Exhaust Gas Recirculation: Description and Operation

Fig. 124 EGR System:

SYSTEM OPERATION

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 solenoid valve, EGR modulator valve, EGR valve position sensor (California only), and the engine control unit (ECU).

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. It consists of a vacuum diaphragm and spring loaded valve. When vacuum is applied to the diaphragm chamber, the valve opens allowing a regulated amount of exhaust into the intake manifold.

EGR MODULATOR VALVE

The EGR modulator valve regulates the vacuum acting on the EGR control valve, depending on the exhaust backpressure. When the exhaust pressure is low (indicating low speed/no load operation) the modulator valve vents the EGR control vacuum to the atmosphere, reducing the amount of vacuum applied to the EGR valve. This causes the valve to remain closed. When the exhaust pressure increases (indicating high speed/moderate to heavy load), the vent closes. The EGR valve then gets full control vacuum to increase the amount of exhaust gas recirculated.

EGR SOLENOID VALVE

The EGR solenoid valve is a normally open vacuum valve, that closes when energized. It gets power from the main relay when the ignition is turned "ON" and becomes energized when the ECU completes the circuit to ground. In its normal state the solenoid valve applies control vacuum to the EGR modulator valve and EGR valve. When energized, the solenoid valve blocks vacuum from the source and vents the other side to the atmosphere, preventing EGR system operation.

ENGINE CONTROL UNIT

The engine control unit monitors signals from the distributor (engine speed), throttle position sensor (compared with engine speed signal determines engine load), and coolant temperature sensor. When operating conditions are correct for EGR operation, the ECU de-energizes the EGR solenoid valve, allowing the system to operate. The ECU de-energizes the solenoid under the following conditions:

1. Engine is at normal operating temperature (coolant temperature greater than 158° F (70° C)
2. Engine speed greater than 1,500 rpm.
3. Engine load moderate to heavy.

EGR POSITION SENSOR (California only)

The EGR position sensor is a variable resistor (potentiometer) that measures how far the EGR valve is open. The ECU monitors the change in resistance to determine whether or not the EGR valve is operating properly.