Exhaust Gas Recirculation: Description and Operation

PURPOSE
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.

Linear EGR Valve Operation:

Linear EGR Valve - Cross Sectional View:

EGR Control System Circuit Diagram:

OPERATION
The linear EGR valve operates on a duty cycle signal ("ON" time vs "OFF" time). The solenoid assembly gets battery power when the ignition is turned "ON" and becomes energized when the signal modifier completes the circuit to ground. The signal modifier generates the duty cycle signal by alternately grounding and opening the EGR valve solenoid circuit. The ECM outputs a varying frequency signal to the EGR valve signal modifier module, which varies the pulse width of the signal to the EGR valve solenoid to raise and lower the pintle. The longer the pulse width, the farther the valve opens and the more exhaust is metered into the intake.