Chevrolet Light Duty Truck 1973, Service Manual

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E M I S S I O N C O N T R O L S Y S T E M S 6 T -7
EXHAUST GAS RECIRCULATION SYSTEM
DESCRIPTION A N D OPERATION
The Exhaust Gas Recirculation (EGR) system is used to
reduce oxides of nitrogen (NOx) emitted from the
engine exhaust (fig. lc). Formation of NOx takes place at
very high temperatures, consequently, it occurs during
the peak temperature period of the combustion process.
To reduce and control NOx formation only a slight
reduction in peak temperatures is required. This
reduction can be accomplished by introducing small
amounts of an inert gas into the combustion process. The
end products of combustion provide a continuous supply
of relatively inert gases, therefore, it becomes a matter
of utilizing those gases in the correct proportion.
The EGR valve is positioned in the left front corner of
the inlet manifold in front of the carburetor on all Mark
IV engines. The internally cast passages permit exhaust
gases to flow to the combustion area through the inlet
manifold (fig. 5c).
Not all engine operational modes are critical as far as
NOx emissions are concerned, the EGR valve and
vacuum source is designed to vary the amount of exhaust
gases recirculated. At idle, no recirculation is needed,
consequently, the opening, or vacuum source, for the
tube that connects the EGR valve is placed just above
the throttle blade.
Therefore, at idle (throttle blade closed) the EGR valve
is closed. When the throttle is opened to accelerate, the
EGR valve begins opening at approximately three inches
of mercury pressure drop in the throttle body.
Valve metering is constant above five inches of mercury
pressure differential. The valve is quickly brought to full
open when accelerating from the throttle closed
condition. Approaching full throttle, when manifold
vacuum is below three inches of mercury the EGR valve
closes. Only a very small portion of the exhaust gases is
used with maximum flow occuring during the 30 to 70
miles per hour cruise condition.
Fig. lc —Exhaust Gas Reciculation System Schematic
To tap this continuous supply of inert gases, without
external pipes or connection to the exhaust system,
additional exhaust gas passages are cast into the complex
runner system of the inlet manifold. Separating the two
EGR passages is a vacuum modulated shut-off and
metering valve, referred to as the “EGR valve” (fig. 2c).
The EGR valve contains a vacuum diaphragm, which is
operated by intake manifold vacuum. The diaphragm
vacuum signal supply port is located in the carburetor
throttle body, and is exposed to engine manifold vacuum
in the off-idle and part throttle to wide open throttle
operation. A .030 orifice in the valve vacuum tube serves
to modulate flow.
On In-Line engines the EGR valve is located on the inlet
manifold adjacent to the carburetor. Exhaust gases are
diverted from the manifold heat box through the EGR
valve and into the inlet manifold through a drilled hole
(fig. 3c).
Small V
8
engines have the EGR valve located externally
on the right rear side of the inlet manifold adjacent to
the rocker arm cover. Internally cast passages direct the
exhaust gases to the area just below the carburetor
throttle plates (fig. 4c).
VACUUM TUBE CONNECTED
V A L V E S E A T .
V A L V E C H A M B E R ..
T O IN T A K E
m a n i f o l d
\ J
___________m
Fig. 2c—EGR Valve-Sectional View
V A L V E O P E N
E X H A U S T G A S
IN L E T P O R T
LIGHT DUTY TRUCK SERVICE MANUAL