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