is the rapid oxidation of a fuel (burning). The products of an ideal combustion process are water (H2O) and carbon dioxide (CO2); if combustion is incomplete, some carbon is not fully oxidized, yielding carbon monoxide (CO). A stoichiometric combustible mixture contains the exact quantities of air (oxygen) and fuel required for complete combustion. For gasoline, this air-fuel ratio is about 15:1 by weight. If the fuel concentration is too rich or too lean relative to the oxygen in the mixture, combustion cannot take place.

Also see explosive limits, internal combustion engine.