the engine, the ignition is turned on and the fuel
valves are opened. The sequence of the starting
procedure is important. At the time the fuel/air
mixture is ignited, enough airflow must pass
through the engine to support combustion.
After the engine has reached its self-sustaining
or self-accelerating speed, the starter can be
deactivated. If the starter is cut off below the
self-sustaining speed, the engine may decelerate
because it doesn't have enough energy to
overcome its own friction and operating losses.
It may also suffer a "hung start" in which it idles
at a speed so low that it is unable to accelerate
enough to obtain proper operating parameters.
A hung-start engine will overheat because of a
lack of cooling air. The starter must continue to
boost engine speed well above self-sustaining
speed to avoid hot or hung (false) starts, or a
combination of both. In a hot start, the engine
lights off, but because of a lack of adequate
cooling and combustion air, the exhaust gas
temperature exceeds the allowable limit for the
engine.
At the proper points in the starting sequence,
the starter and, usually, the ignition system will
cut off. The higher the rpm before the starter cuts
out, the shorter will be the total time required for
the engine to attain idle rpm. This is because the
engine and the starter are working together.
All GTE starters must be able to produce
enough torque to start the engine properly. The
GTEs must reach a certain minimum idle rate for
a start to be satisfactory. This requires the
torque characteristics of an acceptable starter to
exceed by a good margin the amount of torque
needed to overcome friction.
Figure 1-40.--Spark igniter.
The GTEs use three basic types of starters
and starter systems--electric, hydraulic, and
pneumatic. Pneumatic (air-turbine) starters are
SPARK IGNITER SYSTEM
the most commonly used on all except smaller
engines, which generally use electric starters. Some
Once adequate airflow has been established
marine GTE installations use hydraulic starters.
through the combustion area, fuel can be injected
and the spark igniters start the burning process.
Another type of starter system is the air
The spark igniters are high-voltage electrical spark
impingement system. Bleed air from another GTE
producers powered from the ignition exciter
is used directly in the HP turbine assembly to
circuits.
rotate the GG. Due to the volume of air required,
The ignition exciter derives its input power
the air impingement system is used primarily in
from the ship's service 60-Hz, 115-volt electrical
starting aircraft engines and will not be covered
system. Its function is to produce a high-energy
in any further detail. We will describe the
spark at the spark igniter in the engine. This must
pneumatic starter system in chapter 2 of this
be accomplished with a high degree of reliability
TRAMAN.
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