Chapter 10 Ignition Systems
191
Ignition System
Components
The following sections detail the components
commonly used in small engine ignition systems.
An understanding of the construction and opera-
tion of these individual components will help you
better understand the various systems discussed
later in this chapter.
Ignition Coil
The ignition coil used in a magneto system
operates like a transformer. The coil contains two
separate windings of wire insulated from each other
and wound around a common laminated iron core.
See Figure 10-3. The primary winding is heavy-gage
wire with fewer turns than the secondary winding,
which has many turns of light-gage wire.
When electrical current is passed through the
primary winding, a magnetic fi eld is created around
the iron core. When the current is stopped, the
magnetic fi eld collapses rapidly, cutting through
the secondary windings. This rapid cutting of the
fi eld by the wire in the coil induces high voltage in
the secondary circuit. The high secondary voltage,
in turn, causes a spark to jump the spark plug gap
and ignite the air-fuel mixture.
Spark Plugs
A spark plug is a device inserted into the
combustion chamber of an engine that ignites the
compressed air-fuel mixture. At fi rst glance, an
assortment of spark plugs may look very much
alike. Actually, there are many variations. Using the
correct spark plug for a given engine application
can greatly increase the effi ciency, economy, and
service life of the engine.
Figure 10-4 shows the major parts of a typical
spark plug. The terminal nut is the external contact
with the ignition coil. Some terminal nuts are
removable, others are not. Many of the major parts
of the spark plug are used to identify the actual
type of the plug. Other considerations include
construction, heat rating number, and fi ring
end construction. To learn how to identify spark
plug types, use the spark plug symbols chart in
Figure 10-5.
The spark plug insulator is usually an alumi-
num-oxide ceramic material, which has excellent
insulating properties. The insulator must have
high mechanical strength, good heat conducting
quality, and resistance to heat shock. Generally,
ribs on the insulator extend from the terminal
nut to the shell of the plug to prevent fl ashover.
Flashover is the tendency for current to travel
down the outside of the spark plug instead of
through the center electrode.
The center electrode carries the high voltage
current to the spark gap. If the electrical potential
is great enough to cause the current to jump the
plug gap, the side electrode will complete the
circuit to ground.
The sillment seal is a compacted powder that
helps ensure permanent assembly and eliminates
compression leakage under all operating conditions.
The inside gasket also acts as a seal between the
insulator and the steel shell.
Goodheart-Willcox Publisher
Figure 10-3.
An ignition coil consists of two windings. The coil functions as a step-up transformer to produce high voltage and
low amperage from low voltage and high amperage.
Primary lead to
switching device
To switching device
Lead wire
Metal needle To spark plug
High-tension lead
to spark plug
Secondary
winding
Primary
winding
Grounded
to core
Insulation
Laminated
iron core
Copyright Goodheart-Willcox Co., Inc.
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