Starting and ignition - RELIGHTING

want to read about : Starting and ignition - IGNITION


RELIGHTING

Fig. 11-14 A typical flight relight envelope.

28. The jet engine requires facilities for relighting should the flame in the combustion system be extin- guished during flight. However, the ability of the engine to relight will vary according to the altitude and forward

Starting and ignition - IGNITION

want to read about : Starting and ignition - Hydraulic


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;

Starting and ignition - Hydraulic

want to read about : Starting and ignition - Gas turbine


Hydraulic
17. Hydraulic starting is used for starling some small jet engines. In most applications, one of the engine-mounted hydraulic pumps is utilized and is known as a pump/starter, although other applications may use a separate hydraulic motor. Methods of transmitting the torque to the engine may vary, but a typical system would include a reduction gear and clutch assembly. Power to rotate the pump/starter is provided by hydraulic pressure from a ground supply unit and is transmitted to the engine through the reduction gear and clutch.  The starting system is controlled by an electrical circuit that also operates hydraulic valves so that on completion of the starting  cycle  the  pump  /starter  functions  as  a  normalhydraulic pump.

Starting and ignition - Gas turbine

want to read about : Starting and ignition - Air
Gas turbine
14. A gas turbine starter is used for some jet engines and is completely self-contained. It has its own fuel and ignition system, starting system (usually electric or hydraulic) and self-contained oil system. This type of starter is economical to operate and provides a high power output for a comparatively low weight.

Fig. 11-9 A gas turbine starter.

15. The starter consists of a small, compact gas turbine engine, usually featuring a turbine-driven centrifugal compressor, a reverse flow combustion system and a mechanically independent |free-power turbine. The free-power turbine is connected to the main engine via a two-stage epicyclic reduction gear, automatic clutch and output shaft.  A typical gas turbine starter is shown in fig. 11-9.





16. On initiation of the starting cycle, the gas turbine

Starting and ignition - Air

want to read about : Starting and ignition - Iso-propyl-nitrate
Air

Fig. 11-6 An air starter motor.

9. Air starting is used on most commercial and some military jet engines. It has many advantages over other starting systems, and is comparatively light, simple and economical to operate. 10. An air starter motor transmits power through a reduction gear and clutch to the starter output shaft which is connected to the engine. A typical air starter motor is shown in fig. 11-6.



11. The starter turbine is rotated by air taken from an external ground supply, an auxiliary power unit (A.P.U.) or as a cross-feed from a running engine. The air supply to the starter is controlled by an elec- trically operated control and pressure reducing valve that is opened when an engine start is selected and is automatically closed at a predetermined starter speed. The clutch also automatically disengages as the engine accelerates up to idling r.p.m. and the rotation of the starter ceases. A typical air starting system is shown in fig. 11-7.

Starting and ignition - Iso-propyl-nitrate

want to read about : Starting and ignition - Cartridge


Iso-propyl-nitrate

Fig. 11-5 An iso-propyl-nitrate starting system.

8. This type of starter provides a high power output and gives rapid starting characteristics. It has a turbine that transmits power through a reduction gear to the engine. In this instance, the turbine is rotated by  high  pressure  gases  resulting  from  the combustion of iso-propyl-nitrate. This fuel is sprayed into a combustion chamber, which forms part of the starter, where it is electrically ignited by a high- energy ignition system. A pump supplies the fuel to the combustion chamber from a storage tank and an air pump scavenges the starter combustion chamber of fumes before each start. Operation of the fuel and air pumps, ignition systems, and cycle cancellation, is electrically controlled by relays and time switches. An iso-propyl-nitrate starting system is shown in fig.11-5.

Starting and ignition - Cartridge

want to read about Starting and ignition - Electric


Cartridge

Fig. 11-3 A low voltage electrical starting system.

7. Cartridge starting is sometimes used on military engines and provides a quick independent method of starting.

Starting and ignition - Electric

want to read about :Starting and ignition - INTRODUCTION

Electric

Fig. 11-2 An electric starter

5. The electric starter is usually a direct current (D.C.) electric motor coupled to the engine through a reduction gear and ratchet mechanism, or clutch, which automatically disengages after the engine has reached a self-sustaining speed (fig. 11-2).
6. The electrical supply may be of a high or low voltage and is passed through a system of relays and resistances to allow the full voltage to be progres- sively built up as the starter gains speed. It also provides the power for the operation of the ignition system.  The  electrical  supply  is  automatically cancelled when the starter load is reduced after the engine has satisfactorily started or when the time cycle  is  completed.  A typical  electrical  starting system is shown in fig. 11-3.

Starting and ignition - INTRODUCTION

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.

METHODS OF STARTING

Fig. 11-1 A typical starting sequence of aturbo-jet engine.