Distributor Making Adventures

[petertha]

Design 2 was a 3DP part that keeps the sensor captive in a 4-sided slot. The holder slides in laterally through a window milled in the side & is retained by 2x M1.6 screws. The holder has a rounded profile to match the distributor cup OD flush. I used the same 3DP part as a drill & tap guide because the mounting holes enter distributor cup at a slight contact angle so kind of finicky. I like this holder because everything is positioned & it can readily be swapped in & out. But there is less positioning latitude. Another potential advantage TBD is using the same concept in a shutter plate distributor where one 3DP part holds the sensor & a similar one holds the magnet. These are spaced apart straddling the shutter plate in the middle between them.

[petertha]

Rotor
The rotor sits on top of the mag wheel, retained on the shaft with a set screw so it can be clocked independent of the mag wheel. Because if triggering occurs closer to the leading or trailing edge of the magnet, you want the ability to adjust the conductor strip angle to center relative to the terminal.

The rotor needs to be an electric insulator. I tried some variations. I made a similar 3DP mold & cast one from the same urethane as the cap. It had an integral recess for the brass conductor strip. I made another from Delrin or Nylon. And another from a slug of PEEK GF30 I was gifted. They all work & it’s not a difficult part to make either way. As mentioned, some plastics don’t take to common adhesives so I feel like the conductor strip has to be either screwed in or reside in a slot. And softer plastics are also not as resilient for the set screw threads. I shaped my rotor more like a bar but it could be a full disk to help block any rouge arcing to the mag disk I suppose.

Some guys elect to have a sprung contact strip that makes continuous electrical connection to the center high-tension terminal & air gap between the outboard terminal ends. Others have an air gap on both sides. Pros & cons to both so I’ve read. I tried the conductor method with brass & then bronze when it finally arrived. It sparked fine just finger spinning. I also tried entirely air gap & that worked well too. After maybe an hour of finger spinning, I see the faintest arc impression on the strip adjacent to HT lead, nothing on the outboard end. Not quite sure what to make of that. If air gap works on an actual engine, that would be my reference because it’s easier to make IMO. I did not test increasing the air gap to a limit, the current setup is about 0.005” but one was slightly over 0.010”. Maybe things can move over time & there is some safer distance, TBD.

Another consideration is how far the shaft extends into the rotor. The closer it gets to the underside of conductor strip, the more potential to arc down into the shaft? My rotor is only 8mm in height so I drilled the shaft hole blind to keep insulation material on top, only as deep as necessary for the set screw to engage. On materials where you could trust the adhesive, maybe the conductor strip could just be glued on top of the rotor vs. in a recessed trench. I tried this on a test piece, it withheld skimming it flat in the lathe so might be fine in a distributor.

Machined rotor pics

[petertha]

Cast urethane rotor pics

[petertha]

Shutter Plate
This work is still ongoing so not much to show. I made a 4-cylinder mockup which is sparking. The inter distance between magnet & sensor is a little more than the mag disk style, just under 3mm. The shutter plate is 1mm thick 12L14 leaving ~0.5mm air gap either side of 3DP holders. I’ve only tried a 3mm diameter x 2mm thick magnet, but there is probably room to increase size if required. I took a guess at minimum shutter window width just to make it work. Attached is a 3DCAD rendition of the George.B shutter distributor which uses more of an inverted cup shape with side windows vs my plate.

As mentioned, one can adjust the shutter window width to affect dwell duration on a coil-based system. within the limits of disc size & number of cylinders. I think on a CD (capacitive discharge) ignition module, spark is initiated when the sensor first sees the magnet & passes the signal, so the window width probably does not factor? Some of this was discussed on this Sage thread so I won’t repeat.
https://www.homemodelenginemachinist.com/threads/sage-gedde-clarification-confusion.37273/page-4#post-429847

Until the mag wheel vs shutter plate variant gets installed on a running engine, no sense speculating on which way to go. One style isn’t more difficult than the other in the overall scheme, now that I’ve come this far.

[petertha]

Ignition Modules
I haven’t mentioned ignition modules much & will keep this part short because there are smarter people in the room. I was utterly confused by most of the forum discussions, now I’m upgrading myself to ‘mostly confused’ LOL. I bought an S/S (Magnum) module from Dan once our postal strike drama ended. Coincidentally, around the same time I was able to get my hands on a nice, fully completed Sage board with a Coil on Plug from another ME. I was aware of this design from the 2015 MEB Issue #34 article, but figured building it was way over my head. Then I was gifted a Chinese DIY module by another ME out of the blue. So, within a short span I went from zero modules to a nice inventory to try. The Sage & the Magnum S/S are sparking fine. So did the DIY for quite some time, until unfortunately it stopped. Not sure what’s going on there. The sensors check fine, the distributor rotors are the same. Either the smoke found a way out or hardware issue? The board components look perfect but is cast in a block of resin so not much I can diagnose assuming I knew how. Further investigation required.

[petertha]

Conclusion
When I get some more spark plugs, I’ll take the distributor up to max 8 or 9 terminals & post any progress. Meanwhile fire away with any comments or questions. I am living proof of the benefits of information sharing. I’m also going to focus on some related ancillaries like molded boots using 3DP methods & <gasp> shop made spark plugs.

[Kim]

Wow!  That's a wonderful and extensive write-up on how you built your distributors.  Incredibly informative! 

Thank you!
Kim

[Dave Otto]

That is an impressive amount of work and very nice results!

Dave

[RReid]

I'm gonna have to find some time to go through that more carefully. There's a lot there!

[Vixen]

That's a very comprehensive study and report into model engine distributor design and manufacture. It is an excellent reference work and knowledge base. We need to bookmark it, so that we can easily return to it from time to time.

I am pleased to have been able to contribute a little.

Cheers 

Mike

[nj111]

This is very useful research for most of us, thank you for taking the time to present it all!
Nick

[petertha]

Thanks for the nice compliments. And especially to folks mentioned & other forum contributors who made their designs, pictures & ideas available. Otherwise this never would have happened.

Mike, I feel bad that the DIY module you gifted me sparked just fine until it didn't because it was the first one I tried which brought me the the initial 'IT WORKS!' elation. I wish I could further diagnose it. Nothing looks smoky inside the module & new hall sensors did not remedy the situation. I had conversations with a fellow on the other forum who purchased several of these modules over time. Apparently some did not fire right out of the box, some for a while, some zero issues ever & they happily running multi-cylinder engines. I can only speculate, QC issues, or something within the design? On a completely different discussion about ignition modules, one responsible for design mentioned integrating some safety/timeout features. I'd have to check my notes, I can't recall if those were intending to protect the Hall sensor and/or components within the module. Its impossible to know (for me at least) if the DIY incorporates anything equivalent or not. It got me thinking, maybe finger spinning the shaft on the bench was related. Randomly stopping at sensor on for an extended time, or turning it too slow, or....? Might have contributed to a problem it was not designed for?

I'll leave these pictures just for icing on the cake. I recently made a 1/4-32 spark plug (beige color insulator). It leverages off a Steve Huck surface mount design. Some more construction refinement work required on my part, but it seems to be sparking just fine alongside the RCEXL's. I made a quick shop video & managed to grab some frame stills from it. Now that's not the same as successfully running in a hot engine engine environment, but I'll take this as a positive step in that direction. Back to the Bat Cave to work on this stuff.

[Admiral_dk]

To me that sounds like the Designer don't know enough about High Voltage Electronics / Components .... there are a number of pitfalls that ends up like descriped :

Not working at all, working for a short while, working long enough to make you think the construction is ok and a few that apparently survive ....

If the design is in the Public Domain - send me a link and I will see if I can spot remedies (no guaranties - but I have some experience in the field) ....

Per         

[petertha]

The DIY brand ignition module (circuit) discussed in prior post is not in the public domain AFAIK. Its a commercial product. The board is semi potted in epoxy so no easy way to circuit trace beyond identifying certain components. I don't want to throw the module under the bus because I'm working from a very limited data pool. And as mentioned, perhaps my 'finger spinning' bench test had something to do with its demise?. The modules are used on Chinese table top engines.

https://www.enginediy.com/search?type=article%2Cpage%2Cproduct&q=CDI*

There has been some ignition module discussion here on MEM, but you might be interested in some others on HMEM forum. One in particular 'a new ignition circuit' the initial collaboration between Sage & Gedde. Well its not new anymore, but lots of technical stuff exchanged (above my pay grade). Ultimately this particular circuit was more formalized by Dave Sage in 2015 Model Engine Builder magazine, issue #35 along with screens (is that the right word?) to make the board. It lends itself to different kinds of coils. Now there have been many other discussions & board designs both before & since then, I'm just saying you might find this one interesting with your background knowledge.

[petertha]

Getting off topic from distributors to ignition drivers but I just can't help myself. Dropping this little YouTube gem I'd forgotten about from Roger Moore. Not the guy who played 007, someone MUCH more important to our hobby haha. Micro COP, Coil-On-Plug (or Coil-Over-Plug I've heard both. Somebody track him down & get him on the forum!

https://www.youtube.com/@rooj65