Exhaust Gas Recirculation: Description and Operation

Schematic Diagram:

DESCRIPTION
The exhaust gas recirculation (Linear EGR) system is used to decrease the emission levels of oxides of nitrogen (NOx). NOx is a group of chemical compounds containing nitrogen and varying amounts of oxygen that can have harmful environmental effects in large quantities.

NOx forms during the combustion process in amounts that is dependent on the concentration of oxygen in the combustion chamber and the duration that the combustion process temperatures exceed 822°C (1500°F). Decreased NOx levels are accomplished by reducing the peak combustion temperatures through dilution of the incoming fuel/air charge with exhaust gas. When combusted, exhaust gas (largely non-reactive carbon dioxide and water vapor) acts to absorb a portion of the combustion energy, resulting in lower temperatures throughout the combustion process and yielding lower amounts of NOx.

Desired amounts of EGR depend upon the geometry of the combustion chamber and the operating condition of the engine. Extensive laboratory and vehicle tests are used to determine optimal EGR rates for all operating conditions. Too little EGR can yield high NOx, while too much EGR can disrupt combustion events. The linear EGR valve is computer controlled to precisely regulate the amount of EGR delivered to the engine for all operating conditions.

OPERATION
The linear EGR system consists of the PCM, and the linear EGR valve associated with its wiring.

The linear EGR system is designed to offer more precise EGR flow metering, improved emission control and driveability than a typical back pressure system. The PCM monitors the following to control the linear EGR valve.

^ Engine coolant temperature (ECT) sensor
^ Throttle position (TP) sensor
^ Manifold absolute pressure (MAP) sensor
^ Intake air temperature (IAT) sensor
^ Revolutions per minute (rpm)
^ Vehicle speed sensor (VSS).

Based on the inputs to the PCM a pintle position (pulse width) is commanded to the EGR valve.

The linear EGR valve also contains a position feedback circuit which provides a variable voltage (0-5 volts) signal back to the PCM. This signal provides for the control of EGR to be a closed loop system because the actual position of the EGR pintle can be communicated back to the PCM so that corrections can be made. Fuel and spark compensation for EGR are determined from actual EGR position.