compression in cylinder

[bwalker]

Must be?

 

[Crofter]

Thrust from rod angularity has an influence on piston rocking and offset of pin can have an effect on the dynamics involved. If the load were straight down I am sure there would be less rocking with the pin centred. Offset of bore makes the load straighter down thus more effective on crank plus less force wasted on thrust. With either pin or bore offset, there is a different number of crank degrees from piston top of travel to bottom of travel than there is from bottom of travel to top. This because the angularity is slightly different. timing duration will be slightly different depending on which direction you rotate. Depending on transfer port arrangement and angle of induction into the crankcase, crank rotation is very important in moving the charge. Some engines will run very poorly at anything beyond low rpms because of this. Even if you changed the timing they would not run WELL in the opposite direction. Granted there are some arrangements that would be little affected. I just dont believe a blanket statement that two cycles dont care which direction they run is valid.
I do not know whether it is standard practice on saws to offset anything, but on a lot engine designs piston pin offset is very common and bore to crankshaft centerline offsets of nearly a half an inch were used on some designs. It is just interesting to know such small changes in geometry can affect efficiency and wear patterns.

 

[bwalker]

crank rotation is very important in moving the charge.

Thats what a lot of hack engine builders use to say when they where trying to sell turbo vaned cranks in the 70's and early 80's. It was bs then and still is. The primary motavation for charge motion is the rising and falling of the piston.

With either pin or bore offset, there is a different number of crank degrees from piston top of travel to bottom of travel than there is from bottom of travel to top. This because the angularity is slightly different. timing duration will be slightly different depending on which direction you rotate.

You sure on this, Frank? I really cant see how the distance between TDC and BDC can change based on the direction of rotation. Humor me here.

 

[Crofter]

Ben, I did some work on a weed wacker that I put on a large RC plane. There was one transfer up the rising side of the crank. The induction was via single reed on the other side that directed the charge very much directionally with respect to rotation of crank. Not the same beast at all as a dual loop scavenged with transfers either side at centreline of crank. I am just trying to get you to take the blanket off your statement, Lol! That reversing engine on the snowmachine uses some pretty clever electronics to sense position and give one very early firing way before tdc and start auto reverse direction and then establish proper timing on the other side of top dead centre. some two strokes such as the old railroad handcars required you to stop the engine, switch the timing and then crank her up the other way to get reverse. That engine was symetrical and a true pushmepullyou!
I remember struggling with the concept of the different duration when the bore is offset. Just think what would happen if you offset your crank an inch. Piston velocities are then not the same on an equal number of crank deg. rotation on the up stroke as on the down stroke. Piston travel is the same, but velocities are different. We could get Spacemule to give it to us in calculus, I suppose, but I sure as heck don't speak that language either!

 

[Crofter]

As to leakage at ring gap being potentially greater depending on rotation. I dont think there is any question that the piston presses a different face depending on rotation. With a cold and worn piston and rings just to exaggerate things, the ring gaps and locating pin will be pressed flush with the piston side and leakage will have a harder path than it would in the orher scenario with the rings standing out from the piston side. There would be a straight shot down thru the gap and the locating pin will fill a smaller percentage of the gap between ring ends. I think you would have to have a pretty worn engine to see this difference. I dont think Offset would change compression because stroke after port closing is the same either direction regardles of a different number of crank rotation to accomplish it.

 

[bwalker]

I dont think Offset would change compression because stroke after port closing is the same either direction regardles of a different number of crank rotation to accomplish it.

Which is what I have been saying all along.

As for the piston canting inside the bore in relation the the rings locating pin. To make any sizeable differance the skirt to wall clearance would have to be huge. With the piston banging around the bore like that failure is right around the corner.

 

[Crofter]

Ben, you asked me to humour you; I tried, I tired.

 

[bwalker]

Frank, Is it just me or did you do a 180? First you said " am not sure whether it is common now to offset either the piston pin or the center line of the bore to crank center slightly, but this gives a very slight difference in port timing and effective compression ratio dependant on direction of rotation."
The you say " dont think Offset would change compression because stroke after port closing is the same either direction regardles of a different number of crank rotation to accomplish it."
Is this one of those posts that you are arguing for the sake of argument? What say you?

 

[ccicora]

All's I want is a picture of you two shaking hands at the GTG now that would be a good post.

 

[Dennis]

I have witnessed several chainsaws running in reverse, granted not very well, due to timing. I am still (hmm...2 years now..?) working on a set of Super 930 jonsereds in which one will run in reverse on a double ender bar. One day I will finish them...lol

 

[bwalker]

Chris, I think you are detecting some anomosity that simply isnt there.

 

[Crofter]

"Oh what a tangled web we weave when first we practice to deceive".
I wondered where this might have gone while I was sleeping. There should be some pretty obvious flaws. I am being straight on the ring gaps and rotation affect and it is a fact that not all two strokes will run as well in either direction. It is also fact that piston pin offset is common and cylinder bore to crank centre offset is being used. I dont know if it is being used on todays saws. I got to thinking about that possible effect on the operation of a two stroke in its opposite to design direction.
Timing of two stroke events can be described in different ways. Duration can be described in different terms from absolute time to crank degrees. Throwing in offset skews some of these events and velocities of piston at a given position is not the same if direction of rotation is reversed. Piston accelleration is not the same either side of top and bottom dead centre either because of non symetrical rod angularitity before and after. What seems obvious some times is not. I generally do not like blanket statements and I wanted to see if Ben would do his homework on the effects of offset.

 

[ccicora]

Obviously Ben you have over looked the statement infront of you... It is just nice to see two people discuss an issue no matter how moot the point really is and still be able to shake hands and be friends in the end. But I figured that would be very easy for you to see.

 

[WRW]

This is secondhand information, I can't comment on validity. I've been told that the Stihl string trimmers using the same engine for bent shafts and straight shafts just run the engines in reverse to get the opposite head rotation. FWIW.

 

[Crofter]

Chris, Ben and I have lots of Pm's back and forth. We do a lot more posturing here on the forum. We can find lots of things to discuss and I look forward to meeting him at the get together. I try not to jump to conclusions, even so I manage to fool myself occasionally. Makes things interesting if that kind of thing interests you. Lol!