Magneto Work

How Does a Magneto Work?

Most small lawn mowers, chainsaws, mowers, and other small gasoline engines don't need batteries. Instead, they actually power the spark plugs with magnets. Magnets are also used on many small aircraft (the Cessna 152 seen in How Airplanes Work, for example) because they are extremely reliable.  

The idea behind any ignition system is to generate extremely high voltages - around 20,000 volts - at the right time. The voltage causes the spark to jump through the spark plug gap and the spark to ignite the fuel in the engine.

See How a car engine works or How a two-stroke engine works for more details. The magnet is the white block in the following photo (it's the magnet for the chainsaw): The idea behind a magnet is very simple. It is basically a generator that has been tuned to generate periodic high voltage pulses instead of direct current.

Generators (or magnets) are the opposite of electromagnets (see How electromagnets work for more details). In an electromagnet, there is a coil of wire wound around an iron rod (armature). When you apply current to the coil of an electromagnet (with a battery, for example), the coil creates a magnetic field in the armature.

In a generator, you reverse the process. You move a magnet through the armature to induce current in the coil. 

A magnet consists of five parts: The frame. In the magnet above, the armature is in a capital "U" shape. Both ends of the U are towards the steering wheel. A primary coil perhaps 200 turns thick is wound around one leg of the letter U .A secondary coil of perhaps 20,000 turns of very thin wire wound around the primary coil A simple electronic control unit often referred to as an "electronic ignition" (or a set of circuit breakers and capacitors) A pair of strong permanent magnets is mounted in the flywheel.  

When the magnets pass through the U-shaped armature, they create a magnetic field in the armature. This field induces a small amount of current in the primary and secondary coils. However, what we need is extremely high voltage.

Therefore, when the magnetic field in the armature reaches its maximum, a switch in the electronic control unit opens. This switch interrupts the current flowing through the primary coil and causes a voltage spike (perhaps 200 volts). 

The secondary coil, which has 100 times more turns than the primary coil, amplifies this voltage to about 20,000 volts, and this voltage powers the spark plug.  Many riding lawn mowers have batteries if they have accessories like headlights and electric starters. Even so, motors can use magnets because magnets are simple and reliable.