261 M-Tronic - What makes it tick.....

[FergusonTO35]

They make one that is OBD-II compatible and I'm on board!

 

[RI Chevy]

Lol. That would be just too convenient!

 

[bryanr2]

Just Checking with this thread and if there have been any new findings?
@breese

Found this one in the mail recently. That count?

 

[DND 9000]

What makes it tick? Prüfrex and Stihl make it tick.

http://opeforum.com/threads/looking-for-ms261-info.15593/#post-819380

 

[RI Chevy]

:BUMP: Anything more to report???
@breese

 

[RI Chevy]

Bump.
What happened to this???

 

[Brutus]

Hi guys,

Let me introduce myself: I am a modeler (RC aeroplanes) and I do not even own, let alone use IC powered chainsaws.
I was reading this thread because somebody pointed it out to me.

I have very little understanding of the Stihl M-tronic system, and most of what I have read here is abracadabra to me, but here's the thing: I am approaching the matter from the other side. So maybe some of the things I found might help you guys reverse engineering, or possibly develop your own control system.

I wanted to run my normally methanol fuelled toy engines on regular gasoline, and the crude carbs these little engines have, are hopelessly unsuitable for that, so I figured I could use the solenoid valve of the Stihl M-tronic system, to control fuel supply related to throttle opening. Had to figure out how to do that basically from scratch.
All I knew (told to me by a Stihl Service guy living close to me) is that the valve opens and closes at approx 30Hz so I kept approximately that frequency. The solenoid has a resistance of about 33 Ohm, so 5V is a decent working voltage, resulting in a continuous current of around 130 mA.
I found, the valve is open when de-energized, and closed when energized.

I had a friend make a small pulse generator with controllable duty-cycle, that I could hook up to a throttle position signal.
I manually set fuel feed for each throttle position, once this curve is set, it seems to be pretty stable and reliable, given that I only use throttle position, no engine temp or anything, even ignition timing is "fixed" (it has an auto advance curve, but the ignition is a stand-alone unit, set at 28 deg BTDC at >4000 RPM).

What I think Stihl is doing, is to set the fuel jets to "slightly rich" so that the engine can be started with a de-energized system, and as soon as voltage is present and the ECU comes up, the solenoid starts "tailoring" the fuel mixture. How, I do not know, I guess it memorizes the last settings from the previous run, or maybe factory pre-set values.

That was just a guess from my side, but it seems to work, because I set my engines slightly rich on the high speed jet, and program the solenoid to modulate fuel delivery to what is needed (as said, I do that manually).

I have now 6 engines ranging from 5 till 65 cc (all fourstrokes) and a few small 2-strokes, running on a 10:1 gasoline/oil mix.
My engines run basically smokeless (occasional puff of smoke after prolonged idle, but no more than that) despite the fat mix.
I cannot run WOT all the time, because these engines are designed for a cooler burning fuel, but for what I need them to do, there is enough headroom.

Here's a few of them running:
fourstroke, 8,5 cc

2-stroke, 15 cc

I know these are not chainsaws, but engines are engines, and maybe some of the things I learned can be of use to you guys.

Brgds, Bert

 

[Mastermind]

Hi guys,

Let me introduce myself: I am a modeler (RC aeroplanes) and I do not even own, let alone use IC powered chainsaws.
I was reading this thread because somebody pointed it out to me.

I have very little understanding of the Stihl M-tronic system, and most of what I have read here is abracadabra to me, but here's the thing: I am approaching the matter from the other side. So maybe some of the things I found might help you guys reverse engineering, or possibly develop your own control system.

I wanted to run my normally methanol fuelled toy engines on regular gasoline, and the crude carbs these little engines have, are hopelessly unsuitable for that, so I figured I could use the solenoid valve of the Stihl M-tronic system, to control fuel supply related to throttle opening. Had to figure out how to do that basically from scratch.
All I knew (told to me by a Stihl Service guy living close to me) is that the valve opens and closes at approx 30Hz so I kept approximately that frequency. The solenoid has a resistance of about 33 Ohm, so 5V is a decent working voltage, resulting in a continuous current of around 130 mA.
I found, the valve is open when de-energized, and closed when energized.

I had a friend make a small pulse generator with controllable duty-cycle, that I could hook up to a throttle position signal.
I manually set fuel feed for each throttle position, once this curve is set, it seems to be pretty stable and reliable, given that I only use throttle position, no engine temp or anything, even ignition timing is "fixed" (it has an auto advance curve, but the ignition is a stand-alone unit, set at 28 deg BTDC at >4000 RPM).

What I think Stihl is doing, is to set the fuel jets to "slightly rich" so that the engine can be started with a de-energized system, and as soon as voltage is present and the ECU comes up, the solenoid starts "tailoring" the fuel mixture. How, I do not know, I guess it memorizes the last settings from the previous run, or maybe factory pre-set values.

That was just a guess from my side, but it seems to work, because I set my engines slightly rich on the high speed jet, and program the solenoid to modulate fuel delivery to what is needed (as said, I do that manually).

I have now 6 engines ranging from 5 till 65 cc (all fourstrokes) and a few small 2-strokes, running on a 10:1 gasoline/oil mix.
My engines run basically smokeless (occasional puff of smoke after prolonged idle, but no more than that) despite the fat mix.
I cannot run WOT all the time, because these engines are designed for a cooler burning fuel, but for what I need them to do, there is enough headroom.

Here's a few of them running:
fourstroke, 8,5 cc

2-stroke, 15 cc

I know these are not chainsaws, but engines are engines, and maybe some of the things I learned can be of use to you guys.

Brgds, Bert

Welcome to ope sir.

Thank you for sharing.

 

[Brutus]

Welcome to ope sir.

Thank you for sharing.

Thanks for the welcome.
I don't pretend to be able to contribute much, since I don't know much about cutting wood, but I am a bit of an engine nutter (Marine engineer by profession).
But the little I can, I will happily share. I picked up a thing or two on mixture, RPM and load. No expert on things like detonation, and only a little on ignition timing.

Brgds, Bert

 

[RI Chevy]

Cool stuff. Welcome.

 

[Mastermind]

Thanks for the welcome.
I don't pretend to be able to contribute much, since I don't know much about cutting wood, but I am a bit of an engine nutter (Marine engineer by profession).
But the little I can, I will happily share. I picked up a thing or two on mixture, RPM and load. No expert on things like detonation, and only a little on ignition timing.

Brgds, Bert

What you accomplished here is way above my pay grade. I'd like to be able to understand how.....but at this point in my life, I'm just glad I can still see well enough to port the upper transfers. LOL

 

[Brutus]

What you accomplished here is way above my pay grade. I'd like to be able to understand how.....but at this point in my life, I'm just glad I can still see well enough to port the upper transfers. LOL

It is way simpler than it looks.
If you want to get a mental image: imagine a chainsaw with a conventional carb, with high and low jets (and high and low needles).
Now imagine that you make a linkage between the throttle lever, and the high and low speed needles, with adjustable leverage ratio, so the fuel will be metered in accordance with the throttle opening.

I basically did that, I just did it via an electronic device. What I do, does not take RPM or CHT or EGT into account. It is nothing more than a fixed relation between fuel delivery and throttle opening, but because it is electronic, it is WAY more accurate than can be done with any form of conventional carburation: you are not bound by a straight lever ratio, but you can program a "curve" that can literally have any shape needed. And there lies the secret of the M-tronic: not so much in the fact that it is self learning or that it is taking multiple parameters into acount, but simply in sheer improved resolution and greater ability to remain close to stochiometric. A Walbro carb can have its fuel/air ratio vary over a very wide range, and you would not know it did: it is only designed to give a decent idle and a usable WOT. Everything else is irrelevant. This solenoid simply keeps that ratio much better under control, even if it was not ECU-controlled.

Another factor that I found, makes a HUGE difference, is that the solenoid causes an intermittent fuel flow, which greatly promotes fuel evaporation because instead of a steady flow, the fuel is introduced as defined droplets.
To explain that, is a long story, for another time perhaps, but this feature is perhaps even more important than the fine fuel control.

Those two features, the high accuracy of the mix and the improved evaporation, are IMHO the main advantages of the M-tronic.
Has to be, otherwise my engines would run worth crap without an engine control system taking all parameters into account.

 

[smokey7]

Wow very cool you were able to make the solenoid work for you in this application. I think the mystery with the module (coil) to mess with timing and do so only by rpm temp (plug resistance) and fuel control. The whole system is quite elegant. I don't think I will ever buy one but I do like the idea of someone not able to fry a saw by setting it too lean.

 

[RI Chevy]

@breese
Any updates here?

 

[RI Chevy]

@breese
Any updates here?

@breese

Anything more on your progress sir?