Section 3 Engine Systems
190
outside primary source of electricity. They serve as
simple and reliable ignition systems. Basic parts of
a magneto system include:
• Permanent magnets.
• Spark plug.
• Spark plug wire.
• Ignition coil.
• Switching device.
A simplifi ed magneto ignition system is shown
in Figure 10-2. Note that the magnets are mounted
in the fl ywheel and rotate past the coil assembly as
the fl ywheel spins. In Figure 10-2A, the switching
device is closed. As the magnets move past the coil,
current is induced in the coils primary windings.
This current causes a magnetic fi eld to form around
the primary windings. As the engine’s piston nears
TDC on its compression stroke, the switching
device opens and the magnetic fi eld in the primary
windings collapses rapidly, inducing a high-
voltage current in the secondary windings. The high
voltage current travels to the spark plug, where it
arcs across the spark plug gap and ignites the air-
fuel mixture. See Figure 10-2B. An actual magneto
ignition system is much more complex than the
model shown here, but the basic operating principles
are the same.
Volodymyr Krasyuk/Shutterstock.com
Figure 10-1.
The ignition system of a small engine works hard to
produce enough voltage to force electrons to jump the
spark plug gap.
Spark between electrodes
ignites air-fuel mixture
Goodheart-Willcox Publisher
Figure 10-2.
The major parts of this small engine magneto system
are the switching device, coil, flywheel magnets, spark
plug wire, and spark plug.
A
B
Switching
device
closed
Coil assembly
Magnetic field
Flywheel
Magnet
Arc created
at the
spark plug
N
S
N
S
Magnetic
field
collapses
Switching
device
opens
High voltage
is induced
in the
secondary
windings
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