EXHAUST GAS RECIRCULATION (EGR) SYSTEM

When the temperature of the combustion flame exceeds approximately 2,500°F, the nitrogen that is present in the intake air begins to combine with oxygen to produce oxides of nitrogen (NO_X). The exhaust gas recirculation (EGR) system (fig. 4-53) helps to control the formation of oxides of nitrogen by recirculating a portion of the exhaust gases back through the intake manifold, resulting in cooler combustion chamber temperatures.

A basic EGR system is simple, consisting of a vacuum operated EGR valve and a vacuum line from the carburetor. The EGR valve usually bolts to the engine intake manifold or a carburetor plate. Exhaust gases are routed through the cylinder head and intake manifold to the EGR valve.

The EGR valve consists of a vacuum diaphragm, a spring, an exhaust gas valve, and a diaphragm housing. It is designed to control exhaust flow into the intake manifold.

Although there are minor differences between systems, the basic operation of an exhaust gas recirculation system is as follows:

• At idle, the throttle plate in the carburetor or fuel injection throttle body is closed. This blocks off engine vacuum, so it cannot act on the EGR valve. The EGR spring holds the valve shut, and the exhaust gases do NOT enter the intake manifold. If the EGR valve were to open at idle, it could upset the air-fuel mixture and the engine would stall.
• When the throttle plate is swung open to increase speed, engine vacuum is applied to the EGR hose. Vacuum pulls the EGR diaphragm up. In turn, the diaphragm pulls the valve open. Engine exhaust can enter the intake manifold and combustion chambers. At higher engine speeds, there is enough air flowing into the engine that the air-fuel mixture is not upset by the open EGR valve.

There are two different methods of supply vacuum to the EGR valve. The first method uses a vacuum port into the carburetor throat located just above the throttle plate. As the throttle begins to open, vacuum will begin to be applied to the port and operates the EGR valve. The valve will continue to operate fully until approximately half throttle is reached. As the throttle is open past the halfway point, exhaust gas recirculation gradually will diminish to zero, as the throttle approaches the fully opened position.

The second method uses a vacuum port that is directly in the carburetor venturi (fig. 4-53). The carburetor venturi provides vacuum for the EGR valve any time the engine is running at high speed. The problem with using venturi vacuum is that it is not strong enough to open the EGR valve. So to make it work, manifold vacuum is used to operate the EGR valve through a vacuum amplifier. The vacuum amplifier switches the manifold vacuum supply to the EGR valve whenever venturi vacuum is applied to its signal port. At times of large engine loading (wide, open throttle), manifold vacuum will be weak, producing the desired condition of no exhaust gas recirculation.

An engine coolant temperature switch may be used to prevent exhaust gas recirculation when the engine is cold. A cold engine does not have extremely high combustion temperatures and does not produce very much NOx . By blocking vacuum to the EGR valve below 100°F, you can improve the drivability and performance of the cold engine.