Thursday, February 23, 2012

Starting and ignition - INTRODUCTION


1. Two separate systems are required to ensure that a gas turbine engine will start satisfactorily. Firstly, provision must be made for the compressor and turbine to be rotated up to a speed at which adequate air passes into the combustion system to mix with fuel from the fuel spray nozzles (Part 10). Secondly, provision must be made for ignition of the air/fuel mixture in the combustion system. During engine starting the two systems must operate simul- taneously, yet it must also be possible to motor the engine over without ignition for maintenance checks and to operate only the ignition system for relighting during flight (para. 28).

2. The functioning of both systems is co-ordinated during a starting cycle and their operation is auto- matically controlled after the initiation of the cycle by an electrical circuit.  A typical sequence of events  during the start of a turbo-jet engine is shown in fig.11-1.


Fig. 11-1 A typical starting sequence of a
turbo-jet engine.
3. The starting procedure for all jet engines is basically the same, but can be achieved by various methods. The type and power source for the starter varies in accordance with engine and aircraft require- ments. Some use electrical power, others use gas, air or hydraulic pressure, and each has its own merits. For example, a military aircraft requires the engine to be started in the minimum time and, when possible, to be completely independent of external equipment. A commercial aircraft, however, requires the  engine  to  be  started  with  the  minimum disturbance to the passengers and by the most  economical  means.  Whichever  system  is  used,reliability is of prime importance.
4. The starter motor must produce a high torque and transmit it to the engine rotating assembly in a manner that provides smooth acceleration from rest up to a speed at which the gas flow through the engine  provides  sufficient  power  for  the  engineturbine to take over.

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