Exhaust Gas Recirculation: Description and Operation

EGR System Operation

The EGR system routes exhaust gases from the exhaust manifold to the intake manifold during specified engine operating conditions. The EGR valve is opened by ported manifold vacuum to let exhaust gas flow into the intake manifold. The exhaust gas then moves with the air/fuel mixture into the combustion chamber. Since there is less air/fuel mixture to burn in the combustion chamber during EGR operation, the temperature is reduced enough to limit NOx formation. If too much exhaust gas enters combustion will not occur. Because of this, very little exhaust gas is allowed to pass through the valve, especially at idle. The valve should only open when the engine is at operating temperature and above idle speed.

EGR Control Valve:

EGR CONTROL

To regulate EGR flow, a solenoid, controlled by the Spark Control Computer (SCC), is used in the vacuum line. This is a normally open solenoid vacuum valve which is closed when the Spark Control Computer (SCC) completes the ground. The normally open position (de-energized), allows vacuum to pass to the EGR valve. In the closed position (energized) vacuum to the EGR is vented to the atmosphere. Some calibrations use a delay valve between the solenoid and the EGR valve, (refer to Vehicle Emission Control Information Label, located in the engine compartment, for specific application).






VENTURI VACUUM CONTROL SYSTEM

The venturi vacuum control system utilizes a vacuum tap at the throat of the carburetor venturi to provide a control signal. This vacuum signal is amplified to the level required to operate the EGR control valve. Elimination of recycle at wide open throttle is accomplished by a dump diaphragm which compares venturi and manifold vacuum to determine when wide open throttle is achieved. At wide open throttle, the internal reservoir is ``dumped,'' limiting output to the EGR valve to manifold vacuum. The valve opening point is set above the manifold vacuum available at wide open throttle. This system is dependent primarily on engine intake airflow as indicated by the venturi signal, and is also affected by intake vacuum and exhaust gas back pressure.







CHARGE TEMPERATURE SENSOR (CTS)

This sensor, located in the intake manifold runner, monitors engine temperature. The sensor signals the computer when to operate the air switching, EGR, and spark advance systems.





EGR DELAY SYSTEM

Some vehicles are equipped with an EGR delay system to prevent exhaust gas recirculation for approximately 60 seconds after the ignition is turned on. This is accomplished by the Spark Control Computer controlling the EGR solenoid operation.