Aerospace Propulsion Study For Shenyang Aerospace University by Lale420 (mid_term_7)
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Transcript of Aerospace Propulsion Study For Shenyang Aerospace University by Lale420 (mid_term_7)
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Supercharging
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Supercharging/Turbosupercharging
Purpose
To increase the mass airflow to an engine above that available from ambient air pressure
The manifold pressure of a normally aspirated engine is limited to just below the value of ambient atmospheric pressure
A supercharger is an air pump that “packs” extra air molecules into the induction system
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Supercharging/Turbosupercharging
The purpose of supercharging is to increase volumetric efficiency
This deals with how much fuel/air mixture can actually be placed into the cylinder during the induction stroke using an air pump
The mechanism by which the air pump is driven can be broken into two groups
• Supercharging
• Turbo supercharging
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Supercharging
A supercharger air pump is driven via the engine crankshaft
Superchargers usually compress the fuel/air mixture after it leaves the carburettor
The amount of supercharging that can be done is restricted by the temperatures produced, to avoid the problems of pre-ignition and detonation
A super charger may have one or several levels of pressure increase called stages
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Supercharging
The possible pressure increase will be limited to the physical structure of the engine
If 29.92” Hg manifold pressure is ambient, 40” Hg manifold is feasible for an engine to handle
As altitude increases supercharger speed may be increased.
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Turbo-Supercharging
Turbochargers, like superchargers deliver compressed air to the engine
Turbochargers derive their power from the energy of the engine exhaust gases directed against a turbine wheel
The turbine wheel is connected by a shaft to a compressor which then packs air into the inlet manifold
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Turbo-Supercharging
To derive energy to operate a supercharger some power is removed from the crankshaft which is now not available to the propeller
Using engine exhaust to power a turbocharger is almost like getting something for nothing but there are some disadvantages
The turbine and compressor routinely rotate at very high speeds and the turbine which drives the entire unit is subject to extremely high engine exhaust temperatures
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Turbo-Supercharging If exhaust gases operate the compressor there needs to be a system to control the free wheeling turbine and the resultant air pressure at the inlet manifold
This is accomplished with a wastegate which controls the amount of exhaust that hits the turbine rotor
Opening the waste gate directs the exhaust gases away from the turbocharger turbine wheel and no extra air pressure is obtained
Closing the waste gate directs the exhaust gases to the turbocharger turbine wheel and the energy derived form the wheel is converted to energy in the inlet manifold
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Turbo Supercharger
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Turbo Supercharger
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Turbo Supercharger
A significant problem with turbochargers is the compressed air being pumped into the intake manifold being hot Hot air both destroys engines (detonation) and robs them of power
To reduce the extent of the heat problem intercoolers are placed between the compressor discharge and the intake manifold to cool the hot compressed air before it goes into the engine
The disadvantages of using intercoolers are increased weight, and drag (use of ram air)
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Turbo Supercharger Operation
The control of power provided by a turbocharger is via control of the wastegate
The wastegate is either controlled manually or automatically
Manual control is usually accomplished with a vernier control in the cockpit connected by cable directly to the wastegate
During climb the wastegate would be progressively closed as the outside air density decreased; and as manifold pressure reduced, intake pressure could be made up back to the desired value
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Turbo Supercharger Operation
Automatic wastegate control is accomplished by regulating the wastegate between the fully open and fully closed positions to maintain a constant power output
The position of the wastegate is controlled by oil pressure acting on the wastegate valve
When oil pressure is increased on the piston the wastegate valve moves towards the closed position and intake pressure increases
Conversely when oil pressure is decreased (or oil pressure is lost) the wastegate valve moves to the open position and power decreases
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Turbo Supercharger Operation
Every power setting has a critical altitude
Critical altitude is where an increase in height results in a power loss because the wastegate has gone to the fully closed position
Modern turbochargers have over boost protection but do not rely on it as wastegates are known to jam
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Supercharging of an aircraft engine increases its power output by
A. Increasing the mass of mixture entering the cylinders during each cycle
B. Allowing a briefer compression stroke
C. Increasing the mass of mixture entering the cylinders during each stroke
D. Increasing the temperature of the fuel-air mixture
……. and the answer is ………….
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Whilst increasing the volumetric efficiency of an engine, supercharging
A. increases the density and decreases the temperature of the fuel-air mix
B. increases the density and temperature of the fuel- air mix
C. increases the temperature and decreases the potential energy of the charge
D. increases the pressure of the charge by dropping its temperature
……. and the answer is ………….
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What effect does turbo supercharging have on compression ratio?
A. none
B. increases the compression ratio as the wastegate is closed
C. increases the compression ration as the wastegate is opened
D. increases the compression ratio whenever power is increased regardless of wastegate operation
……. and the answer is ………….
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If there is an appreciable leak in the exhaust pipe downstream of the turbocharger wastegate the result would be
A. loss of power output
B. excessive power output
C. no effect on power output
D. fluctuating power output
……. and the answer is ………….