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Piston Engine Propulsion
Thrust Augmentation
• Air enters the carburettor at atmospheric pressure and is mixed in the carburettor with the fuel.
• The fuel/air mixture leaves the carburettor at near-atmospheric pressure, is compressed in the supercharger to a pressure greater than atmospheric, and then enters the engine cylinders..
• The power required to drive the supercharger impeller is transmitted from the engine crankshaft by means of a gear train.
• Because of the high gear ratio, the impeller rotates much faster than the crankshaft
• The higher an aircraft climbs, the less oxygen is available to the engine for combustion. Therefore, as a reciprocating engine powered aircraft climbs, the power output of the engine decreases.
• To help prevent this loss of engine power, more oxygen must be forced into an engine. One method of getting more air into the engine is with a supercharger.
SuperchargingThe main purpose of supercharging an aircraft engine is to increase the manifold pressure above the pressure of the atmosphere in order to provide high power output for take-off and to sustain the maximum power at high altitudes.Increased manifold pressure increases the power output in two ways:
• It increases the weight of the fuel/air charge delivered to the cylinders of the engine.
• It increases the compression pressure. The compression ratio for any given engine is constant; hence, the greater the pressure of the fuel/air mixture at the beginning of the compression stroke, the greater will be the compression pressure
• Most superchargers used on conventional aircraft are alike in that an impeller rotating at high speed is used to compress either the air before it is mixed with the fuel in the carburettor or the fuel/air mixture, which leaves the carburettor.
• Air enters the carburettor at atmospheric pressure and is mixed in the carburettor with the fuel.
• The fuel/air mixture leaves the carburettor at near-atmospheric pressure, is compressed in the supercharger to a pressure greater than atmospheric, and then enters the engine cylinders.
• The power required to drive the supercharger impeller is transmitted from the engine crankshaft by means of a gear train.
• Because of the high gear ratio, the impeller rotates much faster than the crankshaft.
AIR COMPRESSORSUPERCHARGER – DRIVEN BY THE ENGINE
GEAR OR BELT DRIVE
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
Turbocharger• A drawback of gear driven superchargers is
that they use a large amount of the engines power output for the amount of power increase they produce.
• This problem is avoided with a turbo supercharger, or turbocharger, because an engine’s exhaust gases power turbochargers.
• In other words, a turbocharger recovers energy from hot exhaust gases that would otherwise be lost.
• Another advantage of turbochargers is that they can be controlled to maintain an engine’s rated sea- level horsepower from sea level up to the engine’s critical altitude.
• Critical altitude is defined as the maximum altitude under standard atmospheric conditions that a turbocharged engine can produce its rated horsepower.
• In other words, when a turbocharged engine reaches its critical altitude its power starts to decrease just like a normally aspirated engine
COMPRESSED
AIR TO ENGINE
TURBOCHARGER – DRIVEN BY THE ENGINE EXHAUST GASES
EXHAUST GASES
FROM ENGINE
THROUGH
EXHAUST PIPE
TURBINE BLADES
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
CARB.
AIR
FILTER
ENGINE
THE PARTIAL VACUUM TENDS TO WORK AGAINST THE PISTON MOVEMENT MAKING
IT HARDER FOR THEM TO PULL AIR IN
INLET
MANIFOLD
AND FLOW RESTRICTION THROUGH THROTTLE VALVE CAUSED BY SUCTION
FROM ENGINE
BETWEEN THROTTLE VALVE AND ENGINE IS A PARTIAL VACUUM
EXHAUST
MANIFOLD
INLET MANIFOLD VACUUM
AND THEREFORE, THE CYLINDER MAY NOT BE FULLY CHARGED 100% WITH
AIR/FUEL MIXTURE
IF AIR COULD BE FORCED INTO THE ENGINE, THERE WOULD BE A GREATER
VOLUME OF AIR/FUEL IN THE CYLINDER
THEREFORE MORE POWER WOULD BE PRODUCED
SO HOW COULD THIS BE ACHIEVED?
BY USING AN AIR COMPRESSOR
LET’S SEE HOW
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
AIR COMPRESSOR
AIR
COMPRESSOR
TAKE THE LID OFF
AND LOOK INSIDE
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
CUT IN HALF AND LOOK AT CROSS SECTION
CROSS
SECTION
ROTORROTOR
ROTOR
VANES
STATOR
VANES
OUTLET
DUCT
TAKE LID OFF AND LOOK INSIDE
SIMPLIFIED PISTON ENGINE FUEL SYSTEM
AIR COMPRESSOR
TYPICAL
ROTOR
AIR ENTERS
THE ‘EYE’ OF
THE ROTORSIMPLIFIED PISTON ENGINE FUEL SYSTEM
AIR COMPRESSOR