To control exhaust emission, two types of systems are used. The air-injection system and the exhaust gas recirculation (EGR) system.
In (EGR) a certain portion of exhaust emission gases are directed back to the cylinder head; where they are combined with the fuel-air mixture and enter the combustion chamber. The recirculated exhaust gases serve to lower the temperature of combustion. A condition that favors lower production of nitrogen oxides; as combustion products (though at some loss of engine efficiency).
In a typical air-injection system, an engine-driven pump injects air into the exhaust manifold; where the air combines with unburned hydrocarbons and carbon monoxide at a high temperature and; in effect, continues the combustion process. In this way a large percentage of the pollutants that were formerly discharged through the exhaust system; are burned (though with no additional generation of power).
Another area for additional combustion is the catalytic converter. Consisting of an insulated chamber containing ceramic pellets or a ceramic honeycomb structure; coated with a thin layer of metals such as platinum and palladium. As the exhaust gases are passed through the packed beads or the honeycomb; the metals act as catalysts to induce the hydrocarbons, carbon monoxide; and nitrogen oxides in the exhaust to convert to water vapor, carbon dioxide, and nitrogen.