Why do oxides of nitrogen form in car engines?

Oxides of nitrogen are formed in car engines primarily due to the high temperatures present during the combustion process. Under these extreme conditions, nitrogen and oxygen, which are normally quite stable and do not readily react under standard atmospheric conditions, can combine to form various nitrogen oxides, such as nitric oxide (NO) and nitrogen dioxide (NO₂).

The temperatures in a car engine can exceed 1,000 degrees Celsius, providing the necessary energy to overcome the activation energy barrier for this reaction. This formation of nitrogen oxides is a significant concern as they contribute to air pollution and acid rain.

Other factors, like fuel composition and the presence of additives, do influence combustion characteristics, but the formation of nitrogen oxides is fundamentally driven by the reaction of nitrogen and oxygen at elevated temperatures during combustion.