Exhaust Gas Recirculation: Description and Operation

PURPOSE
The Exhaust Gas Recirculation (EGR) system is used in automotive engines to decrease the emission levels of oxides of nitrogen (NOx). NOx defines 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 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 main elements of the system are the EGR valve and EGR solenoid. The vacuum operated EGR valve is mounted on the intake manifold. The EGR solenoid is mounted on the engine valve cover and controls the amount of vacuum to the EGR valve.

OPERATION
The EGR valve is opened by vacuum to let exhaust gas 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, the engine might misfire. For this reason, very little exhaust gas is allowed to pass through the valve, especially at idle. The EGR valve is usually open under the following conditions:

^ Warm engine operation as determined by both engine coolant and manifold are temperature thresholds.
^ Above idle speed but below wide open throttle.

The Powertrain Control Module (PCM) controls the vacuum to the EGR valve with a solenoid valve. A constant 12 volts is applied to the positive terminal of the EGR solenoid. The vacuum supply to the EGR valve is regulated by the PCM controlling the EGR solenoid ground. The percentage of time that the PCM grounds the EGR solenoid is called the solenoid duty cycle. The duty cycle is the time the solenoid is on divided by the tie it is off. A duty cycle of 0% will enable EGR (turn EGR full "ON") since a de-energized solenoid allows vacuum to pass to the EGR valve. A duty cycle of 100% will turn EGR full "OFF" since the solenoid will be energized and will not allow vacuum to pass to the EGR valve. The EGR pulse width is regulated by the PCM depending on the engine load conditions. When the engine is cold, within specified load range and above a specified rpm, the PCM sends 100% duty cycle to the solenoid and blocks vacuum to the EGR valve. When the engine is warm, the PCM sends a duty cycle to the solenoid to allow EGR.