Chapter 10 Ignition Systems
199
Magnets are usually cast into the fl ywheel and
cannot be removed. They are strong permanent
magnets made of Alnico (aluminum, nickel, cobalt
alloy) or a ceramic magnetic material.
The breaker points in the MBI system are
mechanically actuated, opened by the cam and closed
by the breaker point spring. As the fl ywheel turns,
the magnets pass over the legs of the laminated
core of the coil. When the north pole of the magnet
is over the center leg of the coil, the magnetic lines
of force move down the center leg through the coil,
across the bottom of the lamination, and up the
side leg to the south pole. See Figure 10-17.
As the fl ywheel continues to turn, the north
pole of the magnet comes over the side leg and the
south pole is over the center leg of the core. Now
the lines of force move from the north pole down
through the side leg, up through the center leg and
coil, and to the south pole. At this point, the lines
of force have reversed direction.
control circuit, base of transistor (T2), and emitter
of transistor (T2).
When transistor (T2) begins to conduct current,
the base current fl ow is cut. This causes the collector
circuit to shut off, and the transistor (T1) stops
conducting current.
When transistor (T1) stops conducting, current
stops fl owing through the primary of the ignition
coil. This causes the primary magnetic fi eld to
collapse across the secondary windings of the
ignition coil. High voltage is then induced into the
secondary winding to fi re the spark plug.
The secondary circuit includes the coil secondary
windings, spark plug wire, spark plug, and common
grounds returning to the coil secondary.
When the ignition switch is off, the primary
circuit is grounded to prevent the plug from fi ring.
Diode (D1) is installed in the circuit to protect the
TCI module from damage.
The ESG circuit shown in Figure 10-16 is used
to retard the ignition timing. At high engine rpm,
the ESG circuit conducts. This bypasses the trigger
circuit and delays when the current reaches the
base of transistor (T2).
Operation of the Mechanical
Breaker Point Ignition (MBI)
System
For many years, the mechanical breaker point
ignition (MBI) system supplied the ignition spark
on most small engines. Major components and
operation of a typical MBI system are illustrated
in Figure 10-17. The coil, condenser, and breaker
points may be found inside or outside of the fl y-
wheel. This varies with engine type, but the prin-
ciples of operation remain basically the same.
The condenser plays an important part in MBI
system operation. Its primary purpose is to prevent
current from arcing across the breaker point gap
as the points open. If arcing were to occur, it would
burn the points and absorb most of the magnetic
energy stored in the ignition coil. Not enough
energy would be left in the coil to produce the
necessary high voltage surge in the secondary
circuit. The condenser absorbs current the instant the
breaker points begin to separate. Since the condenser
absorbs most of the current, little is left to form an
arc between the points.
Goodheart-Willcox Publisher
Figure 10-17.
Typical MBI system. As the magnets on the flywheel
align with the legs on the core, a magnetic field is
established through the core.
Flywheel
direction
Magnet
Magnet
N
S
Primary coil
Ground
Condenser
Soft iron
laminated core
Ground
Breaker points
Ground
Breaker point
spring
A
B
Cam
Ground
Stop switch
Spark
plug
High
tension wire
Secondary coil
Ground
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