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CD Ignition and its Superiority in Overcoming Shunt Resistances

Discussion in '206/246' started by Fred Winterburn, Dec 6, 2018.

  1. Fred Winterburn

    Fred Winterburn Karting

    Jan 27, 2015
    58
    Folks, I posted this on a Porsche forum recently and was lambasted for spelling and grammar without much technical feedback which I found surprising. I've cleaned it up a little so it should be more readable. Anyway, I had an impromptu experiment that solved many a paradox when it comes to ignition systems and in particular the superiority of CDI in overcoming spark plug fouling. This has not been published by anyone before to my knowledge, but finally explains the real reason why CDI is superior in overcoming a shunt resistance compared to even high energy inductive ignitions. I hope you enjoy reading the attached PDF. Fred
     

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  2. Bluebottle

    Bluebottle F1 Rookie
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    I am sure I would have enjoyed reading it if I could understand it, but, alas, it is way over my head, but that is my fault:(

    It seems a shame that, when you have obviously gone to a lot of trouble to share your expertise, you get a load of s**t for spelling and grammar, instead of the thanks that you deserve. Sharing knowledge like this is what these forums should be all about and I hope it will not deter you from doing so in the future.

    All the best.
     
    Skippr1999 and racerboy9 like this.
  3. DinoLasse

    DinoLasse Formula Junior
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    Fred, it is not an easy read, but I did enjoy it.

    The concept of inductive and capacitive loads with lagging vs. leading currents is not easy or intuitive to grasp for those casually interested in electronics. Electrical engineers get it but hardly anyone else does. That is probably why you have not had any comments on the technical aspects.

    Your reasoning sounds correct, though, and you are making a good point. A CDI and an inductive system may put out voltage pulses of similar rise times, but they will behave very differently when encountering a capacitive load (spark gap) in parallell with a resistive one (fouled plug). The CDI system with its leading current should in fact fire the spark gap before it has time to cut tracks through the more resistive carbon deposits. The inductive system with its lagging current may or may not, depending on how bad the fouling is.
    The theory is certainly correct. How all this plays out in an engine under real conditions may not be as clear or obvious, but is certainly seems to give an extra feather in the cap for the CDI system.

    Thanks for sharing you observations and theories. Most interesting. / Lars
     
  4. Fred Winterburn

    Fred Winterburn Karting

    Jan 27, 2015
    58
    lars,
    Thanks for that. I now have two EEs that believe my theory is correct. Another fellow that repairs CDIs in California was certain it was fast voltage rise time until I sent him the short article, but realized he had been fooled too, and he has been doing this for a great many years. I have read hundreds of articles including several SAE papers, including articles by Bosch, and none of them attribute the ability to overcome fouling to anything but fast voltage rise times. The exception is an article by a Mr Holden, but although he didn't believe it was fast voltage rise times per se, he assumed that inductive ignition was just as good because the rise times are similar. The reason he drew the wrong conclusion in his article is that he did not complicate his experiment by having a spark gap in parallel with resistances. (I know this through correspondence with him 3 years ago) Instead, he simply loaded the secondary of a coil with various resistances and called them 'shunt resistances', when they weren't actually shunting anything. So he drew the wrong conclusion. I have included his article here for reference. It does show how even the most brilliant can be fooled.
    Even my father, who was a real electronics expert, from radio, to radar, to power electronics, never once mentioned anything other than fast voltage rise in any of his CDI notes. He did mention inductive lag, but that was only referring to the ability of CDI to work well at high rpm while the Kettering system does not. He was not an EE, but was building radios when he was 8 years old and teaching electronics in the RCAF at age 21. Not bad for a high school dropout. I'm not an EE either, and quite frankly I wouldn't have made the power factor connection if I hadn't spent several years in the control room of a nuclear power plant. Fred
     

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  5. TonyL

    TonyL Formula 3

    Sep 27, 2007
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    very interesting Fred, thank you.
    Don't understand it though but get the general idea, however any benefits if you don't get spark plug fouling, lets face it SPF is a symptom of a poorly set up or high mileage knackered engine so does it improve it.?
     
  6. Fred Winterburn

    Fred Winterburn Karting

    Jan 27, 2015
    58
    Tony, It will help with any type of shunt resistance not just plug fouling such as moisture on spark plug insulators or wires. Any time there is a misfire that finds a track to ground is not only lost combustion, but also begets more misfires as the tracking gets worse and components need to be changed out more often to restore performance. So yes, it does improve things even if the plugs aren't prone to fouling, and that doesn't take into account any other advantage of CDI. Fred

     
  7. TonyL

    TonyL Formula 3

    Sep 27, 2007
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    Tony
    Thanks Fred, always welcome your technical knowledge.

    I was always under the impression that with fouled plugs the spark has to go somewhere and that it will find the weakest part in the HT system, with good condition spark gaps / clean electrodes etc the earth track has less resistance so it will naturally follow the easiest path and basically bypass (up to a point) any deficiency in the HT wires.

    What type of HT leads do you recommend? Also I use the NGK BP7ES in my 246 and they work perfectly, if I substitute with the iridium plug I get fouling!

    I use the standard distributor with the Superformance S4 ignition module (changed from the useless Black Stallion module) which I fitted into the old dinoplex box. Hot and cold starting is fine and runs good to 7000pm on the dyno.

    does your new discovery alter the unit you sell?

    Kind Regards

    Tony
     
  8. Fred Winterburn

    Fred Winterburn Karting

    Jan 27, 2015
    58
    Tony,
    Yes, with fouled spark plugs, if the energy doesn't go through the spark gap it will go through the fouling deposits instead. That won't hurt insulation, but it surely won't help ignite fuel either. Wet insulation on the other hand, or poor insulation that provides a shunt resistance will damage insulation while at the same time preventing ignition in the cylinder. And then eventually the plugs foul too from successive misfires.
    The ignition energy doesn't always have to find its way to ground on a misfire. It can be contained within the system and dissipate as heat rather than punching through insulation, but with an inductive system, or a very high voltage CDI (which is why too much voltage is bad) that is more likely to happen. With a high voltage CDI, on a misfire the voltage will rise to the available voltage determined by the voltage the discharge capacitor is charged to and the turns ratio of the coil. With an inductive system, the voltage will rise until the energy runs out or is clamped by a zener diode, or the condenser in the Kettering system. The difference is, CDI is less likely to create a misfire because of the higher immediate power and the power factor advantage that helps even when there is no fouling.
    This discovery of mine does not alter anything, but has solved several experimental observations that seemed contradictory. This theory ties it all together and things make sense now. I am going to re-write my article on voltage overshoot because this ties in with that as well.
    I don't own a Dino, but I like Taylor Spiral wound suppression HT wires. Just don't use the carbon wick variety. I use Taylor wire on my Volvo 1800. On my Morgan I use cheap bumble copper conductor wire and it contains the energy of the relatively low voltage CDI really well. I recommend making sure that secondary side resistance is no higher than 10 thousand ohms from the wires, spark plugs, spark plug boots(if they are the resistive type) etc.
    If you are getting fouling with the iridium plugs, you could try opening up the gap just a little. The fine wire reduces the voltage required to fire the plug and this can lead to fouling more quickly. Try another 5 thou and see if it doesn't work with your inductive system. Fred

     
  9. Fred Winterburn

    Fred Winterburn Karting

    Jan 27, 2015
    58
    Tony, regarding the iridium plugs I should have mentioned that opening up the plug gap will increase the intensity of the spark (higher power) with inductive ignitions and will blast off fouling deposits more easily. On old tractors or cars that used very narrow plug gaps, carbon would build up and make the plug gap narrower resulting in a weak spark, plus cause some shunt resistance to compound the problem. Misfires would occur. Pulling a plug lead and making a second gap would raise the voltage threshold and sometimes get the plug to fire. It was a good way of tracking down a miss to find out which cylinder was at fault. Fred

     
  10. TonyL

    TonyL Formula 3

    Sep 27, 2007
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    Thanks Fred, I am using a good quality silicone lead at the moment and all good so far.

    Question - does all this affect spark (burn) duration?
     
  11. Skippr1999

    Skippr1999 Formula 3
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    Dec 22, 2009
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    #11 Skippr1999, Dec 8, 2018
    Last edited: Dec 8, 2018
    Thanks for the info Fred. I haven’t installed the box you sent me yet but will soon.
    Should I keep the original ignition coil or do you recommend a different one ?
    My plugs are NGK
    Bpr7Eix

    Skipp
     
  12. Fred Winterburn

    Fred Winterburn Karting

    Jan 27, 2015
    58
    Tony, Siicone is the jacket, but wha type of conductor is inside? With an inductive ignition two things affect the spark duration. Energy is the first, so if it is wasted in high resistances, that will result in a shorter spark. The second is plug gap, so the wider the gap the more energy that is consumed at the start and so less is available during the long trailing glow phase (that is probably useless anyway) Fred

    "TonyL, post: 146341396, member: 54594"]Thanks Fred, I am using a good quality silicone lead at the moment and all good so far.

    Question - does all this affect spark (burn) duration?[/QUOTE]
     
  13. Fred Winterburn

    Fred Winterburn Karting

    Jan 27, 2015
    58
    Skipp, the retrofit is designed to be plug and play for your original box, so you can use the original type coil. If it ever fails, you can replace it with something less expensive or available. Old coils can be failure prone. I would use a 1.5 ohm inductive type canister coil if that happens. There will be less energy transferred(although I have never tested an original two terminal Dino coil) but you won't notice the difference in performance. Fred

     
  14. Skippr1999

    Skippr1999 Formula 3
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    Dec 22, 2009
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    Thanks Fred
     

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