Exhaust Gas Recirculation: Description and Operation

Exhaust Gas Recirculation (EGR) System Description
The EGR system controls the formation of NOx (Oxides of Nitrogen) emissions by recirculating the exhaust gases into the combustion chamber. NOx emissions increase with higher combustion temperatures. Controlling high combustion chamber temperatures will help limit the formation of NOx emissions. Mixing exhaust gases with the incoming air/fuel mixture modifies the combustion process resulting in lower combustion chamber temperatures. The EGR system accomplishes this by admitting controlled amounts of exhaust gas into the intake manifold to mix with the incoming air.

EGR System Operation
The EGR valve operation is controlled by the Powertrain Control Module (PCM). The PCM uses an EGR Solenoid Vacuum valve to control the vacuum signal to the EGR pressure transducer, which then regulates the EGR valve itself. The EGR Solenoid Vacuum valve is turned ON when the PCM senses various inputs from the engine sensors.

A diaphragm, which opens and closes an internal valve in the EGR pressure transducer, is operated by back-pressure from the exhaust manifold. This exhaust back-pressure, regulates the opening and closing action of the internal valve, which then controls the amount of vacuum that is transmitted from the EGR pressure transducer to the EGR valve.

Under a low load condition such as low speed driving, the exhaust pressure is low. In this state, the diaphragm in the EGR pressure transducer is pushed down by spring force and the EGR pressure transducer valve opens in order to allow air into the vacuum passage from the outside.

This process causes the opening of the EGR valve and the vacuum transmitted to the EGR valve to contract. Thus, a smaller amount of exhaust gas is admitted into the intake manifold.

Under a high load condition, such as high speed driving, the exhaust pressure is high. In this State, the diaphragm in the EGR pressure transducer is pushed up and closes the valve, preventing the air from entering the vacuum passage. This process ultimately causes the opening of the EGR valve along with the vacuum transmitted to the EGR valve to expand, resulting in a larger amount of exhaust gas being admitted into the intake manifold.

Under any one of the following conditions, the PCM closes the vacuum passage of the EGR solenoid vacuum valve. In this state there is no vacuum transmitted to the EGR valve and it remains closed.
^ The engine Coolant Temperature (ECT) sensor is low.
^ The throttle valve opening is less than the specified value (determined by the PCM).
^ When the vehicle is stopped.
^ The engine is running at a high load.

Other than the above, the EGR valve opens and closes in accordance with the EGR pressure transducer operation.

Results Of Incorrect EGR System Operation
Excessive EGR valve flow may cause any of the following conditions:
^ The engine stalling.
^ Rough idle, surging or engine hesitation
^ Lower fuel economy
^ Incomplete combustion and high exhaust emissions

Inadequate EGR valve flow may cause any of the following conditions:
^ Engine Detonation
^ Excessive exhaust emissions