How does a magneto work?

A magneto is a kind of electrical generator used in small gasoline engines in lieu of a battery. Find out how magnetos work in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:00):
Welcome to Brainstuff from how stuff Works dot com where
smart happens him Marshall Brain with today's question, how does
a magneto work? Most small lawnmowers, chainsaws, string trimmers, and

(00:22):
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 session of one fifty two seen in the article
how Airplanes Works uses magnetos because magnetos are incredibly reliable.

(00:47):
The idea behind any ignition system is to generate an
extremely high voltage on the order of twenty thousand volts
at exactly the right time. The voltage causes a spark
to jump across the spark plug 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

(01:10):
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

(01:32):
the electromagnets 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. A
magneto consists of five parts. It has an armature and

(01:53):
a primary coil of wire that's wrapped around that armature.
It might be two d turns of thick wire wrapped
around the piece of iron. Then it has a secondary
coil of perhaps twenty thousand turns of very thin wire
wrapped around that primary coil. And then it has a
simple electrical control unit that commonly goes by the name

(02:16):
electronic ignition. That's a set of breaker points and a capacitor,
and it has a pair of strong permanent magnets embedded
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

(02:40):
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 perhaps two hundred volts.

(03:02):
The secondary coil, having a hundred times more turns than
the primary coil, amplifies this voltage to approximately twenty thousand volts,
and this voltage feeds to the spark plug. Many writing
lawnmowers do have a battery if they have accessories like
headlights and electric starting. Even so, the engine may still

(03:24):
use a magneto because the magneto is so simple and reliable.
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