How does a magneto work?

by | Nov 15, 2009 08:00 PM ET

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Marshall Brain

Hi, I'm Marshall Brain with today's question, how does a magneto work? Most small lawnmowers, chainsaws, string trimmers, and other small gasoline engines don't need a battery. Instead, they actually generate the power for the spark plug using a magneto. Magnetos are also used on many small airplanes. For example, the Cessna 152 seen in the article How Airplanes Work uses magnetos because magnetos are incredibly reliable. The idea behind any ignition system is to generate an extremely high voltage on the order of 20,000 volts at exactly the right time. The voltage causes a spark to jump across the spark plug's gap and the spark ignites the fuel in the engine.

The idea behind a magneto is incredibly simple. It's basically an electrical generator that has been tuned to create a periodic high voltage pulse rather than continuous current like a normal generator would. An electrical generator, or a magneto, is the reverse of an electromagnet. In an electromagnet, there's a coil of wire around an iron bar called the armature. When you apply current to the electromagnet's coil (i.e. with a battery), the coil creates a magnetic field in the armature.

In a generator, you reverse that process. You move a magnet past the armature and you create an electric current in the coil. The magneto consists of five parts. It has an armature and a primary coil of wire that is wrapped around that armature. It might be 200 turns of thick wire wrapped around the piece of iron. Then it has a secondary coil of perhaps 20,000 turns of very thin wire wrapped around that primary coil. Then it has a simple electrical control unit that commonly goes by the name electronic ignition that's a set of breaker points and a capacitor. And it has a pair of strong permanent magnets imbedded in the engine's flywheel.

When those magnets fly past the armature, they create a magnetic field in the armature. This field induces some small amount of current in the primary, and then the secondary, coil. What we need, however, is extremely high voltage. Therefore, as the magnetic field in the armature reaches its maximum, a switch in the electronic control unit opens. This switch breaks the flow of current through the primary coil and causes a voltage spike of perhaps 200 volts. The secondary coil, having a hundred times more turns than the primary coil, amplifies this voltage to approximately 20,000 volts. And this voltage feeds to the spark plug.

Many riding lawnmowers do have a battery if they have accessories like headlights and electric starting. Even so, the engine may still use a magneto because the magneto is so simple and reliable.

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