Exhaust Gas Recirculation: Description and Operation

Fig. 131 EGR System:

The exhaust gas recirculation (EGR) system reduces oxides of nitrogen (NOx) emissions. Atmospheric oxygen and nitrogen do not normally combine except at very high temperatures and pressures, conditions present in the combustion chamber especially during hard acceleration. When the engine is under load, the EGR valve admits a small amount of exhaust gas into the intake manifold to mix with the air/fuel charge. Exhaust gas is essentially inert and reduces peak combustion temperatures and pressures by absorbing some of the heat of combustion without participating in the actual burn. Greater amounts of exhaust gas are metered in as engine speed and load increase.

Primary EGR system components are: EGR control valve, EGR vacuum and vent solenoid valves, EGR valve position sensor, and ECU.

EGR VALVE POSITION SENSOR
The EGR valve position sensor detects EGR control valve opening stroke and sends a signal to the ECU. The sensor is mounted on the EGR control valve.

EGR VALVE
The EGR valve allows a portion of the exhaust gas into the intake manifold to reduce the amount of NOx produced by the engine.

EGR VACUUM AND VENT SOLENOID VALVE
The EGR vent and EGR vacuum solenoid valves control the vacuum signal to the EGR valve.

ENGINE CONTROL UNIT
The engine control unit monitors signals from the distributor (engine speed), air flow meter and throttle sensor (engine load), idle switch, engine coolant temperature sensor, radiator coolant temperature switch, and EGR valve position sensor. The ECU then uses this information to determine when to operate the solenoid valves.