Exhaust Gas Recirculation: Description and Operation

Fig. 22 Exhaust Gas Recirculation System:

Fig. 22a EGR System Operation:

Fig. 23a EGR System Operation:

SYSTEM DESCRIPTION

The exhaust gas recirculation (EGR) system is used to dilute the incoming air/fuel mixture with recirculated exhaust gases. Dilution of the incoming mixture lowers the peak flame temperatures and thus limits the formation of nitrous oxides (NOx). Operation of the system can be seen in Fig. 22a for idle and Fig.23a for mid and wide-open throttle.

EGR VALVE

The Exhaust Gas Recirculation (EGR) valve is a vacuum operated valve which opens and closes a passage from the exhaust manifold to the intake manifold. This allows flame temperature in the combustion chamber to be lowered by diluting the incoming mixture with exhaust gases. The EGR valve is controlled by the vacuum modulator which in turn is controlled by the ECU.

VACUUM MODULATOR

An EGR vacuum modulator is used to control vacuum flow in relation to exhaust gas pressure. At periods of high exhaust pressure, a full flow of vacuum is supplied to the EGR valve. At periods of low exhaust pressure, air is bled into the vacuum circuit through the modulator, causing the EGR valve to close.

OXYGEN SENSOR

The oxygen sensor is an exhaust mounted measuring device for determining the oxygen content of the exhaust gas. Since the amount of oxygen in the exhaust inidcates precisely how complete the combustion of the air/fuel mixture in the cylinders is, it is also a good place to test the air-fuel raito. A basic function test is found in the testing section below and further testing may be found in the "COMPUTERIZED ENGINE CONTROLS" section.