Exhaust Gas Recirculation: Description and Operation

EGR System:

The EGR System controls the formation of NOx (oxides of nitrogen) emissions by recirculating the exhaust gases into the combustion chamber.

The EGR valve is controlled by the ECM through an EGR Solenoid Vacuum (SV) Valve and by exhaust back pressure acting on the modulator. Vacuum is allowed to the EGR valve on acceleration and shut "OFF" upon deceleration or idling. The valve mounted in the EGR modulator is operated by back pressure of the exhaust gas to open and close the modulator valve. By this opening and closing action of the modulator 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, so a less amount of exhaust gas is recirculated to the intake manifold.

Under a high load condition such as high speed driving the exhaust pressure is high. By the high exhaust pressure, the diaphragm in the modulator is pushed up and closes the modulator 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, so a larger amount of exhaust gas is recirculated into the intake manifold.

Under any one of the following conditions, the ECM opens the vacuum passage of the SV valve and vacuum is not transmitted to the EGR valve, but vented to the atmosphere 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 (full throttle).
^ When intake manifold vacuum is low (full throttle).

Other than the above, the EGR valve opens and closes in accordance with the EGR modulator operation.