Starting and ignition - IGNITION

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IGNITION
18. High-energy (H.E.) ignition is used for starting all jet engines and a dual system is always fitted. Each system has an ignition unit connected to its own igniter plug, the two plugs being situated in different positions in the combustion system.
19. Each H.E. ignition unit receives a low voltage supply, controlled by the starting system electrical circuit,  from  the  aircraft  electrical  system.  The electrical energy is stored in the unit until, at a pre- determined value, the energy is dissipated as a high voltage, high amperage discharge across the igniter plug.
20. Ignition units are rated in 'joules' (one joule equals one watt per second). They are designed to give outputs which may vary according to require- ments.  A high value output (e.g. twelve joule) is necessary to ensure that the engine will obtain a sat- isfactory relight at high altitudes and is sometimes necessary for starting. However, under certain flight conditions, such as icing or take-off in heavy rain or snow, it may be necessary to have the ignition system continuously operating to give an automatic relight  should  flame  extinction  occur.  For  this condition, a low value output (e.g. three to six joule) is preferred because it results in a longer life of the igniter plug and ignition unit. Consequently, to suit all engine operating conditions, a combined system giving a high and low value output is favoured. Such a system would consist of one unit emitting a high output to one igniter plug, and a second unit giving a low output to a second igniter plug. However, some ignition units are capable o! supplying both high and  low  outputs,  the  value  being  pre-selected  asrequired.

21. An ignition unit may be supplied with direct current  (D.C.)  and  operated  by  a  trembler mechanism  or  a  transistor  chopper  circuit,  or supplied with alternating current (A.C.) and operated by a transformer. The operation of each type of unit is described in the subsequent paragraphs.

Fig. 11-10 A D.C. trembler-operated ignition unit.

22. The ignition unit shown in fig. 11-10 is atypical D.C.  trembler-operated  unit.  An  induction  coil, operated by the trembler mechanism, charges the reservoir  capacitor  (condenser)  through  a  high voltage rectifier. When the voltage in the capacitor is equal to the breakdown value of a sealed discharge gap, the energy is discharged across the face of the igniter plug. A choke is fitted to extend the duration of the discharge and a discharge resistor is fitted to ensure  that  any  residual  stored  energy  in  the capacitor is dissipated within one minute of the system being switched off. A safety resistor is fitted to enable the unit to operate safely, even when the high tension lead is disconnected and isolated.



23. Operation of the transistorized ignition unit is similar to that of the D.C. trembler-operated unit, except  that  the  trembler-unit  is  replaced  by  a transistor chopper circuit. A typical transistorized unit is  shown  in  fig.  11-11;
 such  a  unit  has  many advantages over the trembler-operated unit because it has no moving parts and gives a much longer operating life.  The size of the transistorized unit is reduced and its weight is less than that of the trembler-operated unit.

Fig. 11-11 A transistorized ignition unit.
and
Fig. 11-12 An A.C. ignition unit.

24. The  A.C. ignition unit, shown in fig, 11-12, receives an alternating current which is passed through a transformer and rectifier to charge a capacitor. When the voltage in the capacitor is equal to the breakdown value of a sealed discharge gap, the capacitor discharges the energy across the face  of the igniter plug. Safety and discharge resistors arefitted as in the trembler-operated unit.

Fig. 11-13 An igniter plug.

25. There are two basic types of igniter plug; the constricted or constrained air gap type and the shunted surface discharge type. The air gap type is similar in operation to the conventional reciprocating engine spark plug, but has a larger air gap between the electrode and body for the spark to cross.  A potential difference of approximately 25,000 volts is required to ionize the gap before a spark will occur. This  high  voltage  requires  very  good  insulation throughout the circuit. The surface discharge igniter plug (fig. 11-13) has the end of the insulator formed by  a  semi-conducting  pellet  which  permits  an electrical  leakage  from  the  central  high  tension electrode to the body.



This ionizes the surface of the 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 due  to  the  progressive  erosion  of  the  igniter electrodes caused by each discharge.
27. The igniter plug tip protrudes approximately 0.1 inch into the flame tube. During operation the spark penetrates a further 0.75 inch.  The fuel mixture is ignited in the relatively stable boundary layer which then propagates throughout the combustion system.