Aircraft Gas Turbine Engines — Free Aviation Powerplant Training

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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 high-temperature 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.

Simplex Fuel Nozzle
Figure 2.15. Wiring Schematic of 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 (cesium-barium 137) and normally require no handling precautions. However, severely damaged units that have been broken open must be handled with forceps or gloves and disposed of in accordance with AR 755-15.

David L. Heiserman, Editor

Revised: June 06, 2015