Powerful ignition systems are integral to the functioning and efficiency of modern aircraft. Gas turbine engines like those seen on the General Electric CF6, depend on capacitor-type ignition systems for combustion. These systems are powered by a low-voltage DC power supply and utilize the same igniter parts to power the engine’s combustion process.
Because of FAA requirements, a gas turbine engine ignition system is a dual system designed with redundancy in the event that a flight-critical part fails. The main structures you’ll come across in a gas turbine engine ignition system are: two exciter ignition units, two transformers, two intermediate ignition leads, and two high tension leads. A few components that are integral to these structures and ignition system functionality are ignition exciters, storage capacitors, and transformer windings.
A standard capacitor-type ignition system feeds low-voltage DC power, often a 24-volt DC input, to an exciter unit. The power supply encounters a filter just before it reaches the exciter unit to prevent interference with the aircraft avionics system. From here, the power is routed to two locations: a DC-motor that powers the multi-lobe cam and single lobe cam, and a storage capacitor.
The breaker points are actuated by the multi-lobe cam. They ensure that flow of current is one directional and channels the supply the auto transformer winding. Here, a magnetic field is created through the opening and closing of the breaker points. When the breaker is closed, the magnetic field of the transformer collapses, inducing a high-voltage feed. This input is passed through a rectifier, which limits flow to a single direction.
At this point, the storage capacitor receives the input power supply. The storage capacitor is attached to the spark plug igniter through a contactor and a dual winding triggering transformer. Once charge has built up on the capacitor, the contactor closes. Some of the high voltage charge flows through the primary winding of the transformer unit to a trigger capacitor. The rest of the high-voltage current is fed to the secondary transformer, where it ionizes the gap at the spark igniter. The spark igniter is now conductive, and the storage capacitor discharges accumulated energy in tandem with electrical charge from the trigger capacitor. This process allows the spark igniter to provide a series of sparks in a small fraction of time.
Once the engine is started, the ignition system is switched off. Combustion is considered self-sustaining at this point. While some ignition systems are manually operated by the pilot, most are manipulated by a Full Authority Digital Control System (FADEC). The electronic controls will auto-engage a continuous ignition process in the event of specified conditions such as a stall warning or flameout. An air-cooling fan airflow system is integrated within the ignition system to keep it operating safely, especially in the event that continuous ignition is needed.
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