CDI capacitive discharge ignition! what tha!?!

Discussion in 'Motorized Bicycle General Discussion' started by joewlms3, Jun 24, 2009.

  1. joewlms3

    joewlms3 New Member

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    referring to the china 2 strokes
    is the ignition really "CDI" cuz it doesnt seem to fit the bill? from what i know of ignitions cdi usually requires a condensor and a point setup to cause the coil to collapse on itself and throw the spark

    it might just be me but looking at it i see alot of those parts arent there! makes me have ?s

    is the coil just really intelligent and knows when fire?
    its just psychic?
    its some system outside of my previous experience?

    probably the last one.

    anyways could someone explain how it works for me?
     
  2. matt167

    matt167 New Member

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    CDI's sometimes use an external electronic trigger, but never off of a points set that I know of.. 2 strokes don't need that kind of timing because they fire every revoloution....

    the magneto creates a signal and power that the CDI can use, the CDI uses a capacitor and a resistor to time the energy transfered into the primary windings, then when it fully discharges, it collapses into the secondarys and sparks..

    2 strokes don't even need any form of ignition box or timing system.. many chain saws and weed eaters just run off the magneto
     
    #2 matt167, Jun 24, 2009
    Last edited: Jun 24, 2009
  3. Cabinfever1977

    Cabinfever1977 New Member

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    has anyone every tried to run engine off of just the magneto on our 2-strokes?
     
  4. joewlms3

    joewlms3 New Member

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    excellent!
    correct me if i drew the wrong conclusion
    the coil times itself because it can only get so full b4 it collapses and fires?
     
  5. matt167

    matt167 New Member

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    the resistor and capacitor are what times it, as well as the position of the magneto coil to the fly magnet, as it will generate electricity only once per revoloution and at the same degree on the crankshaft rotation
     
  6. matt167

    matt167 New Member

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    I don't think the China 2 strokes setup could fire that way, theoretically it is possible, but I'm pretty sure the magneto coils are wound diffrently that those that fire directly.. it would take a diffrent coil I'm pretty sure, but it ~could~ be done
     
  7. joewlms3

    joewlms3 New Member

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    more ?s

    why would the be a 3rd wire?
    how are the wires tapped to the coil?
    does the "lighting" wire have its own windings?
    there is only one magnet in the crankshaft thingy?
     
  8. matt167

    matt167 New Member

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    the third wire is a second ground. designed for a kill switch but it does provide 6v that you can sample from for lighting..

    the crankshaft has 1 magnet.. there is 1 set of windings in the coil, it's all in how it's wired
     
  9. joewlms3

    joewlms3 New Member

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    is the a possibility of rewinding the the magneto for "high performance"

    perhaps more turns around the core or a larger guage of wire?

    what do you think?
     
  10. 2door

    2door Moderator
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    Actually, and this has been proven by disassembly of a magneto coil, there are two seperate windings. The white wire is not a ground but one of the two windings of the coil. The black wire is a ground, the blue the other winding. The rotor/magnet is a two phase input to these coils with an input on every 1/2 crankshaft rotation. Rewinding the magneto coils will have little to no effect on CDI output to the sparkplug. That voltage is determined by the capacitor and triggering circuit in the CDI.
     
  11. Junster

    Junster New Member

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    Exactly what 2door said. The white wire coil when you ground it kills the field in the blue wire coil. It's so you don't need to ground out a high voltage wire for killing the motor. Using the white to kill the motor is the way it was made to work.
     
  12. Norman

    Norman LORD VADER Moderator
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    the white wire is 2.5 layers on the core of the coil frame it is attached at the start of the winding to the coils frame after 2.5 layers of winding the white wire comes out and also attached to the white wire is all of the smaller gauge of wire wrapped around and over the heaver gauge white wires winding and it comes out as on most a blue wire. I lost count of all the smaller gauge windings but it is almost 1/2" deep on the smaller gauge. you can ground out either the white or blue wire and stop the current from reaching the cdi.
    The voltage is ac and the current depends on rpm as it will get higher the faster you crank the engine. There is no trigger on the mag coil I'm thinking the mag coil puts out a pulse and the cdi picks this up to and stores it then triggers the firing coil through the use of the electronics in the cdi.
    On most small engines weather 2 or 4 stroke the spark plug fires on every revolution. One spark on the power stroke and one spark on the exhaust stroke if its a 4 stroke one spark is wasted or called a maverick spark. On the 2 stroke it fires every revolution as every revolution is a power stroke.
    On a briggs you can change the timing by removing the flywheel key or using an off set key. I prefer to find tdc with a degree wheel then by knowing tdc I can set up the flywheel advanced to fire the mag where ever I want it to fire. With that type of ignition system it will retard at high rpms to I add a little more advance to the setting. I get away from using the key by lapping the flywheel to the crank and using about 75 ftlbs of torque on the flywheel nut. You can't lap the mag rotor to the crank on the china engine but you can use an off set key if you want to experiment with the timing.
    Norman
     
    #12 Norman, Jun 26, 2009
    Last edited: Jun 26, 2009
  13. joewlms3

    joewlms3 New Member

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    AHA!

    the pulse frequency as a componet of the AC wave triggers the spark!

    great note on the timing norman! i hadnt even considered it as of yet i havent actually run my motor yet and it is my first one but i had opened the mag cover and looked and my first thought was
    "hmmm no timing advance device"
    i may just have to mess with that at a later date

    im enjoying the forum greatly btw most of the time when i get to discuss engine stuff with someone all the other party ever says is can you fix this for me
     

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