Piston Engine Ignition Circuits: Magnetos and Impulse Coupling

021-10-07Pablo Asensio Martínez2026-03-272 min

A magneto is a self-contained, engine-driven electrical generator designed to supply high voltage to the spark plugs. It does not require an external battery to operate, making it a reliable, independent ignition source for aircraft engines.

Construction and Operation

The magneto consists of a rotating permanent magnet and two windings:

  • Primary Winding: A few turns of thick wire (Low Tension).
  • Secondary Winding: Many turns of thin wire (High Tension).

Working Principle

  1. As the magnet rotates, it induces current in the primary winding.
  2. A contact breaker (cam-operated) interrupts the primary circuit.
  3. This interruption causes a rapid collapse of the magnetic field.
  4. The collapsing field induces a very high voltage in the secondary winding.
  5. A distributor directs this high voltage pulse to the correct spark plug via high-tension leads.

Impulse Coupling

At low engine speeds (during start), a magneto rotates too slowly to produce a strong spark. An Impulse Coupling is used to solve this:

  • Mechanism: A spring-loaded clutch between the engine drive and magneto shaft.
  • Function 1 (Retard): It delays the spark to ensure ignition happens after Top Dead Center (preventing kickback/backfire).
  • Function 2 (Accelerate): It winds up the spring and releases it rapidly, spinning the magneto fast enough to generate a strong spark for starting.
  • After the engine starts, centrifugal force disengages the coupling.

Ignition Switch and Grounding

Reference is often made to the "Mag Check" or "Dead Cut" check.

  • Switch OFF: The ignition switch works by grounding (earthing) the primary circuit. This stops the high voltage generation.
  • Switch ON: The ground connection is broken, allowing the points to control the circuit.
  • Broken Ground Wire: If the ground wire breaks, the magneto is permanently ON ("Hot"). The engine will not stop when the switch is turned off.
    • Danger: The propeller is live! Moving it could start the engine.
    • Action: If the engine doesn't stop with the key, stop it by cutting the fuel mixture (Idle Cut-Off).