fuel ratio

The mass AFR is the number engine people quote. Petrol lands near 14.7, which is why that figure is stamped on so much tuning documentation, and this calculator returns about 15.1 for pure octane, the difference being that real petrol is a blend rather than a single hydrocarbon. Methane comes out near 17.2, and hydrogen at about 34, because hydrogen is so light that a given mass of it needs an enormous mass of air.

Running rich (less air than stoichiometric) makes more power and runs cooler, which is why engines enrich under full load. Running lean improves economy but raises combustion temperature, and above roughly 1,800 K the nitrogen in the air itself starts oxidising into nitrogen oxides.