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 TM 5-685/NAVFAC MO-912
INSERT
COVER
FRONT
NOSE HOLES
STAGE
Figure 3-26. Turbine vane cooling air f l o w .
(4) Scavenging. Scavenging is accomplished by
b. Fuel. As the engine is accelerated by the
a multi-element lubrication and scavenge pump.
starter, fuel is supplied when a specified rotational
One element is used for pumping. The other ele-
speed is attained. When this speed is attained, the
ments are used for forward and aft scavenging of
compressor and engine-driven fuel pump will de-
the B-sump and C-sump. Oil in the A-sump drains
liver sufficient air and fuel, respectively to the com-
by gravity into the accessory gearbox.
bustion chamber to sustain satisfactory combustion.
(5) Venting. S ome lubrication systems are
c. Ignition system. An ignition system, consisting
vented. To maintain high differential pressure
across the carbon seals to prevent oil leakage, a
and igniter plugs, is required. Fuel ignition is en-
high sump vent capacity is required. The A and C
sured by one or two igniter plugs connected to the
sumps vent through the engine output shaft and
exciter by the separate igniter leads. The plugs are
vent collector to ambient. The B-sump vents to the
located in the combustion chamber. Each plug con-
turbine exhaust gas stream.
sists of center and outer electrodes with a semicon-
ductor surface coating at the tip between the two
3-19. Starting system.
electrodes. The semiconductor ma.terial is used as a
Gas turbine engine starters must be capable of ro-
shunt to aid in ionizing the air gap between the two
tating an engine up to a speed-at which it becomes
electrodes so that the plugs will fire. An air shroud
self-sustaining. The starter must provide sufficient
covers the end of the plug immersed in the air
torque to accelerate the engine from a standstill to a
stream for cooling.
self-sustaining speed within a specified time. Al-
d. Specialized system. Starting systems are
though it must continue to assist the engine in ac-
highly specialized and are usually applicable to a
celerating up to a predetermined speed.
given i nstallation or site. Refer to supplier's on-site
a. Electric motor. An electric starter motor is usu-
technical literature for details.
ally used for a gas turbine engine in service as an
3-20. Governor/speed control.
auxiliary generator prime mover. The starter ro-
tates the engine compressor shaft via the gear train
a. Engine operation. The engine is started by an
in the accessory gearbox. In most installations the
external power source. Once the engine reaches idle
starter can be energized either automatically or
speed, it is self-sustaining. All it needs is adequate
manually.
supplies of air and fuel. Combustion gas drives the
3-35
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