Exhaust Gas Recirculation: Description and Operation

Fig. 124 EGR System:

SYSTEM OPERATION
The EGR system reduces 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 present in combustion chambers 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 (contains no fuel or oxidizer) and reduces peak combustion temperatures and pressures by absorbing some of the heat of combustion. Greater amounts of exhaust gas are metered in as engine speed and load increase.

Primary EGR system components are: EGR control valve, EGR vacuum solenoid valve, EGR modulator valve, EGR valve position sensor (Calif. only), and ECU.

EGR CONTROL VALVE
The EGR valve allows a portion of the exhaust gas into the intake manifold to reduce NOx emissions. 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 vacuum acting on the EGR control valve, depending on exhaust backpressure. When exhaust pressure is low (indicating low speed/no load operation), the valve vents EGR control vacuum to atmosphere, reducing vacuum applied to the EGR valve. This causes the valve to remain closed. When 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. When open, the valve passes control vacuum to the EGR modulator valve and EGR valve. When energized, the valve blocks vacuum and vents to atmosphere, preventing EGR system operation.

ENGINE CONTROL UNIT
The ECU 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:

^ Engine at normal operating temperature (coolant temperature greater than 158° F (70° C)
^ Engine speed greater than 1500 rpm.
^ Engine load moderate to heavy.

EGR POSITION SENSOR (California only)
The EGR position sensor is a variable resistor (potentiometer) measuring 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.