A turbine engine ignition system uses a capacitor discharge (CD) exciter unit to fire the igniter plugs. Which statement best describes how this system produces the high-energy spark required for turbine ignition?
- AA rotating magnet continuously induces alternating current directly into the igniter plug without any intermediate storage
- BA transformer-rectifier charges a storage capacitor, which is then discharged through a trigger transformer to the igniter plug in a single high-energy pulse
- CA set of mechanical breaker points interrupts current flow to produce a continuous stream of low-energy sparks
- DAn impulse coupling mechanically advances the spark timing at low engine speeds
Show answer & explanationAnswer & explanation
Correct answer: B. A transformer-rectifier charges a storage capacitor, which is then discharged through a trigger transformer to the igniter plug in a single high-energy pulse
In a capacitor discharge ignition (CDI) exciter unit, input power (AC or DC) is stepped up and rectified to charge a storage capacitor to a high voltage; when a triggering circuit fires, the capacitor rapidly discharges through a trigger transformer, delivering a very high-energy, high-voltage pulse to the igniter plug. This design is used on turbine engines because it produces a much higher energy spark than a magneto, necessary to ignite fuel reliably under adverse conditions such as altitude, cold, or wet starts. Magnetos (with rotating magnets and breaker points) and impulse couplings are reciprocating engine ignition components, not part of turbine CD ignition systems.
Why the other options are wrong
- A. This describes a magneto, not a capacitor discharge system, and lacks the required energy storage.
- C. Breaker points and continuous low-energy sparks describe older reciprocating ignition, not turbine CD systems.
- D. Impulse couplings are reciprocating engine magneto accessories, not part of turbine ignition exciters.
Capacitor Discharge (CD) Ignition Exciter
A turbine engine ignition unit that charges a storage capacitor to high voltage and rapidly discharges it through a trigger transformer to produce a high-energy spark at the igniter plug.
- Produces much higher spark energy than reciprocating magnetos
- Used because turbine fuel ignition needs a strong, reliable spark
- Capacitor charges continuously and discharges intermittently at set intervals
- Only used during starting and, in some cases, continuous ignition for icing/turbulence
Memory trick: Charge the capacitor, then unleash a lightning bolt