Multi cylinder ignition systems

[gbritnell]

Wires: I mentioned early on that I used test lead wire. That was before Roy Sholl (S@S engines) and Jerry Howell. Roy sold 2 sizes of wire, .100 (10KV) and .125 (20KV). The wire is black. Jerry Howell sells a white wire .125 (15KV). I'm sure there are other sources available. The peculiar thing about this wire is even with the wires in close proximity to each other and metal parts on the engine I have never visually seen a spark jump through the insulation, even given the small diameter.
For the distributor end I place the wire in the cap and slide a short piece of shrink tubing over the wire and up against the distributor cap terminal. I then shrink it in place. I then take a larger piece of shrink tubing, one that will fit over the terminal post and long enough to cover the other piece of shrink tubing and shrink it in place. This provides a boot to hold the wire in place and insulate it at the same time.
For the spark plug end I use a commercial part made by Dorman. It was brought to my attention by a fellow modeler, Ron Colonna years ago. It's a vacuum elbow. I trim it down on both ends. I make a brass insert to go inside to hold the wire and snap over the plug. (Drawing attached)
There is a set screw in the center so that when the wire is inserted it can be tightened to hold the wire in place. These caps and boots will fit the Rimfire type plugs. For larger plugs and home made plugs I use a screw on type terminal. I make up my own wire terminal ends and solder them onto the wire.

[petertha]

I know someone started a thread on the subject but I couldn't find it so I'm starting this one. This will be a comprehensive, in depth thread about all the facets of building an ignition for a multi-cylinder ignition system. That being said it will be for an engine of two but in most cases more than two.

Hi George. Yes I believe this thread got initiated as a dedicated topic offshoot from some ignition posts in Mike (Vixen) engine construction thread, so you are definitely in the right spot. Fantastic pics & description, thanks! Actually I'm a bit late to my own party because I was going to upload a generic design I've been plunking away on, in order to solicit comments. Now I'm going to re-read your material in conjunction with some contributions others were kind enough to share. I SHALL ULTIMATELY BUILD A FUNCTIONING MULTI-ENGINE DISTRIBUTER!! haha

[RReid]

The postings George has contributed are a very useful and practical collection of information and experience, and directly relevant to my current project.  I now think I can “downsize” my initial distributor design significantly.

Thank you very much for all that good stuff, George.

[gbritnell]

Spark plugs:
The last link in the ignition chain but undoubtedly the most important is the spark plug. The plug for model engines was developed in the late 20's. AC, maker of auto plugs was one of the first, later followed by Champion. My first introduction to I.C. engine was with the hit and miss type. I bought castings for one and it came with a Remco spark plug. I couldn't find much history other than they made engines and spark plugs starting in the early 50's. Picture #3
The next plug, a Stitt, Picture #2 was made by the Stitt company which made commercial plugs for large stationary engine.
The next pictures are of 2 Champions, a 1/4-32 and a 3/8-24. These were made by the Champion spark plug company up until the advent of the glow plug which killed the market for them. Late 40's
AC also made model engine plugs. The one shown is a 3/8-24
In the mid 90's Paul Knapp started marketing his own line of miniature spark plugs under the name of Rimfire. (last picture)
The NGK CM-6 is available but only for larger hit and miss type engines with a short reach.
When I got into into I.C. engines, late 70's to early 80's I only had access to the Remco plugs but being as they were intended for model airplane use the reach was quite short so if you needed something longer than .187-3/16 you had to make your own.
My first plugs used Teflon for the insulators. There are two types of Teflon, mechanical and Virgin. I found the mechanical material wasn't as homogeneous as the Virgin and would break apart when machining. I found that as the diameters got smaller the Teflon would flex so the first thing I would do was to drill the hole for the electrode and push a piece of wire in to stiffen it up.
Years later I heard that some fellows were using Corian so I tried it and that is what I use today. It has good heat and electrical properties and machines well.
There are machinable ceramics but the material is expensive and fragile. I was given a piece of it years ago but it's still in my box. I never tried it.
Now with the advent of the Chinese makers getting into everything you can buy their miniature plugs.   

[gbritnell]

I have made spark plugs from 1/4-32 thread size all the way down to 6-40. They all work but with the tiniest ones you have to be careful with your fuel mixture as any richness will foul the plugs very easily.
The last 3 pictures are of the NGK CM-6 plug.

[gbritnell]

When I make my 1/4-32 plugs I came up with an idea to be able to reuse the plug when the center electrode wear down. I drill and tap the top of the insulator for a 3-48 thread. I turn up a brass fitting with the #3 thread on one end and a 0-80 thread on the other for attaching the plug wire terminal. When the electrode wears down I simply unscrew the electrode wire, heat it to soften the solder and pull the wire out a little. I then regrind the length to the proper size.
Attached is the drawing for the spark plug for my radial engine.
I turn the Corian to a close slip fit on both the smaller tip extension and the body diameter. I don't worry about gluing the insulator in. I leave a small ring around the top of the spark plug body and after inserting the insulator I crimp the ring over with a simple pressing fixture. 

[petertha]

George, thanks so much for this info. I have lots of questions but I'll start here. I'm practically electronics illiterate, so bear with me. I have seen distributers like this sketch. They contain a disc attached to the shaft which holds small magnets, one per cylinder. When the Hall sensor sees the magnet it tells the CDI/ignition module to fire. That event aligns to the rotor position much like your 'upstairs' assembly. Energy comes down the center wire, along the rotor arm conductor & out to a cylinder wire.

But your system (if I understand) has slots, no magnets? So is the sensor seeing the presence of metal 'frames' between the window cutouts & that signals the module? I've heard the term 'shuttered' trigger, is maybe what this refers to? If so, is the timing dictated by the edge to edge of the metal portion? If so, what type/class of hall effect sensor would a person be looking for because this would seem to be quite different than a magnet sensor? If I'm already way off base then stop me now, otherwise what are the pros & cons of this system vs magnet sensing?

If I wanted to drive the ignition use a CDI module like S/S or others (as opposed to a coil) would your system work as described? Or is the module designed to expect a particular kind of signal in terms of on/off & that must be considered beforehand?

[gbritnell]

Actually if you will look back to reply 15 you'll see that I do have a magnet, but just one. The shutter wheel exposes the magnet to the Hall sensor and turns it on. As far as ignition modules I use 3 types, an S&S, a Jerry Howell and a Sage Gedde. They all work fine with my design.

[petertha]

Sorry George, I misunderstood. So the shutter wheel / openings would be orientated & rotating something like this?

- Is the hall sensor specific to the S/S kit? Is there a preferred offset distance or that's in the specs?
- Can you elaborate on the Sage Gedde system? I think I saw it mentioned on HMEM. Maybe I'm confused with something else, I thought it was a design requiring making a board from scratch?
- Have you tried any of the Chinese CDI modules like what Stirling offers for tabletop engine kits?
- I recall some discussion, maybe on one of Terry Mayhugh's engines? on HMEM, where the 'normal' S/S module had some recommended discharge limit where you had to factor cylinder count and rpm. If it approached or exceeded some amount then S/S recommended a different module. I don't want to put you on that spot speaking to that product, but did you go down that path with your V8?
- and to confirm, in the systems you have used, none of these have advance/retard integrated into the module like (to the best of my knowledge) RCEXL type CDI systems for RC engines? You are accomplishing this by swiveling the distributer body?

[petertha]

I forgot to ask, what are these 2 openings for on the shutter wheel?

[gbritnell]

Hi Peter,
I'll answer the easy one first. The two windows in the top. One is to access the set/Groban screw and the other is just for balance. More could actually be added.
The Hall sensor isn't specific to any particular ignition module. I don't have the model number handy for it but I will look up my last order. There is a supplier out of England, Minimag, that sells ignition components. The Sage-Gedde ignition module is one that was developed and improved by the named designers.  They posted a schematic for it. It is a robust module with safety features built into it. The drawback is that you would have to build it.
My 302 has 8 cylinders and will rev to 7,800 rpm with all three ignition modules , although it starts to run out of breath with the S&S. By that I mean at the higher rev range it's just not as clean as the Sage module. In all fairness the Sage module will operate on 12 volts whereas the S&S is 6 or less.
I bought 2 of the Chinese ignition systems,  one for a single fire and one for a V-twin withe a waste fire. Initially I found that they triggered as soon as the Hall sensed a magnetic field whereas the others trigger when the magnetic Flux is cut off. Similar to a point type ignition.
If you have any other questions please feel free to fire away.

[petertha]

You show your spring plate type contact for the center terminal. Did you determine this was necessary (as opposed to air gap like the cylinder contacts) based on specific arcing problems, or just good practice? Is phosphor bronze chosen for friction or electrical reasons?

I see some distributers (well Terry's in particular) position the cylinder terminals within the cap in order to corner notch them, I think to promote spark direction. It looks like you use flat ends & your overall cap diameters are smaller yet. I'm wondering if its related to air gap distance. You mention keeping the gap distance small - do you have a recommended dimension?

I thought when I make my distributer I would like to bench run it. Is there anything to see or measure in this regard other than number of healthy sparks should equal number of cylinders? haha

[gbritnell]

As far as the terminal design it could go both ways, notched or flat.  The important thing is having the smallest gap possible.  This would mainly depend on the accuracy of the machining.  As far as the Phosphorus Bronze center srip  I just follow automotive practice. One less air gap the better.  Plus the Phos Bronze is springy compared to regular yellow brass.

[Vixen]

I used a similar shutter wheel, one magnet, one Hall sensor system as the CDI/coil trigger on the two 1/3 scale BTH SC-9 cylinder magnetos fitted to my big Bristol Jupiter engine. You can find the details of the design and build of this shutter wheel trigger in post #243 of my "By Jupiter" build log. https://www.modelenginemaker.com/index.php?topic=7559.200
Post #243 is towards the bottom of the page, so you will need to scroll down a long way.

I built this  test rig To test the magnetos and to confirm they worked with a selection of CDI units and traditional ignition coils. Two turns of the handle (simulated crankshaft motion) produces nine fat sparks in that circle of spark plugs. Like George, I discovered some CDI and ignition coils triggered on the leading (rising)edge of the trigger pulse while other triggered falling edge of the pulse.






You will notice the left hand distributor cap and trigger unit cover have both been cut away to let me to see inside the unit and allow the position of the rotor arm and trigger point to be set accurately. Once set, the cutaway items are replaced with working components. I found three pin JR  plugs/ sockets worked well.JR plugs will be familiar to those who also dabble in Radio Control.  I included the newest Bosch 8 cylinder Magneto for the Mercedes engine in the above photo, for a comparison of size.



Cheers

Mike

[Roger B]

A lot of great information     

Rather than a spring contact on the rotor like George I chose to have a sprung centre contact on the cap and a flat rotor contact. The build is here:

https://www.modelenginemaker.com/index.php/topic,7883.150.html