EXHAUST GAS RECIRCULATION (EGR)
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 (NOX).
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
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
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.
are minor differences between systems, the basic operation of an exhaust gas recirculation
system is as follows:
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.
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.
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.
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