8.13 - How engines produce nitrogen oxides
Burning fuel makes an engine hot enough for nitrogen and oxygen from the air to react. This lesson explains how nitrogen oxides form and why this is different from incomplete combustion.
How hot engines produce nitrogen oxides
Engines draw in air containing nitrogen and oxygen. Oxygen reacts with the fuel during combustion, releasing energy and raising the temperature. Nitrogen and oxygen normally react very slowly at room temperature, but at the high temperatures inside an engine more particles have enough energy for reaction.
Oxides of nitrogen
Compounds containing nitrogen and oxygen, including nitrogen monoxide, NO, and nitrogen dioxide, NO₂. The group is often called NOₓ.
Nitrogen monoxide forms in the hot engine:
It can then react with oxygen to form nitrogen dioxide:
Both equations conserve nitrogen and oxygen atoms. NOₓ is a group label, not the formula of one molecule.
The nitrogen in this explanation comes from air, not the hydrocarbon fuel. Lack of oxygen can cause incomplete combustion to produce carbon monoxide or soot; it is the high temperature that enables nitrogen oxides to form.
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