Limited Coils?

[Ozarker]

There's a lot written about limited coils, but does anyone know how they're made to be limited, and how it is that they come into play at certain rpms? Is this all a matter of electronic circuitry, or is there some other method employed?

Does anyone know at what rpm they kick in? I would expect that this would vary somewhat between models, and manufacturers but perhaps not?

 

[Ozarker]

Here's a file "that will look at how a magneto works, and demystify the 'magic' involved in creating the spark."

There's an interesting tidbit in the final paragraph on Page 8.

Final paragraph on Page 10 yields a clue as to timing advances built into the coils.

Good chart and explanation on Page 11.

Eureka! Pages 12 - 13.

Spark-plug basics, Pages 14 - 15.

 

[Landmark]

I am sorry. My knowlege is 'limited' on this subject.

 

[Ozarker]

You, me, and everyone else, apparently!

 

[XP_Slinger]

You, me, and everyone else, apparently!

That's damn amazing no one chimed in to explain them....they must be truly black magic components

 

[Ozarker]

I thought so, too, XP Slinger. There's some Dark Arts to them, no doubt. Guess we just have Jedi, and no Sith Lords of the Dark Side here.

 

[Terry Syd]

Ozarker, nice find! I was stunned to see at the bottom of page 12 that most of the modules produced in the last 10 years have a CDI variable ignition advance. Hell, I haven't heard that before.

Now, where can we find the ignition curves for the various modules? Crikey, I want to get rid of the old set timing magneto system on a couple of saws and adapt a newer ignition module to the saws. Easier starting and more power!

It may be that one of the tell-tale signs of a newer module would be a 'limited' coil produced in the last 10 years.

EDIT: It is interesting to see the curvature of the advance. Notice that the set ignition timing is a compromise of 28 degrees, which sometimes creates hard starting. However, up in the powerband the engine requires about another 5 degrees for a total of 33 degrees advance. The variable timing allows the best of both worlds. Using a CDI is also desirable as a capacitor discharge system has a very fast and strong spark.

 

[XP_Slinger]

I can only assume that limiters are a function of lower resistance the doesn't allow the coil to build and discharge beyond a certain RPM. Now I'm curious and will start digging on how this is accomplished mechanically. There must be a capacitor formula out there that engineers use to set these limiters. Resistance is a precision game and would explain why the same model saw with the same limited coil almost always has slightly different cut out RPM.

 

[mdavlee]

Most limited coil saws use resistor plugs.

 

[Terry Syd]

I'm going back more than 40 years to my electronic days, but timing circuits can be made from resistor/capacitor circuits. The capacitor bleeds down through a resistor, when it hits a certain voltage level it can set a leg on a 'gate' (and, nand, or, nor, whatever). With the gate set, when the ignition trigger comes along on another leg and passes the gate - and the ignition fires. If the RPM gets too high the capacitor keeps getting refreshed and the circuit voltage never drops to set the gate - thus, we never get the ignition trigger past the gate and the engine misfires.

Electronic components have different tolerances, I remember a brown band being withing 20%, silver 10% and gold 5%. Thus with the various tolerances of the resistor and capacitor, the timing can vary - which accounts for the variation in RPM for the limiter.

It's been a long time and 4 beers later, if some current electronic technician can correct me, please do.

 

[Ozarker]

I can only assume that limiters are a function of lower resistance the doesn't allow the coil to build and discharge beyond a certain RPM. Resistance is a precision game and would explain why the same model saw with the same limited coil almost always has slightly different cut out RPM.

I'm going back more than 40 years to my electronic days, but timing circuits can be made from resistor/capacitor circuits. The capacitor bleeds down through a resistor, when it hits a certain voltage level it can set a leg on a 'gate' (and, nand, or, nor, whatever). With the gate set, when the ignition trigger comes along on another leg and passes the gate - and the ignition fires. If the RPM gets too high the capacitor keeps getting refreshed and the circuit voltage never drops to set the gate - thus, we never get the ignition trigger past the gate and the engine misfires.

Electronic components have different tolerances, I remember a brown band being withing 20%, silver 10% and gold 5%. Thus with the various tolerances of the resistor and capacitor, the timing can vary - which accounts for the variation in RPM for the limiter.

It's been a long time and 4 beers later, if some current electronic technician can correct me, please do.

Knowing very little about electronics, these strike me as being most probable. I wonder, too, if there's not some programming involved these days, rather than relying solely on the circuitry?

Most limited coil saws use resistor plugs.

Perhaps another clue, since the resistor plugs are designed to "help(s) reduce the electrical noise from sparking and protects any nearby electronic circuits from interference." Then again, my CS400 is said to have a limited coil, but the manual included does not list that, yet it does, in fact, use a resistor plug, while my MS660 and CS620 are said to have unlimited coils, yet the MS660M uses a resistor plug, while the CS620P does not. And, the CS590 is said to be limited, but does not use a resistor plug, according to the manual.

Still, what you're saying makes sense, for obvious reasons. It may just be that Stihl uses resistor plugs, no matter the configuration of the coil?



Thanks for the input, folks! Muchly appreciated.

 

[plcnut]

The controller board inside this ignition module is digital, with a
microprocessor and a quartz clock built in, and it gets the power it needs to
operate from the electricity created from the magnet in the flywheel.

This clearly indicate's that the timing is cotrolled by a quartz clock rather than the older r/c timers. The presence of the microprocessor also indicates that the timing advance is controlled by an actual program.

 

[Ozarker]

Yeah, that's what I was getting at, picnut, as I did recall reading that. But, I'm trying to learn here, so asked the question. Thanks for answering!

 

[plcnut]

Have any of you attempted to disassemble one without destroying the internals?

 

[XP_Slinger]

Have any of you attempted to disassemble one without destroying the internals?

I'm changing my limited coIL out of my 372 this sunday. So long as I'm happy with its unlimited replacement I could tear it apart.

 

[Terry Syd]

I'm changing my limited coIL out of my 372 this sunday. So long as I'm happy with its unlimited replacement I could tear it apart.

I wish you were in Oz as I wouldn't mind a limited coil on my 365BB. Any Ozzies out there with a spare limited coil sitting in the shed?

plcnut got it right, I remember reading that comment in the manual (it must have been the beer that caused the brain fart).

I don't know how to 'cut and paste' from a PDF file, perhaps someone could cut and paste the timing graph from the manual. The timing figures would be worth discussing and a visual aid would help.

 

[XP_Slinger]

I wish you were in Oz as I wouldn't mind a limited coil on my 365BB. Any Ozzies out there with a spare limited coil sitting in the shed?

plcnut got it right, I remember reading that comment in the manual (it must have been the beer that caused the brain fart).

I don't know how to 'cut and paste' from a PDF file, perhaps someone could cut and paste the timing graph from the manual. The timing figures would be worth discussing and a visual aid would help.

Be happy to send it to you if it didn't cost an arm and leg to get there.
Hopefully my saw likes the advance of the replacement coil. I've read a couple places that the limited coils have a better advance curve. Whether that's true or not I don't know

 

[Terry Syd]

Heck, I'll send you the money. It would still be cheaper than purchasing a coil in Oz. If you're serious about sending it, contact me via private mail and give me your bank particulars.

I've tried to copy and paste the graph on page 11 (ignition curve), but even downloading to my computer the file is still 'password protected'.

If you go to the graph on page 11 you will see that a set ignition advance of 28 degrees crosses over the ignition curve line at about 5,500 RPM. Below that RPM the 28 degrees is too much advance and above it the advance is inadequate.

If we had the advance curves for various modules, we might be able to match a module for a modded saw. The graph on page 11 looks like it may be for a saw that power peaked at 12,000. If someone ported a saw for more RPM, we may need a module for a smaller saw that would rev to 13,000.

 

[Ozarker]

If I understood electronics, I'd love to pull one apart, then figure out the engineering, the program, and how to re-program. Alas, others will get to make millions from that endeavor. Prime business opportunity here for some adventurous person.

Here ya' go, Terry. I had to do a screen-shot, crop, then save, then crop again, to get it.

 

[Terry Syd]

Thanks Ozarker. From the diagram you can see how advancing the ignition has some drawbacks on a set ignition timing engine. If we move the timing from 28 degrees to 30 degrees, it helps up in the top of the powerband, but decreases the power at slightly lower RPM (plus increase the heat to the engine).

In the engine/diagram shown, a two degree advance moves the cross over point of optimum timing from 5,400 to 6,600. If you made a 4 degree change in order to get maximum power, you would move the cross over point all the way up to 8,000. In other words, you sacrifice torque down low to get power up top.