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This controls the amount of exhaust through the turbine.
Waste gate position is actuated by engine oil pressure.
TURBOCHARGER
The turbocharger incorporates a
turbine, which is driven by exhaust
gases, and a compressor that
pressurizes the incoming air.
THROTTLE BODY
This regulates airflow
to the engine.
INTAKE MANIFOLD
Pressurized air from the
turbocharger is supplied to
the cylinders.
EXHAUST MANIFOLD
Exhaust gas is ducted through
the exhaust manifold and is
used to turn the turbine which
drives the compressor.
AIR INTAKE
Intake air is ducted to
the turbocharger where
it is compressed.
Figure 11-5.Turbocharging system.
Ch 11.qxd 5/7/04 8:50 AM Page 11-7
11-8
the manifold pressure decreases as additional altitude
is gained. Ground boosting, on the other hand, is an
application of turbocharging where more than the
standard 29 inches of manifold pressure is used in
flight. In various airplanes using ground boosting,
takeoff manifold pressures may go as high as 45
inches of mercury.
Although a sea level power setting and maximum
r.p.m. can be maintained up to the critical altitude,
this does not mean that the engine is developing sea
level power. Engine power is not determined just by
manifold pressure and r.p.m. Induction air
temperature is also a factor. Turbocharged induction
air is heated by compression. This temperature rise
decreases induction air density which causes a
power loss. Maintaining the equivalent horsepower
output will require a somewhat higher manifold
pressure at a given altitude than if the induction air
were not compressed by turbocharging. If, on the
other hand, the system incorporates an automatic
density controller which, instead of maintaining a
constant manifold pressure, automatically positions
the waste gate so as to maintain constant air density
to the engine, a near constant horsepower output
will result.
OPERATING CHARACTERISTICS
First and foremost, all movements of the power
controls on turbocharged engines should be slow and
gentle. Aggressive and/or abrupt throttle movements
increase the possibility of overboosting. The pilot
should carefully monitor engine indications when
making power changes.
When the waste gate is open, the turbocharged engine
will react the same as a normally aspirated engine
when the r.p.m. is varied. That is, when the r.p.m. is
increased, the manifold pressure will decrease slightly.
When the engine r.p.m. is decreased, the manifold
pressure will increase slightly. However, when the
waste gate is closed, manifold pressure variation with
engine r.p.m. is just the opposite of the normally
aspirated engine. An increase in engine r.p.m. will
result in an increase in manifold pressure, and a
decrease in engine r.p.m. will result in a decrease in
manifold pressure.
Above the critical altitude, where the waste gate
is closed, any change in airspeed will result in a
corresponding change in manifold pressure. This is
true because the increase in ram air pressure with an
increase in airspeed is magnified by the compressor
resulting in an increase in manifold pressure. The
increase in manifold pressure creates a higher mass
flow through the engine, causing higher turbine speeds
and thus further increasing manifold pressure.
When running at high altitudes, aviation gasoline may
tend to vaporize prior to reaching the cylinder. If this
occurs in the portion of the fuel system between the
fuel tank and the engine-driven fuel pump, an
auxiliary positive pressure pump may be needed in the
tank. Since engine-driven pumps pull fuel, they are
easily vapor locked. A boost pump provides positive
pressure—pushes the fuel—reducing the tendency to
vaporize.
HEAT MANAGEMENT
Turbocharged engines must be thoughtfully and
carefully operated, with continuous monitoring of
pressures and temperatures. There are two temperatures
that are especially important—turbine inlet
temperature (TIT) or in some installations exhaust gas
temperature (EGT), and cylinder head temperature.
TIT or EGT limits are set to protect the elements in the
hot section of the turbocharger, while cylinder head
temperature limits protect the engine’s internal parts.
Due to the heat of compression of the induction air, a
turbocharged engine runs at higher operating
temperatures than a non-turbocharged engine. Because
turbocharged engines operate at high altitudes, their
environment is less efficient for cooling. At altitude
　
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