Upshur's twin opposed cyl. engine

[Brian Rupnow]

So, here we are, 8 weeks after my knee operation. I'm doing well,--no infection and minimum soreness. I'm just beginning to walk around my house without using a cane. My energy levels are far lower than I had wished, but apparently that is not unusual at this stage of the game. I got a phone call from my supplier today that my #2-56 brass threaded rod is in, (it's for the distributor) and I will pick it up tomorrow. Most of the energy that I do have right now is going towards my powder paint booth. I am setting here surrounded by box fans, furnace filters, and various pneumatic bits and pieces. Riding two projects at the same time has never worked that well for me, but I will keep you posted as I progress.---Brian

[wjh308]

I noticed you posted a video of this engine running! I'm glad your bad luck streak is over.

[Brian Rupnow]

whj308-i'm not sure what you seen, but this engine has never ran.---Brian

[wjh308]

Must have been an old video that popped up. It was a horizontally opposed twin cylinder engine with a big brass cap on top of the crankcase, where you stated, "The idea was to see the internals work but it was just a grey blur". I was mistaken because it looks very much like your current engine.

[Allan Ostling]

Must have been an old video that popped up.

Was it this one?

<a href="https://www.youtube.com/watch?v=R4vfq6FjRGs" target="_blank">http://www.youtube.com/watch?v=R4vfq6FjRGs</a>

[Brian Rupnow]

Yes

[Allan Ostling]

There is some similarity between your earlier engine and this modified-Upshur build. You have now changed to cams with flat sides instead of radii. Are there other significant differences?

[Brian Rupnow]

Not really. I have read that some people insist that the cams must have a flank radius, and I have read that some people say it doesn't really matter. This current engine is built mostly to Dick Upshur's plans and obviously he didn't think that a radiused cam flank was necessary. The original twin opposed that I designed and made the video of had the sparkplugs on the bottom of the cylinder head, and although it ran very well, if you flooded it when trying to start it there was no way that it would clear itself to start. The current engine has the sparkplugs positioned on the top SIDE of the cylinder head, which I have not seen before, but there are many videos on Youtube of this engine running, so it must work.---Brian

[Jasonb]

Your one based on the Bobcat has cams at 102.5 deg. So as I said on HMEM why can't you see that this engines cams will be fine if set at 102deg.

[Allan Ostling]

I'm confused by your switch to flat flanks on the cams. On HMEM you posted the following in 2023:

"If you don't know---a cam with a roller tappet works just fine with no radius on the flanks at all. But--A cam with no flank radius can't run properly with a normal round, flat bottomed tappet.--Every time the flat non radiused side of the cam hits the flat bottom of the tappet, it "slaps" it. This is bad ju-ju. Although some people get away with it, I prefer to do it the right way. My method requires a number of simple steps, and I will try to walk you thru it."

But on this current engine you do not have roller tappets -- your photo in reply#39 shows flat-bottomed tappets.

[Brian Rupnow]

Allan---I'm standing in two worlds. In "Theoretical world" I am absolutely sure of what I write. I am also standing in "Reality world" which sometimes confuses the Hell out of me. I firmly believe every thing I said in that post that you have quoted. However, Dick Upshur was nobodies fool, and his engine design without a flank radius seems to run very well in videos I have seen of it running on Youtube. I don't have a reason for this. I am not the world's leading authority on cam design. I believe that on a full scale engine which has to give maximum torque under varying conditions that the cams absolutely need to have a flank radius. On small scale versions of this engine, it probably isn't that big a deal.---Brian

[Allan Ostling]

. . . However, Dick Upshur was nobodies fool, and his engine design without a flank radius seems to run very well in videos I have seen of it running on Youtube.. . .

I was unaware that Upshur's design had flat cam flanks. I agree, practicality trumps theory, especially in the thicket of cam curve specs.

[Allan Ostling]

I dug out the textbook for an ME course I took in 1962: Kinematic Analysis of Mechanisms, by Shigley. The author has this to say about circular arc cams:

"At the blending points of the cam the acceleration changes abruptly because of the instantaneous change in radius. This means that the jerk [the first derivative of acceleration] is infinite at those points . . . an indiction of infinite jerk is of value to the designer in that it enables him to forecast phases of the mechanism which are likely to be troublesome as regards excessive wear, high surface fatigue stresses, and noise."

A cam with flat flanks is actually a special case of the circle arc cams, with two segments having zero curvature. Both types have the same number of instantaneous jerk spikes.

So much for theory. Model engine cams need not be too constrained by it.

[Charles Lamont]

With a flat flanked tappet and cam you theroretically get infinite acceleration, which is a whole lot worse than infinite jerk. Of course, the elasticity of the parts means you don't actually get infinte acceleration. In model sizes you can evidently get away with it, but it will result in more wear and mechanical noise.

Brian, the curvature of the flank has as much to do with the dynamics of the valve mechanism as with the performance of the engine. The two considerations are conflicting, and the designer has to compromise. Flatter flanks improve breathing, more curvature reduces accelerations and so mechanical things like wear, noise, and valve bounce. 

[Jasonb]

With the flat being shorter than the diameter of the tappet then it is only running on the flat for a split second so really running on a curved surface just about all the time.