EXHAUST GAS RECIRCULATION (EGR)
by Kyle McFadden

Exhaust Gas Recirculation is used to control the formation of NOx by lowering peak combustion temperatures using exhaust gas to dilute the air/fuel charge to the engine. Since exhaust gas is essentially non-combustible, it is considered an inert gas. By introducing an inert gas into the air/fuel charge to the cylinder, a portion of the charge is displaced, causing a slower explosion in the cylinder and lower combustion temperatures. There are two basic EGR systems used on vehicles today. They are the vacuum actuated valve and the electronically operated valve and any auto repair shop will be able to service both. Operation of both systems is normally managed by the engine control module. The EGR system is not designed to operate during idle on gasoline engines. Operation of the EGR system at idle speeds can cause stalling or rough idle.

Three types of vacuum operated EGR valves used are ported, negative backpressure, and positive backpressure. All three types contain a spring loaded diaphragm that is operated by a vacuum signal. On computerized systems, the vacuum signal to the EGR valve is regulated by the control module. The ported EGR valve has been in use since EGR systems were first introduced. The valve contains a plunger that is held in a closed position by a spring loaded diaphragm. When vacuum is applied to the diaphragm, the spring tension is overcome causing the plunger to move away from its seat. Exhaust gas is allowed to flow into the intake manifold starting the EGR process. EGR Flow control is regulated by that amount of vacuum applied to the valve and the size of the port used to introduce exhaust gas into the intake manifold. Typically the vacuum signal used to open this valve, is a ported vacuum source. Ported vacuum is a vacuum source that is placed on the inlet side of the throttle plate. Vacuum is only available when the throttle is opened; be sure to take this into consideration when diagnosing and troubleshooting an EGR system.

Negative and positive backpressure EGR valves operate in a similar fashion as the ported EGR valve. However, these types of valves use engine exhaust backpressure to regulate the amount of EGR flow by modifying the response of the diaphragm to the vacuum signal. Computerized control of vacuum operated EGR valves is accomplished using solenoids to regulate the vacuum signal to the valve. The control module monitors engine speed, load and vehicle speed to determine the desired EGR flow rate. The control module then activates the solenoids to provide vacuum to the valve accordingly. Some EGR valves contain a position sensor that provides feedback information to the control module relative to the amount of EGR flow. This can be used by the control module to adjust fuel delivery to compensate for EGR valve opening, as well as provide EGR diagnostic information and flow rate.

Electronic operated EGR valves are opened using solenoids rather than manifold vacuum. The electronic EGR valve can contain from one to three solenoids, each actuating a valve to control exhaust gas flow. When the solenoid is commanded on, the valve retracts, uncovering a port. Exhaust gas can then flow through the port into the intake manifold. The electronic EGR valve contains a automotive EGR position sensor that sends a signal that is used by the control module to calculate EGR flow. This type of EGR system allows the control module to quickly and accurately tailor EGR flow to match engine load. If taking your vehicle to an auto repair shop, be sure to mention this to the mechanic for accurate diagnosis.

(Kyle has an affinity for Pale Ale and tooling on his 1956 Chevrolet Nomad Station Wagon.)