Two Cylinder Uniflow

[astroud]

I suggest trying with one of your existing engines, try to turn it over by pushing/pulling the valve rod.

Andrew

[crueby]

I suggest trying with one of your existing engines, try to turn it over by pushing/pulling the valve rod.

Andrew

Good idea. Pick one with a large throw on the valve rod. 

[PaulR]

Thanks for the suggestions!

The engine I've just finished can be turned over by pushing/pulling the valve rod but it's certainly not as easy as using the crank or turning the flywheel which proves Chris's point perfectly. I only have one other engine with an eccentric which is very small and thus difficult to get hold of the rod to apply any force but I can move it a little (that engine is a bit tighter anyway, probably as it's been standing unused for over a decade.

I'll think about making a crank instead but that has the knock-on effect of needing more complicated con rods. For these basic, mostly experimental engines I'd like to keep the number and difficulty of parts down to avoid wasting time and materials in case the design is flawed.

[PaulR]

Oh yeah, the other thing about trying to pull/push the valve rod is I'm also having to drag the piston and rod etc which wouldn't be the case with the original concept so maybe it would be so bad. The other advantage is that the timing could be adjusted. Perhaps I'll be a bit more realistic and try it with two cylinders...

[crueby]

If you can lengthen the stroke of the cam version, that would give it more leverage while keeping the simplicity of  construction.

[PaulR]

If you can lengthen the stroke of the cam version, that would give it more leverage while keeping the simplicity of  construction.

I'm sure I could, I have 8ft of tube to play with after all 

[PaulR]

Might be better to scratch the guide fork idea which might be tricky to line up properly and make the eccentric a bit more traditional with a sheave or even just a little tab to  keep the rod on track. Just making the smaller diameter (spigot part) longer might work just as well though.

I think a two cylinder version is definitely the best place to start. Updating the thread title!

[Charles Lamont]

I would worry less about the friction and more about the lever arm. At mid stroke, the force would be centered halfway out the cam, rather than out where the crank pin would have been. Much less torque  transfer.

Oh, good grief. For the same stroke S, the lever arm is exactly the same S/2, whatever the diameter of the crankpin.

But compare the moment arm about the crankshaft at which the frictional resistance acts between pin an big end with the moment arm in the case of the eccentric and bigger end.

[crueby]

I would worry less about the friction and more about the lever arm. At mid stroke, the force would be centered halfway out the cam, rather than out where the crank pin would have been. Much less torque  transfer.

Oh, good grief. For the same stroke S, the lever arm is exactly the same S/2, whatever the diameter of the crankpin.

But compare the moment arm about the crankshaft at which the frictional resistance acts between pin an big end with the moment arm in the case of the eccentric and bigger end.

Wrong, Charlie Brown (good grief pun). The lever arm is at the middle of the crankpin. With the pin size in the normal crankshaft design range, its a very small difference. But, for the way this one is drawn, its a huge difference. Look at the picture in reply 12 again!

[internal_fire]

Wrong, Charlie Brown (good grief pun). The lever arm is at the middle of the crankpin. With the pin size in the normal crankshaft design range, its a very small difference. But, for the way this one is drawn, its a huge difference. Look at the picture in reply 12 again!

You missed the part about the same stroke. (Of course the proposal that started this tangent did not have the same stroke for the two styles.)

Gene

[crueby]

Wrong, Charlie Brown (good grief pun). The lever arm is at the middle of the crankpin. With the pin size in the normal crankshaft design range, its a very small difference. But, for the way this one is drawn, its a huge difference. Look at the picture in reply 12 again!

You missed the part about the same stroke. (Of course the proposal that started this tangent did not have the same stroke for the two styles.)

Gene

I did not miss that at all. Don't confuse stroke distance with lever length at midstroke. Build a simple mockup of both crankshaft and eccentric with equal throws, push on the conrod, and see how much torque you feel on the crankshaft. It will NOT be the same.

[Charles Lamont]

Gene, I have read the whole thread again, and I have still missed anything about two styles, let alone with different strokes.

Chris, here is a diagram. The lever length for the same stroke is the same in both cases. It is the throw of the crank or eccentric. You are right that it will not work the same, but that is because of the radius at which the friction acts. Hint: imagine with a plain bearing and then with a needle roller.

[crueby]

Gene, I have read the whole thread again, and I have still missed anything about two styles, let alone with different strokes.

Chris, here is a diagram. The lever length for the same stroke is the same in both cases. You are right that it will not work the same, but that is because of the radius at which the friction acts. Hint: imagine with a plain bearing and then with a needle roller.

Okay, I  like that diagram better and agree. The one in reply 12 had me thinking the crank center was in a different place.   And by two styles, I  read that as pin vs cam style?

[internal_fire]

Gene, I have read the whole thread again, and I have still missed anything about two styles, let alone with different strokes.

I probably did not state it very clearly. I was agreeing with you 100%.

Gene

[PaulR]

Glad you're all in agreement, shame I have only the vaguest idea of what you're all talking about 

I'll draw up a plan and put it here for comments before getting stuck in to building it.

Thanks all for the help