Cessna 402C Pilot Operating Handbook page 244

SECTION 7
AIRPLANE & SYSTEMS DESCRIPTIONS
1. Engine induction air is taken in through the induction air inlet
(1), located in the bottom of the engine nacelle, at which point it
passes through a filter and then into the compressor (2).
2. The compressor compresses the induction air.
3. The pressurized induction air from the compressor then passes into
the cylinders through the induction manifold (3).
4. The air and fuel are burned and the exhaust gases are then routed
to the turbine through the exhaust manifold (4).
5. The exhaust gases drive the turbine (5) which, in turn, drives the
compressor.
6. The turbine has enough power to allow the engine to operate in
excess of the maximum 39.0 inches Hg. manifold pressure. There-
fore, in order not to exceed 39.0 inches Hg. manifold pressure, a
bypass or waste gate (6) is used so the excess exhaust gas will be
expelled overboard instead of passing through the turbine.
It can be seen from studying steps (1) through (6) that anything that
affects the flow of induction air into the compressor, or the flow of
exhaust gases into the turbine, will increase or decrease the speed of the
turbocharger. This resultant change in flow will have no effect on the
engine if the waste gate is still open, because the waste gate position
will automatically change to hold compressor discharge pressure constant.
The waste gate automatically maintains allowable compressor discharge
pressure when below 12,000 feet with full throttle and full RPM. Above
12,000 feet, the throttles must be retarded to maintain the manifold pres-
sure within the allowable limits. When the waste gate is closed, any
change in the turbocharger speed will mean a change in engine operation.
Anything that causes an increase or decrease in turbine speed will cause an
increase or decrease in manifold pressure. If turbine speed increases, the
manifold pressure increases; if the turbine speed decreases, the manifold
pressure decreases. Any change in exhaust flow to the turbine or ram
induction air pressure, whether it is an increase or decrease, will be
magnified approximately 8 to 10 times by the compression ratio and the
change in flow through the exhaust system.
Manifold Pressure Variation With Altitude
At full throttle your turbocharger is capable of maintaining the maximum
allowable 39.0 inches H9. manifold pressure, well above 16,000 feet;
however, engine operating limitations establish the maximum manifold pres-
sure that may be used. From 16,000 feet to higher altitudes, the throttles
must be retarded to maintain the manifold pressure within the allowable
limits.
Manifold Pressure Variation With Airspeed
When the waste gate is open at low altitude, changes in airspeed have
little or no effect on manifold pressure. However, at high altitudes when
the waste gate is closed, manifold pressure will vary with variations in
airspeed. This is because any change in pressure at the compressor inlet
is ma9nified 8 to 10 times at the compressor outlet due to compression
ratio and exhaust flow changes.
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