Why do oxides of nitrogen form in car engines?

Oxides of nitrogen, commonly referred to as NOx, are primarily formed in car engines due to the high temperatures achieved during the combustion process. In gasoline or diesel engines, the combustion chamber can reach extremely high temperatures, often exceeding 2000°C (3632°F). At these elevated temperatures, nitrogen and oxygen, which are abundant in the air, can react to form various nitrogen oxides, such as nitrogen monoxide (NO) and nitrogen dioxide (NO2).

This reaction is facilitated by the energy provided in the high-temperature environment, breaking the strong triple bond in molecular nitrogen (N2) to allow for the interaction with oxygen (O2). Therefore, the formation of nitrogen oxides in car engines is a direct consequence of the extreme conditions present during combustion, rather than other processes such as the burning of fossil fuels in general, incomplete combustion, or the operation of catalytic converters.