Chapter 10 Ignition Systems
195
Operation of Capacitive
Discharge Ignition (CDI) System
The capacitive discharge ignition (CDI) system
is a solid state ignition system. It is standard equip-
ment in many applications and has improved the
reliability of modern small gasoline engines. The
only moving parts in a CDI system are the perma-
nent magnets in the fl ywheel. Figure 10-11 shows
a CDI module installed on a small gasoline engine.
Refer to Figure 10-12 to progressively trace current
fl ow through the various electronic components in
a typical CDI system.
As the fl ywheel magnets rotate across the CDI
module laminations, they induce a low voltage
alternating current (ac) in the charge coil. The ac
passes through a rectifi er and is changed to direct
current (dc), which travels to the capacitor, where
it is stored.
When the silicon controlled rectifi er is triggered,
the 300V dc stored in the capacitor travels to the
spark coil. At the coil, the voltage is stepped up
instantly to a maximum of 30,000V. This high voltage
current is discharged across the spark plug gap.
In Figure 10-13, the fl ywheel magnets rotate
approximately 351° before passing the CDI module
laminations and inducing a small electrical charge
Note
Figure F gu 10-1 1 1 10 F F i 1 1 1 0 1 10 compares m the es e three types of magneto h h f
ign ig ni tio on system s s ms . Study the em carefully!
AC Spark Plug Div.; GMC
Figure 10-8.
Spark plug heat transfer determines whether the plug is hot or cold. Heat is controlled by the insulator nose.
Hot
plug
A B C D
48 46 44
Cold
plug
Insulator nose Heat transfer
42
Heat transfer
Goodheart-Willcox Publisher
Figure 10-9.
Two common spark plug wire connectors. A—Exposed
type. B—Neoprene-boot type. An exposed clip connector
can be used in conjunction with a metal strip stop switch.
Exposed clip-type connector
A
B
Boot-type connector
Copyright Goodheart-Willcox Co., Inc.
Previous Page Next Page