Exhaust Gas Recirculation: Description and Operation

DESCRIPTION
The Exhaust Gas Recirculation (EGR) System controls the formation of (NOx) (Oxides of Nitrogen) Emissions by recirculating the exhaust gases into the Combustion Chamber. This is done through the Intake Manifold.

Exhaust Gas Recirculation Schematic:

OPERATION
The Exhaust Gas Recirculation (EGR) is controlled by the Engine Control Module (ECM) through an EGR Vacuum Switch Valve (VSV) and Back Pressure Transducer (BPT). Vacuum is allowed to the EGR Valve at acceleration and shut "OFF" upon deceleration.

The Diaphragm mounted in the EGR BPT is operated by back pressure of the exhaust gas to open and close the Valve. By this opening and closing action of the Valve, the EGR BPT controls the vacuum transmitted 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 BPT is pushed down by the spring force and the BPT Valve opens to allow the air into the vacuum passage from the outside. As a result, the vacuum transmitted to the EGR Valve becomes smaller and so does the opening of the EGR Valve. Thus, less amount of exhaust gas is recirculated to the Intake Manifold.

Under a high load condition such as high speed driving, on the other hand, the exhaust pressure is high. By the high exhaust pressure, the Diaphragm in the BPT is pushed up and closes its Valve. As the air does not enter the vacuum passage in this state, the vacuum transmitted to the EGR Valve grows larger and so does the opening of the EGR Valve. Thus, larger amount of exhaust gas is recirculated to the Intake Manifold.

Under any one of the following conditions, the Engine Control Module (ECM) closes the vacuum passage of the VSV. In this state, as the vacuum is not transmitted to the EGR Valve, it remains closed.
- When Engine Coolant Temperature is low.
- When Throttle Body is at idle position.
- When Engine is running under high load.
- When Intake Manifold Absolute Pressure is low. Other than the above, EGR Valve opens and closes in accordance with EGR BPT operation.