2The Jet Engine The jet engine used in this section is the Rolls-Royce Avon.It is a relatively simple engineknown as a ‘Turbo-jet’.
3The Jet Engine External Features IntakeAnti-Icing TubeRear Compressor CaseCombustion Chambers (Cans)Jet Pipe (Exhaust)Oil TankTurbine SectionFront Compressor CasingCarries hot air from the rear of the compressor,to the outer casing, front vanes bullet,as ice can build up and restrict airflowto the engine, reducing efficiency and thrust.Accessory GearboxFuel, Oil & Hydraulic Pumps, Electrical Generators,RPM Governor.
4The Jet Engine External Features IntakeAnti-Icing TubeCompressor Handling Bleed ValveRear Compressor CaseCombustion Chambers (Cans)Jet Pipe (Exhaust)Oil TankTurbine SectionFront Compressor CasingAccessory GearboxAllows compressor air to bleed overboard.This prevents engine surgingin what is known as the‘Off Design Condition’Fuel, Oil & Hydraulic Pumps, Electrical Generators,RPM Governor.
5The Jet Engine External Features IntakeAnti-Icing TubeCompressor Handling Bleed ValveFront Engine MountRear Compressor CaseCombustion Chambers (Cans)Jet Pipe (Exhaust)Oil TankTurbine SectionRear Engine MountFront Compressor CasingAccessory GearboxUsed to fix the engine into the aircraft,the front mount is flexible to allow expansionand contraction due to temperature changes.Fuel, Oil & Hydraulic Pumps, Electrical Generators,RPM Governor.
7The Jet Engine The Static Assembly FRONT BULLET& SUPPORT VANESCOMBUSTION INNER CASINGCOMBUSTION OUTER CASINGSCOMPRESSOR OUTER CASINGSCOMBUSTION FLAME TUBESREAR CONE AND SUPPORT VANESThe casing is the structural frame work of the engine.It has to be strong enough to carry the weight of all the externalfitments and accessories and to withstand the internal temperaturesand pressures (‘hoop-stress’).
8The Jet Engine The Static Assembly SUPPORT BEARINGSSTATOR VANESNOZZLEGUIDE VANES (NGV’S)The power to weight ratio is very important, the heavier the engine,the less payload the aircraft can carry.Designers strive to get he most power out ofthe lightest engine design.
9The Jet Engine The Rotating Assembly Compressor BladesTurbine BladesShaft AssemblyThe rotating assembly - compressor, shaft and turbine –is carried on bearings and is known as a ‘spool’.Located in the casings via the three bearing assemblies,the shaft rotates at whatever rpm the combustion gases drive it.Compressor blades generally use a ‘Dove-tail’ root fixing,and turbine blades generally use a ‘Fir-Tree’ root fixing.
10The Jet Engine The Rotating Assembly Compressor BladesTurbine BladesShaft AssemblyThe compressor and turbine ‘drums’ are a series of discsfixed together at the rims by bolting or welding together.The turbine absorbs just sufficient energy from the gas streamto drive the compressor,leaving the remainder to provide the thrust.
11The Jet Engine The Gas Path and Temperatures The shaded area is called the ‘GAS PATH’.As the air passes through the engine,its temperature is changed,first by compression, then by adding and burning fuel,the burnt gasses then pass through the turbine section,and finally through the exhaust to the atmosphere.
12The Jet Engine Problem ! The Gas Path and Temperatures Ambient temp450 Deg C2,500 Deg C1,000 Deg CAbove ambientProblem !Temperature changes cause the shaftand casings to expand and contract.But by different amounts !When the engine is started, the casings expandbefore the rotor does.
13The Jet Engine The Gas Path and Temperatures Ambient temp450 Deg C2,500 Deg C1,000 Deg CAbove ambientThese sections expand firstAnd this could put tremendousunwanted forces on the casings, shaft and rotor,leading to failures.
14The Jet Engine The solution is in the bearings The Gas Path and TemperaturesFront Bearing ChamberCentre Bearing ChamberRear Bearing ChamberRoller BearingBall Bearing (Location Bearing)Roller BearingThe solution is in the bearings
15Centre Bearing Chamber The Jet EngineThe Engine BearingsRemember the ProblemTemperature changes cause the shaftand casings to expand and contract.CasingFront Bearing ChamberCentre Bearing ChamberRear Bearing ChamberShaftFront Bearing ChamberRear Bearing ChamberCentre Bearing ChamberThe solution is in the bearings
16The Jet Engine The Engine Bearings Ball Bearings provide positive Axial Locationand don’t allow lateral movement between the casings and the rotorCasingShaftThe bearings are selected to allow expansion(and contraction on cool down)without causing undue stresses.
17The Jet Engine The Engine Bearings These bearings use Rollers instead of ball bearings.The outer track prevents the rollers from moving laterally.The inner track allows lateral movement without undue stress.CasingShaftThe bearings are selected to allow expansion(and contraction on cool down)without causing undue stresses.
18Then, with ‘heat soak’ the shaft expands The Jet EngineThe Engine BearingsThe casing expands first, pushing the rear outer bearing with itCasingHot sectionThen, with ‘heat soak’ the shaft expandsShaftShaft and Casing have no differential movement at this point
19Then, with ‘heat soak’ the shaft expands The Jet EngineThe Engine BearingsThe same happens at the front with pressured air and heat sink fromthe combustor heating the hardwareThe casing expands first, pushing the rear outer bearing with itCasingHot sectionThen, with ‘heat soak’ the shaft expandsThe engine is actually designed to dimensionsbased on normal operating temperaturesShaftShaft and Casing have no differential movement at this point
20Check of Understanding The compressor handling bleed valveallows compressor air to bleed overboardand so prevent engine surging.What is this surging known as?Hoop StressOff Design ConditionRoot Fix ConditionInduced Flow Condition
21Check of Understanding What are the front and rearengine mounts used for?To fix the engine into the aircraftTo hold the shaft assembly in placeTo allow expansion without stressTo stop ice build-up in the engine
22Check of Understanding Where would you expectthe fuel, oil & hydraulic pumps,electrical generators and RPM governorto be situated on a jet engine?In the front bulletIn the accessory gearboxIn the rear compressor caseIn the turbine section
23Check of Understanding Which of these does not form part ofthe static assembly?Rear support vanesCombustion flame tubesCompressor bladesStator vanes
24Check of Understanding The casing the engine has to be strong enoughto withstand the internal pressures.What are these pressures known as?Ambient PressureFir Tree PressureRoot Fix StressHoop Stress
25Check of Understanding The shaft assembly and turbine bladesare part of which engine assembly?Rotating AssemblyOuter Casing AssemblyStatic AssemblyBearing Assembly
26Check of Understanding Which of these statements applies to thecentre bearing chamber?Allows lateral movementwithout undue stressPrevents rollers frommoving laterallyBall Bearings providepositive axial locationUse rollers insteadof ball bearings