JET PROPULSION Part 3 The Jet Engine

The Jet Engine The jet engine used in this section is the Rolls-Royce Avon. It is a relatively simple engine known as a ‘Turbo-jet’.

The Jet Engine External Features Intake Anti-Icing Tube Rear Compressor Case Combustion Chambers (Cans) Jet Pipe (Exhaust) Oil Tank Turbine Section Front Compressor Casing Carries hot air from the rear of the compressor, to the outer casing, front vanes bullet, as ice can build up and restrict airflow to the engine, reducing efficiency and thrust. Accessory Gearbox Fuel, Oil & Hydraulic Pumps, Electrical Generators, RPM Governor.

The Jet Engine External Features Intake Anti-Icing Tube Compressor Handling Bleed Valve Rear Compressor Case Combustion Chambers (Cans) Jet Pipe (Exhaust) Oil Tank Turbine Section Front Compressor Casing Accessory Gearbox Allows compressor air to bleed overboard. This prevents engine surging in what is known as the ‘Off Design Condition’ Fuel, Oil & Hydraulic Pumps, Electrical Generators, RPM Governor.

The Jet Engine External Features Intake Anti-Icing Tube Compressor Handling Bleed Valve Front Engine Mount Rear Compressor Case Combustion Chambers (Cans) Jet Pipe (Exhaust) Oil Tank Turbine Section Rear Engine Mount Front Compressor Casing Accessory Gearbox Used to fix the engine into the aircraft, the front mount is flexible to allow expansion and contraction due to temperature changes. Fuel, Oil & Hydraulic Pumps, Electrical Generators, RPM Governor.

The Jet Engine The Static Assembly

The Jet Engine The Static Assembly FRONT BULLET & SUPPORT VANES COMBUSTION INNER CASING COMBUSTION OUTER CASINGS COMPRESSOR OUTER CASINGS COMBUSTION FLAME TUBES REAR CONE AND SUPPORT VANES The casing is the structural frame work of the engine. It has to be strong enough to carry the weight of all the external fitments and accessories and to withstand the internal temperatures and pressures (‘hoop-stress’).

The Jet Engine The Static Assembly SUPPORT BEARINGS STATOR VANES NOZZLE GUIDE 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 of the lightest engine design.

The Jet Engine The Rotating Assembly Compressor Blades Turbine Blades Shaft Assembly The 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.

The Jet Engine The Rotating Assembly Compressor Blades Turbine Blades Shaft Assembly The compressor and turbine ‘drums’ are a series of discs fixed together at the rims by bolting or welding together. The turbine absorbs just sufficient energy from the gas stream to drive the compressor, leaving the remainder to provide the thrust.

The 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.

The Jet Engine Problem ! The Gas Path and Temperatures Ambient temp 450 Deg C 2,500 Deg C 1,000 Deg C Above ambient Problem ! Temperature changes cause the shaft and casings to expand and contract. But by different amounts ! When the engine is started, the casings expand before the rotor does.

The Jet Engine The Gas Path and Temperatures Ambient temp 450 Deg C 2,500 Deg C 1,000 Deg C Above ambient These sections expand first And this could put tremendous unwanted forces on the casings, shaft and rotor, leading to failures.

The Jet Engine The solution is in the bearings The Gas Path and Temperatures Front Bearing Chamber Centre Bearing Chamber Rear Bearing Chamber Roller Bearing Ball Bearing (Location Bearing) Roller Bearing The solution is in the bearings

Centre Bearing Chamber The Jet Engine The Engine Bearings Remember the Problem Temperature changes cause the shaft and casings to expand and contract. Casing Front Bearing Chamber Centre Bearing Chamber Rear Bearing Chamber Shaft Front Bearing Chamber Rear Bearing Chamber Centre Bearing Chamber The solution is in the bearings

The Jet Engine The Engine Bearings Ball Bearings provide positive Axial Location and don’t allow lateral movement between the casings and the rotor Casing Shaft The bearings are selected to allow expansion (and contraction on cool down) without causing undue stresses.

The 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. Casing Shaft The bearings are selected to allow expansion (and contraction on cool down) without causing undue stresses.

Then, with ‘heat soak’ the shaft expands The Jet Engine The Engine Bearings The casing expands first, pushing the rear outer bearing with it Casing Hot section Then, with ‘heat soak’ the shaft expands Shaft Shaft and Casing have no differential movement at this point

Then, with ‘heat soak’ the shaft expands The Jet Engine The Engine Bearings The same happens at the front with pressured air and heat sink from the combustor heating the hardware The casing expands first, pushing the rear outer bearing with it Casing Hot section Then, with ‘heat soak’ the shaft expands The engine is actually designed to dimensions based on normal operating temperatures Shaft Shaft and Casing have no differential movement at this point

Check of Understanding The compressor handling bleed valve allows compressor air to bleed overboard and so prevent engine surging. What is this surging known as? Hoop Stress Off Design Condition Root Fix Condition Induced Flow Condition

Check of Understanding What are the front and rear engine mounts used for? To fix the engine into the aircraft To hold the shaft assembly in place To allow expansion without stress To stop ice build-up in the engine

Check of Understanding Where would you expect the fuel, oil & hydraulic pumps, electrical generators and RPM governor to be situated on a jet engine? In the front bullet In the accessory gearbox In the rear compressor case In the turbine section

Check of Understanding Which of these does not form part of the static assembly? Rear support vanes Combustion flame tubes Compressor blades Stator vanes

Check of Understanding The casing the engine has to be strong enough to withstand the internal pressures. What are these pressures known as? Ambient Pressure Fir Tree Pressure Root Fix Stress Hoop Stress

Check of Understanding The shaft assembly and turbine blades are part of which engine assembly? Rotating Assembly Outer Casing Assembly Static Assembly Bearing Assembly

Check of Understanding Which of these statements applies to the centre bearing chamber? Allows lateral movement without undue stress Prevents rollers from moving laterally Ball Bearings provide positive axial location Use rollers instead of ball bearings

Questions?

JET PROPULSION End of Presentation