SUPER CHARGER
The mixture once it has entered the inlet manifold under pressure from the impeller is normally referred to as Manifold pressure.
Manifold pressure gauge – indicates atmospheric pressure when a/c is parked on ground with engine off.
A pressure gauge in the cockpit sometimes graduated in lbs per sq. inch or inch of mercury indicates the pressure above or below the sea level atmospheric pressure known as Boost gauge or manifold press gauge).
Note – Standard atmosphere pressure is indicated as zero on the boost gauge.
When engine with Automatic Boost control is started, the induction manifold pressure will fall to a low value this is due to the increase in velocity of mixture flow caused by supercharger operation.
Rated Altitude – is altitude at which a supercharged engine with Automatic Boost Control can maintain its sea-level rated power & above which the power will decrease.
OR
Rated Altitude is altitude at which a Super charged engine with Automatic Boost Control is operating at throttle valve fully open & maintains the maximum Take off boost at altitudes above sea level without change in power.
If a/c climbs further the boost pressure will decrease as with normally aspirated engine.
Boost gauge – Lbs per square inch.
Manifolds pressure gauge – pressure in inches of mercury.
Full Throttle height – is altitude up to which a given boost setting can be maintained at a given rpm. The lower the boost pressure selected, the greater altitude can be maintained by the Automatic Boost Control.
Example
In an A/c at its Rated altitude & still climbing, the butterfly will be fully open at its rated boost, Any climb above this altitude will cause boost to decrease.
As A/c climbs boost pressure decreases, the butterfly will still be opening through the action of the aneroid capsule & the pressure will be maintained until the butterfly can open no further.
This is known as Full Throttle height for that particular boost & rpm.
The speed of engine rpm controls speed of supercharger impeller, at higher than normal rpm the Full throttle height. will be raised.
Note – The Rated altitude. is full Throttle height at Rated boost & normal rpm for that boost.
Maximum Efficiency or minimum Specific fuel consumption, is obtained when the butterfly valve is fully open & the rpm is as low as can be obtained without detonation.
In practice this occurs at the Full throttle height for the power settings in use.
The power is obtained by using the highest possible weak mixture boost in conjunction with the lowest practicable rpm to give the required airspeed.
Note – Supercharger with two gear ratios are normally termed as two speed superchargers.
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Turbo Charger
The Turbo charger compressor increases the pressure in the inlet manifold thereby increase the weight of charge of mixture entering the cylinders.
The Turbocharger turbine is located in the exhaust system & the compressor element is situated between the air-intake & the carburetor or injector, unlike the supercharger, which is normally located between the carburetor & inlet manifold.
Turbine is connected to the Exhaust.
Compressor is connected to the Inlet.
Fuel/Air enters the inlet duct & is compressed by compressor vanes. These vanes convert velocity energy into pressure energy.
For any particular power output the turbocharger delivers a fixed weight of air to the engine in a given time, & since the density of air decreases with increase in the altitude a greater volume of air is compressed & the impeller rotates at a faster speed at high altitude, .
At low altitude impeller rotates slow.
Turbine speed is controlled by waste gate controller.
When the waste-gate is fully open nearly all the exhaust gases pass to the atmospheric (it occurs at low altitude). But as the altitude increases waste-gate closes & exhaust gases are directed to the turbine, & the maximum turbine speed is reached when the waste gate is fully closed.
The spring tends to open the waste gate & engine oil pressure tends to close it.
Oil pressure in the actuator is controlled to regulate the position of waste-gate according to engine requirements.
Absolute pressure controllers are designed to prevent the turbo-charger outlet pressure from exceeding a maximum specified limit.
Note – As altitude increases impeller speed increases, temperature of charge also increases & this will decrease power output for a given manifold pressure & engine speed.
Engine oil & cylinder temperature will also increase as a result of the higher combustion temperature.
Variable pressure controller controls the opening & closing of waste gate.
Dual control unit system
On some ground boosted turbo chargers, dual units are installed.
1) Density Controller – is designed to prevent the turbocharger output from exceeding the limiting pressure. It regulates oil pressure only at full throttle & up to turbo charger critical altitude.
2) Differential Pressure controller – controls waste-gate at all positions of throttles other than fully open.
The bleed valve is fully open at full throttle, when pressure drop is least & gradually opens as throttle is closed & pressure drop increases.
The controller therefore opens the waste gate as throttle is closed & decreases turbocharger outlet pressure in accordance with the power selected.