Exhaust gas recirculation (EGR) systems were introduced in the early 1970s to reduce the amount of nitrogen oxide (NOx) created by an engine during operating periods that result in combustion temperatures in excess of 1,370 °C (2,500 °F). At such high temperatures, nitrogen and oxygen combine in the combustion chamber to form nitrogen oxide which, when combined with hydrocarbons (HC) and sunlight, produce smog.
An EGR system reduces the formation of NOx by recirculating small amounts of exhaust gases into the intake manifold, where they mix with the incoming fuel-air mixture. The dilution of the incoming fuel-air mixture reduces peak combustion temperatures and pressures, resulting in a reduction of NOx.
The purpose of an EGR system is to regulate the recirculating exhaust gas flow rate under all operating characteristics. As well, an EGR system is able to override flow under conditions that would otherwise compromise good engine performance.
In other words, an EGR system works to maintain a balance between very good NOx control and good engine performance. Too much exhaust gas recirculating and mixing with the fuel-air mixture can result in reduced engine performance. Too little exhaust gas recirculating and mixing with the fuel-air mixture can result in exceeding allowable NOx emissions levels.
The EGR system is monitored and controlled by the vehicle's electronic engine control unit (ECU). The ECU accomplishes this by using an exhaust gas temperature sensor and oxygen sensor to determine the appropriate level of EGR flow.
Also on the eTV website