The document discusses capacitive discharge ignition (CDI) systems. CDI systems work by storing energy from a high voltage supply in a capacitor and then discharging the capacitor through an ignition coil and spark plug to generate a spark. There are two main CDI topologies - one uses an SCR to control discharge of the capacitor through the ignition coil primary, while the other uses both an SCR and free-wheeling diode. CDI systems offer advantages like fast voltage rise and insensitivity to resistance, making them suitable for small engines and improving cold starting. However, their short spark duration can limit reliability in some applications.

2. INDEX
1. Introduction
2. CDI Principle
3. General CDI Block Diagram
4. HV Supply
5. Capacitor
6. Switch
7. Sensor
8. Conditioning
9. Ignition Coil
10. Sparking Plug
11.First CDI Topology
12.Second CDI Topology
13.Advantages
14.Disadvantages
15.Conclusion

3. INTRODUCTION
 In the world of small motor, such as mopeds
or lawn movers the ignition system design is
based exclu-sively on CDI.
 In automobile CDI was in the past only used
in the replacement module market.
 Today, due to new standards of pollution
control, the CDI system is becoming one of
the most efficient choice available.
 The purpose of this paper is to analyze the
behavior of the CDI, the solutions we propose
today in small motor applications and the
state of the art in automotive.

4. CDI PRINCIPE
 The spark necessary to ignite the
air/petrol mixture in the combustion
chamber is produced by the CDI
module. This system consists of 7
stages.

5. General CDI block diagram
HV
SUPPLY
CAPACITOR SWITCH IGNITION COIL SPARK
PLUG
SENSOR
CONDITIONING

6. HV supply
 The HV supply differs from small motors
to automobiles. The small one or two
cylinder motors one or two stroke have a
fly-wheel which includes a supply
winding.
 This coil produces, after rectification, a
positive voltage variable between 100V
and 400V.
 The HV for an automobile CDI is
supplied by a DC/DC converter. This
stage produces generally 400V from the
12V battery voltage.

7. CAPACITOR
 The capacitor between 0.47 and 2µF
is used firstly, to store the charge from
the HV supply. During the second
phase of the ignition cycle the
capacitor is discharged through the
ignition circuit.

8. SWITCH
 The switch transfers the energy stored
in the capacitor to the primary of the
ignition coil. This function is carried
out by a SCR or a triac. The switch is
generally linked to a diode for the
reverse current.

9. SENSOR
 The goal of the sensor is to
synchronize the spark with the engine
rotation.
 For the small motor the sensor detects
a bump at each engine revolution.
 For car modules the sensor system
gives a pulse for each cyl-inder
ignition point.

10. Conditioning
 The conditioning is a very important stage which
must assume the following functions:
 Optimisation of the SCR gate current for all the
RPM range.
 Filtering of parasitic strikes occurring on the
sensor signal.
 For the most sophisticated small engine and all
the car systems, it has to ensure the correct lead
angle.
 This stage is realized using few passive
components for small motor modules, while for
automotive man-agement systems a
microprocessor is needed.

11. ignition coil
 The ignition coil is a step up
transformer which delivers high
voltage to the spark plug.
 This value can be between 5 and
20kV depending on the working
conditions.

12. spark plug
 The spark plug is the final element of
the ignition chain. High engine
efficiency and a complete gas com-
bustion are linked to a good spark
quality.
 Generally we estimate a minimum of
20 millijoules is necessary at spark
plug.

13. HOW DOES IT WORK?
 Two different topologies are possible.

14. First CDI topology
Figure shows the first possibility of discharge
circuit.

15.  When spark is needed a
current Ig is injected to the
SCR gate which then fires
the SCR. The SCR firing
initiates the capacitor
discharge which generates
an alternative current.

 The SCR conducts during
all the positive phases of
the discharge current while
the diode D acts for the
negative parts.

16. Discharge current through the
circuit
 CURRENT THROUGH
 THE IGNITION COIL
 PRIMARY 10A/div.
 SCR CURRENT
 10A/div.

 DIODE CURRENT
 10A/div.

17. Second topology
 C PR SC
ID
HV
SUPPLY
SCR D IPR SPARK
IG PLUG

18.  In the 2nd topology shown by figure
the SCR acts during the first part of
the current cycle until the capacitor
voltage reverse.
 Then the free wheeling diode D
conducts as long as there is energy
remaining on the primary coil.

19. Discharge current through the
circuit
SCR CURRENT
10A/div.
DIODE CURRENT
10A/div.
CURRENT
THROUGH
THE IGNITIONCOIL
PRIMARY
10A/div.

20. SMALL ENGINE SYSTEM
 Figure shows the topology we have chosen for
the small motor CDI module.
 The supply coil generates an alternating voltage,
in which the positive parts are rectified by D2 and
the negative parts are clamped by D3.
 This circuit configuration allows the designer to
use 400V diodes for D2 and D3 instead of 1000V
user in other design.
 The capacitance C1 is loaded by the positive
rectified current.

21. Small motor DCI principle
circuit
 STOP
D1 D2
supply coil D3
fly wheel
R
sensor c2
C1
SCR
spark
plug
D4

22. Advantages
 A CDI system has a short charging time, a fast
voltage rise (between 3 ~ 10 kV/μs) compared to
typical inductive systems (300 ~ 500 V/μs) and a
short spark duration limited to about 50-80 µs.
 The fast voltage rise makes CDI systems
insensitive to shunt resistance, but the limited
spark duration can for some applications be too
short to provide reliable ignition.
 The insensitivity to shunt resistance and the ability
to fire multiple sparks can provide improved cold
starting ability.

23. Disadvantages
 Since the CDI system only provides a
short spark, it's also possible to combine
this ignition system with ionization
measurement.
 This is done by connecting a low voltage
(about 80 V) to the spark plug, except
when fired. The current flow over the
spark plug can then be used to calculate
the temperature and pressure inside the
cylinder

24. CONCLUSION
 Capacitive discharge ignition systems
are the only choice for the small
engines.
 They are also found fre-quently in
racing car engine management
computer.
 For the future the CDI could be the
solution meet the new anti-pollution