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hydraulic valves so that on completion of the starting
cycle the pump /starter functions as a normal
hydraulic pump.
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 predetermined
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
Starting and ignition
127
Fig. 11-7 An air starting system.
give outputs which may vary according to requirements.
A high value output (e.g. twelve joule) is
necessary to ensure that the engine will obtain a satisfactory
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 as
required.
Starting and ignition
128
Fig. 11-8 Air impingement starting.
Fig. 11-9 A gas turbine starter.
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.
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.
Starting and ignition
129
Fig. 11-10 A D.C. trembler-operated ignition unit.
Starting and ignition
130
Fig. 11-11 A transistorized ignition unit.
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 are
fitted as in the trembler-operated unit.
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.
　
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