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.

Capacitor Discharge Ignition Circuit (CDI)

The CDI ignition circuit produces a spark from an ignition coil by discharging a capacitor across the primary of the coil. A 2uF capacitor is charged to about 340 volts and the discharge is controlled by an SCR. A Schmitt trigger oscillator (74C14) and MOSFET (IRF510) are used to drive the low voltage side of a small (120/12 volt) power transformer and a voltage doubler arrangement is used on the high voltage side to increase the capacitor voltage to about 340 volts. A similar Schmitt trigger oscillator is used to trigger the SCR about 4 times per second. The power supply is gated off during the discharge time so that the SCR will stop conducting and return to it's blocking state. The diode connected from the 3904 to pin 9 of the 74C14 causes the power supply oscillator to stop during discharge time. The circuit draws only about 200 milliamps from a 12 volt source and delivers almost twice the normal energy of a conventional ignition circuit. High voltage from the coil is about 10KV using a 3/8 inch spark gap at normal air temperature and pressure. Spark rate can be increased to possibly 10 Hertz without losing much spark intensity, but is limited by the low frequency power transformer and duty cycle of the oscillator. For faster spark rates, a higher frequency and lower impedance supply would be required. Note that the ignition coil is not grounded and presents a shock hazard on all of it's terminals. Use CAUTION when operating the circuit. An alternate method of connecting the coil is to ground the (-) terminal and relocate the capacitor between the cathode of the rectifier diode and the positive coil terminal. The SCR is then placed between ground and the +340 volt side of the capacitor. This reduces the shock hazard and is the usual configuration in automotive applications.

Capacitive Discharge Ingition (CDI) Operation.

Petrol engines require a spark at the right time to fire the air/fuel mix which in turn pushes a piston (or spins a rotor or wobbles a wobbleboard or...) There are several ways to make a spark - kettering points charged coil, magneto, flint and a stone - but I'm talking CDIs.
Lets get straight to it. Have a look at the diagram. This is all you'll ever need for your motorbike to run be it 2 stroke or four stroke. Nothing else. That's right. No battery, no regulator, no rectifier - all that junk is for lights and other boring stuff.

Notice that there are a couple of coils (or windings). These literally are wire wrapped around iron. What does wire wrapped arond iron do?
Quick in-a-nutshell physics lesson: When you have a coil with electricity flowing thorugh it this creates a magnetic field. And vice versa - if you have a magnetic field near a coil it will make electricity flow in it. Straight away you can see what the Exciter Coil is doing then. That grey blob underneath it is actually a magnet on the rotor in your engine and it spins around the exciter coil making electricity.

So the first thing that happens is that the engine, spinning a magnet, makes the power for the CDI. How much power? In fact, the faster the engine spins the more voltage is produced to feed the CDI. This may be in the order of 100VAC at peak revs.

Pulse sensor

another coil, but only a small one. It too has a magnet spinning near it - but the magnet only passes by the coil once per revolution and it's a real small magnet. This means that most of the time there is not electricity flowing in the pulse sensor - but when the magnet passes by the coil you get a tiny spike of electricity. This tells the CDI to make a spark as the magnet is cunningly positioned so that the spike hits the CDI at just the right time

There are another 2 windings being The Coil. As in the ignition coil, the one the spark plug connects to. Expanded in-a-nutshell physics lesson: the more turns in a winding, the more voltage; less windings, more current. The "coil" is actually 2 windings, the left hand side only a few turns and the right hand side with lots of turns.

If you put voltage in the few turns side, this makes a magnetic field (remember, electricity = magnetic field) and then the other coil with lots of turns gets electricity in it because of the magnetic field from the first one. Because of the many turns the voltage is much higher but the current much lower. Confused?
Essentially the coil takes a low voltage and turns it into a high voltage. This is to create a spark, which is actually just electricity managing to travel through air (_everything_ will conduct electricity if the voltage is high enough; a compressed air/fuel mix requires at least 10,000 volts).

The CDI itself is really just an amplifying switch with time delay. It sits there storing up electricity in a capacitor (ie the "C" in "CDI"), then the pulse sensor spikes the input, the CDI waits a small amount of time and then puts the burst of electiricity it stored into the coil. That small amount of time represents the advance curve of an engine, ie at idle an engine may need 10 degrees advance but at full revs it may need 40 degrees advance. By delaying the signal and having the pulse sensor in the right place, the CDI can make sure the engine is getting the right advance curve.
Rounding it off, the CDI is connected to ground (so that electricity can flow) and an Engine Kill Switch will short out the CDI resulting in no spark and thus the engine will stop.
The actual internals of the CDI is another story, if you need to know email me and I might write a story about the one built from scratch and fitted to a RM250...

Timing circuit

Based on a PIC16F84

CDI Electronics Circuit

Voltage converterBased on a free running oscillator, it has a very low part count, and make 200-200 volts available for the coil.It uses an ferrite core from an old computer as base for the voltage converter.A description of how to make the transformer

RC CDI Ignition

Features::-

As simple as possible-Programmable advance control -Use no exotic hard-to-find components-Use a normal 4.8V RX battery-Integrated spark test function, no signal generator is needed.(makes 4 or 140 sparks/second without the need for a trigger input)-Spark control can be separated from the voltage board(to be used as a front end for a CDI without advance)-Integrated strobe light-May be extended with a failsafe/engine stop module. (not designed yet)

Before to lern CDI Build for Bigbike

A Simple Motorcycle Electronic Ignition Replacement.
Although these are guides, you must never read this, nor even consider building such devices nor act upon this information. This is because the dogs of law lie around every corner, and everyone should just live a quiet, pastoral life for fear of litigation. (As lawyers get richer, society gets poorer.) If you want freedom, then always vote to keep lawyers and assholes out of politics.
There are many motorcycles which use electronic ignition systems and all have one thing in common, the black box costs a fortune.
For the poor in Britain, or those with small bikes, the costs of replacement can be minimal, as they all have the same basic design. The following simple modification has been run quite happily on Honda NS125, KTM 125 and Yamaha TZR125. All had no electronic ignition systems other than the contents of the generator on the engine. Most wires were burnt flush with the blackened crankcase. This will also work on many larger and smaller machines, with or without wheels.
A little theory. Four strokes (four cycle, Otto cycle,) engines, when spinning fast, need the spark to happen many degrees before the piston reaches the top of the cylinder. This allows time for the flame front to pass through the fuel and air mixture in the cylinder to be at its full potential as the piston begins to descend on the power stroke. But at low speeds, the flame front still happens at the same speed, so at low revs it is therefore necessary for the spark to happen a little later, so the maximum power in the fuel and air mixture still happens as the piston begins to descend. To allow this to happen, most four stokes have an 'advance and retard' unit, which senses the engine speed and times the spark at the right place according to the engine revs.
The Honda C90, ('stepthru' as its normally called) does not have an advance unit, and because its a small engine with plenty of flywheel, it can run happily without an advance and retard unit. As many C90's are scrapped each year, there is a glut of C50/70/90 CDI units and these are the core of this monograph. Many small four strokes can also run happily without an advance and retard unit. But larger four stokes cannot and may damage the piston and crank bearings if the timing is not ideal.
Therefore most four strokes need an advance and retard unit. On early machines, up to around 1985's this is a centrifugal mechanical device, but on modern machines, this is often incorporated in the electronics box. The mechanical designs are usually easy to modify. The latest designs are not easy to do and not discussed here. So if you do not have a mechanical advance unit, then your machine will have to look elsewhere for a solution. Some possible options are described later.

Two Strokes.
Two stroke (two cycle) engines, because of their design, do not need the spark to advance with increasing engine revs. Tick over on a two stroke works quite happily at the fully advanced setting used for full power, and as such, there is no need to have an advance and retard unit. This makes it very easy to fit almost any electronic ignition unit.
Typical set-up.
Inside the basic flywheel rotor are usually two main coils. One coil is fairly large and supplies about fifteen to thirty volts AC to the rectifier for the battery and lights. The other coil is a smaller, more finely wound coil to supply a hundred or so volts to the CDI unit. Outside, or sometimes inside is a small, finger tip sized pulser coil, which triggers the CDI at the correct time for the spark.
This picture shows a C90 rotor with the pulser interrupter as a piece of metal sticking out on the outer edge, which causes the timing pulse at the right place, and the pulser is the black lump between the rotor and stator. Lying flat is the stator plate with the pale lighting and dark CDI generator coils.
There are four or more magnets inside the rotor (it rotates), such that as they pass the stator (static) coils, they induce a changing north - south - north - south alternating magnetic field in the iron plates of the coils, thereby generating electricity in the copper coils of wire.
As can be seen, the four poles at the ends of the coils means that the magnets in the rotors are four, so the N-S-N-S field flows strongly through the iron cores. This is acceptable for a low power machine, but sometimes a little more electrical power is needed in such a small space, so six poles can be used, with six rotating magnets. My old Ducati V twin used about twenty permenanat magnets.
Now we are six. The black and white picture shows another popular arrangement, where the lighting coils are the five coarsely wound coils, while the generator is the obviously different one nearest the viewer. Again the pulser is outside. You will also notice that as a six pole stator, the rotor should have six magnets. You can just make out the size and layout of the six magnets inside the rotor.
Assuming a badly damaged machine.
You may be looking at a burnt out wreck of an engine, I often do, as they are the only ones I can afford. Remove the rotor. Either use the correct tool called a 'puller', or borrow one and always return it in perfect condition. If you do not have a puller, then uncrew the central nut on the flywheel until it is flush with the end of the crankshaft.
With a friend, place two tyre levers (tire irons) behind the back of the flywheel, so that they place a pulling force outwards, levered carefully against the crankcase. Hit the central nut sharply with a small hammer until the flywheel loosens. By placing the nut level with the end of the crankshaft, then the hammering will not damage the crankshaft threads. Do not allow the tyre levers to dent the crankcase. A single, firm, sharp tap should suffice to release most flywheels. Remove the nut and flywheel.
Inspect the wiring to each coil and make a note of the colours to each. Locate the fine wound high voltage coil inside the flywheel generator and inspect the components carefully. Where any wire to the wiring loom is damaged, simply desolder and replace with a new wire. The preferred colours are mentioned below. Ensure the insulating sleeve is securely replaced and will not slide off to expose the bare soldered joint. I usually remove the backing plate the bike, be it a PE suzuki enduro, TZR road racer, or my own specials, then inspect each coil and if suspect, repair as deemed suitable, taking the opportunity to use the old Honda C90 wring loom to fit new wires exiting from the engine casing.
Eventually you will have nice new wires exiting the engine, an earth wire, and a wire or two from the high voltage coil, a wire or two from the heavier coil for the lighting and battery charging, plus a wire or two from the small pulser unit. In some cases, the small pulser coil and the high voltage coil may not be earthed, so will have two wires each, one from each end of their coil of copper wire. As these components normally use an earth, then this can be done by extending the wires and terminating them to an earth tag outside the crankcases. If this does not work, then they are probably connected inside the original CDI unit, but for the C90 unit, they can be earthed.
The C90 unit.
The spark from a C90 ignition will jump the gap in a 50cc to a 1500cc bike, as all spark plugs are essentially the same. There is no need to have a big CDI unit for a big bike. (If the cost of four C90 set-ups is one tenth the cost of the genuine article, then at least have a try at fitting four sets.)
The electronic ignition of many motorcycles and similar machines with electronic ignition can be run using the cheapest and most easily available electronic ignition in the world. Honda C90 parts are available from all second hand bike dealers and breakers, usually for a few pounds, always try to get the wiring loom as well, or at least the part of the wiring loom between CDI and engine. The parts needed are the CDI unit, the wiring loom connector and a foot or so of attached wire, and the ignition coil. I often use the C90 stators inside the flywheel for a supply of suitable copper wire for rewinding this set - up onto other machines.
The simple and commonly available Honda C90 CDi unit works very well with most small two strokes. I can also work with many four strokes which use a mechanical advance and retard unit. To check for a mechanical advance unit, it looks like a set of bob weights restrained by small springs which will fly out when running. This is common on most four strokes with points and can also be a conversion for some older machines. See later.