Exhaust Gas Recirculation: Description and Operation

EGR SYSTEM
The EGR system helps to reduce oxides of nitrogen by recycling exhaust gas through the combustion cycle by admitting exhaust gas into the intake manifold. The EGR valve, located at the point of discharge into the intake manifold, controls exhaust gas flow which is the result of the pressure differential between exhaust system and intake manifold.

Fig. 15 EGR control valve. Diesel engine:

EGR CONTROL VALVE
The EGR control valve, Fig. 15, modulates the amount of exhaust gas admitted into the intake manifold. The amount of gas that flows through the valve is directly proportional to the valve opening which is regulated by amount of vacuum applied to the valve. The valve is closed at wide open throttle and open at closed throttle. At intermediate throttle positions, the valve opening is proportional to throttle position or movement which is transferred to a vacuum regulator valve. The vacuum regulator valve, mounted on the injection pump throttle shaft, generates a vacuum signal proportional to control lever travel.

EGR CONTROL UNIT
The EGR control unit monitors engine speed and coolant temperature and produces a corresponding output signal to open or close the EGR valve. During engine warm-up, when less oxides of nitrogen are generated, the control unit de-energizes the EGR control solenoid which shuts down the vacuum signal to the EGR valve, allowing the valve to close. The control unit also de-energizes the EGR valve during high and low engine speeds to maintain acceptable driveability.

EGR VACUUM REDUCER SOLENOID VALVE
The EGR vacuum reducer solenoid valve reduces the vacuum signal to the EGR valve a specified amount around wide open throttle position to maintain acceptable driveability. An accelerator switch energizes the valve when throttle is wider than a preset opening. The valve also reduces the vacuum signal at high altitude to reduce exhaust smoke.