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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

Preface

Acknowledgements

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

Appendix

Glossary

Index

Superchargers
A supercharger is an engine-driven air pump or compressor
that provides compressed air to the engine to provide
additional pressure to the induction air so the engine can
produce additional power. It increases manifold pressure
and forces the fuel/air mixture into the cylinders. The higher
the manifold pressure, the more dense the fuel/air mixture,
and the more power an engine can produce. With a normally
aspirated engine, it is not possible to have manifold pressure
higher than the existing atmospheric pressure. A supercharger
is capable of boosting manifold pressure above 30 "Hg.

For example, at 8,000 feet a typical engine may be able to
produce 75 percent of the power it could produce at mean sea
level (MSL) because the air is less dense at the higher altitude.
The supercharger compresses the air to a higher density
allowing a supercharged engine to produce the same manifold
pressure at higher altitudes as it could produce at sea level.
Thus, an engine at 8,000 feet MSL could still produce 25 "Hg
of manifold pressure whereas without a supercharger it could
produce only 22 "Hg. Superchargers are especially valuable
at high altitudes (such as 18,000 feet) where the air density
is 50 percent that of sea level. The use of a supercharger in
many cases will supply air to the engine at the same density
it did at sea level. With a normally aspirated engine, it is not
possible to have manifold pressure higher than the existing
atmospheric pressure. A supercharger is capable of boosting
manifold pressure above 30 "Hg.

The components in a supercharged induction system are similar
to those in a normally aspirated system, with the addition of
a supercharger between the fuel metering device and intake
manifold. A supercharger is driven by the engine through a
gear train at one speed, two speeds, or variable speeds. In
addition, superchargers can have one or more stages. Each
stage also provides an increase in pressure and superchargers
may be classified as single stage, two stage, or multistage,
depending on the number of times compression occurs.

An early version of a single-stage, single-speed supercharger
may be referred to as a sea-level supercharger. An engine
equipped with this type of supercharger is called a sea-level
engine. With this type of supercharger, a single gear-driven
impeller is used to increase the power produced by an engine at
all altitudes. The drawback with this type of supercharger is a
decrease in engine power output with an increase in altitude.

Single-stage, single-speed superchargers are found on many
high-powered radial engines and use an air intake that faces
forward so the induction system can take full advantage of the
ram air. Intake air passes through ducts to a carburetor, where
fuel is metered in proportion to the airflow The fuel/air charge
is then ducted to the supercharger, or blower impeller, which
accelerates the fuel/air mixture outward. Once accelerated, the
fuel/air mixture passes through a diffuser, where air velocity
is traded for pressure energy. After compression, the resulting
high pressure fuel/air mixture is directed to the cylinders.

Some of the large radial engines developed during World
War II have a single-stage, two-speed supercharger. With
this type of supercharger, a single impeller may be operated
at two speeds. The low impeller speed is often referred to
as the low blower setting, while the high impeller speed is
called the high blower setting. On engines equipped with a
two-speed supercharger, a lever or switch in the flight deck
activates an oil-operated clutch that switches from one speed
to the other.

Under normal operations, takeoff is made with the
supercharger in the low blower position. In this mode, the
engine performs as a ground-boosted engine, and the power
output decreases as the aircraft gains altitude. However, once
the aircraft reaches a specified altitude, a power reduction is
made, and the supercharger control is switched to the high
blower position. The throttle is then reset to the desired
manifold pressure. An engine equipped with this type of
supercharger is called an altitude engine. [Figure 6-14]

Power output of normally aspirated engine compared to a single-stage, two-speed supercharged engine.
Figure 6-14. Power output of normally aspirated engine compared
to a single-stage, two-speed supercharged engine.

Turbosuperchargers
The most efficient method of increasing horsepower in an
engine is by use of a turbosupercharger or turbocharger.
Installed on an engine, this booster uses the engine's exhaust
gases to drive an air compressor to increase the pressure of
the air going into the engine through the carburetor or fuel
injection system to boost power at higher altitude.

 

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