Advancing the spark timing in a CDI Ignition system

Discussion in 'Automotive Electronics' started by solmillin, Aug 26, 2016.

  1. solmillin

    Thread Starter New Member

    Aug 18, 2016
    22
    1
    I have an 80cc Chinese two stroke engine on my bicycle and wish to experiment with changing the spark timing. Apparently the magneto gives off an alternating voltage of around 100 volts which is used both to power the CDI unit and as the spark trigger signal. The alternating voltage (and trigger signal) frequency is directly associated with a single rotation of the motor and rotating magneto magnet which is attached directly to shaft of the piston crank.

    It would appear that the alternating signal is stored in a capacitor in the CDI unit until the spark trigger signal is processed (once per rotation), whereupon the charge in the capacitor is release into the ignition coil causing the spark.

    If anyone can give me a circuit to adjust the spark trigger signal timing I will be most grateful.
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,229
    You could delay (retard) the signal electronically, but not advance it.
    To advance it you will have to move either the magnet or the trigger coil.
     
    #12 likes this.
  3. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,908
    378
    Agree.
    Out of the box idea? Run a PLL from the trigger signal so you can predict when the next trigger will be then you can issue an earlier trigger. It would need some careful precautions to avoid generating a spark at a stupid time. If you don't value the engine highly it might work.
     
  4. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,004
    1,525
    Then couldn't the spark be mechanically advanced to the new desired setting and then electronically retarded for starting and low speed?
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,229
    Certainly.
     
    shortbus likes this.
  6. solmillin

    Thread Starter New Member

    Aug 18, 2016
    22
    1
    Assuming the positive signal in the alternating voltage is the trigger to discharge the capacitor and create the spark, if we changed the phase of the incoming signal to the CDI, surely this would change when the spark is produced?

    Has anyone got a circuit which can adjust the phase of a fairly hefty incoming (from the magneto) alternating signal, perhaps 100volts and an amp or two before it enters the CDI?

    Original question:
    I have an 80cc Chinese two stroke engine on my bicycle and wish to experiment with changing the spark timing. Apparently the magneto gives off an alternating voltage of around 100 volts which is used both to power the CDI unit and as the spark trigger signal. The alternating voltage (and trigger signal) frequency is directly associated with a single rotation of the motor and rotating magneto magnet which is attached directly to shaft of the piston crank.

    It would appear that the alternating signal is stored in a capacitor in the CDI unit until the spark trigger signal is processed (once per rotation), whereupon the charge in the capacitor is release into the ignition coil causing the spark.

    If anyone can give me a circuit to adjust the spark trigger signal timing I will be most grateful.
     
  7. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,034
    1,633
    Just pull the flywheel and use a offset key on the crankshaft.

    You will be time, money and simplicity ahead plus have far less of a chance for the thing to destroy itself too.
     
    shortbus likes this.
  8. solmillin

    Thread Starter New Member

    Aug 18, 2016
    22
    1
    I think you're right. A mechanical solution certainly would work.
     
  9. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,034
    1,633
    It can be done electronically but to do so reliably requires considerably more electronics than most people would care to try and design and build in order to make it work. Especially so for a odd or old engine that does not support anything but the bare basics for an ignition system like yours.

    From personal experience and experimenting done years ago in college the most effective 2 cycle motorcycle ignition design I could come up with was a simple high voltage flyback transformer (from an old CRT monitor or TV I junked out) based design that was set up so that whenever the points were open the flyback transformer primary side driver circuit (basic two transistor push-pull oscillator) would fire up at whatever resonance frequency it wanted to run at (25- 35 KHZ more or less) and stay on until they closed giving the system a hot spark with very long dwell time that did wonders for an old two cycle engines operation. :cool:

    It worked surprisingly well for it's simplicity! Good starting, idle and running power from old junk parts. :D

    Then one week while I was at school my brother and his friends ran it on straight gas and destroyed the engine beyond reasonable repair. :mad:
     
  10. SLK001

    Well-Known Member

    Nov 29, 2011
    812
    225
    It used to be that magnetos has some degree of adjustability with their mounting. Like two screws through two oblong holes - gave a degree or two of advance/retard of the spark.
     
  11. solmillin

    Thread Starter New Member

    Aug 18, 2016
    22
    1
    These cheap Chinese engines are absolutely solid with their stator and rotor mounting. No adjustment at all. The rotor is fixed to the shaft by a pin and the stator with four bolts to the body of the motor.
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    They always used to have a stator plate that you could slacken 3 screws and rotate it a small amount.

    With most ignition sensors of this type, automatic advance is inherent in the design - as the rotor "pip" approaches the trigger coil faster; the voltage risetime increases.
     
  13. solmillin

    Thread Starter New Member

    Aug 18, 2016
    22
    1
    Yes, it's interesting, the 'pips' at the end of the rotors. It would be good to understand why this effect happens.
     
  14. solmillin

    Thread Starter New Member

    Aug 18, 2016
    22
    1
    Here's an interesting thought... using a transformer to change the phase 180 degrees... I'm sure by including other elements in the circuit we could make the phase variable, and thus vary the spark timing through the CDI... Voila.

    Has anyone got more thoughts on this and/or can add some specific electronic detail to enable variable phase shift and thus spark timing?

    I believe the alternating drive voltage from the magneto stator is around 100 v a/c.

    I wonder if there's any relationship between phase change through a transformer and frequency (ie. speed of the engine and hence the magneto)? All thoughts gratefully appreciated.
     
  15. solmillin

    Thread Starter New Member

    Aug 18, 2016
    22
    1
    blank
     
  16. solmillin

    Thread Starter New Member

    Aug 18, 2016
    22
    1
    blank
     
  17. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,034
    1,633
    Goose? o_O
     
  18. solmillin

    Thread Starter New Member

    Aug 18, 2016
    22
    1
    Can't quite work out how to delete a comment once it's started... hence the 'blank' 'blank'

    Anyone know how?
     
  19. GopherT

    AAC Fanatic!

    Nov 23, 2012
    5,997
    3,756
    For some weird reason, the ability to delete posts is disabled after some minutes.
     
  20. GopherT

    AAC Fanatic!

    Nov 23, 2012
    5,997
    3,756
    A mechanical solution is possible and can be done by adding a small piece of metal to the steel rotating in front of the magneto - depending how it is set up in the little motor.
     
Loading...