Download - Basic Gas Turbines - Starting
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The Gas Turbine Engine: Starting Systems
Although the Gas Turbine Engine is one of the most complicated machines
ever engineered by man, the concept is actually fairly simple. Air is
drawn into a compressor through an air intake, fuel is burned in acombustion chamber, the hot gas drives the turbine and the exhaust
leaves the engine through a nozzle. The turbine, which is directly coupled
to the compressor, provides the power to compress the air; the gas
leaving the nozzle provides the thrust to propel the aircraft. The Jet
Engine works in a self-sustaining cycle; to stop it, the fuel supply is simply
cut. Interestingly, when the engine starts, there is a minimal rotational
speed to achieve so that the turbine can power the compressor.
The starting procedure is basically always the same: a source of powerprovides the high torque needed to rotate the compressor and the
turbine up to a speed at which adequate air passes into the combustion
system. There are several types of starting systems and they are used in
accordance with engine and aircraft requirements.
The Starting Cycle.
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Starting Procedure (extract from a three crew B727)
Starting Methods
Electric: This method consists in driving the engine with a direct current
(D.C.) electric motor. It is coupled to the engine through a reduction gear
and ratchet mechanism, or clutch, which automatically disengages afterthe engine has reached the self-sustaining speed.
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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.
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Cartridge: This starting system is usually used on military engines since it
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 reductiongear and an automatic
disconnect mechanism
to rotate the engine.
Iso-propyl-nitrate: This
starting system provides a
high power output and gives
rapid starting
characteristics. It is the
liquid fuel version of theCartridge one. In this
instance, high-pressure gases,
resulting from the
combustion of iso-propyl-
nitrate, rotate the turbine.
The fuel is sprayed into a
combustion chamber, which
forms part of the starter,
where it is electrically ignitedby a high-energy 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.
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Air: This starting system is light, simple and economical to operate so it
is used on most commercial and some military jet engines. Like the other
systems, the air starter drives a turbine, which transmits power to the
engine through a reduction gear and a clutch. But, unlike the other
systems, the energy used by the starter is external: indeed the turbine isrotated by air taken from a ground supply, an auxiliary power unit (A.P.U.)
or as cross-feed from a running engine. When an external supply of air is
not available, a combustor starter is fitted to the engine to provide the
air required by the air starter. This 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. The fuel/air
mixture is ignited in the combustion chamber and the resultant gas is
directed onto the turbine of the air starter. However, some turbo-jet
engines are not fitted with starter motors at all, but direct the airobtained from an external source, or from an engine that is running,
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Gas turbine: This 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
which drives the engine. To initiate this starter, it is fitted with its own
starter electric motor.
These are the most used methods of starting Gas Turbine Engines. Each
has its own merits and is appropriate to a particular engine and aircraft in
a particular situation.