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).




DIESEL ENGINES

Exhaust gas recirculation is controlled by a single diaphragm, vacuum operated valve. Operating vacuum is supplied by an engine driven vacuum pump and regulated by a Vacuum Regulating Valve (VRV), mounted on the injection pump throttle lever. During normal operation, the EGR valve opening is controlled by vacuum signal strength through the VRV, as throttle angle increases, vacuum strength decreases. In addition, vacuum switching and control valves are used to limit system operation to specific engine operating conditions.



1C Engines

Vacuum pump output on these engines is controlled by a Bimetal Vacuum Switching Valve (BVSV), electrically operated Vacuum Switching Valve (VSV) and a Vacuum Control Valve (VCV). When coolant temperature is greater than 147°F and engine speed exceeds 1250 RPM, both the BVSV and VSV are open and vacuum pump output is directed to the VCV. The VCV is open as long as vacuum regulated by the VRV exceeds 5.31 inches Hg, vacuum is applied to the EGR valve and recirculation takes place.
When coolant temperature is less than 122°F, the BVSV remains closed and vacuum pump output is blocked. When engine speed is less than 1200 RPM, the VSV is de-energized by a control computer, closing the valve and blocking vacuum pump output. During normal running, when VRV regulated vacuum within the system drops below 5.31 inches H, the VCV closes and EGR valve operating vacuum is vented to atmosphere. A damper is used to delay VCV closing.



1C-TL & 2C-T Engines

Vacuum pump output on these engines is controlled by a Bimetal Vacuum Switching Valve (BVSV), Electronic Vacuum Regulating Valve (EVRV) and High Altitude Compensation (HAC) valve. When coolant temperature is greater than 147°F and engine speed exceeds 1200 RPM under normal operating conditions, both BVSV and EVRV are open and exhaust gas is recirculated. Exhaust gas is not recirculated under any other operating conditions.