basic gas turbines - starting

8/4/2019 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.

basic gas turbines - starting

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