AC powered CDI ignition advance?

Discussion in 'General Electronics Chat' started by Classictrial, Aug 29, 2010.

  1. Classictrial

    Thread Starter Member

    Aug 23, 2010
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    I wonder is there likely to be any feasible way of advancing the spark position of an AC powered CDI ignition system, so it provides max total advance of something like 30 degrees at 3000rpm?

    The system I am thinking of has an internal rotor, and a trigger coil incorporated into the stator, which works from the magnets in the rotor itself. From tests I have done with these systems, it seems as though they quickly advance 17-18 degrees at around 1000rpm.

    Problem is that for best possible running a 4T motor needs to have max ignition advance at the position where max torque is generated, which for this application is around 3000rpm. These AC systems also seem to suffer from timing scatter as engine speeds increase, and I have found running is sometimes improved by re-fitting points triggered inductive systems!

    Only thing I have seen to help with this is a very crude box which delays the point where the start retard feature is switched off................however while in some cases these improve low speed operation, they seem to drastically effect higher speed running.

    Its pretty much impossible to find any accurate info on the above, and there seems to an enormous amount of confusion over these AC CDI ignitions, even to the extent that some people believe crude analog systems are "digital" and can be programmed in some way!
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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    There are currently two threads running about motorycle CDI systems, search through here and the Projects forums. One is titled "capacitor degradation - can't recall the other one.
     
  3. Classictrial

    Thread Starter Member

    Aug 23, 2010
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    Have spoken to a couple of people about advance curve relating to analog CDI systems, and they advise its not possible to have anything other than a start retard function, and also suffer from the problems outlined in my earlier post.

    Not 100% sure that what they are saying is correct though, as both manufacture DC digital CDI and inductive bike ignition systems, and obviously are not that interested in providing accurate info on AC self generating CDI set ups.
     
  4. enduro250z

    Member

    Jul 6, 2010
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  5. marshallf3

    Well-Known Member

    Jul 26, 2010
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    You keep forgetting - unless you can move the sensor to a maximum advance position then include adjustable retard it gets difficult. Since no amount of electronics can "predict" where the crankshaft is it would have to take the last known firing pulse then delay it by 360* less the amount of advance you want to introduce. It will now fire early, but based on the RPM it was turning during the last revolution.
     
  6. Classictrial

    Thread Starter Member

    Aug 23, 2010
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    On the Honda engine I am developing currently we have a mechanical advance unit, which works in conjunction with pulse generator and CDI............this is a stock Honda part from an earlier model bike, and works far better than the later system which is triggered by a magnetic slug on the outside of the flywheel.

    I take in what you are saying about the 360 degree delay Marshall, but wonder if its possible to incorporate that into a purely analog AC powered CDI system?
     
  7. enduro250z

    Member

    Jul 6, 2010
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    Classictrial, what Honda do you have? I might know of an off the shelf replacement ignition for it or can maybe be of more assistance if i knew what model.
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    I take in what you are saying about the 360 degree delay Marshall, but wonder if its possible to incorporate that into a purely analog AC powered CDI system?

    I really need to get on the road to work but the quick answer is no unless you were to design a power supply circuit that took part of the HV signal and got it down to something useable for powering extremely low current electroics. The HV winding would be hard to harness from as the output voltage is going to vary wildly.
     
  9. Classictrial

    Thread Starter Member

    Aug 23, 2010
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    Its a TLR trials bike..............AC CDI system, with external flywheel pulse generator pulse coil.
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Most are very similar in the way they operate. All I'm saying is that to introduce any sort of electronic control it should be done in the way they do it on bikes that have it.

    The pickup sensor is mechanically placed at the point of maximum advance, the electronic control circuit delays the triggering pulse by a certain amount of milliseconds depending on the RPM the engine is running at.

    My bike ues two HV windings, one is mainly effective up to around 5K or 6K, after that a coil that can output a slightly higher voltage also comes into play. The timing makes a change around that RPM range too and is controlled by a stepped rotor in the separate coils responsible for generating those signals. At lower RPMs only the pulse from the more closely spaced trigger magnet can fire the SCRs, once RPM has increased to the 5K - 6K range enough voltage is induced by the step that's a bit father away to trigger the SCRs a bit sooner.

    As you're finding, it's really pretty hard to modify these very basic setups, thus the reason a lot of us try to get our power gains by modifying the intake, exhaust and carb jetting. Sadly, through tons of experimentation we've found that our oddball series of bike was pretty well designed for maximum power from the factory and whille aftermarket stuff can alter the way the bike sounds or slightly vary the torque curve the net result is really nothing.

    Your bike however may have several options for improvement such as a better tuned exhaust system or switching over to a larger carb, however be prepared to go through a lot of carb rejetting to realize the full effect of any of these add ons.

    Allthough available as a drop-in, we can't even add a K&N air filter to ours. If you vary from the factory paper design it upsets the way the CV carbs work and if you change the exhaust it also greatly affects the mixture.

    At a mere 500 cc it will best any but the newest fuel injected Harleys so why bother? It will also outhandle most of the newest bikes in winding road conditions. I used to mod the heck out of my dirt bikes but a 13 second 1/4 mile on a street bike is plenty fast enough for me. I can also pull the front tire off the ground going into 2nd gear so there's plenty of scare factor because with a 10K redline you're already going pretty fast when that happens.

    If I wanted faster I'd buy a bigger bike but long ago I found that once you've hit around 140 mph it's all a blur anyway.
     
    Last edited: Sep 2, 2010
  11. Classictrial

    Thread Starter Member

    Aug 23, 2010
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    I am currently running bigger carb with divider plates fitted, custom made exhaust system and air-box, different cam, higher compression piston, and modified cylinder head.

    Using the stock AC CDI system which is triggered from a slug on the flywheel, all these alterations didnt work that great. However using a system from an earlier machine, with a proper linear advance curve provided by a mechanical advancer driven by the cam, provides around 30% power increase over the stock motor.

    The older system though is not a simple bolt on, and entails a different cam, and obviously all the parts required from the earlier bike. The stock style tank will not fit either, and must be modified to clear the ignition housing.

    Hence my interest in achieving the same type of running, without the mechanical advance mechanism. Seems the only way to achieve this is through the use of a digitally controlled DC system, which on this application can be supplied from a rectified feed from the bikes alternator.

    Last year I was very close to having a prototype version of a digital inductive system up and running, but the manufacturer had little grasp of the requirement for it to be 100% waterproof, or that the size of the unit was extremely important.

    I have noticed Vortex in Australia supply digital CDI units for MX applications which seem interesting, but cant quite grasp how what seem to be pretty basic AC systems are able to run digital CDI boxes? Needless to say emails to Vortex have been ignored!
     
  12. enduro250z

    Member

    Jul 6, 2010
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    TLR as in the XL250 motor from the 70’s? If so I know guys put all sorts of CDI’s on them for VMX use. I might ask around.

    Some links that may be of interest

    http://www.zeeltronic.com – has magneto CDI
    http://www.ignitech.cz

    Ahh, yes all the modern Motocross bikes, well at last the 2strokes and everything from the 90’s, they all have sort of fancy modern ignitions but they have no battery. And then theres Vortex, they offer programmable CDI’s for bikes with AC systems, how do they run these ignitions??????????????
    Ive thought of buying a cheap used Votex ignition off ebay to pull apart an investigate.

     
  13. Classictrial

    Thread Starter Member

    Aug 23, 2010
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    The TL used a Kettering system triggered by points mounted on the end of the camshaft, so is not the same as the TLR CDI system.

    I would be very interested to know how the Vortex CDI units work, but would suspect they are probably analog units, with switchable internal circuitry to vary the advance curve slightly.............cant see how a digital system can work without a battery or rectified AC power supply?
     
  14. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Given the correct resistors, diodes, protection zeners, filters etc you could *probably* steal a very small amount of current from the HV winding; at least enough to keep a small supercap charged up in order to run a tiny processor.

    Again the problem would arise in the fact the pickup sensor wouldn't be at maximum advance you could retard from, you'd have to use the "360* less" (2 stroke) or "720* less" (4 stroke) method to introduce advance dependent on the RPM the crank turned on its last revolution.
     
  15. Classictrial

    Thread Starter Member

    Aug 23, 2010
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    We had a system like this up and running last year, powered by a rectified AC supply from the lighting coils and capacitor. Only problem being that the manufacturer had little or no grasp of the need for 100% water resistance, and the control unit was simply too big to fit in available space.

    A company in Australia (Vortex) seemingly offers "digital" CDI units, which seem ideal for dirt bike applications. They wont reply to emails though, and there seems to be little technical content on their site which explains how a "digital" CDI works in conjunction with an AC supply....................
     
  16. marshallf3

    Well-Known Member

    Jul 26, 2010
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    If it's the same Australian company I'm thinking of they were also working on a replacement for ours but seem to have lost interest in their projects.

    Yea, if you've got a lighting coil it's easy to get 6V or 12V to work with.

    The absolute major problem we all seem to face is the inability to move the pickup. On some bikes you might be able to drill a new hole and/or custom machine something that would move one or the other side of the equation.

    I feel pretty lucky that the particular oddball run of bikes like I own were initially designed with optimum values. People have tried changing about everything but the only power anyone's ever gained has been psychological or they just moved the torque curve.
     
  17. Classictrial

    Thread Starter Member

    Aug 23, 2010
    30
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    Maybe the way round this is to use either dual pick ups, or toothed wheel type plate?

    Seems like there are plenty of ready made DC digital control boxes available, but personally I have been confused by those such as Vortex who seem to be claiming to offer analog CDI's which do pretty much everything you would expect from a proper digital control system?

    In older off road applications the stock ignition systems are in most cases far from ideal for purpose, and I have found that by simply providing a proper linear advance curve that performance is greatly improved.

    Most motors will run better on inductive rather than CDI systems, and longer spark duration seems in most cases to count for more than the shorter higher intensity spark of the CDI systems.

    There are obviously any number of snake oil sellers providing ignition system updates, which as you rightly say actually do very little to improve performance, and I guess in some cases may mean negative effects.

    I wonder is there any definitive information relating to the design and manufacture of ignition systems, as I for one am very confused over the claims of those such as Vortex, and cannot grasp the seeming direct relationship between analog and digital, that is also advertised by those such as PVL?
     
  18. marshallf3

    Well-Known Member

    Jul 26, 2010
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    You got me. Since you can't really alter the basic laws of physics that leaves you to either mechanically alter or move the pickup sensor setup or use the method I've described which, in effect, is actually retarding the ignition by almost a full compression stroke.

    A company called Ignitech has somewhat successfully come up with a replacement for our CDIs. No change in the timing though, it just eliminates the hassls of trying to replace the old CDI box (very scarce) or replacing the rotor when the HV coil(s) fail.

    In essence they're just using the 12V system to power an inverter which creates the HV then pulsing the new SCRs with the original timing signal.

    Aside from a few simple ones here and there (most of which are just common rubber seals) the three major failures one can expect to see within the first 100,000 or so miles on these bikes is either/or/and

    1) CDI box
    2) Rotor assy (can be the HV or the charging winding)
    3) Cam chain & associated tightening components.

    I've got less than 10K on this bike so the first thing to go will probably be the CDI due to age, and I've already got a spare from a salvage bike. If need be I've got the schematics of the original and can easily duplicate it.
     
  19. Classictrial

    Thread Starter Member

    Aug 23, 2010
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    The guy that made the prototype TLR digital system last year is an ex F1 engineer, and the toothed wheel trigger was used on F1 engines.

    What bike do you have I wonder? On most recent road going machinery the stock systems seem reasonably ok, and probably likely to work acceptably...........Not so when you start to modify though, and running can obviously be improved if you can adjust ignition to work with other changes.
     
  20. marshallf3

    Well-Known Member

    Jul 26, 2010
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    http://en.wikipedia.org/wiki/Honda_CX500
    http://shoc.hu/data/files/news-228/honda_cx500c.jpg

    Specifically a 1979 CX500 Custom almost perfectly restored to stock. The specs in the article are somewhat conservative. There's a lot more history to be found but word has it the bike was conceved under direct supervision of Mr. Honda in some sort of challenge he had with the president of Motoguzzi.

    If you ever find one they're extremely easy to get running but trying to restore one can become challenge due to a lot of the parts being one of a kind. If you can accept less than near perfect they aren't hard to restore at all. To get perfect replica silencers I had to send around $240 to Europe. I have no back rest but will have a mid-size slightly tinted windshield. Color will be a blue very similar to the new blue that Harley came out with this year, it's a GM color called Medium Gulf Blue Effect. It's a perfect bright blue unaltered by the addition of any red or other color, just a very fine metal flake and a touch of pearl.

    While it was quite rideable when I started the restoration it's taken quite a bit of time to get everything right. At present I'm pretty well down to the paint and polish stage.
     
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