|| Hydrocarbons are a major component of motor vehicle emissions. Hydrocarbon emissions react with other compounds in the atmosphere to produce photo-chemical smog.
|Hydrocarbons in exhaust gases
||Hydrocarbon emissions in the exhaust gases can be caused by valve overlap, incomplete combustion and ignition system misfiring.
|Oxides of nitrogen
||Under the high temperatures and pressure of combustion, nitrogen from the air combines with oxygen to produce oxides of nitrogen. Oxides of nitrogen can be produced in any operating internal combustion engine.
|Particulates||In spark-ignition engines, particulates are caused by incomplete combustion of rich air-fuel mixtures. In compression-ignition engines, they are caused by a lack of turbulence and lack of oxygen.
|Carbon monoxide||Carbon monoxide is produced during combustion when there are not enough oxygen molecules around the hydrocarbon molecules. This can be caused by an incorrect air-fuel ratio. Carbon monoxide is an extremely poisonous gas.
|Carbon dioxide||When complete combustion of air and fuel occurs, carbon dioxide and water are produced.
|Sulfur content in fuels||Gasoline and diesel fuels contain sulfur as part of their chemical makeup. When combined with water vapor, sulfuric acid is produced.|