Chapter 10 Ignition Systems
201
Magneto Ignition Systems for
Two-Cylinder Engines
The magneto systems used in two-cylinder
engines must fi re the spark plug in each cylinder at
the correct time. This is accomplished in one of two
ways. Some systems use two coil assemblies to fi re
the plugs. These assemblies are mounted near the
fl ywheel and are located 180° apart. As the magnets
in the fl ywheel move past each assembly, the cor-
responding spark plug fi res at the proper time.
Other two-cylinder engines use a waste-spark
system to fi re the spark plugs. In this type of system,
the coil assembly has two secondary outputs and
fi res both spark plugs at the same time. The spark
occurs when the piston in one cylinder is on its
compression stroke and the piston in the other
cylinder is on its exhaust stroke. The spark that occurs
during a cylinder’s exhaust stroke has no effect
on engine operation and is, therefore, considered a
“waste” spark.
Battery Ignition Systems
The battery ignition system has a low-voltage
primary circuit and a high-voltage secondary circuit.
Like the magneto system, it consists of a coil, solid
state switching device (or points and condenser), and
spark plug. The basic difference is that the source
of current for the primary circuit is supplied by a
lead-acid battery. See Figure 10-21.
When the ignition switch is turned on, current
fl ows from the positive post of the battery to the
ignition coil. Current traveling through the primary
windings of the coil builds up a magnetic fi eld.
See Figure 10-22. During this time, the switching
device is closed. Ignition at the plug is not required,
so the current returns to the battery through the
common ground.
Then, at the exact time when ignition at the plug
is required, the switching device opens. Current
fl ow stops abruptly, causing the magnetic fi eld
surrounding the coil to collapse. See Figure 10-23.
This rapid change of magnetic fl ux causes voltage to
be induced in every turn of the secondary windings.
The voltage built up in the secondary winding
of the coil can become as high as 30,000V. The sec-
ondary windings have approximately 100 times as
many turns of wire as the primary. Normally, the
voltage does not reach this value. Once it becomes
Dwell (cam angle) is the time the breaker points
stay closed during one revolution of the cam. Dwell
is measured in degrees of cam rotation from the
point of closing to the point of opening. There is
an inverse relationship between the breaker point
gap setting and dwell time. With a wider breaker
point gap setting, dwell decreases. A narrower gap
setting increases dwell.
Remember that the cam is driven directly from
the crankshaft. When the breaker points open, the
spark plug fi res. Obviously, then, changing the point
setting can also change spark timing. The engine
manufacturer specifi es which gap setting is best
(usually between .020″ to .030″) and the number
of degrees before top dead center (BTDC) that the
spark should occur.
Some MBI systems have mechanical ignition
advance systems that retard occurrence of spark for
starting. For intermediate- and high-speed operation,
the system causes the spark to occur earlier in the
cycle.
Goodheart-Willcox Publisher
Figure 10-20.
The spark plug fires, and the condenser discharges
voltage back into the primary circuit.
Ground
Ground
Ground
Flywheel
direction
Primary coil
Secondary coil
Ground
Soft iron
laminated core
High
tension wire
Spark
plug
S
N
Breaker points
open
Cam
Breaker point
spring
Condenser
Stop switch
Copyright Goodheart-Willcox Co., Inc.