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7-4-3 BASIC THEORY
(a) Revolution of the flywheel generates electricity on the primary side of the ignition coil (1), and the base
current I
1 flows to the power transistor (2). Current I1 turns the power transistor “ON” and the electric
current I
2 flows. This is the same situation when the contact breaker is closed in a case of breaker point
type ignition system.
(b) At lower engine revolution, the low speed ignition timing control circuit (3) operates to run the gate
current I
3 to turn the control thyristor (5) “ON”, thus the current I1 bypass the thyristor as current I5.
At this moment, the power transistor (2) turns “OFF” and the current I
2 is shut off abruptly resulting
in the high voltage generated in the secondary coil (7) which produces sparks at the spark plug.
The ignition timing at lower engine revolution is less advanced as shown in the above chart.
(c) At higher engine revolution (over 2,000rpm), advancing control circuit (4) operates to run the gate current
I
4 to turn the control thyristor (5) “ON”, thus the current I1 bypass the thyristor as current I5.
At this moment, the power transistor (2) turns “OFF” and the current I
2 is shut off abruptly resulting
in the high voltage generated in the secondary coil (7) which produces sparks at the spark plug.
At over 2,000rpm, ignition timing on each engine revolution is controlled by advancing control circuit (4)
that receive electrical information from revolution sensing resister (6).
The advancing of ignition timing from lower to higher engine revolution changes lineally as shown in
the above chart.
Fig.7-2b
7-4 IGNITION SYSTEM (EX27)
7-4-1 IGNITION COIL INTERNAL CIRCUIT
7-4-2 IGNITION TIMING CHARACTERISTIC
Fig.7-2a
3
4
0
0 500 1000 1500 2000 2500 3000 3500 4000
5
10
15
20
25
30
35
5
2
6
1
7
I3
I4
I1 I2
Low Speed Ignition
Timing Control
Circuit
Base Resister
Advancing Control
Circuit
Control thyristor
I5
Revolution Sensing
Resister
Power Transistor
Primary Coil
Spark Plug
Secondary Coil
IGNITION TIMING
B.T.D.C.( )
LINEAR ADVANCING
ENGINE REVOLUTION (R.P.M.)