Engine Ignition System
After the starter spun up the compressor to an rpm that provides an engine with sufficient air for combustion the main ignition system is activated. Ignition within an engine gas turbine is established by using a high-energy (HE) unit. Attached to the HE unit is an igniter plug that will arc to ignite the fuel/air mixture in the combustion chamber. Normally there are two ignition plugs installed that can operate independently or together. Selection on which igniter plug should be used depends on the position of the ignition selector switch. Ignition is no longer needed after a normal engine start and so the ignition system is deactivated. During specific flight conditions, the ignition system is required to provide a standby protection against in-flight flameout.

High Energy Unit (HE Unit)
The high energy or exciter unit, is powered by the aircraft electrical system, either AC or DC. The electrical energy received is stored until it is dissipated as a high voltage, high amperage discharge at a predetermined value through the igniter plug. An ignition unit carries a joule rating. Joules are defined as power (Watt) multiplied by time (Seconds). This means one Joule equals one watt. The amount of Joules output greatly depends on engine requirements and may vary between different types of engines. A high Joule rating may be necessary for engine start-up or relighting at high altitudes after a flameout occurred. Joule output can be mathematically found as well as why the ignition charge is lethal by the following equation. For example, a turbine engine ignition system is able to store 2.500 VDC with an ionizing voltage of 700 VDC at the igniter plug. The current in the system is 220 amps and the plug arcs in 40 millionths of a second. The Joule rating in this case would be (table 1.1) :

Table 1.1 HE Unit Example Calculation
Watts
=
Volts * Amps
Watts
=
700 * 220
Watts
=
154.000
Joules
=
Watts * Time
Joules
=
154.000 * 0,000040
Joules
=
6,16

The Joule rating calculated above would be similar to those measured in business jet engines. Large turbofan engines, like the CFM56, have Joule ratings between 14,5 and 16 Joules. The ignition exciter is therefore capable of producing a 14,000 to 18,000 volt DC output at a rate of approximately 1 pulse per second.

Igniter Plug
There are two types of igniter plugs; the constricted air gap type commonly used on gas turbine engines and the shunted surface discharge type. As the name implies, the air gap type has a larger gap between the body of the igniter and the electrode. This means that a voltage difference of approximately 25.000 Volts is required to ionize the gap before a spark will occur. Insulation throughout the circuit should therefore be sufficient to withstand these high voltages. The discharge igniter plug also discharges by high intensity flashover from the electrode but only requires a potential difference of approximately 2000 Volts in order to operate.

Spark rate in normal operation lies between 60 and 100 sparks per minute. It is therefore necessary to replace the igniter plugs periodically due to progressive erosion of the igniter electrodes caused by each discharge.

Igniter Selection
Each of the igniter plugs can be selected by means of the ignition selector switch. This switch enables the left, right or both plugs to be activated. The ignition selector switch is used in conjunction with the engine start switches when, for example, engine start-up is initiated. The engine start switch has four positions (table 1.2).

Table 1.2 Igniter Selection
Ground
Both the starter and the selected ignition plugs are energized.
Off
Both the starter and ignition plugs are de-energized, unless an engine flameout occurs, which automatically turns on the ignition.
Continuous
Both the starter and the selected ignition plugs are energized.
Flight
The starter and both igniter plugs are energized regardless of ignition selector switch position.

Flameout Protection
Critical flight conditions like take-off, landing or bad weather operation require the igniter system to serve as back-up against in-flight flameout. This is accomplished by having the ignition system to operate continuously in order to relight automatically should a flameout occur. Using the ignition system in order to relight only requires a low Joule output value from the igniter system. Besides this, lower Joule output results in a longer life of the igniter system and igniter plug. This means that, to suit all engine operation conditions the igniter system should be capable of delivering both high and low Joule outputs. In some cases this means that two separate igniter systems have to be installed, However, there a systems that are able to supply both high and low outputs.
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