Gasoline Engine EGR

Fig. 40 Single diaphragm EGR valve cross sectional view:

Fig. 41 Dual diaphragm EGR valve cross sectional view:

As the throttle valves are opened and the engine speeds up, ported vacuum is applied to a vacuum diaphragm in the E.G.R. valve through a connecting tube. When the vacuum reaches approximately 3 inches Hg, the diaphragm moves upward against spring tension and is in the full-up position at approximately 7-8 inches Hg of vacuum. This diaphragm is connected by a shaft to a valve which closes off the exhaust gas port, Fig. 40. As the diaphragm moves up, it opens the valve in the exhaust gas port which allows exhaust gas to be pulled into the intake manifold and enter the cylinders. The exhaust gas port must be closed during idle as the mixing of exhaust gases with the fuel air mixture at this point would cause rough running.
The dual diaphragm EGR valve, Fig. 41, is designed to provide increased exhaust gas recirculation rates when engine loads increase. Manifold vacuum is used as the signal to indicate the engine load.
The valve is similar to the single diaphragm valve except that a second diaphragm has been added to the valve and is connected to the upper diaphragm with a spacer, thus both diaphragms move together. A manifold vacuum signal is applied to the volume between the two diaphragms. The upper diaphragm has a larger diameter piston than the lower diaphragm, therefore the load caused by the manifold vacuum between the two diaphragms aids the spring load. Thus as the engine load increases and manifold vacuum decreases the combined load of the spring and the vacuum chamber are reduced allowing the valve to open further for a given EGR vacuum signal.
Therefore, for high intake manifold vacuums (such as cruising), the opening is less than for low manifold vacuums obtained during accelerations. The valve now is capable of providing more recirculation on accelerations where loads are higher and the tendency to produce NOx is greater.

Fig. 42 Integrated electronic EGR valve:

Integrated Electronic EGR Valve

This valve functions like a port valve with a remote vacuum regulator, except the regulator and pintle position sensor are sealed in the black plastic cover, Fig. 42. The regulator and position sensor are not serviceable. There is a serviceable filter, that provides clean fresh air to the regulator.
The ECM provides variable current to the vacuum regulator. This correct produces desired EGR flow using inputs from mass air flow sensor, coolant temperature sensor and engine RPM.
This valve can be identified by a part number engraved on the non-removable black plastic top.

Fig. 43 Positive back pressure EGR valve cross sectional view:

Fig. 44 Negative back pressure EGR valve cross sectional view:

Back Pressure EGR Valve

This valve is used on some vehicles to regulate the EGR flow according to engine load. The back pressure EGR valve is equipped with a transducer located inside the valve. The transducer uses exhaust gas pressure to control an air bleed within the valve to modify the vacuum signal from the carburetor.
A small diaphragm controlled valve inside the EGR valve assembly acts as a pressure regulator. The control valve receives an exhaust back pressure signal through the hollow shaft which exerts a force on the bottom of the control valve diaphragm, opposed by light spring pressure. A metal deflector plate prevents hot exhaust gases from flowing directly on the diaphragm.
Vacuum is applied to the EGR valve from the carburetor spark port, to ensure no exhaust gas recirculation during idle. During off-idle operation, manifold vacuum is applied to the vacuum chamber through a restriction in the signal tube. When engine load is light, and back pressure is low, the control valve is open, allowing air to flow from the bleeds in the diaphragm plate, through the control valve orifice, and into the vacuum chamber. The air bleeds off vacuum, decreasing the signal trying to open the EGR valve. If back pressure does not close the control valve, sealing off the air flow, there will not be any vacuum buildup to open the EGR valve for exhaust gas recirculation.
When power demands are made on the engine, and exhaust gas recirculation is required, exhaust back pressure increases, closing the control valve, thereby shutting off air flow through the valve. Vacuum builds up in the vacuum chamber until the spring force holding the EGR valve closed is overcome.
When the EGR valve opens, the exhaust pressure decreases because some of the exhaust gas is flowing into the intake manifold through the EGR passage. In actual operation, the system will reach a balanced condition providing maximum EGR operation.
Any increase in engine load will momentarily increase the exhaust signal, causing the control valve to close, allowing a stronger vacuum signal. The system will then stabilize at a greater EGR flow.
At maximum engine load, when manifold vacuum is nearly zero, there will be no EGR flow momentarily. This is due to the insufficient vacuum required to pull the valve open, even though high exhaust back pressure has closed the control valve.
Two types of back pressure EGR valves are used on General Motors vehicles. A Positive Back Pressure EGR Valve, Fig. 43, is used on most engines. A Negative Back Pressure EGR Valve, Fig. 44, is used on those engines with relatively low back pressure to provide the desired opening point and flow rate is used on those engines with relatively low back pressure to provide the desired opening point and flow rateMost EGR valves can be identified by the raised pattern on the diaphragm plate. Ported vacuum valves have a raised circular rib. Positive Back Pressure valves have a slightly raised ``X'' shaped rib, and Negative Back Pressure valves have a substantially raised ``X'' shaped rib. However, on some engines it is not possible to determine valve type in this manner. On these models, a letter ``N'' for negative or ``P'' for positive is stamped next to the date code below the model number on the top of the valve and no identification letter stamped after model number for ported vacuum valves. Refer to this letter for valve identification.



EGR Control

Vacuum signals to the EGR valve are controlled by either temperature operated valves and switches or by electrically operated solenoids.
On Thermal Vacuum Switch (TVS) controlled models, vacuum signals to the EGR valve are blocked when engine coolant temperature is below switch calibration point. When engine coolant is at or above switch calibration temperature, the switch opens and allows vacuum to reach the EGR valve.
Two types of electrically controlled vacuum solenoids are used to allow Electronic Control Module (ECM) control of the EGR valve operation. A solenoid which operates similar to the TVS and a solenoid that bleeds-off a portion of the EGR valve vacuum signal are used. Depending upon application, either one or both of these solenoids may be used to control EGR.
The EGR solenoid is energized by the ECM to block vacuum signals to the EGR valve. When engine is cold and operating within specified load and RPM range, the ECM completes the solenoid ground circuit. When engine reaches the temperature programmed into the ECM, the solenoid ground circuit is opened and vacuum signals are allowed to reach the EGR valve.
The EGR bleed solenoid is used to bleed-off a portion of the EGR vacuum signal, limiting the amount of recirculation under certain operating conditions. The bleed solenoid is operated by the ECM when engine is cold and/or when the Transmission Converter Clutch (TCC), if equipped, is engaged. Most models use a Pulse Width Modulated (PWM) type bleed solenoid. The solenoid is turned ``on and off'' 32 times per second by the ECM, with the amount of vacuum bleed determined by the solenoid ``on time'' per cycle. This allows the ECM to bleed-off up to one half of the EGR vacuum signal, or allow full vacuum by de-energizing the solenoid.