Section 3 Engine Systems
192
700°F (371°C), fouling or shorting of the plug due
to carbon is likely to occur.
Spark Plug Wire
The spark plug wire connects the output of the
ignition coil secondary windings to the spark plug.
The spark plug wire is heavily insulated because it
carries high voltage. If the insulation deteriorates,
much of the voltage can be lost by arcing to nearby
metallic parts of the engine.
Two common methods of spark plug wire con-
nections are shown in Figure 10-9. Application A
uses the exposed clip, which is satisfactory in uses
where moisture, oil, or dirt will not get on the plug
or can easily be wiped off. The boot type, shown at
B, provides better plug protection.
Switching Devices
Switching devices are used in the ignition
system to control the primary current to the ignition
coil. The switching devices are either mechanical
or electronic.
The ignition systems in some older engines
use mechanical breaker points to control primary
current to the coil. The breaker points generally
consist of two tungsten contacts. One contact point
is stationary, the other is movable. Each contact is
fastened to a bracket. Tungsten is a hard metal with
a high melting temperature. These characteristics
Spark plug reach varies with the type of
spark plug. Some are long, others quite short. See
Figure 10-6. Several standard thread sizes are
commonly used. Threads on some spark plugs are
metric sizes, usually 14mm.
Spark Plug Heat Transfer
Heat transfer in spark plugs is an important
consideration. The heat of combustion is conducted
through the plug as shown in Figure 10-7. Spark
plugs are manufactured in various heat ranges
from hot to cold. See Figure 10-8. Cold running
spark plugs are those that transfer heat readily from
the fi ring end. They are used to avoid overheating
in engines having high combustion temperatures.
In fi guring spark plug heat range, the length of
the insulator nose determines how well and how
far the heat travels. Spark plug A in Figure 10-8,
for example, is a hot plug because the heat must
travel a greater distance to the cylinder head. Spark
plug D is comparatively colder than A. A cold plug
installed in a cool running engine will tend to foul.
Cool running usually occurs at low power levels,
continuous idling, or in start/stop operation.
The tip of the insulator is the hottest part of
the spark plug and its temperature can be related
to preignition (fi ring of fuel charge prior to normal
ignition) or plug fouling. Experiments show that if
combustion chamber temperature exceeds 1750°F
(954°C) in a four-cycle engine, preignition is likely
to occur. If insulator tip temperature drops below
Deere & Co.
Figure 10-4.
A spark plug carries high-voltage current produced by the ignition system. It also must withstand the high
temperatures and shock of combustion, insulate the center electrode against current loss, and seal against
compression leakage.
Ground electrode
Conductive glass seal Insulator
Connection
thread
Terminal
nut
Leakage-current
barrier
Terminal stud
Swaged and heat-shrunk
fitting
Captive outer
gasket
Insulator
tip
Center
electrode
Copyright Goodheart-Willcox Co., Inc.
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