Section 3 Engine Systems
196
coil to energize the silicon controlled rectifi er in
the advanced fi ring position (29° BTDC).
Operation of Transistor-
Controlled Ignition (TCI) System
The individual components that make up the
transistor-controlled ignition (TCI) system are
given in a chart in Figure 10-15. Study the function
of each part carefully.
There are a variety of transistor-controlled
circuits. Each has its own unique characteristics and
modifi cations. Figure 10-16 illustrates a typical
circuit for a transistor-controlled ignition. Refer
to this circuit as its principles are described in the
following section.
As the engine fl ywheel rotates, the magnets on
the fl ywheel pass by the ignition coil. The magnetic
Note
On O On some s m small al engines equipped q with electronic fuel O O O l i i d i h l i f l
injection in ct n n, th he engine g control o unit (ECU) calculates
the th e p p pr oper p p ignition n timing based b on engine operating
conditions. co o d d o In these e system ms , engine sensors feed
inf in fo rmation rm t t on n en ng ine operation r to the ECU. The ECU
the th en prompts o o p the ignition m mo dule to fire the spark
plu p ug s s at t the e proper p e time ba as ed on the sensor inputs.
These T h s s s s sy stems em provide pre ec ise control of spark timing
for fo r impro m m o ov e ed engine e operating t efficiency.
in the trigger coil. At starting speeds, this electrical
charge is just great enough to turn on the silicon
controlled rectifi er (SCR) in a retarded fi ring position
(9° BTDC). This provides for easy starting.
In Figure 10-14, when the engine reaches
approximately 800 rpm, advanced fi ring begins.
The fl ywheel magnets travel approximately 331°, at
which time enough voltage is induced in the trigger
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Figure 10-11.
The CDI ignition module is compact and maintenance
free. The only moving parts in a CDI system are the
flywheel magnets.
Spark plug
Spark plug
wire
Module Laminations Flywheel
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Figure 10-10.
This chart compares mechanical breaker point, transistor-controlled, and capacitor discharge ignition systems.
Comparisons Mechanical Breaker
Ignition System
Transistor Controlled
Ignition System
Capacitor Discharge
Ignition System
Abbreviation MBI TCI CDI
Circuit type Conventional Solid state Solid state
Energy source Primary current of ignition coil Primary current of ignition coil Stored in capacitor
Trigger switch Breaker contacts Power transistor Thyristor
Secondary voltage Standard Standard Higher
Spark duration Standard Standard Shorter
Rise time* Standard Standard Shorter
Maximum operating speed Standard Higher Higher
Maintenance Regap and retime None None
*Rise time–time required for maximum voltage to occur.
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