The exhaust gas recirculation (EGR) system allows burned exhaust gases to enter the engine intake manifold to help reduce NOx emissions. When exhaust gases are added to the air-fuel mixture, they decrease peak combustion temperatures. For this reason, an exhaust gas recirculation system lowers the amount of NOx in the engine exhaust.

A basic EGR system is simple. It consists of a vacuum Operated EGR valve and a vacuum line from the carburettor. This is pictured in Fig. 31-14. The EGR valve usually bolts to the engine intake manifold or a carburettor plate. Exhaust gases are UM routed through the cylinder head and intake manifold to the EGR valve.

The EGR valve consists of a vacuum diaphragm, spring, plunger, exhaust gas valve, and a diaphragm housing. It is designed to control exhaust flow into the intake manifold. See Fig. 31-15.

Basic EGR system operation

At idle, the throttle plate in the carburettor or fuel injection throttle body is closed, Fig. 31-14. This blocks off engine vacuum so it cannot act on the EGR valve. The EGR spring holds the valve shut and exhaust gases do NOT enter the intake manifold. If the EGR valve were to open at Idle, It could upset the air-fuel mixture and the engine could stall.

When the throttle plate is swung open to increase speed, engine vacuum is applied to the EGR hose. Vacuum pulls the EGR diaphragm up. In turn, the diaphragm pulls the valve open.

Engine exhaust can then enter the intake manifold and combustion chambers. At higher engine speeds, there is enough air flowing into the engine that the air-fuel mixture is not upset by the open EGR valve.

EGR system variations

There are several variations of our basic EGR system. The basic function, however, is fundamentally the same for all. Look at Fig. 31-15. It illustrates a backpressure EGR valve. This type valve uses both engine vacuums and exhaust backpressure to control valve action. This provides more accurate control of EGR valve opening.

An engine coolant temperature switch may be used to prevent exhaust gas recirculation when the engine is cold. A cold engine does not have extremely high combustion temperatures and does not produce very much NOx. By blocking vacuum to the EGR valve below about 38°C, the drivability and performance of the cold engine is improved.

A wide open throttle valve (WOT valve) is sometimes connected into the vacuum line to the EGR valve. It opens under full acceleration to provide venture vacuum to the EGR valve. At wide open throttle, intake manifold vacuum is very low, I but venturi vacuum is high. Figs. 31-16 and 31-17 show two other EGR valve designs. Study them!

Fig. 31-18 shows an EGR system with small jets in the bottom of the intake manifold. The small EGR jets meter a small amount of exhaust gases into the air-fuel mixture. The jets are small enough that they do not upset the idle air-fuel mixture.

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