Section 3 Engine Systems
202
ondary windings are connected, and the common
ground of the battery and primary circuit is used
to complete the secondary circuit.
With this type of coil, very little primary current
can fl ow into the secondary circuit because the
secondary circuit is normally open at the spark plug
gap. Primary current is just not great enough to
jump the gap. Therefore, the two circuits function
separately.
The primary winding of the coil consists of about
200 turns of heavy copper wire. The secondary
winding has approximately 20,000 turns of very fi ne
copper wire. Because the magnetic fi eld collapses
great enough to jump the spark plug gap, the volt-
age drops. Usually, the amount required to jump
the gap is between 6000V and 20,000V. The actual
amount of voltage required depends on variables
such as compression, engine speed, shape and
condition of electrodes, spark plug gap, etc.
Ignition Coil
The ignition coil used in battery ignition systems
serves as a step-up transformer. It increases low-
voltage primary current to the high voltage required
to bridge the spark plug gap. The primary and sec-
Kohler Co.
Figure 10-21.
A battery ignition system is similar to a magneto system, except that the battery replaces the flywheel magnets.
Primary Circuit
Secondary Circuit
Ignition
switch
Ignition
coil
Ground
Battery
High-tension lead
Spark plug
Solid state switching device
or points and condenser
Primary-
secondary
connection
Kohler Co.
Figure 10-22.
When the switching device closes in a battery ignition system, primary current builds a magnetic field around the coil.
Primary Circuit
Secondary Circuit
Magnetic Field Expanded
Ignition
switch
Ignition
coil
Ground
Battery
High-tension lead
Spark plug
Switch
device closed
Magnetic field builds
around primary windings
Copyright Goodheart-Willcox Co., Inc.