Page 141 - The Jet Engine

Starting and ignition

Fig. 11-14 A typical flight relight envelope.

pellet to provide a low resistance path for the energy
stored in the capacitor. The discharge takes the form
of a high intensity flashover from the electrode to the
body and only requires a potential difference of
approximately 2000 volts for operation.

26. The normal spark rate of a typical ignition
system is between 60 and 100 sparks per minute.
Periodic replacement of the igniter plug is necessary
Fig. 11-13 An igniter plug. due to the progressive erosion of the igniter
electrodes caused by each discharge.
24. The A.C. ignition unit, shown in fig, 11-12,
receives an alternating current which is passed 27. The igniter plug tip protrudes approximately 0.1
through a transformer and rectifier to charge a inch into the flame tube. During operation the spark
capacitor. When the voltage in the capacitor is equal penetrates a further 0.75 inch. The fuel mixture is
to the breakdown value of a sealed discharge gap, ignited in the relatively stable boundary layer which
the capacitor discharges the energy across the face then propagates throughout the combustion system.
of the igniter plug. Safety and discharge resistors are
fitted as in the trembler-operated unit. RELIGHTING

25. There are two basic types of igniter plug; the 28. The jet engine requires facilities for relighting
constricted or constrained air gap type and the should the flame in the combustion system be extin-
shunted surface discharge type. The air gap type is guished during flight. However, the ability of the
similar in operation to the conventional reciprocating engine to relight will vary according to the altitude
engine spark plug, but has a larger air gap between and forward speed of the aircraft. A typical relight
the electrode and body for the spark to cross. A envelope, showing the flight conditions under which
potential difference of approximately 25,000 volts is an engine will obtain a satisfactory relight, is shown
required to ionize the gap before a spark will occur. in fig. 11-14. Within the limits of the envelope, the
This high voltage requires very good insulation airflow through the engine will rotate the compressor
throughout the circuit. The surface discharge igniter at a speed satisfactory for relighting; all that is
plug (fig. 11-13) has the end of the insulator formed required therefore, provided that a fuel supply is
by a semi-conducting pellet which permits an available, is the operation of the ignition system. This
electrical leakage from the central high tension is provided for by a separate switch that operates
electrode to the body. This ionizes the surface of the only the ignition system.

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