Exhaust Gas Recirculation: Description and Operation

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 CONTROL SOLENOID (DUTY SOLENOID)

The EGR control solenoid is a vented, solenoid operated, vacuum switch. The ECM alternately energizes and de-energizes the solenoid, many times each second, to regulate the vacuum signal to the EGR valve. When the solenoid is energized, the vent opens, allowing ambient air into the vacuum circuit, reducing the vacuum signal to the EGR valve. When the solenoid is de-energized (vent closed), the EGR valve receives full vacuum from the source.