Exhaust Gas Recirculation: Description and Operation

EGR System:

EGR PURPOSE

The Exhaust Gas Recirculation (EGR) system is used to lower NOx (oxides of nitrogen) emission levels caused by high combustion temperature. It does this by decreasing combustion temperature. The main element of the system is the EGR valve operated by vacuum and mounted on the intake manifold. The EGR valve feeds small amounts of exhaust gases back into the combustion chamber.

EGR OPERATION

This system controls the formation of NOx emission by recirculating the exhaust gas into the combustion chamber through the intake manifold. The EGR valve operation is controlled by the ECM. The ECM uses a Vacuum Switching Valve (VSV) to control vacuum to the EGR modulator, then to the EGR valve itself. The EGR valve is turned "ON" when the ECM senses various inputs from sensors. 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 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.

^ On vehicles with California emissions, the operation of EGR valve is monitored by ECM through the signal from the Recirculated Exhaust Gas Temperature Sensor (REGTS). The REGTS measures the temperature in the exhaust gas passage. Should anything abnormal occur, the CHECK ENGINE light turns "ON" to warn the driver.

EGR MODULATOR OPERATION

The diaphragm mounted in the EGR modulator is operated by backpressure of the exhaust gas 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 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 the 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.

EGR VSV:

EGR VACUUM SWITCHING VALVE (VSV)

The EGR valve is controlled by the ECM. The ECM uses a Vacuum Switching Valve (VSV) to control vacuum to the EGR modulator, then to the EGR itself. The EGR valve is turned "ON" when the ECM senses various inputs from sensors.

Under any one of the following conditions, the ECM closes the vacuum passage of VSV. In this state, as the vacuum is not transmitted to the EGR valve, it remains closed.

^ When engine coolant temperature is low
^ When engine is running at high speed (higher than 6000 rpm)
^ When barometric pressure is low (at high altitude)
^ When engine is running at high load
^ When transmission is in lock-up condition (for A/T Federal emission vehicle)
Other than the above, the EGR valve opens and closes in accordance with the EGR modulator operation.