CDI resistance to plug fouling, the real reason.

[Fred Winterburn]

Folks,
I know this is bound to create some controversy, however I decided to post it anyway. I have known for a very long time that a fast voltage rate of rise was not the reason why CDI is far superior to an inductive ignition in overcoming a shunt resistance (moisture or carbon fouling). Scope shots reveal that if a particular coil is used inductively and switched with a transistor the slope of the voltage rise is nearly identical to when a CDI is driving the coil. For a few years now I had assumed the superiority of CDI in overcoming a shunt resistance had to do with sheer power. It turns out that it is done with more finesse, rather than brute strength. Please read the attached article. Please reserve comments to technical content. Fred

I have learnt something that I should have known already with the amount of experimentation I have already done, but somehow I missed this one. I had assumed for quite some time that the ability of CDI to resist fouling and still produce a spark was due to sheer power, but this is not the major factor. Nor is it voltage rise time because it can be proven that voltage rise time is more a function of the coil characteristics and is similar no matter whether CDI is being used or the coil is used conventionally. Voltage rise time certainly must matter and so must available power, but those two factors are not the main reason why CDI is superior in firing fouled spark plugs. I was rather surprised and amazed at the experiment last Saturday. I have never seen this experiment reproduced by anyone which is quite remarkable really considering I am more of a hobbyist than a manufacturer.*

On Saturday the 7th of October, 2018, I was re-testing a CDI before I sent it out to a customer. A sudden moist warm front had come through the area and the inside of my shed where the test equipment is located became dripping wet with condensation. Things were so damp I had to wipe the meter faces on the instruments to read them. As part of the test I have the switch in STD mode to prove the switch works and that the standby Kettering system will work if required. With the CDI switch in STD, and the spark gap on the test machine set at 25kV, there was arcing to ground down the ceramic insulator post for one of the spherical electrodes of the spark gap. Only 50% of the time was the spark crossing the actual spark gap, the rest of the time it was shorting down the wet and dusty ceramic insulator. If I opened the spark gap to 30kV, all of the ignition energy shorted down the ceramic insulator with no spark at all across the electrodes. (the ceramic insulator is about the same size and length as a spark plug insulator)*

I decided to see how much worse or better it would be in CD mode. It was better beyond my expectations. I fully expected with the increased power of the CDI that it would arc down both the ceramic insulator and the spark gap, IE two sparks in parallel, and that it would be able to do so close to the upper voltage limit of approximately 30kV for the CDI (with a standard canister coil). However that is not what happened at all despite the higher wattage available with CDI. Instead, there was absolutely no shorting down the insulator post at any spark gap. All of the sparks were across the spark gap where they should be. Even more interesting, is that with the spark gap opened beyond 30kV and beyond the available voltage of the CDI, there was still no shorting down the ceramic insulator !*

My present working theory is that it has to do with power factor. A spark gap behaves like a leaky capacitor before breakdown occurs, IE it is a capacitive load. Current(amps) leads voltage by something less than 90 degrees because of resistance in the load circuit, however the load is mostly capacitive. That type of load wants a leading power factor, IE leading VARS (volt-amps-reactive). The dirty insulator is also slightly capacitive, and likely also slightly inductive, but mostly resistive until flash-over when the resistance drops suddenly. A unity power factor where current and voltage are in phase with one another is perfect for supplying resistive loads (no VARS either lagging or leading). In Kettering mode, current lags voltage by well over 45 *degrees (exact value unknown and will vary with coils) *as the field collapses. A lagging power factor results that is excellent for supplying inductive loads (lagging VARS), but not efficient for capacitive loads that want leading VARS (IE, the spark gap!). A large capacitor discharging through the primary winding of a coil provides a leading current or power factor, of some angle greater than unity, but not a full 90 degrees leading. It certainly provides enough of a leading power factor (leading VARS) to make the spark gap the preferred load over a resistive load providing its resistance is high enough.*

Another way of looking at it that might be simpler, is that a capacitive load such as the spark gap, prefers a fast rising current rather than a fast rising voltage and with current leading voltage with CDI, the spark gap must break down first. With current leading voltage, the voltage across the shunt resistance doesn't build up until it is already too late. In the interim, the spark gap breaks down and becomes the low impedance load so 99% of the current flow is through the spark gap every time.*

In addition, an inductive system is somewhat self regulating as far as voltage goes. If a spark doesn't form, the voltage keeps on rising until such time that the spark gap does break down, or more likely, that insulation breaks down. Usually with an inductive system the voltage will rise far enough for current(amperes) to flow somewhere, be it through fouling, or a carbon track inside the distributor cap created through previous misfires. To make matters worse, an inductive system unless extremely powerful will have a higher voltage overshoot than CDI. That is, the peak voltage at the coil secondary will be far higher than the actual break down voltage at the spark gap even when there is a spark. The greater voltage overshoot with inductive ignitions is also due to the current lagging the voltage. The voltage must rise to a higher value before there is enough current available to pre-ionize the gap and provide enough current to actually break down the gap. It takes voltage and current to break down a spark gap (IE power). A weak inductive ignition will show a higher voltage overshoot on the scope compared to more powerful inductive ignition, because the voltage must rise even higher to provide the required current. A CDI on the other hand, can be engineered with the voltage controlled at the source, IE the CDI power supply, and as such the voltage cannot rise higher than the turns ratio of the coil dictates. So, if the voltage is controlled to a reasonable value, insulation is protected despite the higher power of the CDI that can breach insulating materials more easily. This is why it is imperative that CDI be voltage controlled at the source.*

The experiment was inadvertent, but quite useful as it showed as nearly as possible how wet spark plugs would behave in a car engine with both ignition types. Plug fouling inside the engine at the electrodes, or wet electrodes would behave similarly with CDI preferring to fire through the spark gap rather than a shunt resistance. The caveat to this is that not all CDIs will be this forgiving. It is only an advantage if the CDI voltage is controlled to a reasonable voltage, which is not the case for most that have been in production. In my case it explains why I never needed to change distributor caps, or wires on any of my old cars using similarly designed CDIs (Hyland and others designed by my late father, Lloyd Winterburn). Fred Winterburn

Edit: Corrected spelling of shear to sheer and removed a couple of commas. Added line spaces between paragraphs. Reversed order of 5th and 6th paragraph. This will have to do for now. Technical content unchanged.

[boyt911sc]

Fred,

I don’t think you reviewed it before posting. It’s difficult to read and understand a technical report in this format. I don’t know when the sentence is completed or ended. Maybe this is intended for some elite people in this special field of work. I could not follow nor understand the flow of information. Since you asked for comment, this is my feedback.

Tony

[HarryD]

Fred. I agree with boyt911sc that article is very hard to read due to formatting.

I suggest you grab the original text and edit into a better format with a text editor and then repost your original post. I need to present technical documents all the time to a variety of audiences and readability/ formatting is critical to the end user understanding.

[manbridge 74]

I could follow it plenty enough. Those guys who designed the CDI and the engineers who built it were after excellence and got it done 50 years ago.

[Fred Winterburn]

Thank-you, I will edit accordingly. Fred

Originally Posted by VitoV

Then God help us all.

Introduction, body, conclusion. Tell them what you're going to tell them, tell them, then tell them what you just told them.

Also, you suck at using commas. I say that as a well-wisher and interested reader.

Lastly, did you mean "sheer power," or is shear power a technical term that we should be familiar with?

Yours grammatically,
V

[Fred Winterburn]

Vito, I have done a quick edit. For now that's as good as it's going to get. Don't expect any more. I could have brought in the mathematicians and really made it a muddle. Best to stick with plain English. I wrote it as more of a diary of what happened, rather than a true technical article. If anyone that reads it doesn't get the apparent impossibility of what I observed, then they were reading too quickly. It was quite the epiphany for me. As far as I know, the reasoning I proposed for the experimental observation has never been published before which is why I invited comment. It does require some electrical power transmission knowledge to make sense of, but that can't be explained in a short article. One needs to study the subject first. Fred

Originally Posted by VitoV

Thanks, Fred. I am very interested in this topic and look forward to your thoughts coming in a more digestible form.

[Jack Stands]

Thanks for posting your actual experiences. I always enjoy learn8ng more about these systems.

[HarryD]

Looks much better. Thank you.

I need some time to digest.

[DanielDudley]

You are saying Bruce Lee's fist hits the face before the arm can move to block it. There is snap in that punch. The arm is moving to block, but the punch has already been delivered.
I seem to recall looking at a regular points fed spark and a CDI spark on a scope many years ago. CDI is broad, square and fat, but being square, it is all delivered at once. Points are slower and weaker, with a sloped slow climb to a lower peak.

I don't know, it's just an analogy. I have been wondering lately how Iridium plugs would work on these cars and electronic ignition. Seems like they might work better than the old copper cores with CDI.

[HarryD]

Originally Posted by DanielDudley

You are saying Bruce Lee's fist hits the face before the arm can move to block it. There is snap in that punch. The arm is moving to block, but the punch has already been delivered.
I seem to recall looking at a regular points fed spark and a CDI spark on a scope many years ago. CDI is broad, square and fat, but being square, it is all delivered at once. Points are slower and weaker, with a sloped slow climb to a lower peak.

I don't know, it's just an analogy. I have been wondering lately how Iridium plugs would work on these cars and electronic ignition. Seems like they might work better than the old copper cores with CDI.

Confused. Points are a switch. Are you comparing the wave form of a CDI to a Kettering set up?

[Fred Winterburn]

Daniel,
I like your analogy. You might have been comparing Kettering systems using their intended coils to most CDIs that use low inductance coils. In that case, you will notice that CDI has a much quicker voltage rise, probably in the order of ten times faster, especially if you are looking at small engine CDIs that need very low inductance coils due to the small capacity discharge capacitors. Another reason that people think that it is voltage rise is the comparison between Kettering and CDI when using the same coil and there is a condenser at play. You will observe the voltage rate of rise is about 1.5 times faster with CDI. The condenser in the points system changes the natural frequency, so the no-load sine wave frequency is slowed accordingly. And thus a myth is born that it is a fast rising voltage that overcomes a shunt resistance (moisture on a plug insulator or carbon fouling as examples).

If you take that same Kettering coil and switch it with a transistor (pertronix for example), such that the condenser is no longer in the circuit, you will see that the slope of the voltage rise is close whether the coil is used inductively or driven by a CDI. Furthermore, despite the voltage rise being quicker without the condenser, the condenser actually helps with overcoming fouling by a couple of percent over pertronix when all other factors are considered, such as the 1V, semiconductor voltage drop of the transistor switch. From those two observations, it can be deduced that it is not the rate of voltage rise that makes CDI many times better at overcoming a shunt resistance. Also, you can take a modern e-core type coil meant for inductive systems that is more efficient than a canister style coil, and it will still be very poor at overcoming a shunt resistance. With one GM e-core type coil I tested, the ability to overcome fouling was even worse than a standard canister type coil and that was with 8 amps through it. So it is not voltage alone, or energy storage, or voltage rate of rise that is the main contributor to overcoming a shunt resistance.

That's why the experiment was so revealing, and why I stated that it was done with finesse rather than brute force. The only explanation that solves all of the questions is that it is power factor. Fred

Originally Posted by DanielDudley

You are saying Bruce Lee's fist hits the face before the arm can move to block it. There is snap in that punch. The arm is moving to block, but the punch has already been delivered.
I seem to recall looking at a regular points fed spark and a CDI spark on a scope many years ago. CDI is broad, square and fat, but being square, it is all delivered at once. Points are slower and weaker, with a sloped slow climb to a lower peak.

I don't know, it's just an analogy. I have been wondering lately how Iridium plugs would work on these cars and electronic ignition. Seems like they might work better than the old copper cores with CDI.