Combustion is a chemical reaction where a fuel reacts with oxygen to create CO₂ and water, and a lot of heat energy.

• Fuel and Oxygen are the reactants
• Carbon dioxide and water are the products

A "stoichiometric" (or "chemically correct") ratio is the exact ratio of reactants that combine perfectly to form the products. For AVGAS or petrol (both gasoline based fuels), the chemically correct fuel:air mass ratio is roughly 1:14.7. This means for every 1 kg of fuel, exactly 14.7 kg of air are required to perfectly burn it into just CO₂ and water, releasing the maximum amount of heat. Most of the time, aircraft engines run either lean or rich of this mixture ratio:

• Rich mixture is rich in fuel, meaning that there is more fuel (or less air) than in the ideal ratio. An example of a rich fuel:air ratio would be 1:13
• Lean mixture is one with less fuel (more air) than required for a perfect reaction. For example, 1:17

The air:fuel ratio is based on mass, as the volume varies depending on density. the mass is a direct measure of the number of molecules of each reactant, which is what matters for the chemical reaction.

It is often ideal to run piston engines slightly rich of the ideal mixture. This is to prevent the engine from overheating, both by having a slightly imperfect reaction which produces less heat, and also by using uncombusted fuel to cool down the inside of the engine. An engine operating at the perfect mixture setting will have the hottest possible exhaust gas temperature (EGT) - and this is how you can adjust the fuel mixture during flight.