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Many small gasoline-powered tools like lawn mowers, chainsaws, trimmers, and similar engines operate without a battery. Instead, they produce the necessary spark for ignition using a magneto. This same technology is found in various small aircraft, such as the Cessna 152, as detailed in How Airplanes Work, due to its exceptional reliability.
The core principle of any ignition system is to produce a very high voltage — typically around 20,000 volts — at precisely the right moment. This high voltage triggers a spark that jumps across the gap of the spark plug, which then ignites the fuel within the engine. More information can be found in How Car Engines Work or How Two-Stroke Engines Work.
The magneto is the white component shown in the following image (it is the magneto for a chainsaw):
A magneto operates on a straightforward concept. Essentially, it functions as an electrical generator that is calibrated to emit a periodic high-voltage pulse, as opposed to continuous current. An electrical generator (or magneto) works in the reverse manner of an electromagnet (see How Electromagnets Work). In an electromagnet, a coil of wire is wrapped around an iron bar (called the armature). When current is applied to the electromagnet's coil (e.g. through a battery), it generates a magnetic field in the armature. In a generator, the process is inverted — a magnet is moved past the armature to generate an electric current in the coil.
A magneto is made up of five key components:
- An armature, which in this case is shaped like the letter "U", with the arms of the U positioned toward the flywheel.
- A primary coil, typically consisting of around 200 thick wire turns, wrapped around one leg of the U-shaped armature.
- A secondary coil, containing approximately 20,000 very thin wire turns, wrapped around the primary coil.
- An electronic control unit, often referred to as "electronic ignition" (alternatively, it could be a set of breaker points and a capacitor).
- A pair of robust permanent magnets, embedded within the flywheel of the engine.
Here is an image of the two magnets used in this system:
As the magnets move past the U-shaped armature, they create a magnetic field within it. This magnetic field induces a small current in both the primary and secondary coils. However, to achieve the required high voltage, when the magnetic field in the armature peaks, a switch in the electronic control unit opens. This break in current flow through the primary coil creates a voltage spike (around 200 volts). Since the secondary coil has 100 times more turns than the primary coil, this voltage is amplified to about 20,000 volts, which is then sent to the spark plug.
Many riding lawn mowers are equipped with a battery, especially if they feature accessories such as headlights or electric starting. Nevertheless, the engine might still use a magneto due to its simplicity and reliability.
You can explore more about magnetos through the following resources:
- Magnetos - an excellent introductory resource
- Old engine magnetos and ignition systems
- BMW Ignition systems
- Magneto and spark plugs
- Ignitions
- Lawn Mower Questions and Answers
- Spark coil
