When does the magneto generate a spark?

The correct answer is that the magneto generates a spark when the primary circuit opens. In a magneto ignition system, the operation relies on the principle of electromagnetic induction. The magneto consists of a rotating magnet and a coil of wire. When the engine is cranked or started, the magnet rotates, creating a magnetic field.

As the magnet continues to turn, the primary circuit—which includes a contact breaker or points—is closed, allowing current to flow through the primary winding of the ignition coil. When the engine reaches a certain point in its rotation, the contact breaker or points open, interrupting the current flow in the primary winding. This sudden interruption generates a high-voltage pulse in the secondary winding of the ignition coil, which is subsequently delivered to the spark plug, resulting in an ignition spark.

Understanding this mechanism emphasizes the significance of the primary circuit's state in the operation of a magneto, making it clear why the spark is generated specifically when the primary circuit opens.