Exhaust Gas Recirculation: Description and Operation

Digital EGR Valve:

Digital EGR Valve Assembly:

DESCRIPTION
The Exhaust Gas Recirculation (EGR) system is used to lower Oxides of Nitrogen (NOx) emission levels caused by high combustion temperature. It does this by reducing combustion temperatures.

The main element of the system is the digital EGR valve. The EGR valve feeds small amounts of exhaust gas back into the combustion chamber. With fuel/air mixture thus diluted, combustion temperatures are reduced.

OPERATION
The digital EGR valve is designed to accurately supply EGR to an engine independent of intake manifold vacuum. The valve controls EGR flow from the exhaust to the intake manifold through three orifices which increment in size to produce seven combination. When a solenoid is energized, the armature, with attached shaft and swivel pintle is lifted, opening the orifice.
The flow accuracy is dependent on metering orifice size only, which results in improved control.
The swivel pintle feature insures good sealing of the exhaust gas, reducing the need for critical assembly alignment. The effects of EGR leakage on idle quality are reduced because the shaft and seals are exposed to the exhaust pressure instead of manifold vacuum. The shafts are sealed from the exhaust chamber by floating seals held in place by the seal spring. These springs also hold the upper seals that seal the armature cavity in the solenoids.
The solenoid coils are fastened together to maximize reliability and to seal the coils from the environment. The coils use a common power terminal with individual ground terminals.
The digital EGR valve is opened by the PCM Quad Driver, grounding each respective solenoid circuit. This activates the solenoid, raises the pintle, and allows exhaust gas to flow into the intake manifold. The exhaust gas then moves with the air/fuel mixture into the combustion chamber. If too much exhaust gas enters combustion will not occur. Because of this, very little exhaust gas is allowed to pass through the valve, virtually none at idle. The EGR valve is usually open under the following conditions:

- Warn engine operation.
- Above idle speed.

EGR CONTROL
To regulate EGR flow, the PCM controls the EGR solenoids to vary the amount of EGR flow. The PCM uses the information from the the following sensors to control the flow:
- Engine Coolant Temperature (ECT) Sensor.
- Throttle Position (TP) Sensor.
- Manifold Absolute Pressure (MAP).