SUPERCHARGING AND TURBOCHARGING

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SUPERCHARGING AND TURBOCHARGING

• HIGH PERFORMANCE AIRCRAFT ENGINES

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POWER MANAGEMENT

• The power created by a reciprocating engine is a
  product of MAP (manifold absolute pressure) and
  rpm.
• If rpm remains constant and MP is increased,
  power output will be increased.
• If MP remains constant and rpm is increased,
  power output will be increased.

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FACTORS AFFECTING POWER

• Humidity- water vapor in the air takes the place
  of oxygen molecules. The molecular weight of
  water vapor is less than oxygen as a result
  moist air is less dense than dry air. Less dense
  air means decreased performance.
• Temperature- temperature affects air density
  which affects performance T?, d?, P?
• Mixture- fuelair ratio
• Ambient Pressure- the pressure altitude at the
  aerodrome affects air density and performance
  p?, d?, P?

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FACTORS AFFECTING POWER

• Friction loss- as the air flows through the
  intake system it loses pressure due to skin
  friction and rounding corners. This factor is
  based on intake system design.
• Altitude- as an aircraft climbs the air becomes
  less dense and power decreases as a result. As we
  climb the throttle must be opened further to
  maintain climb power.
• Critical altitude- the altitude where the
  throttle is fully open in order to achieve the
  desired power setting.(normally aspirated engine)

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POWER SETTINGS

• The power setting used for cruise flight is a
  trade-off between fuel economy, engine longevity,
  and speed.
• 100 power is only used for takeoff and initial
  climb. 100 power will only be available under
  certain altitude and temperature conditions.
• Normal cruise power setting is usually 65.

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RULES TO PROLONG ENGINE LIFE

• Always observe manufacturer operating
  limitations.
• Make throttle, propeller rpm, and mixture changes
  slowly and smoothly. Abrupt changes put large
  stresses on engine components.
• Keep rpm high during power changes to avoid high
  cylinder pressures and stresses.
• Power increase MPT
• Power decrease TPM

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RULES TO PROLONG ENGINE LIFE

• Be aware of thermal shock. Avoid large power
  reductions. Reduce power in increments. Manage
  cowl flaps correctly.
• Periodically warm the engine in prolonged power
  off descents.
• Use full rich mixture when operating at full or
  near full power.

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RULES TO PROLONG ENGINE LIFE

• Be aware of conditions conducive to the formation
  of carb ice.
• Preheat a cold engine before start. Idle at low
  rpm until the oil pressure and temp. is within
  operating parameters.
• Allow turbocharged engines to idle for a few
  minutes before shutdown to allow components to
  cool.

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GIVE ME MORE AIR

• Normally aspirated engine power is limited by the
  ambient air density.
• By utilizing supercharger or turbocharger systems
  we can increase engine power output through
  larger range of atmospheric conditions.
• This is done by supplying the engine with higher
  manifold pressures than normal aspiration.

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WHY FLY HIGH?

• Get above weather, icing, turbulence, and
  terrain.
• Utilize stronger winds aloft.
• Lower traffic density.
• Ensure radar coverage for ATC assistance.
• Go faster. Aircraft fly faster at altitude given
  the same power setting. The thinner air produces
  less parasite drag.
• Conserve fuel. Aircraft engines require less fuel
  at altitude due to the less dense air.

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SUPERCHARGERS

• Usually compress the fuel/air mixture after it
  leaves the carburetor.
• A supercharger is driven directly from the
  engine.
• Some of the power created is offset by the power
  required to drive the supercharger.
• The amount of supercharging done is limited by
  the temperatures produced to avoid detonation
  problems.

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SUPERCHARGERS

• Each increase in air/fuel mixture pressure is
  called a stage.
• Single-stage, two-stage, multi-stage.
• Superchargers may also be geared to operate at
  variable speeds.
• Single-speed, two-speed, variable-speed.
• EX single-stage, two-speed supercharger.
• Multi-speed superchargers are used to control
  supercharger output at different altitudes.
  (higher output for higher altitudes)

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SUPERCHARGERS

• Superchargers are usually built as an integral
  part of the engine.
• There most common aviation application is on high
  powered radial engines.
• The air entering the induction system is
  controlled by the throttle valve.
• The fuel is mixed with air in the carburetor.
• The fuel/air mixture enters the supercharger,
  where an impeller (centrifugal compressor)
  compresses the mixture.
• This compressed mixture is fed to the cylinders
  via the intake manifold.

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SUPERCHARGERS
EXHAUST GASES
FUEL/AIR MIXTURE
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ADVANTAGES/DISADVANTAGES

• Advantages
• Improved performance at altitude.
• More power for take-off.
• Disdvantages
• Power gain is offset by power used by engine to
  drive supercharger.
• Increased temperature of fuel/air mixture
  increases risk of detonation.

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TURBOCHARGERS

• Turbochargers deliver compressed air to the inlet
  side of the carburetor or fuel control unit.
• Unlike a supercharger, they are driven by the
  exhaust gases produced by the combustion process.
• In this way turbochargers harness some of the
  unused energy contained in the hot exhaust gases.
• A ground boosted turbocharged engine will produce
  MP on the ground higher than ambient pressure in
  order to achieve its rated power.
• A turbo-normalized engine will maintain sea level
  performance to higher altitudes.

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INTAKE AIR
EXHAUST
CARBURETOR
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TURBOCHARGERS

• The turbocharger consists of a compressor
  assembly, exhaust gas turbine assembly, and a
  pump and bearing casing.
• The compressor assembly is made up of a housing
  which directs air flow and a compressor wheel
  (impeller).
• The exhaust gas turbine assembly is made up of a
  housing which directs exhaust gas flow and a
  turbine wheel.
• The center casing contains a housing which
  directs cooling oil around the shaft linking the
  turbine and compressor. The shaft is suspended by
  bearings which reduce the heat created by
  friction.

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TURBOCHARGERS

• The impeller/compressor, turbine wheel, and
  connecting shaft together are called the rotor.
• At no time in the process do the exhaust gases
  come into contact with the compressed air.
• Turbocharger output is controlled by the
  wastegate.

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WASTEGATE

• The wastegate controls the amount of exhaust
  gases directed to the turbine wheel of the
  turbocharger.
• If the wastegate is open all the exhaust gas is
  vented overboard through the exhaust system
  bypassing the turbocharger. (zero boost)
• As the wastegate is closed, more and more exhaust
  gas is directed to the turbocharger until the
  wastegate is fully closed. (max. boost)
• There are manual and automatic wastegates.

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FIXED WASTEGATE

• Without a wastegate, the amount of boost that a
  turbocharger creates varies with the pressure of
  the engine's exhaust. This happens because
  exhaust pressure varies with relation to the
  engine's speed (measured in RPM's). This implies
  that as an engine reaches higher RPM's,
  increasing amounts of boost will be created by
  the turbocharger. The problem with this is that
  an engine can only accommodate a given amount of
  boost.
• With this type of installment the pilot is
  responsible for maintaining MP within limits
  through throttle setting.

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MANUAL WASTGATE

• A manual wastegate is controlled by the pilot
  through linkage and a flight deck control.
• On some installations it is possible to overboost
  the engine during takeoff if the wastegate is not
  managed properly.
• As the aircraft climbs to its normally aspirated
  critical altitude, the pilot begins to close the
  wastegate through the climb.
• The altitude at which the wastegate is fully
  closed and MP pressure can no longer be
  maintained is the critical altitude.
• During descent the pilot must open the wastegate
  in increments to ensure engine operating
  parameters are not exceeded.

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AUTOMATIC WASTEGATE

• Automatically adjusts wastegate position based on
  throttle position.
• This is achieved through a density controller and
  a differential pressure controller.
• Engine oil pressure is used to control a
  wastegate actuator.
• Oil pressure moves the wastegate to the closed
  position, while a spring moves it to the open
  position. The density controller and differential
  pressure controller adjust the amount of oil
  which is bled away form the actuator to control
  wastegate position.

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AUTOMATIC WASTEGATE

• Deck pressure- pressure between compressor
  discharge and the throttle valve.
• Manifold pressure- pressure in the intake
  manifold downstream from the throttle valve.
• Critical altitude- The altitude at which the
  wastegate is completely closed, and manifold
  pressure will start to drop if the climb is
  continued. From this point it acts as a normally
  aspirated engine.

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DENSITY CONTROLLER

• The density controllers job is to limit maximum
  MP preventing overboost.
• It limits deck pressure while the aircraft is
  below the turbochargers critical altitude.
• If deck pressure becomes too high, the density
  controller bleeds more oil from the wastegate
  actuator causing the wastegate to open.

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DIFFERENTIAL PRESSURE CONTROLLER

• The differential pressure controller senses the
  air pressures on either side of the throttle
  plate (deck pressure/manifold pressure) and acts
  to maintain an optimum balance between a low
  turbocharger workload and a quick spool-up time.
• It controls oil bled from the wastegate actuator
  to maintain a pre-set pressure differential of
  approx.2-3.
• This ensures the system will respond quickly and
  smoothly to throttle changes. Helps control
  bootstrapping.

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TURBOCHARGED ENGINE TRAITS

• Bootstrapping- any change in engine rpm or
  temperature will change the amount of exhaust gas
  flowing to the turbine. This will cause an
  increase or decrease in boost. The resultant
  fluctuation of MP is called bootstrapping. It is
  most pronounced when the wastegate is fully
  closed.
• Overboost- manifold pressure exceeds the limits
  of the engine.
• Overshoot- a turbocharged engine is more
  sensitive to throttle changes than a normally
  aspirated engine. Smooth throttle control is
  needed to avoid MP drift.
• Cool down- the rotor of a turbocharger is subject
  to intense temperatures due to the high rpm. A
  cool down period of idle operation before
  shutdown is necessary with most installations.

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INTERCOOLER

• The compression of air by the turbocharger
  creates a temperature rise in the induction air
  which could result in detonation.
• An intercooler cools the induction air after
  compression.
• The intercooler acts as a air to air heat
  exchanger.

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TURBO/SUPERCHARGED
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PRESSURIZATION

• The compressed air form a turbocharger is
  commonly used to provide a source of pressurized
  cabin air.