Essentiality and control of Oxygen in engine operation.

Air is approximately 21% oxygen. If there is not enough oxygen for proper combustion, the fuel will not burn completely and will produce less energy. An excessively rich air fuel ratio will increase pollutants from the engine. The fuel burns in three stages. First, the hydrogen burns to form water vapour. Second, the carbon burns to carbon monoxide. Finally, the carbon monoxide burns to carbon dioxide. This last stage produces most of the power of the engine. If all of the oxygen is consumed before this stage because there is too much fuel, engine's power is reduced.

There are a few exceptions where introducing fuel upstream of the combustion chamber can cool down the incoming air through evaporative cooling. The extra fuel that is not burned in the combustion chamber cools down the intake air resulting in more power. With direct injection this effect is not as dramatic but it can cool down the combustion chamber enough to reduce certain pollutants such as nitrogen oxides (NOx), while raising others such as partially decomposed hydrocarbons.

The air-fuel mix is drawn into an engine because downward motion of the pistons induces a partial vacuum. A compressor can additionally be used to force a larger charge (forced induction) into the cylinder to produce more power. The compressor is either mechanically driven supercharging or exhaust driven turbocharging. Either way, forced induction increases the air pressure exterior to the cylinder inlet port.