Quantcast
Jump to content

  • Do you own an ATV? Join our Forum!

    As a member, you can post in our forums, upload your photos and videos, use and contribute to our downloads, create your own member page, add your ATV events, and even start your own ATV club to host your own club forum and gallery.  Registration is fast and you can even login with social network accounts to sync your profiles and content.

Frank Angerano

What does CDI stand for

Recommended Posts

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! 

https://www.elprocus.com/capacitor-discharge-ignition-cdi-system-working/

 

https://en.wikipedia.org/wiki/Capacitor_discharge_ignition

 

 

 

  • Like 1

Share this post


Link to post
Share on other sites


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.  

Edited by Admin
Edited per member request
  • Like 1

Share this post


Link to post
Share on other sites

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." 

Share this post


Link to post
Share on other sites


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 😀

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites


Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Similar Topics

    • By Old_ATV
      I have a mid 2000 sportsman 700 that only works in low gear.  Any thoughts?
    • By John Torbus
      I am trying to figure out what the resistors do in this circuit.  From what I have read in the manual the head light/tail light circuit is A/C.  I am adding LED headlights/taillight so I want to convert it to D/C.  My assumption is the resistors in the circuit bleed off some power since the incandescent bulbs are low wattage.  Any input would be greatly appreciated.  I am not putting a Baja LED light kit or light bar, just LED bulbs in the stock location and a small cube flood mounted on the handle bars.  I already have the LED bulbs installed in the stock locations but they flicker badly.
       
      Cheers,
      John

      Pred 50 Wiring Diagram.pdf
    • By Thehick344
      I have a 2001 650 quest that ran great. Shut it off, wont start. Got tinkering at a buddies, it ended up in a snowbank. Lost my train of thought since then.  I have a black, red, green  top with two plug in wires yellow and yellow and red I believe.. I have a new battery. Just dunno what to do. I tried hooking the wire coming up to the solenoid on the far side, then directly across I put the positive cable and it instantly started turning over without the button being pressed until I took the cable off.  The little black resistor box has also been eliminated.   Any help would be greatly appreciated.. 
    • By BuckBilly
      Good morning to all !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
      Let's see if we can make this a daily event.
    • By Brent Wright
      What's a good size to start a 10 year old on?
  • Recently Browsing   0 members

    No registered users viewing this page.


×
×
  • Create New...