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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 simultaneously,
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 automatically
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.
11: Starting and ignition
Contents Page
Introduction 121
Methods of starting 122
Electric
Cartridge
Iso-propyl-nitrate
Air
Gas turbine
Hydraulic
Ignition 127
Relighting 131
121
Fig. 11-1 A typical starting sequence of a
turbo-jet engine.
METHODS OF STARTING
3. The starting procedure for all jet engines is
basically the same, but can be achieved by various
methods. The type and power source for the starter
varies in accordance with engine and aircraft requirements.
Some use electrical power, others use gas,
air or hydraulic pressure, and each has its own
merits. For example, a military aircraft requires the
engine to be started in the minimum time and, when
possible, to be completely independent of external
equipment. A commercial aircraft, however, requires
the engine to be started with the minimum
disturbance to the passengers and by the most
economical means. Whichever system is used,
reliability is of prime importance.
4. The starter motor must produce a high torque
and transmit it to the engine rotating assembly in a
manner that provides smooth acceleration from rest
up to a speed at which the gas flow through the
engine provides sufficient power for the engine
turbine to take over.
Electric
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 progressively
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
122
Fig. 11-2 An electric starter.
Starting and ignition
123
Fig. 11-3 A low voltage electrical starting system.
Cartridge
7. Cartridge starting is sometimes used on military
engines and provides a quick independent method of
starting. The starter motor is basically a small
impulse-type turbine that is driven by high velocity
gases from a burning cartridge. The power output of
the turbine is passed through a reduction gear and
an automatic disconnect mechanism to rotate the
engine. An electrically fired detonator initiates the
burning of the cartridge charge. As a cordite charge
provides the power supply for this type of starter, the
size of the charge required may well limit the use of
the cartridge starters. A triple-breech starter is
illustrated in fig. 11-4.
Iso-propyl-nitrate
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 highenergy
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.
　
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