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In a gasoline engine, a mixture of air and fuel vapor is compressed and then ignited. Air is about 20 percent oxygen and about 80 percent nitrogen. Gasoline is mostly hydrogen and carbon. In the engine, oxygen atoms unite with the hydrogen atoms in the gasoline to form water (H₂O). Oxygen atoms also unite with carbon atoms to form carbon dioxide (CO₂). During combustion, temperatures may get as high as 6,000°F. At such high temperatures, nitrogen combines with oxygen, producing nitrogen oxides (NOx) that contribute to air pollution.

In ideal (perfect) combustion, all the hydrocarbons in the gasoline would be made into H₂O and CO₂. However, engines do not produce ideal combustion. Some hydrocarbons are left over. Also, some poisonous carbon monoxide (CO) is produced instead of CO₂. These combustion products also contribute to air pollution.