The automobile, powered by piston-type internal combustion engine, is so widely used that it has become the dominant source of air pollutants, particularly of photochemical smog, in large, urban cities.

Modern internal combustion engines operate through the Otto cycle, which involves rapid batch-burning of petroleum vapors. The combustion inside the cylinder is initiated by a spark and proceeds outward through the gas volume until it reaches the cylinder walls where it is cooled. Close to the cylinder wall, where combustion is quenched, a fraction of the fuel remains unburnt. In the next cycle of the engine the hot combusted gases and unburnt fuel vapor are forced out through the exhaust system of the automobile.

Automotive engines generally operate on "fuel rich" mixtures, which means that there is not quite enough oxygen to completely burn the fuel. As a result there is an excess of unburnt hydrocarbons...