Ignition Unit

2.22.

IGNITION UNIT

Usually, gas turbine engines are equipped with two or more

igniter plugs; however, the smaller engines like the T63 have only

one igniter plug, sometimes called the spark plug. Igniter plugs

serve a purpose similar to the spark plug in a reciprocating engine,

although operation of the ignition system and the igniter plugs is

necessary only for a short period during the engine starting cycle.

On many installations, ignition is initiated simultaneously with the

starter. The ignition cycle takes place several times per second and

continues to operate as long as the ignition switch is on.

The term "high energy" is used in the section to describe the

capacitor type of ignition system. However, the amount of energy

produced is very small. The intense spark is obtained by expending a

small amount of electric energy in a very short time. Energy is the

capacity for doing work. It can be expressed as the product of the

electrical power and time. Gas turbine ignition systems are rated in

joules. The joule is also an expression of electric energy, being

equal to the amount of energy expended in one second by an electric

current of one ampere through a resistance of one ohm. All other

factors being equal, the temperature of the spark is determined by

the power level reached. A hightemperature spark can result from

increasing the energy level, or by shortening the duration of the

spark. Increasing the energy level requires a heavier, more bulky

ignition unit, since the energy delivered to the spark plug is only

about 30 to 40 percent of the total energy stored in the capacitor.

Also the higher the current flow, the higher the erosion rate on the

igniter plug electrodes. Furthermore, much of the spark would be

wasted, because ignition takes place in a matter of microseconds. In

a capacitor discharge ignition system, most of the total energy

available to the igniter plugs is dissipated in 10 to 100

microseconds, with up to 80, 000 watts with a spark duration of 50

microseconds. Figure 2.15 shows a wiring schematic of a typical

ignition unit.

WARNING:

When working around the ignition unit of the

engine, disconnect the input lead to the

ignition exciter unit. Remove the igniter

plugs from the combustion chamber and ground

them to the engine. You do this to

dissipate any charge that might be left in

the exciter unit.

Some ignition exciter units contain a

very small amount of radioactive material

(cesiumbarium 137) and normally require no

handling