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Jet Engine Pneumatic Starter

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To start a gas turbine engine, we need to rotate the engine up to a certain percentage rpm of the operating speed of the engine, and at the same time to supply fuel and ignition in the combustion chamber with a predetermined sequence. This speed level can be from 15 to 25 per cent of the normal operating speed of the engine, or about 50 per cent of the idle speed of the engine. If the engine has more than one shafts, then we speak for the shaft of the gas generator, or the engine core shaft. The starting procedure stops when the engine can self sustain and increase it's speed overcoming the friction, aerodynamic and mechanical loads from the accesories mounted on the gear box of the engine and they are necessary to suport the operation of the engine itself and the rest airplane systems, such as generator, hydraulic pump, fuel pump, air bleed etc. The starting system provide the mechanical energy needed to rotate the engine, either on the ground, or in the air, in case we need to assist an engine to start after engine flame out. The main component of the starting system of an engine, is the engine starter. There are various types of engine starters: Pneumatic (air turbine) starter, Electric motor starter, Cartrige (gas turbine) starter, Hydraulic motor starter, Gas injection to the turbine starting, and Auxiliary gas generator starting. We will describe here the Pneumatic (air turbine) engine starter, which is the most commonly used to start the large civil Jet engines for decades now. Lately there is a tendency to turn to electric engine starters for large engines as well.  The pneumatic starter converts the thermodynamic energy of the compressed air coming from the air system of the airplane or an external air source, into mechanical energy output on a shaft to rotate the engine through the engine accesory gear box.

On the photo on the left we can see the Allied Signal  P/N 3505716-5 pneumatic starter installed on a CFM56-3 engine. You can see the turbine rotor blades through the exhaust of the starter, the case which incorporates the air inlet of the unit and on top the starter air valve, an electricaly controled and pneumaticaly operated valve which controls the air upstream of the starter. This valve can be controled manualy as well when the electropneumatic function is in fault.

A typical jet engine pneumatic startrer incorporates a single stage turbine into it's housing. On the photo on the right you can see the housing of jet engine starter made by Allied Signal with Part Number 3505716-5 made of aluminum alloy. The compressed air is geting into the housing in Radial direction and it is sirculating in the housing before it passes through the guide vanes of the stator shown on the left of the picture, which guides and delivers the air to the turbine wheel with the optimum angle speed and pressure for maximum efficiency.




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