Exhaust Gas Recirculation: Description and Operation

EGR VALVE
The Exhaust Gas Recirculation Valve controls the flow of exhaust gases being recirculated into the induction system. The EGR Valve operation is controlled by the Thermo Valve, the Vacuum Regulating Valve (VRV) and port vacuum.




THERMO VALVE
The thermo valve is threaded into the intake manifold and senses the engine coolant temperature. Until the engine coolant has warmed up to operating temperature, the thermo valve is open venting manifold and port vacuum to the atmosphere. This improves driveability while the engine is still cold. As the engine temperature increases, the wax pellet within the thermo valve expands until the vent passage is blocked. Port vacuum is then applied to the EGR valve chamber.

Vacuum Regulator Valve:

VACUUM REGULATOR VALVE

The VRV is used to modify the port vacuum to the EGR Valve, after the engine has reached operating temperature.

While the engine is at idle, ported vacuum is not strong enough to open the EGR Valve, so it remains closed by spring pressure. This provides a smooth and stable idle.
As engine load increases, port vacuum increases, causing the EGR Valve to open. As the valve opens and exhaust gases begin to flow into the induction system, a vacuum is created across the EGR Valve venturi. The vacuum from the venturi is applied to the lower portion of the VRV, causing the poppet valve inside to open. When the poppet valve opens, the port vacuum being applied to the EGR Valve is bled back to the carburetor which enables the EGR valve spring to close the valve. The reduced flow across the EGR Venturi reduces the vacuum being applied to the VRV, which causes the poppet valve within the VRV's lower chamber to move toward the closed position. The resulting closed loop control of the EGR Valve improves driveability while maintaining a low NOx emissions output.
During a heavy engine load, ported vacuum is applied to the middle chamber of the VRV, as well as the EGR valve. The vacuum being applied to the middle chamber of the VRV aids in keeping the poppet valve closed and prevents the escape of port vacuum to the atmosphere.
Under a WOT. condition, ported vacuum is not strong enough to open the EGR Valve, so it remains closed by spring pressure. This provides the engine with an undiluted mixture to give the engine more power.