Nitrogen and oxygen
Since free nitrogen (N2) and free oxygen (O2) are both major components of atmospheric air they are naturally preset in the exhaust gas and are not regarded as air pollutants. The content of the two gasses in the atmospheric air is about 78.1% nitrogen and 20.95% oxygen (by volume).
Most nitrogen that passes through the engine does not react with oxygen during the combustion process. A minor portion does however become oxidized to nitrogen oxides which will be described in the following.
A portion of the oxygen in the intake air is consumed during the combustion process depending on the excess air ratio.