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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Fri Sep 04, 2015 3:08 pm 
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Those are sympathetic vibrations. Ultimately, the root cause of vibration is the part of the machine that's actually moving: the engine itself.

If you want to solve isolated oscillations due to sympathetic vibrations, then tackle those specific items. There are bar snakes and end weights (usually called "dampeners. The stock bars have some very light ones) for the handlebars, though for as long as I can remember, filling the handle bars has solved the problem for most folks. Material can vary: some like sand; some like lead shot. The most popular, believe it or not, has always been concrete.

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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Fri Sep 04, 2015 10:03 pm 
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jsonder wrote:


How about some Test Data, on the Offset Cylinder Bore or Crankshaft.

Link : http://papers.sae.org/2004-01-0606/

How long has this Topic been Explored? Would about 90 years be soon enough. On the Link below, tap the Page Down Key 4 times, will start showing the Patents and Dates.

Link : http://www.google.com/patents/US5816201

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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Sat Sep 05, 2015 6:00 am 
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Thanks for the links SS. It's interesting that actually testing the concept showed no conclusive benefit in the SAE paper. The patent was a bit ridiculous in suggesting a 24 inch connecting rod to minimize the drawbacks of the offset. I'd expect the bulk and weight of such an engine would pretty much negate any small horsepower increases in an automotive application.

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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Sat Sep 05, 2015 7:24 pm 
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This thing!

The problem here-- the reason this comes up once a generation or so-- is that it _looks_ like it should work. After all, most engines today fire just a tiny bit BTDC, right? So are the explosive forces not being hampered by the still-up-moving piston? Are they not reduced in efficiency because they start pushing straight _into_ the crankshaft instead of rotating it?

Then you look at that offset cylinder design and you think "this is precisely how I push my kids on a merry-go-round at the park! I push "sideways," taking full advantage of the rotation!"

Thus, it "looks" completely obvious that this is a far more efficient design.


But for the same reasons that you can't tune by ear, you can't engineer by eye:


What people are _not_ looking at is that this design allows the cylinder to "push" for only about 2/3 as much rotation. Then it's just being dragged along the up-cycle. In extreme designs, it's only pushing for 1/2 as long.

They are also not considering the need for a much longer connecting rod, meaning longer (and therefore slower) effective stroke. Slight drop in ponies when trying to build RPM.

What it amounts to is precisely what Duckster said: "If there is a gain, it's small enough that the extra detriments of the design completely balance it out."

There are a lot of actual science articles buried all over the place that back this up (and I'm assuming SS posted at least one; don't have time to read the links right now, but I will try to get it to before the weekend is out).

This false 'obviousness' has taken more hold than usual of late, simply because a couple of really powerful, really fast motorcycles _are_ using this design. The problem is that people assume they are using this design for this "free" power boost.

Remember the heart of all aspects of the universe, Kiddies: TANSTAAFL! TANSTAAFL now; TANSTAAFL then; TANSTAAFL forever and always!

"Windmills do not work that way!"

What the proponents who are currently pointing to the smattering of motorcycles using this design are not noticing is that there are _other_ reasons to use this design, most specific to motorcycles is compactness. The resulting shape changes to the castings of the engine allow the engineers to wedge a few more cubes into a space that must, more or less, fit into the "hug radius" of a human being who is already doting on a fuel tank. It allows the crank to push further back and the cylinders to not have to run up quite so far. If the connecting rods are mounted at alternating points on the rotation, it even allows the cylinders to brought much more closely together than possible without it.

But none of those things will provide any sort of "free" power or "power boost." TANSTAAFL. It's simply not possible.

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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Tue Oct 27, 2015 1:03 am 
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The thing with the asymmetrical piston design, is that only the stroke (explosion, and entry of air/fuel) is counted. The counter stroke (compression, and where the piston is rotating past the vertical center points, and exiting the exhaust gasses), is not considered a stroke.

In essence, the engine would be a larger cc engine, where only a smaller amount of the stroke is used; meaning, if the engine would be symmetric, it would considered be eg: a 320cc, with 160cc per cylinder used as stroke, and 160cc used as counter stroke.
But asymmetric only 2x125cc is measured as stroke, and the remaining 195cc is counted as counter stroke.
Only the right amount of fuel of a 250cc would be injected, but the performance should be closer to a 275cc as the fuel mixture can burn more efficiently, and performance levels are somewhere right in the middle between the 250 and 320cc.
The numbers are generic serving as example, not to be taken literally.

The Rebel 250 does not have an asymmetric design.
I don't know if their newer models CB 300/500 F/R/X, or even their newer 750s do.


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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Tue Oct 27, 2015 10:45 am 
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I've read your post 3 times now, and still can't make any sense of it. Perhaps you need to review the basic concepts of a piston engine before trying to explain it to others.

Regardless of whether the cylinder bore is concentric with the crankshaft, the engine displacement equals the swept volume of the pistons, i.e. the cyindrical volume swept by the piston top travelling from the top of its stroke to the bottom, regardless of the position of the crankshaft and connecting rods below the cylinder.
In a conventional engine, the stroke dimension is simply the diameter of the circle traced by the crankshaft throw since the connecting rod is vertical at both the top and bottom of the stroke. Multiply that by the cross sectional area of the piston and you have piston displacement.
It gets a bit more complex when you offset the cylinder axis ahead of the crankshaft axis. The top of the stroke and bottom of the stroke still occur when the con rod is aligned with the centerline of the crankshaft. At both top and bottom of the stroke the con rod will no longer be vertical . The more offset you introduce, the more assymetry you build into the piston strokes with respect to crankshaft rotation. the upstroke takes fewer crankshaft degrees and the downstroke takes more. For a constant RPM, the piston speed (and Piston Friction) is higher on the upstrokes to travel the same distance in fewer crankshaft degrees of rotation.
Please note however, that although the stroke lengths do increase with more offset of the cylinders, the up stroke and down stroke lengths remain identical to each other as with any piston engine, and from the fuel injection standpoint, the only thing that matters is RPM and displacement. The carburetor can't see anything that is happening below the piston tops.
You do get increased piston displacement with increased cylinder offset from a given crank throw dimension, but at the expense of more rod angle and piston speed on the upstrokes. net benefit? Apparently most manufacturers don't think so.

If you're interested in the science of it rather than the voodoo you just made up, read the abstract of the SAE paper on the subject researched by Soul Searcher previously.

http://papers.sae.org/2004-01-0606/

The following is a quote from this abstract.

"....a single-cylinder variant of a production GM 4.2L I6 engine was designed and tested. Cylinder pressure acquisition equipment and dynamometer torque measurements were used to quantify differences in combustion performance and firing friction between an offset crankshaft design and a standard baseline configuration. Extensive testing on the single-cylinder engine in both configurations found no significant frictional or thermodynamic differences to exist. Multiple tests of both design configurations were run in an effort to increase statistical confidence and test data showed good repeatability and precision. Analysis of cylinder pressure data clearly showed the offset crankshaft architecture had no performance benefit attributable to combustion performance changes. Friction measurements based on dynamometer torque readings however were somewhat less conclusive, as the level of torque change measured, although repeatable, were within the boundaries of experimental error. Also in contrast to previously mentioned literature, the data consistently showed a slight increase in friction with the offset crankshaft configuration used for this study. "

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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Tue Oct 27, 2015 5:37 pm 
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I could be completely wrong, but I was under the impression that he was referring to the fact that not all the bore surface is technically cylinder, as the offset wobble of the crankshaft must be made room for, or something along those lines. Thus, there is a "short side" in there. Technically, there's no reason for this area to be bored at all, but it results in a cylinder that externally appears to be longer than it really is, what with it setting off to one side, etc.

Granted, it's still something of a stretch, but that's the way I took it. The numbers didn't really add up; I just assumed they were examples to illustrate the point.

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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Tue Oct 27, 2015 5:57 pm 
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Would you be able to elaborate on that Duke, because I'm still getting nothing here. What do you mean by "offset wobble of the crankshaft"? I can see needing a little cutout at the bottom of the cylinder to allow for the offset conrod, but all of the cylinder wall above the piston skirts is going to be still active.
The "short side" might be the idea that the up strokes will take less than 180 degrees of crank rotation and the down strokes will take more than 180 resulting in a higher piston speed on the upstrokes than on the downstrokes.
I actually set up a spreadsheet with easily varible parameters for the engine that shows engine displacement increases with offset of cylinders and this is more pronounced with shorter conrods.

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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Tue Oct 27, 2015 6:59 pm 
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I really can't. Evidently I missed his point entirely.

But let me do this: I can explain the point that I _thought_ he was trying to make:

As you know, it's impossible to "offset" from the center of rotation if the cylinder is facing (oriented along an axis that is ) pointed directly toward the axis of the crankshaft. You can move it to any point along the 160 degrees; the only way to "offset" the cylinder is to turn its longitudinal axis such that a line descending directly through the center of it passed by but misses the rotational axis of the crankshaft.

In such a design, there needs to be clearance for the connecting rod which will now "wobble" further to maintain connection to the "offset" crankshaft, which is further way at a quarter rotation than it is 180 degrees from that point. I wish I could draw with words, because I sure as heck can't do it any other way! :lol:

All that being the case, the cylinder housing, from the outside, at least, can appear longer than it really is, but only on one side. It's possible to construct such an engine that one side (the side to which the cylinder is "offset" can be nearly flat all the way to line defined by the rotational axis of the crank. Further, if you want, but it's not necessary. However, this can't be done on the other side, simply because the cylinder wall will at some point have to stop to stay out of the crankshaft's way.

Again, it's just what I got from what he said. I can't say it made a lot of sense--- let me rephrase that: it made perfect sense, but in that "well yeah; I thought we had already ceded that point" sort of way. So I took away, particularly once he started throwing up numbers.... No; not quite. After I spent a few minutes trying to find relevant calculations in which the numbers worked, I eventually got the impression that he was tossing out examples to explain the above:

While the combustion chambers _look_, at least on one side, like they belong on a 750, there is in fact only 500cc displacement. That sort of thing.

To reiterate my disclaimer in the previous post:

It didn't make a lot of sense; it is simply what I got out of it.

Yes: we pride ourselves on keeping misinformation off the board and sound science for good information, but given the general trend of this thread and the unworkability of what was stated in that post, I just didn't feel that it was necessary to comment on it. I couldn't fathom anyone taking that for reality.

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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Tue Oct 27, 2015 7:42 pm 
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Duke wrote:
given the general trend of this thread and the unworkability of what was stated in that post, I just didn't feel that it was necessary to comment on it. I couldn't fathom anyone taking that for reality.


No argument there, but perhaps a technical newbie might just feel dumb because he doesn't understand it, whereas you and I know it's just technobabble.

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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Wed Nov 11, 2015 8:10 am 
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Without the right terminology, there's no way I could write it any better than I did, since my main language isn't English, even less Mechanical English, so I'd suggest to overlook the wrong terminology used.

The numbers also where just examples.

I'll try to reword it again,
If you get the principle I'm trying to explain, is already good enough!

As far as the performance of an offset cylinder goes, it is more in case the stroke ends up being longer than the original; comparing the same length of the explosion stroke of both a standard piston, and an offset one.
After all, only the stroke where the explosion occurs, is counted as productive, the stroke where the cylinder goes back to top is considered waste, and longer than the productive stroke; where as on a regular engine the explosion and compression cycle (or stroke) size are identical.

On an identical engine, with only difference an offset cylinder, the same amount of air/fuel mixture can be compressed,
Not from top to bottom, but under an angle,
but the 'explosion' part of the cycle is shorter, thus it's performance is lower. Though the engine will run more efficiently.
To put up generic numbers again, a regular engine could be 249cc, but an identical offset engine have 249cc of compression, but only 234cc of productive stroke (explosion). Again, numbers are to serve as an example.

I realize without a picture I'm probably not making a lot of sense.


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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Wed Nov 11, 2015 8:37 am 
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What's your first language?

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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Wed Nov 11, 2015 5:50 pm 
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No; actually that makes perfect sense. It sounds like I got it the first time, then. :)

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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Wed Nov 11, 2015 6:00 pm 
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Language barrier or not, you are talking nonsense, MeeLee.
The piston STROKE increases slightly with cylinder offset. However the "down" stroke (explosion, if you like) and the up stroke (compression) are still identical lengths. If the piston travelled further down than it did up, it would quickly end up in the crankcase.
However, the down stroke would now take a little more than 180 degrees of crankshaft rotation and the up stroke a little less. Obviously for a constant crankshaft speed, the down stroke would require slightly higher piston speed than the upstroke. Those that have studied the matter found no power or efficiency advantage to the layout as noted earlier in the thread.

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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Mon Nov 16, 2015 7:49 pm 
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The explosion stroke (down) is measured from between 1 to 2 o'clock on the rotation, to about 5.
Thus, the explosion is smaller than the upstroke, which is measured from 5 to 11.
From 11 to 2 there's really nothing going on. The gas gets compressed a bit, and released a bit, before detonation at 2 o'clock.


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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Mon Nov 16, 2015 8:46 pm 
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Language barrier or not, you are STILL talking nonsense, MeeLee.
The piston STROKE increases slightly with cylinder offset. However the "down" stroke (explosion, if you like) and the up stroke (compression) are still identical lengths. If the piston travelled further down than it did up, it would quickly end up in the crankcase.
However, the down stroke would now take a little more than 180 degrees of crankshaft rotation and the up stroke a little less.

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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Tue Nov 17, 2015 6:34 pm 
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I would not say about you, that you speak nonsense, Duckster, so I would appreciate you treating me with the same respect.

Regardless, on an offset piston, the explosion stroke is shorter than on a regular design engine, as only a fraction of the 180 degrees is used.
The return is about as large as a regular design, 180 degrees.
The remaining (whatever should have been part of the 'explosion' part of the stroke, is actually a dead part, where the piston pivots around top dead center, until it reaches the point that can account for the "explosion stroke".
It makes perfect sense to me.

The downstroke can never take more than 180 degrees, as you say. It is shortened, through the offset.
The offset is at the side of the 'explosion', not at the side of the compression.

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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Tue Nov 17, 2015 6:59 pm 
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No. No, no, no, no, no. Each phase of the cycle is still the same length, just represented by different degrees of the crankshaft rotation. They are the same length. Intake stroke equals compression stroke equals combustion stroke equals exhaust stroke.

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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Tue Nov 17, 2015 8:11 pm 
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I stand corrected, looking at the picture.

I was of presumption that the compression stroke started around 2 o clock on the illustration above, but it actually starts between 12 and 1 o clock, and the compression stroke starts at 7 o clock, not at 5, as I had previously assumed.


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 Post subject: Re: a little 2 stroke oil in the fuel
PostPosted: Sun Dec 27, 2015 1:58 am 
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Very interesting vid about engine cleaning, and lubrication in the upper piston,
One of the reasons to always add a bit of 2 stroke oil in the fuel (or this):

https://m.youtube.com/watch?v=kbo3nZm88Q4


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