Exhaust Gas Recirculation: Description and Operation

Exhaust Gas Recirculation System (EGR):

SYSTEM DESCRIPTION

The 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 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.

Typical EGR Valve:

EGR VALVE

The EGR valve meters exhaust gas into the intake system. The EGR valve is normally closed. When the system is operating, throttle valve ported vacuum is applied to the diaphragm chamber of the EGR valve, causing the valve to open during acceleration and cruising conditions.


EGR GAS TEMPERATURE SENSOR - CALIF. ONLY

The EGR temperature sensor is used to provide 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.

Back Pressure Transducer:

EXHAUST BACKPRESSURE TRANSDUCER

The exhaust backpressure 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 is used to block the vacuum signal to the EGR valve when the engine is cold. The thermal vacuum valve (closed at ambient temperatures) has a bimetallic spring that distorts when heated. When the engine temperature approaches normal operating range the spring distorts and opens the valve.