Engine emissions
Spark-ignition engine

Perfect combustion of a hydrocarbon leads to the formation of carbon dioxide and water. This ideal pattern is rarely realized in an engine: exhaust gases are made up of a large number of molecules, some of which are identified pollutants, such as carbon monoxide (CO), nitrogen oxides (NO, NO₂, N₂O, represented as NO_x) and unburned hydrocarbons (HC). The determining parameter for the production of these pollutants is the richness of the carburetted mixture in which combustion has developed (figure 30). Rich mixtures produce CO and unburnt hydrocarbons; CO disappears below the stoichiometric ratio and reappears...