Exhaust Gas Recirculation: Description and Operation

Fig. 131 EGR System:

The exhaust gas recirculation (EGR) system is used to reduce oxides of nitrogen (NOx) emissions. The atmosphere is made up of mostly Nitrogen, with a smaller percentage of oxygen, and a mixture of other gases. Oxygen and Nitrogen do not normally combine except at very high temperatures and pressures, conditions which are 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. The exhaust gas is essentially inert (contains no fuel or oxidizer) 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 are increased.

The primary components of the EGR system are: the EGR control valve (EGR valve), EGR vacuum and EGR vent solenoid valves, EGR valve position sensor, and the engine control unit (ECU).


EGR VALVE POSITION SENSOR

The exhaust gas recirculation valve position sensor detects EGR control valve opening stroke and sends a signal to the engine control unit. The sensor is mounted on the EGR control valve.

EGR (CONTROL) 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.