CAPACITIVE DISCHARGE IGNITION

1. INTRODUCTION

In the world of small motor, such as mopeds or
lawn movers the ignition system design is based
exclusively on CDI (1). In automobile CDI was in
the past only used in the replacement module mar-
ket. Today, due to new standards of pollution con-
trol, the CDI system is becoming one of the most
efficient choice available.
The purpose of this paper is to analyze the behav-
ior of the CDI, the solutions we propose today in
small motor applications and the state of the art inautomotive

2. CDI PRINCIPE

The spark necessary to ignite the air/petrol mixture
in the combustion chamber is produced by the CDImodule.

This system consists of 7 stages.

HV supply

The HV supply differs from small motors to auto-
mobiles.The small one or two cylinder motors one
or two stroke have a fly-wheel which includes a
supply winding. This coil produces, after rectifica-
tion, 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.

APPLICATION NOTE

Capacitor
The capacitor between 0.47 and 2ตF 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 cir-
cuit.

Switch
The switch transfers the energy stored in the ca-
pacitor to the primary of the ignition coil. This func-
tion is carried out by a SCR or a triac. The switch is
generally linked to a diode for the reverse current.

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 cylinder ignition point.

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 realised using few passive components for small motor modules,

while for automotive management systems a microprocessor isneeded.

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

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

3. HOW DOES IT WORK ?
Two different topologies are possible.

3.1. First topology
Figure shows the first possibility of discharge
circuit.

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.