Exhaust Gas Recirculation: Description and Operation

EGR System Description and Operation

Note: All engines except 4.8l and 7.4l FED

Purpose

The EGR system is used to lower NOx (oxides of nitrogen) emission levels caused by high combustion temperatures. It does this by decreasing combustion temperature.






The EGR system has a negative backpressure EGR valve, hose from a ported manifold vacuum source and a method to control the vacuum source. On an engine without an ECM, a coolant temperature TVS is used to control vacuum and on an engine with an ECM, a solenoid is used to control vacuum.


Operation






On an engine without an ECM, the thermostatic vacuum switch (TVS) blocks vacuum to the EGR valve during cold engine operation.






On an engine with an ECM, a vacuum solenoid energized by a signal from the ECM block vacuum to the EGR valve during cold engine operation, cranking and at wide-open throttle.

When the engine is warm, the TVS opens or the solenoid de-energizes which opens the vacuum source to the EGR valve. At idle or wide-open throttle, there is little or no vacuum from the vacuum source, the EGR pintle is closed and there is no exhaust gas recirculation. When the engine is above idle speed, the pintle valve rises and a small amount of exhaust gas goes into the combustion chamber.

The negative backpressure valve varies the amount of exhaust gas flow into the manifold depending on manifold vacuum and variations in exhaust backpressure.

The diaphragm on the valve has an internal air bleed hole which is held closed by a small spring when there is no exhaust backpressure.
Engine vacuum opens the EGR valve against the pressure of a large spring. When manifold vacuum combines with negative exhaust backpressure, the vacuum bleed hole opens and the EGR valve closes. This cycle occurs approximately forty times per second during normal engine operation.

If there is enough vacuum signal from the vacuum source to operate the EGR valve, the pintle valve cycles at a greater valve opening when exhaust manifold pressure is high, and intake manifold vacuum is fairly low, and will cycle at a lower valve opening when exhaust manifold pressure is low, and intake manifold vacuum is high.

On engines with an ECM, the EGR System is called "Pulse Width Modulation" (PWM), which means the ECM turns the solenoid "ON" and "OFF" many times a second and varies the amount of "ON" time (Pulse Width) to vary the amount of exhaust gas recirculation.

EGR Bleed Solenoid

On 5.0L and 5.7L Federal engines with light duty emissions and automatic transmission, an EGR bleed solenoid is connected in the transmission converter clutch circuit. When the transmission converter clutch applies, the solenoid is energized and the vacuum to the EGR valve is reduced to about half normal value.