Page 137 - The Jet Engine

Starting and ignition

Fig. 11-7 An air starting system.
operates and shuts down the gas turbine starter. As cycle the pump /starter functions as a normal
the starter runs down, the clutch automatically hydraulic pump.
disengages from the output shaft and the main
engine accelerates up to idling r.p.m. under its own IGNITION
power.
18. High-energy (H.E.) ignition is used for starting
Hydraulic all jet engines and a dual system is always fitted.
17. Hydraulic starting is used for starling some Each system has an ignition unit connected to its
small jet engines. In most applications, one of the own igniter plug, the two plugs being situated in
engine-mounted hydraulic pumps is utilized and is different positions in the combustion system.
known as a pump/starter, although other applications 19. Each H.E. ignition unit receives a low voltage
may use a separate hydraulic motor. Methods of supply, controlled by the starting system electrical
transmitting the torque to the engine may vary, but a circuit, from the aircraft electrical system. The
typical system would include a reduction gear and electrical energy is stored in the unit until, at a pre-
clutch assembly. Power to rotate the pump/starter is determined value, the energy is dissipated as a high
provided by hydraulic pressure from a ground supply voltage, high amperage discharge across the igniter
unit and is transmitted to the engine through the plug.
reduction gear and clutch. The starting system is
controlled by an electrical circuit that also operates 20. Ignition units are rated in 'joules' (one joule
hydraulic valves so that on completion of the starting equals one watt per second). They are designed to

127