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Magnetos are devices that generate electricity by using magnets to create high voltage pulses of Alternating Current (AC). Magnetos were the earliest and simplest type of spark ignition. They were used in early automobiles, as well as most other internal combustion engines. Their function was to provide power to the spark plugs, but are now only used in a few devices, including the lawnmower. While some lawnmowers also use a battery to power lights, and other functions, the lawnmower engine still utilises a magneto, due to their simplicity and reliability.
The operation of any internal combustion engine requires a spark, which can be produced in any number of different ways. In an engine using a magneto, the magneto produces high voltage pulses, which power the spark plugs. The voltage arcs across the spark plugs, creating a spark, and igniting the fuel to provide power to the engine.
Magnetos have five separate parts. The first part is a pair of strong permanent magnets that are embedded in the flywheel of the engine. The second is an electric control unit, or a capacitor and a set of breakers. The third part is an armature, around which the coils are wrapped. The fourth and fifth parts of a magneto are the primary coil, wrapped around the armature, and the secondary coil, wrapped around the primary coil.
The process of moving the magnets past the armature creates electricity in the primary and secondary coils. The electrical control unit breaks the flow of electricity through the primary coil. This action amplifies the current moving through the secondary coil, until it is sufficient to power the spark plugs. A lawnmower magneto can produce up to 20,000 volts of electricity.