An older reciprocating engine ignition system includes a vibrator (booster/induction vibrator) unit used only during starting. What is the primary function of this vibrator unit?
- ATo advance the ignition timing automatically as engine RPM increases past idle
- BTo synchronize the firing order between the left and right magnetos during normal operation
- CTo interrupt battery current rapidly, producing a series of high-voltage pulses to fire the spark plugs when the magneto is turning too slowly for adequate spark
- DTo reduce spark plug fouling by supplying a continuous low-voltage current during cruise flight
Show answer & explanationAnswer & explanation
Correct answer: C. To interrupt battery current rapidly, producing a series of high-voltage pulses to fire the spark plugs when the magneto is turning too slowly for adequate spark
During cranking, magneto rotor speed is too low to generate an adequate spark. A vibrator (booster coil/induction vibrator) rapidly interrupts battery DC current through a coil, inducing a rapid series of high-voltage pulses that are fed to the distributor and spark plugs (often called a 'shower of sparks') to ensure reliable ignition until the engine reaches a speed at which the magneto alone produces sufficient spark. It is not used for automatic timing advance, continuous cruise operation, or magneto synchronization, all of which are unrelated functions.
Why the other options are wrong
- A. Timing advance during cranking is handled by the impulse coupling, not the vibrator.
- B. Magnetos are timed to each other via engine timing marks, not by a vibrator unit.
- D. The vibrator operates only during starting, not continuously during cruise.
Ignition Vibrator (Booster/Induction Vibrator)
A starting aid that rapidly interrupts battery current to produce a series of high-voltage pulses ('shower of sparks'), compensating for low magneto output at cranking speed.
- Used only during engine starting
- Produces a rapid series of sparks, not a single spark
- Works with battery power, independent of magneto rotor speed
- Often paired with, or superseded by, impulse couplings on modern engines
Memory trick: Vibrator buzzes battery power into a shower of sparks