Turbine Ignition Maintenance
Most aircraft engines rely on an electrical ignition system to create a spark, which in turn initiates or continues the engine's combustion process. Many laypersons and technicians are familiar with ignition systems used on internal combustion engines for automobiles and general aviation aircraft; however, ignition systems for gas turbine engines are probably the least understood and discussed. of all ignition systems. Compared to automotive systems, the spark discharge of a turbine ignition system is much larger, and, in some cases, the spark is over 100 times more powerful than the spark in an automobile engine.
The function of gas turbine ignition systems is to convert energy from some source to a high intensity electrical discharge (i.e., sparks). One good spark will initiate combustion of the fuel/air mixture in a turbine engine's combustor, however, several sparks are usually provided to ensure fast and reliable engine starts. Once the turbine engine is lit, the ignition system's job is finished unless the engine needs to be relit during an in-flight shutdown, or until the next operational cycle of the aircraft's turbine main engines or auxiliary power unit (APU) begins again.
Although the basic elements of every gas turbine ignition system are the same, even the sophisticated observer has trouble delineating amongst the various types. Actual performance, design, types and the physical appearances of most turbine-powered aircraft ignition systems differ from application to application. Engine operational requirements, combustor designs and performance parameters, operating environments, mounting considerations, FAA requirements, and differing design philosophies associated with providing reliable ignition are just a few of the reasons for the many models of turbine engine ignition systems in service today.
Though more reliable, today's turbine ignition systems are also more complex, requiring that aircraft technicians possess a higher degree of knowledge and understanding of turbine ignition system theory. Familiarity with a few basics will enable technicians to develop proper insight into standard operation, troubleshooting, and maintenance practices.
Turbine Ignition Systems Overview
The typical gas turbine engine ignition system consists of an exciter or exciters, an ignition lead or set of ignition leads, and an igniter plug or set of igniter plugs. Furthermore, the different types of ignition systems can be broken down into two categories: low-tension ignition systems and high-tension ignition systems.
The heart of every turbine ignition system —both high and low tension — is the exciter, an engine- or airframe-mounted hermetically-sealed electrical box. The exciter uses input power from an airframe electrical bus or an engine-mounted generator to convert relatively low voltage to more useable, high-voltage energy pulses that fire an igniter plug and in turn ignite the fuel/air mixture. In order to deliver a high-energy discharge given the relatively low input power, a capacitor is charged up, and then all of the energy stored in the capacitor is released at once.
Conventional exciters use a spark gap as a switch to discharge the capacitor after it reaches a predetermined voltage level. An alternative to the older yet ever popular spark-gap-switch technology is the solid-state exciter. Invented and patented by Unison Industries, a solid-state exciter uses an electronic solid-state switch to regulate the energy discharge from the storage capacitor, resulting in precise control of discharge voltage, energy, and spark rate. For instance, a solid-state exciter can be designed to deliver a high spark rate during the engine starting cycle, then to reset itself to a lower spark rate during normal engine operation. This multi-mode capability ensures that sufficient energy is delivered to the air/fuel mixture to start the engine, and reduces wear and tear of the igniter plugs during normal engine operation.
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