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Exhaust Gas Recirculation (EGR) System Used In IC Engines

Excessive Nitrogen Oxides (No2) form when peak combustion temperature exceeds 1950*C. To lower the combustion temperature, many engines have EGR system. It recirculates about 10% of the inert gas back into the intake manifold. The cooler exhaust gas absorbs heat from the much hotter combustion process. It reduces peak combustion temperature and lowers the formation of  No2. The EGR system provides a passage between exhaust manifold and inlet manifold. An EGR valve provided on this line opens and closes the passage.




It consists of a spring-loaded vacuum diaphragm linked to a tapered valve. This valve controls the passage for the exhaust gas. The chamber is connected by a tube to a vacuum port in the throttle body. When there is no vacuum at this port, the spring will push the diaphragm down and keep the passage closed. Therefore, no exhaust gas recirculates. It happens during the idle, when No2 formation is at a minimum. Also EGR could stall an idling engine. 

When the throttle is opened from the idle position, vacuum applied will gradually open the tapered valve. It causes the exhaust gas to flow into the intake manifold. At wide-open throttle, the intake manifold vacuum is low and EGR valve is closed by the spring. Thus, EGR valve systems do not affect full power operation. It is thus seen that exhaust gas is recirculated only in this system when the engine operating conditions are from NO2.

Many engines have thermal vacuum switch. It prevents EGR until engine temperature reaches 38*C. This switch is mounted in the engine water jacket where it consciences coolant temperature. The switches closes when the engine is cold. It prevents EGR just after a cold engine starts. After the engine warms up, the switch is opened. 

 

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