Work out combustion air requirement instantly with clear inputs, formula shown and shareable results.
Complete combustion of CxHy needs x + y/4 moles of oxygen, and since dry air is 20.95% oxygen by mole, the air requirement is that divided by 0.2095. Converting both sides to mass with 28.96 g/mol for air gives the air-fuel ratio: octane comes out near 15.1:1, the familiar petrol figure. Excess air scales the requirement without changing the stoichiometry.
Oxygen requirement
O2 = x + y/4 per mole of fuel
Air-fuel ratio by mass
AFR = (O2 / 0.2095) x 28.96 / (12.011x + 1.008y)
Mixing is never perfect, so a stoichiometric charge leaves unburnt fuel. Boilers typically run 10-20% excess to complete combustion without wasting too much heat up the flue.
Yes, in the air figure - the 0.2095 division brings the accompanying nitrogen along, which is what carries heat out of the exhaust.