Engine design sanity-check

[Floydism]

I'm new here (although I've lurked for quite some time!) and I'd love some input on my latest project. I've built a couple of engines before, a copy of the ubiquitous ebay coffee-cup LTD, and a horizontal gamma built from a kit. But I think it's time to build one to my own design (or how else can i really call it mine). I'm not looking to re-invent the wheel, I just want to make sure it'll actually work as intended!

So far I've decided that a Beta-type with rhombic drive would suit me best, after many hours of trolling the interwebs and mumbling to myself, I decided that I needed to write some code to start playing with the geometry of the thing, and I've arrived at a design that I think should work well, but I'd really like a grown-up to give it a look-over.

Hot-cap/cylinder bore - 18mm
Displacer length - 60mm
Displacer stroke- ~16mm
Displacer dia 17mm (0.5mm per side clearance to cyl)
Piston Stroke ~10mm
Calculated disp swept vol to piston swept vol ratio 1.42
Phase angle is 91@piston TDC, 87@piston BDC (I've added an offset of 60deg at the crankshaft to get something close to 90degrees)
Connecting rod lengths - 30mm
Crankshaft throws - Piston 5mm, Displacer 8mm

The piston will be graphite, in a brass cylinder liner wrapped in an aluminium "cooling-jacket"
Displacer and hot-cap will be 304 stainless, machined as thin as i can manage (I'm aiming for 0.5mm, maybe less)
Con-rods and yolks will be aluminium, with ball-raced big-ends and bronze-bushes for the little-ends, with 4 piston rods (2 per side) and 2 displacer rods.
The crankshaft will be a series of steel discs, drilled at appropriate places to take press-fit 3mm silver-steel journals. they will be connected via 30-tooth module 1 spur-gears.
I'll cast a nice enclosed bottom casing for it all to live in, and keep grit/dust out of the works, which also leaves the possibility of pressurising the engine in future should I feel the need to.

I intend to use this engine to power a fairly small R/C boat so the plan is to lay the engine down at a fairly shallow angle, with the hot-end a little higher than the drive-end, which would also mean plenty of cooling water available just under the cylinder. the drive arrangment isn't totally worked out yes but I've got a few ideas (it's really nice when an engine has 2 possible drive outputs rotating in opposite directions!)

My questions are;

Am I mad for thinking that an engine this size could power a small boat, even slowly?

Does my rhombic drive make sense? especially the offset piston/displacer angle?

Is there anything I've not mentioned that i may need to consider?

As far as actually making the thing, i'm a competent machinist, with a reasonable workshop (and have produced a few nice IC engines from scratch), I also have a nice little foundry setup for making any castings required, So I'm confident in my ability to produce a usable engine, as long as the design itself is good!

[staska]

Hello. Will it be atmospheric or pressurised ? For 100 cc at 3-5 bar you would in 100-200 watts of power if you heater and cooler is right.

[Floydism]

While It could be pressurised, I'm not intending to run it under pressure, at least not initially. I've not put too much time into looking at the seals etc needed to make it work. But I've left the design of my gear-cover as a simple flat plate so if the need arises to seal a rotating shaft,It would only mean modifying 1 part and shouldn't interfere with the rest of the design. I'm sure there are many other things that will end-up being changed over time to try to squeeze more power from it, I've got a few ideas involving integral displacer/regenerator and may end-up making a few different crankshafts to try other phase-angles. but for the time being, I'd just like a running engine, hopefully capable of turning a prop!

[staska]

Ok. But for un pressurised you need some 1000-2000 cc to power a boat.

Here is main viewtopic.php?f=1&t=5223&start=15

[Floydism]

That's a nicely made unit, I'm quite familiar with the difficulties of making something that size on a little chinese lathe, especially stainless parts! I'd like to build a bigger engine next, capable of real work, but ONLY once I've got a better understanding of sirling engines with this one first!

This is much more of a recreational engine, so I'm only planning on powering a very small boat (around 300-400mm) so i really don't think I need a 1000cc+ unit to power it. I could be wrong, but my intiution tells me that my design will at least be within an order of magnitude of what's required.

I've only really managed to get comfortable with the math required for the geometry/kinematic side of the calculations, the math required for a full understanding of thermodynamics (and calculus in general) is as-yet beyond me! (blame my mis-spent youth for that!) although i've got a reasonable understanding of the physics involved.

I've been sketching-out the cranks and con-rods, and working out the actual details of producing them. I think i can actually make it!

[staska]

Ouch. Ok. I was thinking of human size boat :) gor small rc boat - it is fine.

And no, i do not have small chinese lathes. Two Harrisons and Ycm cnc mill :)

[Floydism]

Ah, my mistake, at first glance it looked smaller! Those are lovely machines! Personally, My biggest machine is a little Warco 180 (basically, a polished-up chinese 7x14) and I've definitely pushed it well beyond what such a small (and bendy) machine should be capable of! it even works part-time as my milling machine! Although, to be fair, it does play it's part in earning my living, so it owes me nothing! It's part of the reason for keeping things small for now, I'm going to need a bigger workshop before i can even think about any bigger machinery, and to get a bigger workshop, there's many more flying sparks/swarf that need to be made to pay for it!

My stainless just arrived in the post, i couldn't resist giving it a test cut to see how it behaves, it's only 304 so I wasn't expecting it to be a complete nightmare, and it wasn't, i'm going to play a bit more later and see just how thin i can go before it falls apart! I might make a new hot-end to replace the glass one on another of my engines. I'll probably machine the inside first, then stick it on a close-fitting mandrel to do the outside. but I may just turn it to od then use a collet-chuck (if i can find the right size in my alarmingly dis-organised cupboard of tooling!)

[staska]

Write mo your mail in pm, will send some of my books on model size stirlings. Lots of technology.

[Floydism]

I have returned from the shed slightly dejectedly, as my 18mm collet is apparently, not very round, or square for that matter, and the 19mm I intended to use for the actual engine itself seems worse! I've somehow not had occasion to use those particular ones yet, and they're cheap imported jobbies, the chuck itself is equally cheap, but only runs-out about half a thou all across the taper, and my other collets are nearly as good, apart from the sizes i actually need!

The steel cut lovely though! at least the bits of it I actually cut turned out nice, and on-dimension with no dramas. My previous run-ins with other grades of stainless have been less fun, so that's a bonus.

[Floydism]

Much headscratching tonight over how to accurately make the crankshaft webs, it's probably the single most important part to get right, and I only really wish to make them once! I've got the method sussed now and I'll get on with it just as soon as rmy 5mm silver steel (for journals and to make reamers) arrives in the post. It'll be a built-up crank with press-fit journals, that I shall mount MR85 bearings on to as I assemble it. the rods will be split-cap affairs so i can put them on later (and just in-case I'm being optimistic by using aluminium!), I'm really looking forward to making those, it's usually my favorite bit of engine-building.

[Tom Booth]

Would love to see some photos and/or video on your progress.