Section 3 Engine Systems
194
more dependable than a mechanical type because
it has no moving parts to wear or burn out.
Magneto Ignition Systems
There are several types of magneto systems used
on small engines. These systems are classifi ed by
the type of switching device they use to control
primary current to the coil. Mechanical breaker
point ignition (MBI) systems use mechanical
breaker points to control current in the ignition
coil. This type of system was used exclusively until
the development of the solid state ignition system.
Today, all ignition systems in late-model engines
are of the solid state type. Solid state systems use
electronic devices (transistors, capacitors, diodes,
etc.) to control various ignition system functions.
Solid state ignition systems provide many advan-
tages over mechanical systems:
• Since there are no moving parts, mechanical
adjustments are not required.
• No breaker points to burn, pit, or replace.
• Increase spark plug life.
• Easy starting, even with fouled plugs.
• Higher spark output and faster voltage rise.
• Spark advance is electronic and automatic. It
never needs adjusting.
• Electronic unit is hermetically sealed and
unaffected by dust, dirt, oil, or moisture.
• System delivers uniform performance
throughout component life and under adverse
operating conditions.
• Improves idling and provides smoother
power under load.
The following are the three general classifi ca-
tions of magneto ignition systems.
1. A capacitive discharge ignition (CDI) system
is a solid state (no moving parts) system that
stores its primary energy in a capacitor and
uses semiconductors for timing or triggering
the system.
2. A transistor-controlled ignition (TCI)
system is an inductive system that does not
use mechanical breaker points. It utilizes
semiconductors (transistors, diodes, etc.) for
switching purposes.
3. A mechanical breaker point ignition (MBI)
system is a fl ywheel magneto inductive system
commonly used for internal combustion engines
until the mid-1980s. It employs mechanical
breaker points to time or trigger the system.
are needed to withstand the continual opening
and closing that takes place and the eroding effect
of the arc that occurs when the points break (start
to open).
The ignition systems in most engines use
electronic switching devices to control the primary
current to the coil. An electronic switching device is
Goodheart-Willcox Publisher
Figure 10-6.
Spark plug reach (length of thread) can vary considerably
from one plug to another. Too long a reach can damage
a piston. Too short a reach provides poor combustion.
3/4″
3/8″
Goodheart-Willcox Publisher
Figure 10-7.
The heat of ignition and combustion must be conducted
away from critical parts of the spark plug to prevent
preignition and burning of the electrodes.
9%
To air
4.5%
4.5%
91%
49%
42%
100%
67%
21%
8%
4%
Copyright Goodheart-Willcox Co., Inc.