Exhaust Gas Recirculation System (California)

EGR System (California):

OPERATION

This system controls the formation of NOx emissions by recirculating the exhaust gas into the combustion chamber through the intake manifold. There are two EGR valves included in this system circuit. One is the main EGR valve, which under the control of the BVSV (Bi-metal Vacuum Switching Valve) and the exhaust-pressure-actuated EGR modulator, opens and closes the exhaust gas passage to the intake manifold. The other is the sub EGR valve, which under the control of the ECM and the VSV (Vacuum Switching Valve), opens and closes the bypass passage to the main EGR valve so as to regulate the exhaust gas flow through it. The operation of the EGR valves is monitored by the ECM through the signal from the Recirculated Exhaust Gas Temperature Sensor (REGTS) which measures the temperature in the exhaust gas passage. Should anything abnormal occur, the "CHECK ENGINE" light turns "ON" to warn the driver.

Main EGR Valve 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 supplied to the main 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 supplied to the main EGR valve becomes smaller and so does the opening of the EGR valve. Thus, less 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 supplied to the main EGR valve grows larger and so does the opening of the main EGR valve. Thus, a larger amount of exhaust gas is recirculated to the intake manifold. When the coolant temperature is low, the vacuum passage of the main EGR valve is closed by the BVSV on the intake manifold. In this state, vacuum is not supplied to the EGR valve and it will remain closed. On the other hand when the coolant temperature is normal, the BVSV is opens, so the main EGR valve opens and closes in accordance with the EGR modulator operation.

Sub EGR Valve Operation

The sub EGR valve is controlled by the VSV which opens and closes the vacuum passage to the sub EGR valve and the magnitude of the vacuum. The ECM controls the VSV on the basis of signals from various sensors and closes the vacuum passage of the VSV if any of the following conditions are satisfied:
In this state, as the intake manifold vacuum is not supplied to the EGR valve, it will remain closed.

^ Engine coolant temperature is low.
^ Engine speed is high (higher than 6000 rpm).
^ Barometric pressure is low (at high altitude).
^ Engine load is high.
^ Transmission is in 5th gear (for M/T vehicle) or lock-up (for A/T vehicle).

In any condition other than the above, as the vacuum passage of the VSV is opened, the intake manifold vacuum is applied to the diaphragm of the EGR valve. When the vacuum exceeds the return spring force, the sub EGR valve opens the bypass passage for the exhaust gas flowing to the main EGR valve. If the main EGR valve is open, then more exhaust gas is recirculated to the intake manifold than when the sub EGR valve is closed.