Exhaust Gas Recirculation: Description and Operation

Fig. 27 EGR system.:

Exhaust gas recirculation, Fig. 27 is used to reduce oxides of nitrogen emissions in the vehicle exhaust. EGR uses partial recirculation of the exhaust gases from a port in the cylinder head to a port in the intake manifold. The EGR valve controls the flow of exhaust gas in response to vacuum signals from the thermo valve, vacuum regulator valve, and port vacuum.

Fig. 60 EGR system, idle or wide open throttle:

During idling or full throttle operation 'E' port vacuum, Fig. 60 is low, the EGR valve is closed by spring force and there is no EGR flow.

Fig. 61 EGR system, cold engine:

When the engine is cold the thermo valve opens to allow 'E' port vacuum to vent to atmosphere, Fig. 61. The EGR valve remains closed by spring force.

Fig. 62 EGR system, low to mid throttle:

At operating temperature when the throttle is low to mid-range the thermo valve closes allowing 'E' port vacuum, Fig. 62 to open the EGR valve. As the EGR valve opens venturi pressure in the EGR port falls due to the increased flow of exhaust gas.

Fig. 63 EGR system, closed loop operation:

As the pressure in the EGR port venturi falls the vacuum regulator valve (VRV) opens to vent 'E' port pressure to atmosphere, Fig. 63. The EGR valve then closes slightly reducing the flow of exhaust gases.

Fig. 64 EGR valve system operation:

The EGR valve, VRV, and EGR port venturi act together as a closed loop system to regulate EGR flow rate, Fig. 64.

Fig. 65 EGR system, high load operation:

During high mid-range throttle operation, Fig. 65 oxides of nitrogen (NOx) are produced in large quantities. Full EGR operation occurs when 'A' port vacuum is applied to the VRV to shut off 'E' port vacuum escape to atmosphere, thus preventing EGR valve flow modulation by venturi pressure action.

Fig. 33 Vacuum regulator valve:

VACUUM REGULATOR VALVE (VRV)

The vacuum regulator valve, Fig. 33 is used to modify the EGR valve vacuum control signal at low engine load. The vacuum regulator valve reduces the EGR vacuum signal by bleeding air within the vacuum regulator valve when the pressure applied to the regulator valve is lower than the pre-set value corresponding to low engine loads. The pressure applied to the regulator valve is the sum of exhaust pressure and venturi vacuum.

The air bleed is closed when the pressure applied to the vacuum regulator valve is higher than the pre-set value corresponding to high engine load operation. The EGR valve then operates according to an unmodified vacuum signal.

Fig. 94 Thermo valve:

THERMO VALVE

A thermo valve, Fig. 94 is incorporated in the EGR system for sensing the coolant temperature at the intake manifold. The thermo valve prevents EGR flow until the engine warms up to prevent drivability problems. The thermo valve also holds the purge control valve closed while the coolant temperature is lower than a pre-set value to reduce CO and HC emissions during engine warm-up. Once the engine warms up the thermo valve allows the purge control valve to open.