Exhaust Gas Recirculation: Description and Operation

This 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).




GASOLINE ENGINES

EGR systems on these models are temperature sensitive by means of either a bi-metal vacuum switching valve, a thermostatic vacuum switching valve, or on electrically operated vacuum switching valve. Operation temperatures vary by engine and application. 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. On some models, the air bleed function is performed by a vacuum control valve, which opens the vacuum circuit to the atmosphere during periods of high vacuum. Electric vacuum switching valves are used on some models to open the EGR valve on engine shut down, preventing engine dieseling.
EGR valves are of two basic types, single diaphragm and dual diaphragm. Single diaphragm types have one main port to control exhaust gas flow. Dual diaphragm types have a main port with a built-in restricted bypass, and a secondary port which controls the actual flow to the intake system. This provides greater control over the volume of exhaust gas flow.