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What is the purpose of an impulse coupling in a magneto?

  1. To supply constant voltage

  2. To produce a low momentary speed

  3. To increase the momentary speed of the magneto

  4. To trigger the starter motor

The correct answer is: To increase the momentary speed of the magneto

The impulse coupling in a magneto serves the specific purpose of increasing the momentary speed of the magneto when the engine is being started. When the engine is cranked, the impulse coupling allows the magneto to turn slowly at first. As the engine cranking continues, the coupling engages and transmits a sudden burst of speed to the magneto. This is critical because the increased speed leads to higher output voltage and more efficient ignition of the fuel-air mixture in the engine's cylinders, ensuring a reliable start. This mechanism is particularly important in aircraft engines where successful ignition at start-up can be challenging due to mechanical resistance and low RPMs. The impulse coupling essentially allows the magneto to generate a strong spark that is created by the rapid movement of the magnets past the coils within the magneto, thus ensuring that the engine fires effectively during initial start-up. The other options do not correctly describe the function of the impulse coupling. Supplying constant voltage is not its purpose, as the coupling is not involved in regulating voltage levels. Producing a low momentary speed contradicts its function of increasing speed, and triggering the starter motor is related to the engine starting mechanism rather than the magneto's operational characteristics.