Exhaust Gas Recirculation: Description and Operation

EGR System And Components:

The EGR System controls the formation of NOx (Oxides of Nitrogen) emissions by recirculating the exhaust gases into the combustion chamber. This is done through the intake manifold. The EGR is controlled by the ECM through an EGR vacuum switching valve and modulator. Vacuum is allowed to the EGR Valve at acceleration and shut "OFF" upon deceleration or constant engine speed. The diaphragm in the EGR modulator is operated by variations in exhaust gas back pressure. By this opening and closing of the 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 the exhaust pressure is high. The diaphragm in the modulator is pushed up by high exhaust pressure 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, a larger amount of exhaust gas is recirculated to the intake manifold. Under any one of the following conditions, the ECM closes the vacuum passage of the VSV 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.

EGR MODULATOR

EGR Modulator:

The diaphragm mounted in the EGR modulator is operated by exhaust gas back pressure to open and close the valve. By this opening and closing action of the 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 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 amount of exhaust gas is recirculated to the intake manifold.

EGR VACUUM SWITCH VALVE (VSV)

Vacuum Switch Valve (VSV):

Under any one of the following conditions, the ECM closes the vacuum passage of the VSV. In this state 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.