Exhaust Gas Recirculation: Description and Operation

The Exhaust Gas Recirculation System controls the formation of NOx (Oxides of Nitrogen) emissions by recirculating some of the exhaust gases back into the combustion chamber through the intake manifold.

The EGR is controlled by the ECM through an EGR solenoid vacuum (EGR SV) valve and modulator. Vacuum is allowed to the EGR Valve at acceleration and shut "OFF" upon deceleration or constant engine speed. The diaphragm mounted in the EGR modulator is operated by back pressure of the exhaust gas to open and close the EGR SV valve. By this opening and closing action of the EGR SV valve, the EGR modulator controls the vacuum transmitted to the EGR valve. Under a low load condition such as low speed driving, the exhaust pressure is low. In this state, the diaphragm in the EGR modulator is pushed down by the spring force and the modulator valve opens to allow the air into the vacuum passage from the outside. As a result, the vacuum transmitted to the EGR valve becomes smaller and so does the opening of the EGR valve. Thus, less amount of exhaust gas is recirculated to the intake manifold.

Under a high load condition such as high speed driving, on the other hand, the exhaust pressure is high. By the high exhaust pressure, the diaphragm in the modulator is pushed up and closes its valve. As the air does not enter the vacuum passage in this state, the vacuum transmitted to the EGR valve grows larger and so does the opening of the EGR valve. Thus, larger amounts of exhaust gas is recirculated to the intake manifold. Under any one of the following conditions, the ECM closes the vacuum passage of the EGR SV valve and the EGR valve remains closed:

^ When engine coolant temperature is low.

^ When throttle valve is at idle position.

^ When engine is running under high load.

When intake manifold pressure is low. Other than the above, the EGR valve opens and closes in accordance with the EGR modulator operation.