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This is a good question asked by @Johnny Chancellor. I put it in a new topic so it does not get lost.  We all know we need a cdi box but I bet a lot of the members here don’t know how it works. This is a good explanation of a cdi box including what it does, how it works and what “cdi” stands for. 

Ive posted two links here. Feel free to add to the list! 







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Hey, look, I'm new but a topic I know a lot about!  I'm an ex ignition systems engineer, and I used to work as a test engineer at NGK here in the US. CDI is capacitive discharge ignition.  This i

This is a good question asked by @Johnny Chancellor. I put it in a new topic so it does not get lost.  We all know we need a cdi box but I bet a lot of the members here don’t know how it works. This i

Good call out @Frank Angerano , topic pinned!

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  • 4 weeks later...

Hey, look, I'm new but a topic I know a lot about!  I'm an ex ignition systems engineer, and I used to work as a test engineer at NGK here in the US. CDI is capacitive discharge ignition. 

This is a VERY complicated topic, but I seem to be pretty good at analogies.  Quick background: an ignition coil is an inductive device, you apply voltage to one side of the coil, and you generate a magnetic field, when that field is collapsed very quickly, the secondary side of the coil will generate voltage that is proportional to the winding ratio on the primary and secondary (12volts in, 40,000volts out).  Capacitance is created whenever you have two metal plates within close proximity of one another.  For example, an ignition coil in the head (like in a coil on plug engine) creates capacitance because of the close proximity between the coil and the head.  Capacitance can be added via special boxes or even some spark plugs (pulstar, if they are still around, but don't use them...)  So, what does that mean for spark?  Well, capacative discharge occurs very quickly over a very small amount of time (think nanoseconds), while inductive discharge occurs over a longer time, think microseconds. 

So, analogy time.  Think of a coil as a gear reduction with a flywheel on it.  The flywheel has a clutch that engages and disengages a water pump.  When you apply a 12 RPM input to the gear reduction, you get 40,000RPM on the output, but it takes a while to spin up.  So, when you start spinning the input and the flywheel is spinning, this is like the dwell time.  Once the flywheel is spun up sufficiently, the clutch engages the water pump, and water starts to flow into a hose.  At the end of the hose is a valve that will open at a certain pressure (this pressure changes, and is analogous to the required voltage to breakdown the spark gap).  "There is some stretch in the hose, so when the valve opens there is a little bit more flow for a short time, once that's done, the pump keeps pumping until the flywheel runs down."  The small stretch in the hose is  like the capacitance of a standard ignition system.  Once the valve is opened, the flywheel energy pumps out what it can, and then it's done.  A CDI system is like adding a big pressure tank to the end of the hose.  Once the pressure starts to build, more and more water is stored in the tank.  Once the valve opens, a HUGE tremendous amount of water comes out, but for a very short time.  Once the tank is done, the flywheel/pump still pumps a little bit out.  So, the pressure tank is like capacitive discharge, and the flywheel pump is inductive discharge. 

So, what's the advantage?  Well, the problem with a spark plug is something called quenching.  When spark occurs, there is a very small flame kernel that is susceptible to quenching via nearby relatively cool metal, like the plug, the head, etc. Increasing ignitability reduces this quenching effect, which is actually the benefit of fine wire spark plugs.  CDI helps this by providing a lot more energy in a very short time.  Some CDI systems help even more by allowing more current on the primary side (hence the need for bigger wires).  

So anyway, that's what I can add, hopefully it helps.  

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Need to edit, but I missed my window:

"There is some stretch in the hose, so when the valve opens there is a little bit more flow. 

Should read

"There is some stretch in the hose, so when the valve opens there is a little bit more flow for a short time, once that's done, the pump keeps pumping until the flywheel runs down." 

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2 minutes ago, Russell Senior said:

Need to edit, but I missed my window:

"There is some stretch in the hose, so when the valve opens there is a little bit more flow. 

Should read

"There is some stretch in the hose, so when the valve opens there is a little bit more flow for a short time, once that's done, the pump keeps pumping until the flywheel runs down." 

Edited 😀

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  • 4 weeks later...

Great explanation I would only like to add how that hose valve You talk, is opened  so the spark that ignites the fuel can be generated, Thistask  is performed by the the pickup coil, a  little magnetic sensor  mounted very close to the magneto/rotor .The purpose of this coil/sensor is to send a voltage signal  to the cdi module (also known as ECU) for  spark to ocurr. Everytime the magneto/rotor tab(s) passes by the pickup coil ,there is a variation in  pickup coil magnetic field  producing a voltage change  going  the ECU , this  voltage variation is also known as Hall effect.  usually on the 0 to 5 V range  .The ECU receives this voltage and sends  the ignition coil a signal to generate a spark  at the designated cylinder. But this is  very curious here, because that signal instead of beeing a voltage is just the opposite, It is the suppresion of  voltage sent to the ignition. coil primary winding  that forces the  magnetic field  inside the ignition coil to collapse ,thus producing a high voltage and a spark in the secondary winding which is connected directly to the spark plug. In other words the signal  sent from the pickup coil to  ECU/CDI  grounds  the ignition coil primary circuit , making the  already existing magnetic field inside the ignition coil  to collapse,  creating  a very high voltage inside the secondary winding ,(usually in the 40.000 V range), which jumps to ground via the plug and  so the spark is created.

  Pickup Coils are also known as transducers.These elements convert one type of energy (in this case magnetic) into another (commonly voltage variation).Transducers are  very useful to measure different variables like position, temperature,light,proximity etc.beeing  the more common sensors used in vehicles for their versatility and simplicity.

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Ok, my two cents worth..

First off there is a difference between a cdi and an ecu/ecm. To avoid confusion for anyone looking in a manual it's important to know that some engine control module/units have a cdi system built into them and some don't.  Cdi are purely for firing the spark plug and can be built into an ecu or they can be external, and also, cdi can be fitted to engines that don't have an ecu. An ecu generally only come on fuel injected engines these days and they monitor the throttle position, the engine temp, air temp and revs, and they control the fuel injection, the ignition firing and ignition advance.

There are two different sorts of cdi. There are twelve volt cdi that run off the battery, and there are cdi that run off the crankshaft generator.

The first sort, the twelve volt ones, use battery power which they first run through an electronic circuit which turns it into an alternating current which goes through a small transformer and gets stepped up to approximately 350 volts. That high voltage gets stored in a capacitor and is then discharged through a specially designed ignition coil made to handle the very high voltage and high current which the discharging capacitor supplies. The discharge is controlled by either an ecu which has a crank position sensor, or it's controlled by a trigger coil or hall effect sensor on the crank. If it's controlled by an ecu, the ecu does a calculation of what the ignition advance should be based on revs, temp and throttle position. If it's controlled by a trigger on the crank the timing is advanced by virtue of the fact that at higher revs the magnet(or hall effect disruptor), has more effect as it's speed increases. At low revs the magnet has to get quite close to the sensor before it produces enough induced power to trigger the cdi, but as revs increase the rapidly moving magnet induces more effect and so triggers the cdi at a greater distance before the trigger, hence an advanced firing point for the cdi/ignition.

In the other sort, the crank mounted generator type, they use the alternating current out of the generator and they put it through a circuit comprised of diodes and small capacitors which "doubles" the voltage. They call it a doubler or stacker but it doesn't actually quite double, but  the voltage rises by about 1.73 times. In a twelve volt flywheel generator which has a loaded alternating voltage of about 28 volts, and up to 120 when it's open circuit, that gives about 120 effective volts. The 120 volts is then stored in a capacitor and is discharged when it's triggered by either an ecu or a trigger on the crank, and in either case the ignition advance is controlled in the same way as in the first version.

In both cases the cdi discharges through an ignition coil which is itself a stepup transformer that produces a spark of thousands of volts, 25,000 to 40,000 volts being common

The two advantages of a cdi are that it produces a hotter fatter spark, which can't be extinguished when the compression pressures go up during flame propagation, and they can fire the ignition coil at a higher frequency which allows for higher revs. Cdi systems must have a special ignition coil which can handle the high current and voltage.



Edited by Mech
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The combustion difference between a conventional ignition coil operation and a cdi ignition coil operation is this..

In the conventional system the 12v power goes through the coil and then to a set of points or an electronic switch which allows the power to flow to earth. That flow through the primary windings of the coil slowly(relatively slow), builds up a magnetic field which saturates the secondary windings. When we want spark we cut off the flow by opening the points or electronic switch and the magnetic field collapses quickly(relatively quickly), which induces the high voltage to fire the sparkplug. The spark is caused by the collapsing magnetic field

In the cdi system we discharge the capacitor at high voltage and high current and it induces the spark as the rapidly rising magnetic field is propagated outward through the secondary windings. The spark is caused by the expanding field.

In the conventional system, as the collapsing magnetic field induces the high voltage in the secondary windings, it also induces another lower voltage in the primary windings, and that low voltage is in the opposite direction to the initial voltage. We call this an "counter electro magnetic force", or cemf, and it has the effect of opposing the initial induced voltage in the secondary windings as well. The overall effect of this initial emf, and then the cemf, is to set up a fluctuating high voltage spark, which, if we look at it on an oscilloscope goes high, then a little lower, then up, then lower still, then up, then lower still, It typically fluctuates about six times, each times with a lower voltage, until it dies right out. So a conventional system can give initial ignition, but then as the flame propagates and causes a high pressure, the fluctuating voltage spark can get cut off by the increased pressure and fail to ignite any unburnt fuel  that would benefit from a long lasting spark. That can cause misfires or incomplete combustion.

The cdi spark is one long fat hot spark which doesn't suffer from the cemf as much and so gives good secondary ignition and less misfiring or incomplete combustion.

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