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Pilot's Handbook of Aeronautical Knowledge
Aircraft Systems
Superchargers and Turbosuperchargers

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Turbocharger components.
Figure 6-15. Turbocharger components.

The major disadvantage of the gear-driven supercharger––use
of a large amount of the engine's power output for the amount
of power increase produced––is avoided with a turbocharger,
because turbochargers are powered by an engine's exhaust
gases. This means a turbocharger recovers energy from hot
exhaust gases that would otherwise be lost.

A second advantage of turbochargers over superchargers is
the ability to maintain control over an engine's rated sealevel
horsepower from sea level up to the engine's critical
altitude. Critical altitude is the maximum altitude at which
a turbocharged engine can produce its rated horsepower.
Above the critical altitude, power output begins to decrease
like it does for a normally aspirated engine.

Turbochargers increase the pressure of the engine's induction
air, which allows the engine to develop sea level or greater
horsepower at higher altitudes. A turbocharger is comprised
of two main elements: a compressor and turbine. The
compressor section houses an impeller that turns at a high rate
of speed. As induction air is drawn across the impeller blades,
the impeller accelerates the air, allowing a large volume of
air to be drawn into the compressor housing. The impeller's
action subsequently produces high-pressure, high-density
air, which is delivered to the engine. To turn the impeller,
the engine's exhaust gases are used to drive a turbine wheel
that is mounted on the opposite end of the impeller's drive
shaft. By directing different amounts of exhaust gases to .ow
over the turbine, more energy can be extracted, causing the
impeller to deliver more compressed air to the engine. The
waste gate, essentially an adjustable butterfly valve installed
in the exhaust system, is used to vary the mass of exhaust gas
.owing into the turbine. When closed, most of the exhaust
gases from the engine are forced to flow through the turbine.

When open, the exhaust gases are allowed to bypass the
turbine by .owing directly out through the engine's exhaust
pipe. [Figure 6-15]

Since the temperature of a gas rises when it is compressed,
turbocharging causes the temperature of the induction air to
increase. To reduce this temperature and lower the risk of
detonation, many turbocharged engines use an intercooler.
This small heat exchanger uses outside air to cool the hot
compressed air before it enters the fuel metering device.

System Operation
On most modern turbocharged engines, the position of
the waste gate is governed by a pressure-sensing control
mechanism coupled to an actuator. Engine oil directed into
or away from this actuator moves the waste gate position.
On these systems, the actuator is automatically positioned to
produce the desired MAP simply by changing the position
of the throttle control.

Other turbocharging system designs use a separate manual
control to position the waste gate. With manual control,
the manifold pressure gauge must be closely monitored to
determine when the desired MAP has been achieved. Manual
systems are often found on aircraft that have been modified
with aftermarket turbocharging systems. These systems
require special operating considerations. For example, if the
waste gate is left closed after descending from a high altitude,
it is possible to produce a manifold pressure that exceeds the
engine's limitations. This condition, called an overboost,
may produce severe detonation because of the leaning effect
resulting from increased air density during descent.