Transcript Mechanical Aspiration

Mechanical Aspiration
• The process of mechanically increasing the
manifold pressure of an engine in order to
maintain and/or increase horsepower.
Compressed Air
Ambient Air
Problems of Altitude
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Air density and pressure decrease
1/2 as much air at 20,000 feet as at sea level
Less oxygen
NOTE: Temperature and exhaust back pressure
are decreasing but this is not enough to offset the
decline in density and pressure
Theory
• More fuel and air at a higher pressure can produce
more horsepower within an engine
• A naturally aspirated engine can only burn as
much fuel as it has air to mix with
• Mechanical aspiration increases the density of the
air in the induction manifold so that more fuel can
be added
• Mechanical aspiration increases the pressure in the
combustion chamber to increase power
• The increases in power are limited by the strength,
temperature, and lubrication limits in the engine
1000
Ground boost supercharger
Naturally aspirated
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Theory
• The performance number of the fuel also limits the
amount of boost that can occur (detonation)
• 50 InHg is considered a high boost in modern
engines (approx. 20 above atmospheric)
Supercharging and Turbocharging
• Supercharging is boosting MAP above 30 InHg,
regardless of altitude or method of driving the
compressor
• Superchargers may be internal or external and may
be driven by gears or the exhaust stream
• Internal - compresses fuel and air
• External - compresses air only
Fuel and Air
Air Only
Supercharging and Turbocharging
• Turbocharging is using the exhaust stream to drive
a compressor with MAP at or below 30 InHg.
This is normally for altitude compensation only
• Turbosupercharging is the use of the exhaust
stream to drive a compressor to increase MAP
above 30 InHg
• Normalizers are gear driven compressors that
compensate for altitude (below 30 InHg)
• Ground-boosting is the use of a supercharger to
increase take-off horsepower
Increasing Horsepower
• The supercharger increases horsepower by
increasing the weight/density of the mixture and
by increasing compression pressures
• The supercharger also consumes some horsepower
in order to boost total horsepower output
The Compressor
• The compressor receives air near atmospheric
density and pressure and increases both across its
vanes
• Centrifugal flow compressors are the most
common in modern systems
• The compressor impeller is turned by a shaft
attached to the turbine
• The faster the compressor impeller turns, the more
boost that is available to the engine
Compressor
Turbine and
shaft
Turbine
• The exhaust flow from the engine is directed over
the blades of the turbine to provide the force to
turn the shaft and compressor
• Leaks in the exhaust system before the turbine will
decrease performance
• Combustion deposits may form on the turbine and
reduce efficiency
• Turbine speed is controlled to change the amount
of boost available
Lycoming Turbocharging
• Overview:
This system is used to maintain sea level
performance as altitude increases. It does not
boost above 30 InHg and uses the exhaust to
drive the compressor (turbocharger)
Waste Gate and Exhaust Bypass Valve
• The waste gate is used to control the flow of
exhaust gases through the turbine of a
turbocharger
• The exhaust may flow entirely through the turbine
or some may bypass to the tailpipe
• The exhaust bypass valve moves the waste gate by
pushing or pulling on an actuating arm
• The arm is attached to the waste gate inside the
exhaust pipe. Spring tension holds the waste gate
open until oil pressure begins to close it.
Waste Gate and Exhaust Bypass Valve
• As the waste gate closes, more exhaust is routed to
the turbine
• As the waste gate opens, more exhaust exits the
tailpipe
• The more exhaust that is routed to the turbine, the
faster it spins, and the more boost that is available
• Oil pressure controls the position of the waste gate
by pushing on a piston and opposing spring
pressure inside the exhaust bypass valve
• Each controller senses critical parameters and
adjusts oil pressure accordingly
Density Controller
• This controller reacts or “senses” the temperature
and pressure of the air between the compressor
and the throttle plate
• The air after the compressor and before the
throttle plate is called deck pressure
• The air after the throttle plate is still manifold
pressure (MAP)
• A bellows in the density controller expands and
contracts in response to pressure changes.
• The bellows is attached to a valve that controls oil
pressure
• Varying oil pressure controls the waste gate
Density Controller
• As air pressure in the deck drops, the bellows
expands and restricts the oil from returning to the
sump.
• Oil pressure rises, pushes on the piston in the
exhaust bypass valve and moves the waste gate
closer to closed. This routes exhaust to the turbine
to increase boost
• Dry nitrogen is used inside the bellows to allow it
to sense temperature
• Boost is increased on days that are hot and the air
is less dense
Differential Pressure Controller
• Used to reduce the amount of boost during partial
throttle operation
• Used in addition to the density controller as the
density controller only works at full throttle
• Senses the difference between deck and MAP and
adjusts oil flow in the system
• A diaphragm with deck on one side and MAP on
the other is attached to a oil control poppet valve
• A 2 InHg differential is maintained between the
deck and the MAP (across the throttle plate)
Differential Pressure Controller
• As MAP drops, as in idle operation, the deck
pressure is still high. The controller opens an oil
passage to relieve oil pressure and reduce boost
Variable Pressure Controller
• This controller is used instead of the density
controller and differential pressure controller
• The primary difference is a direct connection of
the throttle to the controller
• Oil pressure is still varied to control the system
• The throttle cable rotates a cam within the
controller that varies the spring tension on the oil
poppet valve
• The higher the throttle position, the higher the
pressure must be around the controller bellows
(deck) to unseat the valve and relieve oil pressure
• Set the throttle and the controller regulates boost
Continental Turbosupercharging
Continental Turbosupercharging
• This system is designed to allow the pilot to select
any desired power output at any time. This
includes boosting at sea level and altitude
compensation
• Three controllers and an actuator control the
turbosupercharger output to the engine
• The fuel air control unit of the GTSIO-520
contains both the supercharger controller and the
fuel injector assembly (engine in lab)
• An inter-cooler is used to cool the induction air
after it is compressed to prevent detonation
• An inter-cooler is an air to air heat exchanger
Continental Turbosupercharging
Continental Turbosupercharging
• All controllers vary the oil pressure to the exhaust
bypass valve (waste gate actuator)
Absolute - Pressure Controller
• This acts as a relief valve for the deck pressure
• At a preset maximum deck pressure, this
controller bypasses oil to the sump and reduces
boost
Rate - of - Change Controller
• This controller limits the rate of boost to 6.4
InHg/sec.
• Excess boost rates will cause the system to
overshoot the selected boost level (momentum)
Pressure - Ratio Controller
• This controller reduces boost at high altitudes to
prevent detonation
• The controller limits deck pressure to 2.2 times
ambient pressure
Variable Absolute Pressure Controller
• This is the controller that Continental uses to
combine all of the previously discussed controllers
into one unit
• Same principles as Lycoming Variable Pressure
Controller (throttle position is a mechanical input)
MAP Relief Valve
• This is a safety valve used in some Continental
systems to relieve MAP at a preset maximum
pressure by venting deck pressure
END