Exhaust Gas Recirculation: Description and Operation

EGR System:

The EGR system functions to reduce oxides of nitrogen (NOx) emissions. When the engine is under load, the EGR valve admits a small amount of exhaust gas into 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.

EGR VALVE

This valve is controlled by a flexible diaphragm which is spring loaded to hold the valve closed. Vacuum applied to the top side of the diaphragm overcomes spring pressure and opens the valve in the exhaust port, allowing exhaust gas into the intake manifold and thus to the cylinders.

Back Pressure Transducer:

BACK PRESSURE TRANSDUCER

The back pressure transducer regulates the vacuum signal to the EGR valve. When the exhaust pressure is low, the transducer opens a small air bleed passage allowing some air into the vacuum circuit. This reduces the vacuum signal to the EGR valve and the valve closes slightly. If the exhaust pressure is very low the EGR valve will close completely. When exhaust pressure is high, the air bleed passage is closed and control vacuum is directly applied to the EGR valve diaphragm.

Thermal Vacuum Valve:

THERMAL VACUUM VALVE

A thermal vacuum valve prevents EGR valve operation when the engine is cold by blocking the vacuum signal to the backpressure transducer. The thermal vacuum valve is closed at ambient temperatures and opens when the engine temperature approaches normal operating range (above 122±7~F, 50±4~C).



EGR GAS TEMPERATURE SENSOR (CALIFORNIA ONLY)

The EGR temperature sensor provides the ECM with information about EGR valve operation. When the EGR valve is open, exhaust gas heats the sensor, and when the valve is closed the sensor cools. The ECM monitors this change of temperature, and memorizes the range across which the temperatures varies under normal operation. If the ECM senses the actual temperature range is too small compared with the normal range (approx. 6~C/11~F) stored in memory , it will record a fault. A trouble code will be set if the fault is recorded on three consecutive measurements. Measurements are taken when conditions exist such that the ECM "knows" the valve should be either fully open or fully closed.