Chapter 10 Ignition Systems
205
Summary
The primary purpose of the ignition system is
to provide suffi cient voltage to discharge a spark
between the electrodes of a spark plug. Many
small engines use magneto systems to supply igni-
tion spark. Magneto systems are self-contained
systems that produce electrical current for ignition
without an outside primary source of electricity.
Basic magneto system parts include permanent
magnets, coil, switching devices, spark plug wire,
and spark plug.
Using the correct spark plug can greatly increase
engine effi ciency and service life. Reach, heat range,
and electrode type must all be considered.
Most small engines are equipped with solid
state ignition systems that use electronic devices
in place of one or more mechanical ignition com-
ponents. These systems do not require mechani-
cal adjustments. The two most common solid state
systems are the capacitive discharge ignition (CDI)
system and the transistor-controlled ignition (TCI)
system.
The CDI system stores primary energy in a
capacitor and uses semiconductor devices to trigger
the ignition system. The TCI system is an inductive
system that utilizes semiconductor devices (tran-
sistors, diodes, etc.) for switching purposes.
The mechanical breaker point ignition system
is a fl ywheel magneto inductive system. It employs
mechanical breaker points to time the triggering
of the ignition system. Dwell (cam angle) is the
amount of time that the breaker points stay closed
during one revolution of the cam. Some small
engines are equipped with mechanical systems that
retard and advance timing.
Instead of a magneto, some ignition systems
use a lead-acid battery to supply primary current.
These systems generally employ an ignition coil.
Because the battery is the only source of energy
for battery ignition systems, a generator is used to
replenish energy in the battery.
Review Questions
Answer the following questions on a separate
sheet of paper.
1. Describe the two major tasks performed by an
ignition system.
2. If a four-cycle engine runs at 3600 rpm, the
number of sparks per minute required at the
spark plug would be _____.
3. Name the main electrical components that
make up the magneto ignition system.
4. The coil acts as a transformer that _____.
A. steps down the voltage and increases the
output amperage
B. steps up the voltage and amperage
C. steps down the voltage and amperage
D. steps up the voltage and decreases the
output amperage
5. In the ignition coil, the primary winding
has _____.
A. many turns of fi ne wire
B. few turns of fi ne wire
C. few turns of heavy wire
D. many turns of heavy wire
6. What is the purpose of the ribs on the spark
plug insulator?
7. Would a cool spark plug have a short or long
insulator nose?
8. Defi ne preignition.
9. When the switching device in the magneto is
closed, _____.
A. current is induced in the primary circuit
by the fl ywheel magnets
B. the spark plug fi res
C. a high voltage is induced in the secondary
circuit
D. All of the above.
10. True or False? Electronic switching devices are
more dependable than mechanical switching
devices.
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