Mechanical Aspiration

• The process of mechanically increasing the manifold pressure of an engine in order to maintain and/or increase horsepower.

Ambient Air

Compressed Air

Problems of Altitude

• 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

0

1000

Ground boost supercharger

Naturally aspirated

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