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11-5.
Air
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 electrically
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
Starting and ignition
124
Fig. 11-4 A triple-breech cartridge starter.
Starting and ignition
125
Fig. 11-5 An iso-propyl-nitrate starting system.
rotation of the starter ceases. A typical air starting
system is shown in fig. 11-7.
12. A combustor starter is sometimes fitted to an
engine incorporating an air starter and is used to
supply power to the starter when an external supply
of air is not available. The starter unit has a small
combustion chamber into which high pressure air,
from an aircraft-mounted storage bottle, and fuel,
from the engine fuel system, are introduced. Control
valves regulate the air supply which pressurizes a
fuel accumulator to give sufficient fuel pressure for
atomization and also activates the continuous
ignition system. The fuel/air mixture is ignited in the
combustion chamber and the resultant gas is
directed onto the turbine of the air starter. An
electrical circuit is provided to shut off the air supply
which in turn terminates the fuel and ignition systems
on completion of the starting cycle.
13. Some turbo-jet engines are not fitted with starter
motors, but use air impingement onto the turbine
blades as a means of rotating the engine. The air is
obtained from an external source, or from an engine
that is running, and is directed through non-return
valves and nozzles onto the turbine blades. A typical
method of air impingement starting is shown in fig.
11-8.
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.
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
starter is rotated by its own starter motor until it
reaches self-sustaining speed, when the starting and
ignition systems are automatically switched off.
Acceleration then continues up to a controlled speed
of approximately 60,000 r.p.m. At the same time as
the gas turbine starter engine is accelerating, the
exhaust gas is being directed, via nozzle guide
vanes, onto the free-power turbine to provide the
drive to the main engine. Once the main engine
reaches self-sustaining speed, a cut-out switch
Starting and ignition
126
Fig. 11-6 An air starter motor.
operates and shuts down the gas turbine starter. As
the starter runs down, the clutch automatically
disengages from the output shaft and the main
engine accelerates up to idling r.p.m. under its own
power.
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
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