Exhaust Gas Recirculation: Description and Operation

The EGR system recirculates a portion of exhaust gasses into the intake manifold, which dilutes the air/fuel mixture, thereby lowering combustion temperatures. The resultant decrease in temperature helps reduce oxides of nitrogen present in the exhaust flow.
The system consists of an EGR Control Valve, a Sub-EGR Control Valve and a Thermo Valve.

Fig. 7 Single type EGR control valve (Typical):

Fig. 8 Dual type EGR control valve:

EGR CONTROL VALVE

The EGR Control Valve controls the flow of exhaust gasses during specific modes of operation. During high load operation, the control valve increases EGR flow to help meet the stringent emissions standards required by law. However, during low load operation, the control valve reduces EGR flow to help improve driveability. Two types of control valves are used.
The single type EGR Control Valve, Fig. 7, is operated by carburetor vacuum in response to throttle valve opening. EGR flow is not available at idle or wide open throttle operation. The dual type EGR Control Valve, Fig. 8, consists of primary and secondary valves which are operated by different carburetor vacuums in response to throttle valve opening. The primary valve controls EGR flow when the throttle valve is slightly open, while the secondary valve controls EGR flow during wider throttle openings. EGR flow is not available at idle or wide open throttle operation.

Fig. 9 Sub-EGR control valve:

SUB-EGR CONTROL VALVE

The Sub-EGR Control Valve, Fig. 9, modulates EGR flow according to throttle opening. It is mechanically connected to the throttle linkage and opens and closes with the motion of the throttle valve.



THERMO VALVE

The Thermo Valve controls the supply of vacuum available to the EGR Control Valve. It senses coolant temperature and closes the control valve when temperatures are below the pre-set value of the valve, which results in improved cold engine driveability. When coolant temperature rises above the pre-set temperature of the valve, the valve opens the control valve and permits exhaust gas recirculation to take place.


EGR CONTROL SOLENOID VALVE


When the solenoid valve is energized by the ECU, the EGR control valve is closed due to throttle ported pressure mixed with the turbocharger pressure. When the solenoid valve is de-energized by the ECU, the EGR control valve is opened with the aid of throttle ported vacuum applied to its diaphragm.