How Magnetos Work

Electric current generators

By on 19 May 2008, 10:52 GMT
Magnetos are generally used to generate extremely high voltages for small internal combustion engines, such as those equipping chainsaws, small cars and even small airplanes. The engine of the Cessna 152 airplane for example, has an ignition system using a magneto to increase engine reliability. The output voltage generated by a typical magneto peaks around 20,000 volts and is precisely tuned with the operation of the engine so that the spark is given by the spark plug just at the right time to light the combustible mix.

Practically, the magneto is an electric current generator that gives a high-voltage pulse. Magnetos consist of five major components: the armature, the primary coil, the secondary coil, the electronic control unit and a pair of permanent magnets.

The armature is U-shaped and is facing the flywheel of the engine, housing the two permanent magnets. The primary coil, made out of thick wire of about 200 turns, is placed on one of the two arms of the armature. On top of the primary coil comes the secondary coil, made of thin wire of about 20,000 turns. The electronic control unit, also known as electronic ignition, controls the whole operation of the magneto.

How it works

As the flywheel of the engine starts to spin, the permanent magnets embedded on it generate a variable magnetic field, which induces current in the primary and secondary coils. When the magnetic field reaches its highest values in the armature, the electronic control unit breaks the flow of current through the primary coil. This determines a spike in the current intensity, generating a powerful magnetic field which is picked up by the secondary coil, which in turn generates an electric current.

The electric current through the primary coil has a voltage of about 200 volts which is amplified about 100 times by the secondary coil, since it has 100 more turns than the primary one, generating 20,000 volts of current fed directly into the spark plug.

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Image of the flywheel and the two permanent magnets of a magneto
   Image of the flywheel and the two permanent magnets of a magneto