Tests of CDI Ignition Modules

[Zeb]

Not sure how to approach that one. Do you have an idea of how you would do it?

I did some poking around. There's a lot more to triggering a CDI circuit than just sinking a pin on a programmable IC board. It seems you either have to modify the triggering circuit inside the CDI or mimic a pickup coil input. Definitely out of my hobby league. Looking forward to reading this and some other threads, as all my previous experience is with mags.

 

[dsage]

Well. Don I'm glad you're on the case. The math has always been over my head. I tend to work on a "try it and see what happens" basis. I think a bit more up-front math is involved in order to head down the right road to a worthwhile experiment. It seems you are the man for that. I hope you get somewhere.
FWIW I've always wondered how many Joules is/are actually required in a CDI for a model. I never had much luck determining that. Maybe you can.
From that requirement I thought I might work back to the HV power supply requirement (watts etc). From there comes the task of winding a transformer / oscillator system to deliver that power. And from there I could probably build something. I have no problem building things.
All that said you still have to get the power to a typical ignition coil unless you come up with a small ignition coil that can deliver the requisite power. I think the problem with the current model CDI's is the HV coils are very limited. Size seems to matter.
One model CDI I am familiar with uses a 2uf cap and a couple of hundred volts which is almost typical and could be useful but it delivers it into a pretty useless tiny HV coil so the output power is marginal. More voltage would be desirable but largely wasted on such a small coil.
So I think you are correct the problem is in the tiny coils ability to deliver the energy properly (long spark).

 

[dsage]

I suspect Bill Noble's circuit from post 16 probably works quite well. I've seen many similar to it and built a couple. A somewhat robust power supply, a reasonable sized capacitor dumped by an SCR - into a regular coil. Pretty typical stuff. But for me, overall size was prohibitive. Ok if your putting it under the hood of a car but not model worthy IMHO.
The key will be to figure out the power actually required for a model and scale the normal ignition coil and primary power supply down to suit.

 

[william_b_noble]

you say it is hard to tell if it puts enough energy into the spark .... would it matter if I told you that the exact circuit powered my 356A for half a million miles without a problem?

WRT size, my unit was built on a scrap of aluminum I had handy, probably 4 or 5 inches long, if size mattered, surface mount parts would bring the size down to about a 2X2X1 inch, and using a ferrite core transformer and increasing the frequency accordingly would cut it more. The particular circuit is not intentionally tuned, so it oscillates at the resonance of the magnetics. If you need something small I think I'd look at modern coil packs for low compression cars wtihout overhead valves, or maybe a moped or something.

for example, how about this 12V to 220V Step UP Power Module 35W DC-AC Boost Inverter Module Dual Chan`-m | eBay, a very smaLL boost converter, just add a capacitor and an SCR and maybe a damper diode and you will have a package that can easily hide inside some pillar or someting. 250 V is about what I ran the capacitor on my CD ignition system to, so then all you need is a small coil.

one more thing - for a small engine, one microfarad is more than enough capacitance, 1/2 CV^2, so 500V is 4 times the energy of 250, but what happens is you saturate the magnetic core and you get no benefit, the extra energy just turns into heat. DO NOT!!! use an electrolytic, it won't work, the ESR is too high, use an oil or mylar or equivalent cap designed for rapid discharge.

 

[dsage]

Your EBay link is bad. But I found it searching by your description. Looks good.
As you point out a SMALL ignition coil would also be required. Many moped coils are not that small either. At least compared to the whole size of the current S/S et.al. modules.
It's a good start though.
Thanks

 

[Vietti]

Totally out of my depth, but I have been searching for a long time for a small commercial coil for my magnetos. About the smallest coil I've found is the coil out of a Stihl chainsaw-be sure to get one for an old one with points. Never tried one in a Kettering type setting. Another small coil, though oddly shaped, is from a BSA Bantam motorbike.

Is anyone aware of a smaller coil??

 

[petertha]

I spent some time looking at all the wonderful machine work in your attachments. As for the problems you are having with oscillations in the two ignition modules, it is not obvious what is going on there. One thing occurs to me: The two modules may be triggering each other by coupling through the common power supply (shared battery). It would be relatively east to try a separate battery for each module to see if the oscillations go away.

Just to be clear, the website I referenced is not mine, nor are the engine / ignition system builds. I was just providing as reference to shop made miniature coils & related ignition components. The builder is Ken-ichi Tsuzuki from Japan.

I noticed you tested a CH. Were there any identifiable disadvantages or limitations to the unit or was the testing exercise was more for other comparative purposes? The reason I ask is CH seems popular among the RC crowd, both retrofitting nitro glow plug engines & standalone gasoline, but maybe not slower stationary type. On the surface what appears appealing is: availability of same thread (small) spark plugs to swap out glow plugs, minimal/no CR modification (call it 6-8 range), relatively compact size/weight of module & drive battery intended to be airborne, multi-cylinder modules available (at increasing cost of course). I'm not endorsing or suggesting. I have never run one myself, just a sideline observer thus far. It is a buy-and-run solution, not a shop made solution. I'm considering building an opposed twin for gasoline/spark ignition (from glow ignition plans) to get exposed to all this. Sorry for the digression, I'll watch what unfolds here, maybe there are other options.

https://ch-ignitions.com/product/cdi-1-4-32-straight-spark-plug-cap-ch-single-cylinder-ignition/
https://ch-ignitions.com/product-category/ignitions/5-cylinders-cdi/

 

[doubletop]

Not sure how to approach that one. Do you have an idea of how you would do it?

I have had a go at this as I was having trouble in getting an RCExcel type ignition to work and wondered how it did it. Especially how they worked out the advance (The problem turned out to be my engine and not the ignition.)

I used a PIC microcontroller with a hall sensor to measure the rev interval of the engine using one of the internal timers. With the time information from the previous rev the advance was determined and the using the timer the spark initiated. The advance could be varied depending on engine revs by using an advance curve lookup table. I had trouble getting meaningful results so I added a servo to set the throttle, controlled by a PC via the PIC. That enabled me to set stable rev settings. I then could vary the advance retard and pulse width on a running engine and re-map the advance curve as I went. Of course the one thing I didn’t have was a way of varying the load on the engine.

For an ignition coil I used an in plug unit from a Subaru. They are interesting and can be triggered by a 5v TTL signal so ideal for integrating with a micro controller.



It came to nothing really, once I realised what the RCExcel units were doing I reverted to using them

Pete

 

[DKGrimm]

Looking forward to reading this and some other threads, as all my previous experience is with mags.

Mine also. I think most people would be surprised to find out just how complex this ignition stuff is in real life.

 

[DKGrimm]

I have had a go at this as I was having trouble in getting an RCExcel type ignition to work and wondered how it did it. Especially how they worked out the advance (The problem turned out to be my engine and not the ignition.)

I used a PIC microcontroller with a hall sensor to measure the rev interval of the engine using one of the internal timers. With the time information from the previous rev the advance was determined and the using the timer the spark initiated. The advance could be varied depending on engine revs by using an advance curve lookup table. I had trouble getting meaningful results so I added a servo to set the throttle, controlled by a PC via the PIC. That enabled me to set stable rev settings. I then could vary the advance retard and pulse width on a running engine and re-map the advance curve as I went. Of course the one thing I didn’t have was a way of varying the load on the engine.

For an ignition coil I used an in plug unit from a Subaru. They are interesting and can be triggered by a 5v TTL signal so ideal for integrating with a micro controller.



It came to nothing really, once I realised what the RCExcel units were doing I reverted to using them

Pete

Pete,

With your engine running that well, there is no doubt that the ignition energy is adequate, judging from the quick starting, the idle performance and the RPM range. That's a good data point. What spark plug gap are you using? Because if I measure an RCExcel unit with that spark gap we can start to answer the question: How much actual spark energy is necessary to make an engine run well?

 

[DKGrimm]

one more thing - for a small engine, one microfarad is more than enough capacitance, 1/2 CV^2, so 500V is 4 times the energy of 250, but what happens is you saturate the magnetic core and you get no benefit, the extra energy just turns into heat. DO NOT!!! use an electrolytic, it won't work, the ESR is too high, use an oil or mylar or equivalent cap designed for rapid discharge.

Bill,

Right on! All the energy in the world won't do anybody any good if it just gets lost in the coil. I think the problem goes beyond core saturation. A solenoid-type coil without a closed magnetic path around it is hard to drive into saturation. There's also stray capacitance and natural resonance of the coil. My understanding of that part is fuzzy. I'm hoping to be able to figure out a mathematical model for that somewhere along the way, so people design a better coil specifically to work with CDI designs.

 

[petertha]

You are probably aware of this book?
http://www.model-engine-plans.com/books/

 

[DKGrimm]

I noticed you tested a CH. Were there any identifiable disadvantages or limitations to the unit or was the testing exercise was more for other comparative purposes? The reason I ask is CH seems popular among the RC crowd, both retrofitting nitro glow plug engines & standalone gasoline, but maybe not slower stationary type. On the surface what appears appealing is: availability of same thread (small) spark plugs to swap out glow plugs, minimal/no CR modification (call it 6-8 range), relatively compact size/weight of module & drive battery intended to be airborne, multi-cylinder modules available (at increasing cost of course). I'm not endorsing or suggesting. I have never run one myself, just a sideline observer thus far. It is a buy-and-run solution, not a shop made solution. I'm considering building an opposed twin for gasoline/spark ignition (from glow ignition plans) to get exposed to all this. Sorry for the digression, I'll watch what unfolds here, maybe there are other options.

OK. I understand. The CH unit I tested was borrowed from Paul Knapp. It was an older model with a single unshielded output, and I returned it to Paul before I had set up the capability to measure actual spark energy, so I can't tell you much. I should probably order one or two of the current production units to test. Your links to the CH web site was helpful. Thank you.

 

[dsage]

You are probably aware of this book?
http://www.model-engine-plans.com/books/

I am aware of it and I have a copy. It's gold as far as windiing your own coils goes. I wound a couple of successful coils from it of a reasonable size. BUT that 20,000 turns of #48 wire (or something like that) quickly discouraged me from making any more. I built a winder but it was too slow. You could farm the winding out to a company to wind them for you but I suspect the minimum qty and cost per - unless you want to run a business of it - would be prohibitive. Bob did great work on that book.

 

[DKGrimm]

You are probably aware of this book?
http://www.model-engine-plans.com/books/

Yes, I've had it for years. Bob was a wealth of practical knowledge and experience and wrote well and turned out a huge amount of really good work. He didn't like to use the word capacitor very much, and he didn't think much of engineers, so I had to maintain a respectful distance.

I built an N/C coil winder from an old Unimat lathe and some stepper motors. I wound a few 7/8-inch diameter coils with near 20,000 turn secondaries, but the wire size was only about 0.001 (including the varnish) and the recommended winding tension was 9 grams. I made a lot of scrap, and then had to vacuum pot the coils to keep them from flashing over internally. Took about 12 hours to do a coil, not counting prep. I thought about making some for sale, but how much do you have to charge for something with that much labor, and how do you provide a replacement guarantee for something that can be zapped by just forgetting to connect a spark plug lead?

Don

 

[Zeb]

Yes, I've had it for years. Bob was a wealth of practical knowledge and experience and wrote well and turned out a huge amount of really good work. He didn't like to use the word capacitor very much, and he didn't think much of engineers, so I had to maintain a respectful distance.

I agree. Good stuff in the book, but I wouldn't call the use of capacitor "vulgar". I'll defer instead to John Schwaner on capacitors (*See pg 16). His aircraft ignition book and engine manual are excellent. Pg. 29 has an interesting section about energy release at the plug.

I'm also much more interested in observing the DOE process than purchasing the end result. Staying tuned here.

 

[74Sprint]

Well I'll put my 2 cents worth in and it's long winded. First Don I'd like to commend you on your research, it's nice to see. The only real problem I have is that your results are more to do with the ignition coils than the actual CDI units.

I have been researching ignition/s, combustion, and deflagration (Wikipedia), for over 20 years. This thread appears to deal with all 3 subjects and I'd like to touch on them.

First combustion:
Basically it is the burning of a fuel. It's not an explosion but, rather, hopefully a controlled process of combining heat, fuel, and oxygen. Dave in that motor.com article I have to disagree when they say it is the separation of a fuel molecule because first you have to combine the oxygen with the fuel and that only happens with heat (ionization). It's only after they combined with a sufficient amount of heat (current now) that the fuel burns and breaks down. I live in Canada on the cold prairies and trust me on a very cold day neither a diesel or a gasoline engine will burn fuel if they get to cold. Oh in electronics and electrical we say voltage hurts but current burns you.

Second deflagration:

Deflagration (Lat: de + flagrare, "to burn down") is subsonic combustion in which a pre-mixed flame propagates through a mixture of fuel and oxidizer.[1] Deflagrations can only occur in pre-mixed fuels. Most fires found in daily life are diffusion flames. Deflagrations with flame speeds in the range of 1 m/sec differ from detonations which propagate supersonically through shock waves with speeds in the range of 1 km/sec.

Because of the racing I do I've spent a lot of time researching this subject. It's more than just cylinder and combustion chamber design. It's also more than just the spark at the end of the sparkplug. I don't want a slow burn, I want one that's as fast as possible to build as much cylinder pressure as possible before the piston goes past 45 degrees from TDC. As a fuel/oxygen mixture burns it expands creating a pressure wave in front of it. It is this wave that compresses the air/fuel mixture even more. Because of the pressure the air/fuel molecules are closer to each other, they combine faster and burn faster, split up faster. Think of gunpowder poured on the ground as compared to being in a gun barrel. When the pressure wave goes supersonic this is when we get what we feel is an explosion. All things from simple burning of black gunpowder to a thermal nuclear device go through the deflagration process. I have found that a very strong inductive spark with multi strikes is the best to ignite the fuel mixture allowing me to take ignition timing of up to 10 degrees out of an engine. Which leads me to ignition.

Ignition and Ignition systems:
So we have basically 2 types of ignition systems here not counting thermal ignitions, inductive and CDI. CDI still uses induction just differently. Since we are looking at CDI I'll try and stay on that topic. As most people know with CDI we have to charge up a capacitor with high voltage, well somewhat high voltage of around 100 volts or higher first. We then send that through a step-up transformer to get our HV spark. Sounds simple right. Well it's not if we want it done right. This type of ignition was first invented by a Canadian Air Force pilot back in the 50's. We have different types of CDI ignition systems also such as AC-CDI and DC-CDI. AC-CDI is more like the CDI ignitions we find on our lawn mowers which, is a pulsed charged CDI. The DC-CDI has a DC to AC to DC system where it takes the DC current and converts it to stepped up AC voltage and then rectifies that back to HV DC which goes to the capacitor. Now both capacitors and inductors have what is know as 'time constants' of charge and discharge. There are 6 time constants but, because the 6th is so small of a change we ignore it and go with just 5. This brings us to 'how fast can I charge a capacitor to the energy level that I want'. There basically 3 things we are concerned with here. 1. The bigger the capacitor the longer it takes to charge it. 2. Most CDIs quit sparking at voltages below 100 volts. 3. The ESR or the internal resistance of the capacitor, the lower the better. 1 & 2 have more to do with the RPM of the engine, more RPM less time to charge the cap. 3 has more to do with spark quality because the faster the discharge the better the spark. But because of the charge/discharge time constants we may not get all the energy in or out of that cap in time. So to compensate we have to use a smaller cap but, now we are decreasing the amount of stored energy.

The primary side inductor or the primary winding of our ignition coil has it's own time constants and we need to or at least try to match the time constants of the capacitor. After that we need to take into account the XC & XL (reactive impedance) of the capacitor and coil to match our RPM. For example the old 4 wire GM HEI coil-in-a-cap circa 1975 was known as a system that couldn't go above 5,500 RPM. Well that was wrong but, it did have a output droop between 5,300 and 5,700 caused by that XL of the coil. I've seen it on my test bench that can take a GM-HEI to 15,000 RPM. So we need not just any coil but, one that has low resistance but, also high enough inductance to create a high current transfer to the secondary side and match our RPM. Not like your going to find one just anywhere. Oh I should mention, you can use a regular automotive induction coil with a CDI but, not a CDI coil with an inductive driver, you'll blow the coil, been there, done that. Because the current transfer of a CDI is so weak, compared to an inductive ignition, the secondary winding is also of low resistance and kind of high inductance.

When a CDI ignition fires it creates a very high voltage spark at the sparkplug but it is low on current. Because air and fuel have a high resistance (dielectric strength) the more we compress that air/fuel mixture the more resistance and the more voltage we need to jump the gap and start the ionization. With all ignition coils the energy stored in it once the spark has jumped the gap gets converted into current, ionization creates low resistance and the field collapses faster. Question is "how much energy is left to convert into current." Remember I said voltage hurts and current burns, I have a few old acquaintances that can attest to that. So is our capacitor big enough? Can we charge it up enough? Can it discharge fast enough? Can our coil work with the capacitor? There are a ton of calculations that can be made to get you in the ballpark but, nothing beats a good old testbench. A proper safe one with a pressure test setup.

Dave you said If they were any good car manufacturers would be using them.

Well some have but, remember GM's ignition lock screw up when they only wanted to spend $1 instead of $1.50. Well 1,000,000 cars times $0.50 was some ones Xmas bonus of $500,000. It's the same thing with ignitions, CDIs cost more than Tx inductive ones. It all comes down to cost and "just good enough wins out".

End of part 1 - The basics.

Ray

 

[william_b_noble]

could you elaborate on what you mean by "6 time constants" this makes no sense to me, particularly in this context- in the CD ignition I designed, the capacitor is charged from a bridge from a power supply that can source a fixed amount of current without saturating the transformer (and the voltage on a transformer with constant current is just the current times time). It is discharged into an ignition coil through an SCR, and the back voltage at the end of the cycle turns off the SCR and is recovered by a diode, as I recall. These are very simple circuits, parts values are non-critical. I found that setting the operating voltage (e.g. the voltage you have on the cap) to a little above the back EMF across the points when they open is sufficient for a hot spark - that is generally around 200V. Maybe you are referring to the non-electronic time constants such as the burn front propagation in the mixture?

 

[DKGrimm]

Well I'll put my 2 cents worth in and it's long winded. First Don I'd like to commend you on your research, it's nice to see. The only real problem I have is that your results are more to do with the ignition coils than the actual CDI units.

I have been researching ignition/s, combustion, and deflagration (Wikipedia), for over 20 years. This thread appears to deal with all 3 subjects and I'd like to touch on them.

First combustion:
Basically it is the burning of a fuel. It's not an explosion but, rather, hopefully a controlled process of combining heat, fuel, and oxygen. Dave in that motor.com article I have to disagree when they say it is the separation of a fuel molecule because first you have to combine the oxygen with the fuel and that only happens with heat (ionization). It's only after they combined with a sufficient amount of heat (current now) that the fuel burns and breaks down. I live in Canada on the cold prairies and trust me on a very cold day neither a diesel or a gasoline engine will burn fuel if they get to cold. Oh in electronics and electrical we say voltage hurts but current burns you.

Second deflagration:

Because of the racing I do I've spent a lot of time researching this subject. It's more than just cylinder and combustion chamber design. It's also more than just the spark at the end of the sparkplug. I don't want a slow burn, I want one that's as fast as possible to build as much cylinder pressure as possible before the piston goes past 45 degrees from TDC. As a fuel/oxygen mixture burns it expands creating a pressure wave in front of it. It is this wave that compresses the air/fuel mixture even more. Because of the pressure the air/fuel molecules are closer to each other, they combine faster and burn faster, split up faster. Think of gunpowder poured on the ground as compared to being in a gun barrel. When the pressure wave goes supersonic this is when we get what we feel is an explosion. All things from simple burning of black gunpowder to a thermal nuclear device go through the deflagration process. I have found that a very strong inductive spark with multi strikes is the best to ignite the fuel mixture allowing me to take ignition timing of up to 10 degrees out of an engine. Which leads me to ignition.

Ignition and Ignition systems:
So we have basically 2 types of ignition systems here not counting thermal ignitions, inductive and CDI. CDI still uses induction just differently. Since we are looking at CDI I'll try and stay on that topic. As most people know with CDI we have to charge up a capacitor with high voltage, well somewhat high voltage of around 100 volts or higher first. We then send that through a step-up transformer to get our HV spark. Sounds simple right. Well it's not if we want it done right. This type of ignition was first invented by a Canadian Air Force pilot back in the 50's. We have different types of CDI ignition systems also such as AC-CDI and DC-CDI. AC-CDI is more like the CDI ignitions we find on our lawn mowers which, is a pulsed charged CDI. The DC-CDI has a DC to AC to DC system where it takes the DC current and converts it to stepped up AC voltage and then rectifies that back to HV DC which goes to the capacitor. Now both capacitors and inductors have what is know as 'time constants' of charge and discharge. There are 6 time constants but, because the 6th is so small of a change we ignore it and go with just 5. This brings us to 'how fast can I charge a capacitor to the energy level that I want'. There basically 3 things we are concerned with here. 1. The bigger the capacitor the longer it takes to charge it. 2. Most CDIs quit sparking at voltages below 100 volts. 3. The ESR or the internal resistance of the capacitor, the lower the better. 1 & 2 have more to do with the RPM of the engine, more RPM less time to charge the cap. 3 has more to do with spark quality because the faster the discharge the better the spark. But because of the charge/discharge time constants we may not get all the energy in or out of that cap in time. So to compensate we have to use a smaller cap but, now we are decreasing the amount of stored energy.

The primary side inductor or the primary winding of our ignition coil has it's own time constants and we need to or at least try to match the time constants of the capacitor. After that we need to take into account the XC & XL (reactive impedance) of the capacitor and coil to match our RPM. For example the old 4 wire GM HEI coil-in-a-cap circa 1975 was known as a system that couldn't go above 5,500 RPM. Well that was wrong but, it did have a output droop between 5,300 and 5,700 caused by that XL of the coil. I've seen it on my test bench that can take a GM-HEI to 15,000 RPM. So we need not just any coil but, one that has low resistance but, also high enough inductance to create a high current transfer to the secondary side and match our RPM. Not like your going to find one just anywhere. Oh I should mention, you can use a regular automotive induction coil with a CDI but, not a CDI coil with an inductive driver, you'll blow the coil, been there, done that. Because the current transfer of a CDI is so weak, compared to an inductive ignition, the secondary winding is also of low resistance and kind of high inductance.

When a CDI ignition fires it creates a very high voltage spark at the sparkplug but it is low on current. Because air and fuel have a high resistance (dielectric strength) the more we compress that air/fuel mixture the more resistance and the more voltage we need to jump the gap and start the ionization. With all ignition coils the energy stored in it once the spark has jumped the gap gets converted into current, ionization creates low resistance and the field collapses faster. Question is "how much energy is left to convert into current." Remember I said voltage hurts and current burns, I have a few old acquaintances that can attest to that. So is our capacitor big enough? Can we charge it up enough? Can it discharge fast enough? Can our coil work with the capacitor? There are a ton of calculations that can be made to get you in the ballpark but, nothing beats a good old testbench. A proper safe one with a pressure test setup.


Well some have but, remember GM's ignition lock screw up when they only wanted to spend $1 instead of $1.50. Well 1,000,000 cars times $0.50 was some ones Xmas bonus of $500,000. It's the same thing with ignitions, CDIs cost more than Tx inductive ones. It all comes down to cost and "just good enough wins out".

End of part 1 - The basics.

Ray

Ray,

Thans for your comments. You have a lot of insight into the complicated world of ignition and your experience far exceeds my own.

My target when I started this project was just to try both inductive and CDI systems one can easily buy and use. My experience and training was as an engineer in an avionics company, where I did a lot of electro-magnetic types of things, including switching power supplies, lightning protection, and interference between systems. I have designed and bult some decent model engine magnetos but am starting essentially from zero knowledge about battery type ignition systems. Your insights are very helpful.

I wrote earlier that I'm struck by how little of the initial energy in a CDI system actually shows up as heat in the spark. As you say, the CDI coil is a key element in this. A coil is a simple little device with just a core and some wires. How hard could it be? The answer is, really hard! Most equivalent circuits representations of transformers are single-frequency models. An ignition pulse is anything but that. Its a broad spectrum package of energy from dc to light. The standard transformer "T" model doesn't really take into account some frequency-dependent effects such as interwiring capacitance, skin effect resistance changes, and even radiation losses. (Try to make a radio work next to an unshielded ignition system.)

As a secondary goal I hope to pursue this along the way, but I'm going to move on right now to test a number of different coils using a Sage-Gedde driver to see how they do. At least I'll have some actual measurements of the differences between the two systems.

 

[william_b_noble]

Ray,

Thans for your comments. You have a lot of insight into the complicated world of ignition and your experience far exceeds my own.

My target when I started this project was just to try both inductive and CDI systems one can easily buy and use. My experience and training was as an engineer in an avionics company, where I did a lot of electro-magnetic types of things, including switching power supplies, lightning protection, and interference between systems. I have designed and bult some decent model engine magnetos but am starting essentially from zero knowledge about battery type ignition systems. Your insights are very helpful.

I wrote earlier that I'm struck by how little of the initial energy in a CDI system actually shows up as heat in the spark. As you say, the CDI coil is a key element in this. A coil is a simple little device with just a core and some wires. How hard could it be? The answer is, really hard! Most equivalent circuits representations of transformers are single-frequency models. An ignition pulse is anything but that. Its a broad spectrum package of energy from dc to light. The standard transformer "T" model doesn't really take into account some frequency-dependent effects such as interwiring capacitance, skin effect resistance changes, and even radiation losses. (Try to make a radio work next to an unshielded ignition system.)

As a secondary goal I hope to pursue this along the way, but I'm going to move on right now to test a number of different coils using a Sage-Gedde driver to see how they do. At least I'll have some actual measurements of the differences between the two systems.

it seems to me, that any voltage above some margin over the breakdown voltage across the plug gap and the operating pressure is just wasted. If I was trying to make a miniaturized spark system, that is where I'd start - use a full size ignition system with a storage scope with a HV probe, and see what voltage you are seeing - the peak you see will probably be just before breakdown of the gap, and if you add 50% to that you should have a relable spark. that would let you set the turns ratio in the coil and probably avoid hte #40 wire issue discussed upthread. A lot of automotive ignitions run in the 2-3KV area, so a 200V primary only needs a 10:1 ratio to give you 2KV at the output - that's not really how inductive ignitions work, I know, but that is a way to make things a lot smaller and then use surface mount parts to create the desired primary voltage