Two Cylinder Uniflow

[PaulR]

Another bonkers plan is emerging... a uniflow engine with four cylinders!

I'm thinking of using a single piece of brass for the cylinder block with some sort of inlet manifold on top. For simplicity, and to enable me to make all the parts in groups (and ensure they're identical):

... the eccentric 'rods' will just be shaped sheet metal with holes at each end.
... the eccentrics will be made from a single piece sliced into four, fixed with grub screws
... a forked guide might be needed for each eccentric 'rod' to stop them wandering left/right - maybe attached to the cylinder block (as shown here) or the standards or somewhere else 
... the shaft and the standards will need to be chunky as there's no central support

Here's the basic concept (ignore the random proportions!), based on the previous experiments it'll probably be extremely noisy...



Hope to make a start on this some time next week.

[Sanjay F]

I like it, another unique engine begins!   

[crueby]

I've never seen  eccentrics  used instead of crank pins on a piston. Offset can be the same, but it seems like the force transferred would be maybe half? Has anyone else  tried it?   

[PaulR]

I've never seen  eccentrics  used instead of crank pins on a piston. Offset can be the same, but it seems like the force transferred would be maybe half? Has anyone else  tried it?   

I hadn't even considered that but now I see that the force isn't being applied to 'one point', the pin, it must be weaker I guess although the energy has to go somewhere. I wouldn't know where to begin trying to calculate it though - the last time I did any mechanical engineering calculations was in 1975 (and I struggled then!).

[Charles Lamont]

Think about the friction.

[PaulR]

Just worked out the contact area between the eccentric and a 'rod' made from sheet and compared it to a big end on a crankpin. Plucked figures out of the air that are typical for the sort of sizes I use. Bizarrely the results are the same but in practice I think the eccentric would be larger and therefore create more friction.

Eccentric 12mm dia:perimeter 2*pi*r = 37.68
Contact area of 1.5mm thick sheet metal 'rod/strap': = 1.5 x 37.68 = 56.52 mm²

3mm pin perimeter: 6.28 x 1.5 = 9.42
Contact area of boss: 9.42 x 6 = 56.52 mm²

[PaulR]

The guides would also add friction but, as far as I can see, there's no need for the rod/strap to surround the eccentric - it could be an open 'fork', reducing contact area by 50%. I suppose it's the same as a cam set up?

[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.

[PaulR]

Hmm, I don't have any suitable brass for the cylinder block and large pieces are eye-wateringly expensive these days  I do have that pig of cast iron but I might need to do some soldering so that's probably out too. What I do have however, is almost 8 foot of 1/2" thin walled brass tube from two towel rails! I'm guessing it's drawn as I can't see any seam but even then I didn't think it would be much use. Seems I was wrong - I made a steel test piston and tried it over a five inch length and it's a fantastically smooth and air tight fit, at least as good as any bore I've been able to produce.

I figured drilling the exhaust ports would be tricky without support and when breaking through as the wall is only about 0.6mm thick so I turned down a tight-fitting dowel and used a simple widget to for a test. Absolutely no issues and the piston is still a great fit.

So, if this idea comes to fruition, I think I'll use four lengths of towel rail for the cylinders (funny old world isn't it?) and solder them carefully between plates at top and bottom. If it doesn't I'll certainly use this tube for several other engines!

[PaulR]

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.

Thanks Chris but I can't quite get my head round that , can you clarify a bit please?

[PaulR]

Ignore that, I've just worked out what you mean.... thinking more.

[PaulR]

I think you mean as per the red arrow rather than the green?

[crueby]

I think you mean as per the red arrow rather than the green?

Exactly.

[PaulR]

Hmm, wonder if it would still run but just be a bit weedy....?

[crueby]

Hmm, wonder if it would still run but just be a bit weedy....?

One sure way to find out...  It could be an interesting experiment to try it with the eccentrics and then swap parts to a normal style crankshaft. Or, mock it up with one eccentric and one crank, and just move it by hand to feel the difference?