Exhaust Gas Recirculation: Description and Operation

Fig. 39 Exhaust Gas Recirculation System (Supercharged):

Fig. 42 Exhaust Gas Recirculation System Operation (Supercharged):

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 temperature limiting the formation of nitrous oxides (NOx). System components include the EGR valve, vacuum modulator, and vacuum switching valve (VSV). The vacuum modulator senses engine load and determines the amount of exhaust gas to recirculate. The electronic control unit (ECU) measures engine speed and temperature and decides when to open the VSV and allow the vacuum modulator to control the EGR valve.

EGR VALVE

The vacuum operated EGR valve opens and closes a passage from the exhaust manifold to the intake manifold. Flame temperature in the combustion chamber is then lowered by diluting the incoming mixture with inert exhaust gases. The EGR valve is controled directly by the vacuum modulator and indirectly by the ECU.

VACUUM SWITCHING VALVE (VSV)

The vacuum switching valve is controlled by the ECU which in turn helps control EGR valve operation. When the vacuum modulator applies vacuum to the EGR valve, the pressure is equalized across the check valve and the vacuum from the vacuum reservoir is allowed to assist in opening the EGR valve. This only occurs when the ECU opens the VSV according to engine rpm.

VACUUM MODULATOR

This valve controls the EGR valve and regulates the amount of recirculated exhaust gases that are allowed into the intake manifold by sensing engine load conditions.