Chapter 6. Electrical; Ignition System; Capacitor Discharge Ignition (C.d.i.) - Yamaha QT 50F Service Manual

CHAPTER 6. ELECTRICAL
6-1. IGNITION SYSTEM
A. Capacitor Discharge Ignition (C.D.I.)
A capacitor discharge ignition (C.D.I.) system
eliminates the need for a mechanical contact
breaker, and its inherent disadvantages. A
simple electronic circuit using a large storage
capacitor and a thyristor (Silicon Control
Rectifier) provides a correctly-timed, high-
intensity voltage to the spark plug.
1. Method of ignition operation
The .voltage generated by the charge
coil is rectified by D1 (diode) and flows
in the direction -» thus charging C
(condenser). On the other hand, the
voltage generated by the pulser coils is
rectified by D2 then applied to SCR as a
gate signal.
When the gate signal reaches the trig-
ger level, SCR becomes conductive,
thus allowing C to discharge its stored
current. The current flows in the direc-
tion—->. This change in the current
generates a high surge of voltage in the
secondary winding of the ignition coil,
thus causing a spark to jump.
1. Charge coil
2. Pulser coil
3. Condenser
4. Ignition coil
5. Sparkplug
2. Generation of pulses
A magnetic circuit is produced by using
the magnet on the rotor, and pulses are
generated, according to the magnitude
of voltage produced in the pulser coil by
the variations in the magnetic flux.
The magnetic circuit is formed by the
pulser core, boss, flywheel and magne-
tic flux varies, and according to the
variations, a voltage is produced in the
pulser coil.
Boss
Flywheel
The voltage is generated in the pulser
coil, and when it reaches the trigger
level, the SCR becomes conductive,
thus causing the capacitor to discharge
and to induce a spark jump at the spark
plug..
Method of ignition advance
This system is equipped with pulser coil
(L2), which generates the pulse voltage
as illustrated below. And its signal has
a rise slope and the voltage of the signal
becomes greater with the increase of
engine speed.
Volt L2
Crank angle
6-2