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Miniature distributor

PeterT

Ultra Member
Premium Member
I want my next engine to be spark ignition (vs glow plugs) so I’ve been working on a miniature, scale-ish but functional distributor. This is a 24mm diameter cap for no particular engine. I chose 4-cylinder because that would be the likely minimum, but working towards higher cylinder count on the same body platform. At this point it a bench runner to test different part layouts, materials & methods. I leveraged off of a few other designs developed by people smarter than me.

Electrons & arc jumping don’t scale, so packing the components into a small body is not without its challenges. No spark, no bang. No bang, no joy LOL. That's why I opted for safe but boring glow plugs on my 5-cyl radial. But those have limitations too. Requires methanol-based fuel vs gasoline, relatively high CR, no real ignition timing control. No ability to dwell/advance.

The body is 6061 aluminum, 8mm bearings on either end, spinny parts mounted on a 3mm shaft. Maybe what is a bit unique on my distributor is selective use of 3DP for some parts, which solves some hard to machine issues. FS distributor caps can be quite a funky shape with the main cup, towers layout, fillets, retention lugs or clasps to the body… My cap was 3DP'd from my CAD design. Then some micro-autobody primer work to clean it up & hide the print striations. From this a female mold as poured using silicone confined by 3DP mold plate & dam. Then the cap itself was cast into the silicone using urethane (hard plastic) resin. This was turned in the lathe to hollow out the cavity & lip. The towers were drilled & they hold 2.5mm OD brass tubes which are RC type 2mm banana plugs. My tubes are a little bit sticky-uppy in these pics as I’m still figuring things out. The wires are 16 AWG stranded wire, soldered to the banana plugs.

What drives the distributor are dedicated ignition boards or modules, kind of a niche product to model engineering developed by electronics wizards in the hobby. There are not many to choose from but I’m just glad that some dedicated hobbyists figured it out so all I have to do is figure out wiring & color codes. Some modules are capacitive discharge, others use coils. There are pros & cons to each. Shout out to Charlie @CWelkie who made some boards from a circuit published in a ME magazine. I was lucky enough to get one of his inventory spares. It uses a very compact COP (coil over/on plug) so the whole ignition system can be hidden in a compact box. Turns out we travelled in similar RC circles in Calgary when we were young, so that was cool. And Brent @eotrfish you may recognize this particular rotor was made from your PEEK GF-30 slug, thanks!

This distributor uses an aluminum magnet wheel: N magnets for N cylinders. A hall effect sensor picks up the magnet pulse & signals the module to fire. I’ve read there are can be practical limits to magnet density before potential issues arise. Another style is a single magnet facing the sensor, but uses a shutter plate between them. The plate has N alternating windows & signal the module that way. And window width correspond to dwell on a coil based system. So less magnets but a bit more finicky construction. I have seen 2 styles of rotors. One with a sprung physical contact on the high-tension terminal. The other is air gap on both HT & cylinder terminal. Again pros & cons to each. I’ve tried both but so far the (0.006”) air gap on both is working.

Now that I have sparks (yaaaay!) the plan is to keep refining. I want to increase the terminal count to see if I can get 8 terminals without interference. I’m also working on a shutter plate version. And next step is to make my own spark plugs on smaller thread formats. The commercial plugs are ¼-32 but I have seen them as small as 8-40. Actually ¼-32 match the engines I have in mind so its more of a morbid curiosity thing.
 

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(Bambu A1) 3DP cap male bodies. I wish I figured out my 0.2mm nozzle sooner, quite a bit better than the 0.4mm which would have saved some primer futzing.
 

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COP module built by @CWelkie The significantly more complex wiring installed by yours truly. That's an inside joke because I managed to get some sensor wires bass-akwards despite very clear instructions. But the magic smoke stayed in, so technically its not an error in my books ;)
 

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I enjoyed talking with you at the meet, there is some very fine work there. Our conversation about some of the different types of ignition systems and operating them in a miniature operation/environment has had me thinking about some things.
I have pulled down an 8 cylinder ford distributor, and am going to compare it to a 6 cylinder one I have. The 8 cylinder one, someone attempted disassembly with a hammer, like what the @#$&! ,! a hammer?? Anyway, will see what’s to be seen.
 
I enjoyed talking with you at the meet
Yes ditto. You got me thinking about FS distributors. So many variations.

I've seen modelers mill the cap from solid Delrin or Nylon or whatever (or CNC if you were so equipped). I was pretty confident I was going to mess a few up trying different things, so I went the mold route. My first male plug was aluminum. Although dimensionally more accurate, that was too much like work machining the lugs & posts & still lacked the fillets & other details. So I tried the 3DP method & got decent enough results (with primer) to carry on that mode. The casting urethane is working pretty good, but I suppose I could also mix up my own goop, glass filled epoxy or whatever.
 

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Impressive stuff Peter! More great applications of 3D printing. You could use a resin printer for the finer detail parts like the boots, even super high res ones are fairly small dollars. I've used that to create silicon molds, works well. I did so for wax but the internet tells me you can cast epoxy parts in the silicon mold quite well
 
Awesome work Peter. Nice use for 3d printing. I've used similar methods for casting fishing lures before. A resin printer is like magic for that job of printing plugs to make molds.
 
One son is messing with resin 3d printing also now, says very fine detail, though some of the resins are very brittle, and some “very $$$$”. Not sure where he is getting the resin from, says have to really watch prices, and age of product. Washing finished product with “iso”? And can reuse a number of times after settling iso. He been filament printing for some years, so this is adding to his “making”. Awaiting his future results and thoughts.
Have also thinking about ignition using the dead cylinder system, spark on 2 cylinders at the same time, one cylinder firing normal time before top dead centre, end of compression stroke, and other cylinder firing at same time, but on exhaust stroke. Less timing lobes needed, in some cases no distributor either.
More thoughts, less time, less sleep, LOL.
 
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