Chinese engine, CDI ignition or what ??

Discussion in '2-Stroke Engines' started by duivendyk, Jun 1, 2008.

  1. duivendyk

    duivendyk Guest

    In a recent post I explained how CDI ignition systems work,this info was based on the CDI's as found in Honda mopeds & scooters.I stated there that I was not familiar with what was exactly in the Chinese engines.
    After looking around in the forums I concluded that their system may not be a CDI setup, after all,but a sort of distributed quasi magneto based affair.
    There is an ignition module that supplies the spark,which interfaces with the engine power coil in the flywheel and is probably just an ignition stepup transformer,nothing more.
    The ignition module has a ground connection (black),and a power coil connection (blue) from the flywheel generator, which energizes the coil's primary,the output is the HV ignition pulse to the plug.
    The reason it cannot be a CDI module is that it lacks the essential trigger pulse connection from the engine flywheel.With the kill switch (blue & red wires) the input to the coil can be shorted to ground,killing the engine.What then about the "white" mystery wire coming out of the engine which people have used to power lights with.Good question,my best guess:it's a "tap"on the power coil, supplying a lower voltage ac. output and can be used to kill the engine like the blue wire but in a safer way.With some diodes & savvy you could probably charge a battery with it,without messing up the ignition
    The RFI (radio frequency interference) attributes of this allovertheplace ignition setup are pretty bad, but not dangerous as some people who could not get their digital multimeters to work, were concerned about.(Unless you are fool enough to grab onto the bare "blue wire")
    These instuments are broadband and use sample& hold techniques which make them quite vulnerable to any kind of interference.The best way to protect them in this polluted environment is the following:twist the test leads around one another as much as possible and put a capacitor across the meter input as close as possible( 0.1 to 1 microfarad,preferably a ceramic cap).An ancient technology analog meter will probably work a lot better.
    I could give some pointers on how to reduce EMI, if anyone caress to know,most of what people come up with (ferrite beads copper jackets etc) is quite ineffective when dealing mainly with low frequency stray magnetic fields,as is the case here.

    Last edited by a moderator: Jun 1, 2008

  2. spad4me

    spad4me Member


    Most motorized bicycle riders want reliable power output.
    A solid metal core sparkplug wire and a non resistor plug.
    A rf friendly coilwire and a resistor plug with some thought to close as possile placement to the plug location will help with rf.

    I enjoy the performance of a solid metal plugwire and a nonresistor plug, I also notice the smell of castor and of fried thigh as I ride merrly along. LOL
  3. BSA

    BSA Guest

    The magnetos output is AC so I would guess that the magneto rotor can only be fitted one way so I would guess that a part of the sine wave is coordinted with TDC(or just slightly before) on the engine, allowing the CDI to know when to fire.
    That would make sense as if you connect the CDI in with the wrong polarity it will spark but at the direct opposite of TDC.

  4. duivendyk

    duivendyk Guest

    It is indeed possible to do that in principle,but not all that easy to implement,getting the timing right would be a challenge.The charging pulse amplitude is proportional to rpm and the duration inversely so.Ideally ignition should occur after the capacitor is fully charged.By the way this ac voltage is not a really sinusoidal,rather it is a single cycle ac pulse (pos/neg or visa versa) during a portion of the revolution,followed be an idle period.You can dream up all kind of circuits that process this input to derive a discharge trigger,for instance when it starts decreasing.What puzzles me is that ignition problems are common with these engines and people report remedies that serve to get more voltage to the plug.(Solid wire cable, no resistor plug,small plug gap).The main claim to fame of CDI systems is that the capacitor discharge produces a very high voltage, short duration pulse that will fire just about any plug regardless of condition.That had me puzzled,hence my query.It may be that the pulse transformer in the ignition unit is not adequately insulated, that can lead to gradual insulation breakdown (carbon tracking),this leakage starts to bypass the discharge path to the plug.This would fit the symptoms.Most people couldn't care less of course what kind of ignition it is,they just want it to work.
  5. eljefino

    eljefino Member

    I find myself looking at briggs & stratton, they got rid of points ignition in 1982 for a real simple module that has a plug wire, a coil, a "brain" and an iron horseshoe all in one unit. The flywheel has magnets that power and time the spark discharge.

    I have to think this chinese system is not too different in design-- except that the coil is "divorced" from the horseshoe by a length of blue wire.
  6. duivendyk

    duivendyk Guest

    I very much doubt that the B&S system was a CDI system.That was pretty much what I was saying essentially in my original post about the Chinese 2 strokes.The key distinction between a magneto or for that matter any coil based ignition system as contrasted with a capacitor discharge system is wether the energy storage device for the spark generation is an inductor (coil), or a capacitor.There are basic differences in the nature of the spark discharge,a capacitor based system generates a intense discharge of short duration,whereas as an inductor based system produces a series of less intense sparks over a considerable longer period.This is the preferable way to reliably ignite the micture.It happens to be cheaper to store energy in a coil (which you need anyway) than a capacitor.Therefore magneto based systems typically have a lot more energy available per discharge,(4/5 times as much).It is not impossible to come up with a capacitor discharge system that provides a longer ignition period (multiple spark systems used in race cars etc),but these are certainly not cheap.They require a seperate high voltage supply.In the B&S system there may have been be a built-in trigger pickup to open the primary circuit.Another thing,good high voltage caps are neither cheap nor small.Why would someone build something rather inherently expensive (because of the high voltages involved) unless there is a good reason to do so ?. But until some one knowledgeable actually dissects one of these black boxes, we'll never know for sure.
  7. jaguar

    jaguar Well-Known Member

    Your assumption about this ignition system is 100% off. This is a true CDI that uses the negative part of the AC output from the magneto stator coil to "trigger" the capacitive discharge to the built in coil. It is exactly the same type as what Kawasaki put on their KDX motorcycles but without the few extra components to retard the timing at high rpm's. This Grubee engine CDI is made for 4 strokes because it continues to advance the spark timing with engine rpm. A true 2 stroke CDI advances the ignition up to a moderate rpm and then little by little retards the ignition to keep from overheating and melting the piston. I have modified the KDX design to match the stator of these engines and, along with a motorcycle ignition coil, achieved around a 10% gain in uphill speed. Also my bike revs oout farther without overheating. Read more about it by clicking my signature link.
  8. Dave C

    Dave C Member

    I got the coil now I'm just waiting for the box :) I know about shipping times, I buy things from Hong Kong and shipping times from there can be as little as 5 days, most likely right a two weeks and if it's something you really need more like a month :p. Even if it gets here tomorrow I'd still have to wait. After installing hundreds of rings I broke my first one ever and I am waiting for new ones to arrive :/