Combustion Analysis Calculator

A combustion analysis calculator finds the balanced chemical equation, air-fuel ratio, and heat released when a fuel burns. It works for common fuels such as methane, propane, and octane, and for custom hydrocarbon fuels entered atom by atom.

How to use the combustion analysis calculator

1. Pick a fuel type, such as methane or propane, or choose "custom" and enter the number of carbon, hydrogen, oxygen, and nitrogen atoms in the fuel molecule.
2. Choose the oxidizer: air (21% oxygen, 79% nitrogen by volume) or pure oxygen.
3. Set the excess air percentage. Use 0% for the exact stoichiometric mixture, or a higher number to add extra air.
4. Read the results: the balanced equation, the air-fuel ratio, the heat released, and the mass share of each product gas.

What is combustion?

Combustion is a chemical reaction between a fuel and an oxidizer, usually oxygen, that releases heat. In complete combustion, a hydrocarbon fuel (a molecule made of carbon and hydrogen) reacts with enough oxygen to form only carbon dioxide (CO₂) and water (H₂O). If there is not enough oxygen, the reaction is incomplete and also produces carbon monoxide (CO), soot, and unburned fuel.

Stoichiometric oxygen formula

For a fuel with the formula CₓHᵧOᵤ, the moles of oxygen (O₂) needed for complete combustion are:

n(O₂) = x + y/4 − z/2

Here x is the number of carbon atoms, y the number of hydrogen atoms, and z the number of oxygen atoms already in the fuel. The balanced equation is:

CₓHᵧOᵤ + n(O₂) → x CO₂ + (y/2) H₂O

How to calculate the air-fuel ratio

The air-fuel ratio (AFR) is the mass of air divided by the mass of fuel needed for complete combustion. To find it:

1. Calculate the stoichiometric moles of O₂ from the formula above.
2. If the oxidizer is air, find the matching moles of N₂ using the 79:21 air ratio: mol N₂ = mol O₂ × (79/21).
3. Convert both gases to mass using their molar masses (O₂ = 32.00 g/mol, N₂ = 28.02 g/mol) and add them together to get the air mass.
4. Divide the air mass by the fuel's molar mass.

Worked example: methane

Methane (CH₄) has one carbon and four hydrogen atoms, so it needs n(O₂) = 1 + 4/4 = 2 mol of oxygen per mole of fuel:

CH₄ + 2 O₂ → CO₂ + 2 H₂O

With air as the oxidizer, 2 mol of O₂ brings 2 × (79/21) ≈ 7.52 mol of N₂. The air mass is (2 × 32.00) + (7.52 × 28.02) ≈ 274.8 g. Methane's molar mass is about 16.04 g/mol, so the air-fuel ratio is 274.8 ÷ 16.04 ≈ 17.1 to 1 by mass.

Heat of combustion

The heat of combustion is the energy released per mole or per kilogram of fuel burned. It is calculated with Hess's law, using the standard enthalpy of formation of each substance:

Heat released = 393.5 × x + 241.8 × (y/2) + ΔHf(fuel)

The values 393.5 and 241.8 kJ/mol come from the enthalpy of formation of CO₂ and water vapor. Because water is counted as vapor and not liquid, this gives the lower heating value (LHV), not the higher heating value (HHV). The HHV is larger because it also counts the heat released when water vapor condenses to liquid, which does not happen inside most combustion equipment.

For methane, ΔHf = −74.8 kJ/mol, so the heat released is 393.5(1) + 241.8(2) − 74.8 = 802.3 kJ/mol. Dividing by methane's molar mass (16.04 g/mol) gives about 50.0 MJ/kg.

Common fuels

Methane is the main component of natural gas. Propane and butane are used for heating, cooking, and portable stoves. Octane stands in for gasoline in these calculations, though real gasoline is a mixture of many hydrocarbons and its own stoichiometric AFR is usually quoted near 14.7:1.

Excess air and combustion efficiency

Excess air is extra air added beyond the stoichiometric amount, given as a percentage. Real burners rarely run at exactly 0% excess air, because imperfect mixing would leave some fuel unburned.

• At 0% excess air, the mixture is exactly stoichiometric. This gives the highest flame temperature but risks incomplete combustion if mixing is imperfect.
• Most burners run with 10–20% excess air, which reliably burns all the fuel while wasting only a little heat.
• Excess air above 50% lowers efficiency further, because the extra nitrogen and oxygen absorb heat and leave with the exhaust. It also lowers flame temperature, which can reduce the formation of nitrogen oxides (NOx).

What affects a combustion reaction

Fuel composition. Fuels with a higher ratio of carbon to hydrogen release less energy per kilogram and produce more CO₂ for the same energy output.

Oxidizer choice. Air dilutes the reaction with nitrogen, which lowers the flame temperature compared with pure oxygen. Pure-oxygen combustion burns hotter but can produce more nitrogen oxides if any nitrogen is present from the fuel or from air leaking in.

Mixing. Fuel and oxidizer must mix well for the reaction to go to completion. Pockets that are too fuel-rich or too fuel-lean burn incompletely, even if the overall mixture is correct on average.

Where combustion analysis is used

Engineers use these calculations to design car and aircraft engines, tune industrial furnaces and boilers, and size residential heating systems. Environmental engineers use the same balanced equations to estimate CO₂ and NOx output for emissions reporting.

Frequently asked questions

What is the difference between complete and incomplete combustion? Complete combustion happens when there is enough oxygen to turn all the fuel into CO₂ and water. Incomplete combustion happens when oxygen is short, producing carbon monoxide, soot, and unburned fuel instead.

Why do combustion systems use excess air? Extra air makes sure every part of the fuel meets enough oxygen, even where mixing is not perfect. Most systems run with about 10–20% excess air to avoid producing carbon monoxide.

How is the air-fuel ratio calculated? Find the moles of O₂ needed from the fuel's formula, add the matching N₂ if the oxidizer is air, convert both to mass, and divide the total air mass by the fuel mass. Methane needs about 17.1 kg of air per kilogram of fuel.

What is the difference between higher and lower heating value? The lower heating value (LHV) assumes the water produced stays as vapor. The higher heating value (HHV) assumes that water condenses back to liquid, releasing extra heat. This calculator reports the LHV, since most combustion equipment lets the water vapor escape as exhaust rather than condensing it.

What is stoichiometric combustion? Stoichiometric combustion is the exact mixture of fuel and oxidizer with no leftover fuel or oxygen in the products. Any more air than this is called excess air; any less is called a fuel-rich mixture.

Does fuel composition affect the exhaust? Yes. More hydrogen in the fuel means more water vapor in the exhaust. More carbon means more CO₂. Fuel that contains nitrogen or sulfur can also produce nitrogen oxides or sulfur dioxide when it burns.