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.

EGR Valve (Digital):

EGR Control Circuit:

OPERATION

The EGR system is digitally controlled and designed to supply regulated amounts of exhaust gas to the intake manifold independently of manifold vacuum. Two solenoids in the EGR valve assembly open and close calibrated orifices in the valve to produce four combinations of EGR flow: No flow (both solenoids "OFF"); small flow - solenoid "B" only "ON"; medium flow - solenoid "C" only "ON"; and high flow - both solenoids "B" and "C" - ON (solenoid "A" is not used.) The solenoids get battery power when the ignition is turned "ON" and are energized when the PCM completes the circuit to ground.