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Setting Up a 2-Sttoke for Direct Drive

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StarJar

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Setting Up a 2-Stroke for Direct Drive

This thread is dedicated to the memory of Mark Stull. May he rest in peace with a new set of wings.------------------------------------------------------------------

Mark's documentation of DD 2-sroke operation cáught a lot of us by surprise. Especially his experience of his DD setup, which he said performed as well as the reduction unit he had been using.
The experiment ended when his MZ-201 engine with about 400 hours of DD use failed due to a rod bearing. (He made a forced landing which he walked away from, but the airplane was badly dammaged.)

Having said, that I wanted to explore the concept a bit more with maybe some further possibilities.
All insights and comments are welcome.

After reading some material suggested by another member (we know who you are) I got the impression than one further thing that could be done, is to re-port the engine for lower rpm with high temperature epoxy.

Also using a carb with a smaller flow to get more mixure accuracy and response in this lower RPM range.

I'll start there and see if anyone has any opinions on this, or more ideas, be they supportive, conflicting or in totally different direction.
 
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TFF

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If you're really to do a low RPM 2 stroke; new sleeve with ports designed for what you need would be better. Here is a place to learn about 2 strokes http://modelenginenews.org there is a design section that takes some looking. Most want to make RPMs go up so reverse mods, ha.
 

StarJar

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If you're really to do a low RPM 2 stroke; new sleeve with ports designed for what you need would be better. Here is a place to learn about 2 strokes http://modelenginenews.org there is a design section that takes some looking. Most want to make RPMs go up so reverse mods, ha.
That's exactly one thing I was looking for but hadn't found yet. Thanks TFF.

I also have a belief that the size and timing of the smaller ports will not only improve the gas actions in the cylinder, but also allow a longer power stroke, and more compression stroke.

But let's not stop there!
 

StarJar

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That's exactly one thing I was looking for but hadn't found yet. Thanks TFF.

I also have a belief that the size and timing of the smaller ports will not only improve the gas actions in the cylinder, but also allow a longer power stroke, and more compression stroke.

But let's not stop there!
Oh I thought the site was going to be blank sleeves. But at least I know they're out their now.
Looks like TFF has more info for me to consider on this site. That should be good too.
 

texas-buzzard

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I talked to Mark many times since we were at the same airpark. When he first had the thought of trying a dd on his mz201 he knew that it would require a small inefficient prop to develope and keep the engine in the power band. His first flight attempt was unsuccessful so the prop was re-pitched and dropped in diameter. His second attempt did enable him to get airborne but fuel consumption was high and the prop was very loud. He continued to try different setups but never was satisfied completely. After his forced landing the dd was scrapped and went back to a reduction driven on a Kawasaki 340. He also talked about re-porting the engine but never did as he became very happy with the 340.

Monte
 

StarJar

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I talked to Mark many times since we were at the same airpark. When he first had the thought of trying a dd on his mz201 he knew that it would require a small inefficient prop to develope and keep the engine in the power band. His first flight attempt was unsuccessful so the prop was re-pitched and dropped in diameter. His second attempt did enable him to get airborne but fuel consumption was high and the prop was very loud. He continued to try different setups but never was satisfied completely. After his forced landing the dd was scrapped and went back to a reduction driven on a Kawasaki 340. He also talked about re-porting the engine but never did as he became very happy with the 340.

Monte
His thread corroborates what you say about his progression to efficient props.
He said thngs also about performance, noise and vibration that may have been colored a bit by his enthusiasm of generally successful flights.
I don't blame him for losing some of the enthusiasm after the engine failure. It's also very hard to say if the dd caused the engine failure. That, to me, is a puzzle. Probability says, both yes and no, IMO, because it did happen on the dd repurposed engine, but it did do it after 400 hours of unoverhauled use and it occured at the rear rod bearing.
That being the case, I've made it a goal to investigate the thrust bearing method of the Rotax or any engine I use, to see what the specs on it might be.
I highly doubt it was a radial load problem because of the way it failed at the rear rod.
But it also looks like there is space on the pto shaft, and case face to put another bearing or bearings there to help with both thrust and radial loads.

I know Mark went through an extreme phase with props going from 38" to 50". Both seem a little extreme, each at different ends of the spectrum.
If I recall correctly a 46" prop's tip does about .92 mach at 4800 rpm.
I don't see any reason to go beyond a 46" prop if your trying to run at the highest RPM.

He was getting into the direct drive at the same time he was building the biplane, being the experimenter that he was, but the direct drive might not have been the best match for that scenario, although he just about made the best of it.

In short I'm interested in using his experience and changing the formula to see if it will work for my proposed low drag ultralight.

Thanks sincerely for your first hand account, though.

PS. To whom it may concern; I also do not plan to test the plane and engine concurrently.
 
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cheapracer

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Re: Setting Up a 2-Stroke for Direct Drive

re-port the engine for lower rpm with high temperature epoxy.

Also using a carb with a smaller flow to get more mixure accuracy and response in this lower RPM range.
Absolutely no need.

Add reed valves and the appropriately designed pipe/expansion chamber is all that's needed.

If you want to go further then grab Gordon Jennings 2 Stroke Tuning book, it's foundations are all taken from Yamaha's great work that they made public and Jennings has made it easy to follow.
 

pictsidhe

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Epoxy filling an exhaust port seems a bad idea to me. Saying that, I once repaired a deep score in a plated cylinder with epoxy and was pleasantly surprised that it actually held up...
The usual way to shorten the timing on a 2 stroke is to lathe a bit of the cylinder base, then fix the squish somehow. John Robinson in the UK did a good 2 stroke tuning book. I rewrote his program for my Casio programmable calculator, lacking a computer back then.
 

pictsidhe

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I suspect that the op wants a little more than 50hp/ton. Saying that, yes, you can tune a 2 stroke for low rpm. But don't expect to get much more torque than a high rpm tune without turbocharging. That means less power.
 

TFF

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I would consider that boat engine running at redline. For its mass, 100rpm is screaming like a .049.
 

cheapracer

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The usual way to shorten the timing on a 2 stroke is to lathe a bit of the cylinder base, then fix the squish somehow.
Waste of time with regards to the engines in question that already has mild exhaust port height/timing. You need movements in the order of 1/4" or 6mm plus to have any real effect, not many barrels allow that sort of reduction (that's not needed).

Better to find a taller piston if you have to, still a waste of time though, the real world effects are negligible, I played around with this stuff for many years.

Nothing has effect like the right designed chamber and port area, it is 90% of it. When you get the area, then you need a reed to stop reversion at lower speeds.
 

Dana

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For what it's worth, some 2-stroke dirt bike engines have variable exhaust timing via a "power valve", which closes off the lower portion of the exhaust port at low rpm for better low end torque. The KX100 engine I converted for paramotor use had such a setup; it was operated by a centrifugal deal that opened the port fully above 4000 rpm or so (this was a 12000 rpm engine). Since I didn't need low end torque and I'd heard stories of the centrifugal mechanism falling apart I removed the mechanism and permanently blocked the power valve in the full open position. I also lengthened the exhaust a bit to lower the resonant frequency a bit to try to lower the peak rpm.

Dana

P.S. If anybody is interested in experimenting with such a setup, I still have the engine gathering dust, be willing to sell it. About 25HP, water cooled, with a 3.4:1 gear reduction and hub for a GSC ground adjustable prop.
 

clanon

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Nothing has effect like the right designed chamber and port area, it is 90% of it. When you get the area, then you need a reed to stop reversion at lower speeds.
What about Lengthening the admission a little...?
 

cheapracer

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What about Lengthening the admission a little...?
No need to bother on a 2 stroke when a reed is an instant answer. Go look at any trials bike, which needs loads of low down grunt rpms, and you will see direct crankcase reed induction (the shortest route possible) on all of them.

Just go to a bike wreckers and buy some reeds and blocks, cheap as, literally every 2 stroke bike has had them since the late 1970's.

Plane engines don't need them as they run in a higher and narrow rpm envelope, but will very much suit the mods talked about here, with the right pipe.
 
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For what it's worth, some 2-stroke dirt bike engines have variable exhaust timing via a "power valve", which closes off the lower portion of the exhaust port at low rpm for better low end torque. The KX100 engine I converted for paramotor use had such a setup; it was operated by a centrifugal deal that opened the port fully above 4000 rpm or so (this was a 12000 rpm engine). Since I didn't need low end torque and I'd heard stories of the centrifugal mechanism falling apart I removed the mechanism and permanently blocked the power valve in the full open position. I also lengthened the exhaust a bit to lower the resonant frequency a bit to try to lower the peak rpm.

Dana

P.S. If anybody is interested in experimenting with such a setup, I still have the engine gathering dust, be willing to sell it. About 25HP, water cooled, with a 3.4:1 gear reduction and hub for a GSC ground adjustable prop.
What price are you looking at?
 

Dana

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I'll take $500 plus shipping, will include the frame and tank too. PM me if you're interested.

Dana
 
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