Tests of CDI Ignition Modules

Home Model Engine Machinist Forum

Help Support Home Model Engine Machinist Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Don,

I want to thank you for working on this project! I always strive to learn something new each day and never really thought that I would be so interested in the spark of a sparkplug, but your presentation has peaked my interest.

They say a picture is worth a thousand words and after viewing the two on the 5 kv versus the 2 kv images all I can say is Wow! The 5 kv looks perky and alert while the 2 kv looks like a sad old Mama. ;)

As a layman in this field, I'm curious as to how the difference between sparkplug types may affect your work? Don't mean to put the cart before the horse here though. Again, thank you for your efforts.
Different sparkplug types, gaps, compression pressure, gas movement (swirl) will all have effects. Besides that, CDI discharges are completely different than coil discharges. I am one of nine blind men describing an elephant.
 
Yeah, I'm aware of the list that you wrote, and the sparkplug is just one item in that list, as they all play into how well an engine performs.
I probably should have better described my thoughts on the plugs better. Earlier in this thread you said, "Hard to say if it gets more energy into the spark". Now that isn't the full quote but the part on how much energy gets to the spark stuck in my mind. It got me to thinking, that if the same amount of energy gets to the plugs, how much of that energy gets to the spark? Every manufacturer likes to make claim of why their plug is the best! No, I am not asking to have you do a spark plug shoot-out. I am just curious if different plugs affect the performance of the CDI or is it the plug itself.

Out of a group of 9 blindmen, I would trust your seat of the pants judgement over the others any day of the week., now if it were 10, hmm?
 
"Hard to say if it gets more energy into the spark"
When I said that I was talking about test firing ignition modules into a resistor, as I recall. That load would be very different from a spark plug. I don't think there's any energy lost in the spark plug itself. Whatever current and voltage go into the terminal on top goes into the spark. I have seen engines that run well on one brand of spark plugs but not at all well on another. I don't know why, but I'll bet it has more to do with electrode and insulator shape, temperature conduction, gap, fouling, and stuff like that.
 
Here is a typical spark waveform from my measurements on an automobile coil. It shows a little bit about the nature of sparks. First, here is one that is “normal” strength, like those you’ve seen on ignition analyzers. The yellow trace is voltage, blue is current. Voltage scale is 500 v/cm; current scale is 10 ma/cm. After the initial spike the arc voltage settles at about 400 volts until the arc quits after 2.7 ms, with ringing of the spark plug voltage at about 2.5 kHz for a few milliseconds with no current flow in the plug. Arc current after the initial spike decays linearly from about 28 ma, ending at about 3 ma. Doing the math on those numbers gives a 6 watt average over the length of the spark, for about 16 mjoules of energy delivered to the arc.

OOOOH !!!, AAAAAH !!!, love those wave forms !!! (I saw similar voltage curves back when I was experimenting, but never thought to add a current sensing circuit, BRAVO !!!)

if/when you have the time, could you also put a scope probe on the primary, I'd like someone to confirm what I saw which was a big spike there too, which I have a hunch may be the true culprit in Hall Sensor Death.

Pete.
 
OOOOH !!!, AAAAAH !!!, love those wave forms !!! (I saw similar voltage curves back when I was experimenting, but never thought to add a current sensing circuit, BRAVO !!!)

if/when you have the time, could you also put a scope probe on the primary, I'd like someone to confirm what I saw which was a big spike there too, which I have a hunch may be the true culprit in Hall Sensor Death.

Pete.
I'll do that when I get a chance, Pete. I think you are 100% right about the threat to the Hall sensor. That spike is so energetic that it couples into everything, and you will definitely see it in the primary. Roy Sholl caught on to this many years ago and advised people to keep grounds short and keep the sensor well insulated from nearby metal.
 
I know I promised at the beginning of this that I would test various CDI and inductive ignition systems and report the results. It turned into a much bigger problem that I though it might be, but I though this would be a good time to do an interim report to show where things stand right now.

At the beginning I had tested all the CDI modules I could lay my hands on and had found they are all similar in output spark energy. They all fell within the range of 0.7 to 1.4 millijoules delivered to a CM6 spark plug with a 0.025 inch gap.

I’ve spent a month or more tuning my test setup to measure inductive coil systems and think I have figured out what I can and can’t measure. I’ve written a report on where that stands – mostly to myself – an you can read it for what it’s worth as an attachment.

The only reportable testing so far is an automobile coil of indeterminate heritage driven by a Sage-Gedde driver with a primary current of 1.2 amps into the same CM6 spark plug with the same 0.025 inch gap. Answer is: just over 3.2 millijoules.

I want to take a break for right now and try to run my engine using this coil, but my plan is to gather up all the different coils I can find and test those for comparison. More results as I get them.

Don
 

Attachments

  • Ig.pdf
    1.3 MB · Views: 4
Last edited:
Great work Don. Much appreciated.
From your report you say you used a 6v battery and limited the current to 1.2 amps for my module.
Does that mean you used a 5 ohm ballast resistor? (assuming the coil is very low resistance).
I've always used about 2 ohms on a 12v system for those typical coils. I think my coil measures about 0.2 ohms.
I wonder how many more joules that would result in at the plug gap?
 
Last edited:
Great work Don. Much appreciated.
From your report you say you used a 6v battery and limited the current to 1.2 amps for my module.
Does that mean you used a 5 ohm ballast resistor? (assuming the coil is very low resistance).
I've always used about 2 ohms on a 12v system for those typical coils. I think my coil measures about 0.2 ohms.
I wonder how many more joules that would result in at the plug gap?
Coil resistance was 1.4 ohms, and I used 2 ohms ballast resistor with a 6 volt sealed cell lead-acid battery. I didn't measure the drop across the driver or the droop in the battery voltage. I measured the current with the 0.1 ohm sampling resistor. Just trying to get a feel for the minimum coil current that would give a good spark.
 
Ah. Ok it seems your coil had a built in resistor. They are available either way it seems. Mine has printed right on it that a ballast resistor is required. It's the big cylindrical type like you show in your setup. Hence my 2ohm ballast at 12V.
Would it be possible to check a Ford two wire COP coil? They are readily available for cheap on Amazon if you search around a bit. I believe I purchased a set of 8 for $40 or so a few years ago.
Many folks are finding them desirable because they are small(er) especially if you strip off the springy rubber plug connector from them.
One could also be had cheap from an auto wrecker I suppose.
It's possible the ones on Amazon are sub-standard but we don't demand peak performance from them either. Mine are working fine for what is required.
 
Ah. Ok it seems your coil had a built in resistor. They are available either way it seems. Mine has printed right on it that a ballast resistor is required. It's the big cylindrical type like you show in your setup. Hence my 2ohm ballast at 12V.
Would it be possible to check a Ford two wire COP coil? They are readily available for cheap on Amazon if you search around a bit. I believe I purchased a set of 8 for $40 or so a few years ago.
Many folks are finding them desirable because they are small(er) especially if you strip off the springy rubber plug connector from them.
One could also be had cheap from an auto wrecker I suppose.
It's possible the ones on Amazon are sub-standard but we don't demand peak performance from them either. Mine are working fine for what is required.
That's in my plan.
 
DK, I've been using "EMGO 24-71543" moped coils with Sage-Gedde drivers, they're easy to find on eBay, but I'll mail you one if you're willing to test it, Peter.

here's the back side of the fire wall of my Merlin engine stand showing a pair of everything...
 

Attachments

  • IMG_0279.JPG
    IMG_0279.JPG
    1.8 MB · Views: 4
DK, I've been using "EMGO 24-71543" moped coils with Sage-Gedde drivers, they're easy to find on eBay, but I'll mail you one if you're willing to test it, Peter.

here's the back side of the fire wall of my Merlin engine stand showing a pair of everything...
Sounds like that should be a good combination. Just ordered a 24-71532 for my tests. Looks the same, but if you want me to run one of yours, I'd be happy to. I'll send an address by private message.
 
It's taken looooong enough, but I finally got some results to put in front of people. The chart is a summary of results testing the 8 coils I have gotten my hands on. I got most of them from John Vietti. Their pedigree was lost in the transfer, but the pictures in the second attachment may help if you run across similar ones on Ebay or elsewhere.

The testing was done with about the lowest primary current one could ever use. The battery was about 6.2 volts, and there was a total of 5 ohms of ballast resistance (1 ohm of that was a primary current sense resistor). There are two tests on each coil, one with a Rimfire Viper Z3 plug with a 0.012 gap and the other with a CM6 with a 0.025 gap. The next round of testing will use lower ballast resistance, more like one would normally use.

With this small primary current some coils could fire either plug, some could fire only the 0.012 gap plug, and some could not do either. I'll skip the measurement details in this post, but I probably need to write up an explanation so people can understand what I did and can critique it. I thought it was interesting that some of the coils put out more spark energy at this low current level than many of the CDI modules tested previously at full power.

All energy results in the table are millijoules. The energy in the primary is calculated as one-half the [inductance] x [current squared]. Energy in the secondary is the sum of all the samples of spark plug [volts] x [current] x [sample period].

The "mj per ampsq" parameter may sound a bit weird. Since primary energy is proportional to amps squared, the ratio of output energy to input current squared may turn out to be a useful parameter to describe the efficiency of the coil. To be continued . . .

Maybe "mjoule sec/pri" a bit more intuitive. That's the ratio of output energy to stored primary energy.

Anyway, this all is a lot more work than I bargained for. I'll follow up in due course with more tests, but I'd be very happy to hear your reactions.
 

Attachments

  • Low Current Tests of 8 Coils.pdf
    224.2 KB · Views: 4
  • Coil Slideshow.pdf
    1.2 MB · Views: 10
Some nice work there Don with a nice selection of coils. That should cover a lot of bases.
I found it interesting the the MSD "Blaster" coil really appears to be better and not just advertising hype.
Much appreciated
Can you make a check for me. My coil driver was designed for 12v. If you run it at 6v there is likely very little overhead for the hall sensor VCC. Most hall sensors have a VCC of 5v. (not all). But if you run them on the edge some may not switch reliably. Also the drive for the IGBT may be lacking. All can be checked with your scope.
Paul Denham of the Bay Area Engine Modelers Club runs a lot of the drivers at 6v and found it useful to put a large (say 100uf 16v or so) capacitor after the on-board supply diode to filter and supply a bit of a reservoir for the hall sensor and electronics.
If you experience any sketchy switching action especially at higher rpms you may want to add the capacitor.
Thanks a lot for your efforts.
 
Some nice work there Don with a nice selection of coils. That should cover a lot of bases.
I found it interesting the the MSD "Blaster" coil really appears to be better and not just advertising hype.
Much appreciated
Can you make a check for me. My coil driver was designed for 12v. If you run it at 6v there is likely very little overhead for the hall sensor VCC. Most hall sensors have a VCC of 5v. (not all). But if you run them on the edge some may not switch reliably. Also the drive for the IGBT may be lacking. All can be checked with your scope.
Paul Denham of the Bay Area Engine Modelers Club runs a lot of the drivers at 6v and found it useful to put a large (say 100uf 16v or so) capacitor after the on-board supply diode to filter and supply a bit of a reservoir for the hall sensor and electronics.
If you experience any sketchy switching action especially at higher rpms you may want to add the capacitor.
Thanks a lot for your efforts.
Right. I remember you telling me this before, but everything works so well as is I just never bothered to look. I'll do so, but first we have a local machinist club meeting on April 1 (yes, really, no fooling). I want to hook up your driver with one of the coils and see if that can make my so-called Atkinson Valve engine run so I can take it along to show-and-tell. Getting that engine to run was the original problem that started this whole exercise.
 
Don, not sure why you are using a ballast resistor? I agree with dsage on the 6 volts. At 6 volts any resistance beyond the primary winding will effect the output of the coil. I would also be curious what your results would look like if the spark plugs were under 30 to 50 psi. of pressure. I have a spark tester that can be pressurized and it does effect the ability of a system to produce a spark, both CDI and coils.

Lots of good work here and I know how time consuming it can be. Thanks for all the effort you have put into this.
 
Don, not sure why you are using a ballast resistor? I agree with dsage on the 6 volts. At 6 volts any resistance beyond the primary winding will effect the output of the coil. I would also be curious what your results would look like if the spark plugs were under 30 to 50 psi. of pressure. I have a spark tester that can be pressurized and it does effect the ability of a system to produce a spark, both CDI and coils.

Lots of good work here and I know how time consuming it can be. Thanks for all the effort you have put into this.
Excellent points. The only reason for the ballast resistor in these tests was to test the coils themselves at different levels of primary current. I will run more tests with smaller ballast resistors, and then with none at all, just to see how strong the spark gets for each case.

Your point about pressurizing the plug for testing is right on. I actually have done this for magnetos and will for coils as well -- in the fullness of time. I've been informed by reliable sources that God created time so that everything doesn't have to happen at once.

Don
 
Back
Top