Exhaust Gas Recirculation: Description and Operation

EGR System:

GENERAL DESCRIPTION
The Exhaust Gas Recirculation (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.

OPERATION
EGR SV Valve
The EGR system is controlled by the ECM through an Exhaust Gas Recirculation 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.

EGR Modulator
The diaphragm mounted in the EGR modulator is operated by back pressure 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.

Low Load Condition
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, a lesser amount of exhaust gas is recirculated to the intake manifold.

High Load Condition
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, a larger amount of exhaust gas is recirculated to the intake manifold.

EGR Closed Conditions
Under any of the following conditions, the ECM closes the vacuum passage of the EGR SV valve. In this state, as the vacuum is not transmitted to the EGR valve, it remains closed:
^ When Engine coolant temperature is low.
^ When the throttle valve is at idle position.
^ When the engine is running under high load.

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