Multi cylinder ignition systems

[Vixen]

There are many ways to make spring contract between the distributor cap and the rotor arm contact. Spring arm or sprung centre contact, as described by George and Roger above

Another option is to use the phosphor bronze contact arm and points, salvaged from an old relay.


Here are four, I made earlier.

Mike

[RReid]

Hi Mike. More great info being collected here, this thread is becoming a real treasure trove for those of us with the need/interest but not the experience!

Just to let you know, the link back to your "By Jupiter" thread back in reply #43 is broken by a gap in the spelling. I got there by cutting/pasting into a new tab and eliminating the gap, but I'm sure you'd want to fix it properly.

[Vixen]

              Fixed

[gbritnell]

Ideally a small piece of carbon rod that is spring loaded for the center contact would be the way to go.  It just adds more height to the rotor and therefore the cap. As with a topic like this it's great to have multiple ideas to work with.

[petertha]

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.

My understanding (maybe wrong) is kv (kilovolt) refers to the maximum voltage rating mostly or entirely related to the sheathing, not the conductor wire itself? For a model distributer if we could wave a magic wand it would be scale diameter for aesthetics, but of course has to meet the electrical requirements & maybe that's the tradeoff. I don't consider ~0.1" too large for my intended application, but I'm just wondering more about these related details. What kind of voltage levels are we expecting or seeing in coils and specifically CDI's (my likely path). How would an under-rated kv rating manifest itself?

But I'm confused by the conductor specs, which I don't think relates to KV? I have a basic understanding of resistance specs & that is related to current carrying capability. Too much current & the conductor heats up, then bad things happen. So by selecting the KV & the nominal wire OD, are we indirectly defining a wire that just so happens to also meet our ignition voltage & current conductor wire requirements? I'm guessing the amperage must be quite low based on the acceptable cross section of the rotor conductor bar & terminals etc? What kind of amperage range is expected?

Another point of personal confusion. I think carbon based core/coated? automotive ignition wires are to help suppress electrical (EMI) / radio (RFI) interference. ie. that the carbon (or whatever it is) doesn't really improve electrical properties. I can understand if conventional stranded wire kicks out 'noise' to the extent of RFI problems for RC models if it interferes with the receiver reception & thus model control. I 'think' that's why we see those big braided cables, metal/grounded plug caps, integrated resistors... all that stuff on RCEXL type CDI modules used in RC applications? So on this basis are we saying conventional stranded wire is fine for a bench running engine? I've heard opinions that as the distributer & related distances shrink on our scale engines, arcing or cross-firing can happen or is likely to happen. But the counterpoint is many examples of small distributer + high cylinder count which seem to run just fine. So is it safe to say there must be other contributors to these problems (maybe gap distance). Or maybe person A used a coil & person B used a CDI brand X?

Why are capacitors used in FS distributers & we dont seem to need them in model distributers?

[gbritnell]

I believe S&S supplied a small capacitor for their ignition when using points rather than a Hall sensor.  My Holt has a points trigger but I have never used the capacitor and haven't had any problems.

[Roger B]

The voltage rating of a wire is complicated. A piece of typical PVC panel wire will withstand around 20kV short term, 6 kV long term but will carry a rating of 1000V. For long term voltage rating the quality of the conductor surface and the lack of an air gap between the conductor and the insulation is important to control the corona discharge which will cause long term breakdown.

Adding some resistance to the ignition wires or plug caps reduces the rate of voltage rise and the amount of RF interference produced.

Capacitors are used with traditional points and coil ignition systems to reduce arcing at the points, as the points open the capacitor holds the voltage across them to zero until it charges up. By the time the capacitor charges up the points should be open enough to withstand the voltage. The capacitor also allows for some resonant oscillation to prolong the spark.

[petertha]

Mike, if I recall your Jupiter distributer correctly, you initially considered both a shutter wheel type & a magnet disc type. You went with a shutter wheel & confirmed it functioned well on a bench rig so ended it there, at least for now. And you have also made a magnet disc type on different project.

Are there pros, cons or limits where one system would be net better performance than the other? My own naïve assumption, at least for a distributer of say 1" diameter or larger & 4-9 cylinders max, is a magnet disc is possibly a tad easier to fabricate. Disc requires N holes for N cylinders equally spaced. Magnets are spit cheap so buying 1 vs buying 7 more for an 8-cylinder doesn't seem like a concern. I've seen neodiums available as small as 1mm dia x 0.5mm thick. Maybe that's too small or maybe the quality / consistency factors in this? The shutter wheel requires a bit more careful milling compared to N spotted holes for a magnet disc, but not a huge difference either way. Maybe crowding too many magnets in too small a space on its disc can create issues? Or spinning an array of magnets in close proximity to the upstairs rotor assembly is less desirable for some reason & requires more vertical offset distance between them?  I think I'm hearing the hall effect sensor is essentially the same if not identical. They are also a relatively inexpensive component & require about the same consideration/methods to mount. I am also considering making 2 types just to see, but wondering out loud why not magnet disc type first?

[gbritnell]

The only issue with a magnet disc is there is no way to control the dwell. With a shutter disc you can control the dwell with the size of the windows.

[petertha]

Thanks George. I have no idea if my sketch is a figment of my own imagination so hopefully its understandable enough for this purpose, or correct me as needed. I've read some hall effect sensor specs & I understand ~20%. Using your V8 shutter & magnet dimensions as example, are you saying that the aperture width would constrain (reduce) the interval time the sensor sees compared to a bare magnet in a disc something like so?

I assumed the way CDI modules worked is they had some kind of internal logic; when the hall tripped ON based on some minimum magnet strength, they applied their own 'on time' spark duration based on that signal point mainly so it met the requirements of the capacitor discharge. But maybe it will attempt to deliver for as long as as it detects ON & that's where shutter controls? Now maybe I'm getting messed up with similar discussion on the RCEXL type modules that keep track of pulses & time & therefore know RPM & how to electronically apply advance/retard... they are a different animal apparently even though they also use hall sensors. I should probably address these questions to S/S but bottom line is I think their base configuration assumes a magnet disc? But I think I've also read where people use a shutter with S/S.

https://www.cncengines.com/ic.html

[Vixen]

Peter,

I am not sure you have got all that Hall trigger stuff quite right.

Lets go back to first principles. The image below represents the magnetic field surrounding a simple bar magnet (dipole). Note: the magnetic field is normal (vertical) to the two end faces (poles) of the magnet, but only in a very small part near the centre of that face. Everywhere else, the magnetic field is inclined at an angle. A Hall effect sensor acts as an electronic switch. It will switch ON, ONLY when an applied magnetic field is a) strong enough  b) normal (vertical) to the face of the sensor c) of the correct magnetic polarity.  One face of a Hall effect switch will only respond to the North pole of a magnet. Flip the Hall sensor over and the reverse face will only respond the the South Pole. Therefore the Hall sensor and magnet's orientation is vital.





We know it is possible to make a Hall effect multi cylinder ignition trigger systems by rotating an appropriate number of small rare earth past a single Hall sensor. Alternatively it is possible to use a single static magnet and Hall sensor with a rotating iron/ steel shutter wheel between. Both will 'sorta' work, but the shutter when has the potential for more precise timing. It seems unreasonable to assume that the 8 'cheapo' rare earth magnets in the rotating disc will all have identical magnetic fields. Some timing dither is to be expected. Which method you use depends on the design restraints and space available for your ignition trigger. The rotating disc could perhaps be smaller, or fit the available space better than the shutter disc. Sometimes it's the other way round.

One advantage of the shutter wheel (as George pointed out) is the ability to control the mark/space ratio (blade to gap ratio) of the output pulse from the Hall effect sensor. The mark/space ratio can be used to alter the 'dwell angle' of your ignition device. The 'dwell angle' gives time, especially at higher RPMs, to fully charge the capacitor in a CDI unit, or time for the field to fully build up in an ignition coil.  However, you need to be cautious. Coil systems fire when the points open i.e. the Hall sensor switches state. Commercial CDI units can fire either when the Hall switches On or when it switches to the OFF state. You never know which until you buy and test one. It's generally not a problem, as you can usually alter the angle where the Hall sensor triggers to suit your chosen CDI or coil.

When it comes to choosing the mark/ space ratio for your shutter wheel, you need to consider how and when the chosen coil or CDI fires. One will favor a longer mark to space ratio, whereas the another will favor a shorter mark and a longer space to give the desired 'dwell angle'. It therefore pays to optimise your design for ONE type of ignition system and stick with it. Swapping between ignition systems and hoping, may not always produce the best results.

Cheers

Mike