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TM 9-8000
draws air into its center and throws it off at its rim. The
4-29.  General. Turbocharging or supercharging is a
air then is pushed along the inside of the circular
method of increasing engine volumetric efficiency (para
The diameter of the housing gradually
2-23) by forcing the air-fuel mixture into the intake rather
increases to the outlet where the air is pushed out. The
than merely allowing the pistons to draw it in naturally.
air from the outlet then is routed to the carburetor via a
Supercharging and turbocharging in some cases will
pressure box (para 4-31e).
push volumetric efficiencies over 100 percent. Engines
must be modified to operate properly in some cases,
c.  Rootes Supercharger (Fig. 4-69).  The rootes
because the extra air-fuel mixture will cause higher
supercharger is of the positive displacement type, and
compression pressures, resulting in detonation (para 4-
consists of two rotors inside a housing. As the rotors are
driven by the engine, air is trapped between them and
the housing. It then is carried to the outlet where it is
4-30.  Turbochargers
(Fig. 4-67).  A turbocharger
discharged. The rotors and the housing in this type of
uses the force of the engine exhaust stream to force the
supercharger must maintain very tight clearances and
air-fuel mixture into the engine. It consists of a housing
therefore are very sensitive to dirt.
containing two chambers.  One chamber contains a
turbine that is spun as hot exhaust gases are directed
d. Vane Supercharger (Fig. 4-70).
The vane
against it. The turbine shaft drives an impeller that is
supercharger operates the same way as the vane oil
located in the other chamber.  The spinning impeller
pump described in paragraph 20-6.  It is a positive
draws an air-fuel mixture from the carburetor and forces
displacement-type supercharger that usually is belt
it into the engine. Because the volume of exhaust gases
driven by the engine.  The air output of the vane
increases with engine load and speed, the turbocharger
supercharger usually is routed to the carburetor via a
speed will increase proportionally, keeping the manifold
pressure box (para 4-31e).
pressure boost fairly uniform.  A device known as a
waste gate is installed on turbocharged engines to
e. Pressure Box (Fig. 4-71). A large percentage of
control manifold pressure. It is a valve that, when open,
superchargers are situated so that they force air into the
allows engine exhaust to bypass the turbocharger
carburetor throat rather than drawing the mixture from
turbine, effectively reducing intake pressure. The waste-
the bottom of the carburetor. This creates a problem
gate valve is operated by a diaphragm that is operated
because it will cause pressure in the venturi that is higher
by manifold pressure.  The diaphragm will open the
than atmospheric.  This, in turn, will cause air to be
waste-gate valve whenever manifold pressure reaches
blown into the discharge nozzle rather than fuel being
the desired maximum.
drawn out. The solution is to enclose the carburetor in a
sealed pressure box.  The outlet of the supercharger
then pressurizes the pressure box, providing the
necessary boost. A relief valve usually is provided on the
a. General. Superchargers are engine driven air
box to prevent pressure in the box from exceeding a
pumps that force the air-fuel mixture into the engine.
desired limit.
They are made in three basic configurations: centrifugal,
rootes, and vane.
b. Centrifugal Supercharger (Fig. 4-68).
centrifugal supercharger has an impeller equipped with
curved vanes. As the impeller is driven by the engine, it


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