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.
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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