After the spark gap or solid-state switch releases electrical energy from the storage capacitor, additional output circuitry in the exciter transforms the electrical energy into a discharge waveform. The exciter discharge is conducted through the ignition lead to the igniter plug where a spark is created that ignites the fuel/air mixture. Because engine combustor designs vary greatly, exciter discharge circuitry can be very complex. Depending on the application, exciter discharge voltages can range from 2,000 volts to over 20,000 volts, discharge current can be unidirectional or oscillatory, and even the shape of the discharge waveform can be modified and controlled.
High- and Low-Tension Ignition Systems
High-tension systems capable of delivering more than 8,000 volts per spark are used with surface-gap igniter plugs on most large commercial aircraft. This type of ignition system has the advantage of surface-gap igniter plug that naturally ceases to fire at high combustor pressures thereby reducing igniter tip erosion. Although the service life of high-tension igniter plugs can be as long as 3,500 engine hours, there is a size and weight trade off required to generate and deliver the firestorm of voltage a high-tension ignition system is capable of transferring. For this reason, some engine manufacturers prefer lighter and smaller low-tension ignition systems.
Low-tension ignition systems — deliver less than 8,000 volts to the engine's combustor. While there is the benefit of lighter weight and re-atively compact size, the voltage produced by these systems is usually not adequate to ionize a normal surface-gap igniter plug, there-fore, semiconductor plug technology is required. These plugs use a semiconductor material to initiate ionization between the igniter plug tip electrodes, resulting in the formation of a spark at a much lower voltage than in a non-semiconductor igniter plug. Low-tension igniter plugs typically do not quench at high combustion pressures requiring the pilot or the digital engine control to turn the ignition system off to prevent premature igniter plug wear. Typical low-tension igniter plug life is normally less than 500 hours.
Troubleshooting Turbine Ignition
Procedures for troubleshooting an ignition system on-wing are as variable as aircraft engines themselves. Some OEM manufacturers offer on-wing test equipment that prevents unnecessary removal and recertification of healthy ignition systems, nonetheless, several of the following techniques are reliable procedures that should be incorporated into the technician's repertoire. Both high- and low-tension ignition systems should be discharged prior to routine inspection of any ignition system component. Most exciters incorporate a bleed-down resistor that will quickly dissipate any residual energy stored in the capacitor, but, as a second fail-safe, it is best to power down the system for a minimum of three minutes before beginning any system checks. Typically, it is not necessary to ground the system prior to performingcomponent inspections; however, such information should be verified against the most current version of an engine's component maintenance manual.
Although an audible check of the ignition system will not provide concrete substantiation of the condition of its components, this type of assessment can serve as a rudimentary tool in determining the likelihood of an ignition system malfunction. Unfortunately, a well-trained ear is necessary to distinguish the sound of a healthy ignition system from a functioning system that requires repair. Fully operational high- and low-tension igniter plugs are characterized by a snapping sound that is audible through an aircraft's exhaust duct.
The majority of engines are equipped with two plugs, and some dual-channel exciters fire these plugs simultaneously. As a result, it is possible to confuse the snapping sound of one operational plug for a healthy system though the second circuit is inoperable. If the igniter plugs fail to fire altogether, input power to the exciter should be verified using a digital multimeter. Most exciter data plates display input voltage, ampere requirements, and input connector polarity; however, an engine's maintenance manual is the most reliable source of specific data related to input power configuration. If the circuit breaker trips each time the ignition system is turned on, it is likely that the exciter has shorted and is drawing excessive current, or the airframe wiring harness has shorted to ground.
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