Carbon monoxide and hydrocarbons are converted to harmless gases by catalytic converters and air injection emission control systems. However, there's a third pollutant in automobile exhaust that neither systems deal with. That pollutant is called oxides of nitrogen. It's the job of the exhaust gas recirculation system, or E.G.R., to lower the level of oxides of nitrogen in your car's exhaust. High temperatures in the combustion chamber, about twenty five hundred degrees or more, cause very high production of oxides of nitrogen. The EGR system lowers combustion chamber temperatures by diluting the air/fuel mixture with a small amount of exhaust gas. The diluted mixture produces a smaller explosion thus a lower temperature is maintained and less oxides of nitrogen are created. The EGR system uses a valve to regulate the amount of exhaust gas introduced to the air/fuel mixture. This control valve may need to be cleaned or replaced periodically. The drawback to this system is reduction in the amount of power the engine can deliver.
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