A Simple Stirling

[vtsteam]

6mm stroke w/15mm displacer = roughly 10.6 cc displacer swept volume

10mm stroke w/9mm power piston = roughly 6.4cc swept volume

10.6 to 6.4 = 1.65 to 1

Seems okay at roughly 6mm stroke on the displacer.

All that dead space around your displacer, though not what I would call "swept" volume is absolutely going to degrade performance, by reducing the overall compression ratio and reduce the heat differential of the overall airmass.  You want to minimize it, generally.  1.25mm is a lot.

[Jasonb]

6mm stroke w/15mm displacer = roughly 10.6 cc displacer swept volume

10mm stroke w/9mm power piston = roughly 6.4cc swept volume

10.6 to 6.4 = 1.65 to 1

I get different figures

0.6 stroke x 1.5 displacer = 1.8cc

1.0 stroke x 0.9 power piston = 0.64cc

1.8 to 0.64 = 2.8 to 1

The Raab I have just finished would work out to 2.17:1

[vtsteam]

6mm stroke w/15mm displacer = roughly 10.6 cc displacer swept volume

10mm stroke w/9mm power piston = roughly 6.4cc swept volume

10.6 to 6.4 = 1.65 to 1

I get different figures

0.6 stroke x 1.5 displacer = 1.8cc

1.0 stroke x 0.9 power piston = 0.64cc

1.8 to 0.64 = 2.8 to 1

The Raab I have just finished would work out to 2.17:1

Decimal problems for me!

But for the first I still get 1.06 then......hmmmm....

[vtsteam]

I still get 1.65 to 1 ratio. 

[PaulR]

OK I think I've been doing this wrong or over-complicating things  - I've been using the volume of air moved by the power piston and subtracting that from the total volume of the displacer cylinder less the displacer (ie the net volume of air in the displacer) and trying to make the ratio between them 1.5x.

Now I understand what's required, so...

Power piston 9mm dia with 10mm stroke, swept vol = 10 x (pi x 4.5 x 4.5) = 636 cubic mm

Displacer 15 mm dia with 6mm stroke, swept vol = 6 x (pi x 7.5 x 7.5) = 1060 cubic mm

Ratio - 1060/636 = 1.666x

That ignores the gap around the displacer and any 'buffer' at the ends to allow for thermal expansion but it near enough ok.

I'm try to get the gap down to 1mm but it just depends on the exact sizes of the various tube materials I have available.

I plan on using just a meths burner. As a first attempt I'm thinking of omitting any cooling - ok it may not run for long but if it runs at all I can modify or build another.


Thank both for the help!

[vtsteam]

Just as a suggestion, Paul, find your displacer stock in your tube supply first -- ideally aluminum. Bore the inside to clean it up. Then turn the outside down (with the tube mounted on a wooden mandrel) to give a thin wall.

Then just make a displacer cylinder from a random oversize dia solid rod and drill and bore it out to about 1.5 mm oversize for the OD of the displacer.

Then put the displacer cylinder onto another temporary turned hardwood mandrel and turn the outside down as thin as you feel comfortable with.

[PaulR]

Just as a suggestion, Paul, find your displacer stock in your tube supply first -- ideally aluminum. Bore the inside to clean it up. Then turn the outside down (with the tube mounted on a wooden mandrel) to give a thin wall.

Then just make a displacer cylinder from a random oversize dia solid rod and drill and bore it out to about 1.5 mm oversize for the OD of the displacer.

Then put the displacer cylinder onto another temporary turned hardwood mandrel and turn the outside down as thin as you feel comfortable with.

As I only re-started model engineering at the beginning of this year I've not got much stock and only have a couple of pieces of tube I've salvaged so options will be limited! Thanks again for the tips.

[PaulR]

Measured various bits of tube to see what's possible and these seem to be the bast match for displacer cylinder/displacer. The one at left is what I'd planned to use for the cylinder, not sure if it's stainless but it has a wall thickness of 0.75mm which seems ok. The other piece is some sort of steel - not sure if I can turn it down to anything less than 1mm thick and that would only leave a gap of around 0.3mm all round which seems far too little.

As I haven't got any aluminium, the only other option is drill/bore a piece of 3/4" steel then turn down to maybe 15.5mm (to leave a 1mm gap all round). At least I've got a spare broom stave I can use for a mandrel

[Jasonb]

You want the displacer to be quite thin to keep the weight down so a bit of carful boring or turning of the OD should do the trick.

I'll post a bit more of my Raab build later and include the displacer in that.

[Dave Otto]

On my Kyko engines I did the displacer cylinder and piston both from solid, starting at post #50 of my build.
https://www.modelenginemaker.com/index.php/topic,11216.0.html

Dave

[PaulR]

Thanks both. I don't know what the material of the planned displacer is but it blunted two HSS tools when I tried to do some test turning of the OD. I took off maybe 0.5mm after which the surface seemed to become incredibly tough, so that idea is scrapped. I'll see what solid rod I have that can be bored out and turned down to suit.

[PaulR]

Change of plan. I've got lots of 1/2" thin walled brass tube (~0.4mm) that I can use for the displacer. For the displacer cylinder I'll have to bore out/turn down some 5/8" mild steel rod. Just did a test on a short piece and was able to turn it down to about 0.4mm with no issues, hopefully I can get the same result on a longer piece. Probably not ideal material but it'll have to do. Need to redraw my plan to suit these new sizes.

[vtsteam]

You can use either silver or brass braze for the displacer cylinder end if it is of steel. Stainless would have required silver.

I usually braze the end on before final turning/thinning of the displacer cylinder. I leave a small step at the very end so the end piece maintains a reasonable joint width.

If you are making the cylinder from solid, you can leave the end closed, and not have to braze one on. Boring to a thin end can be tricky, but of course do-able. Or you can bore to an end on long stock, then carefully cut it off close -- with careful measurement -- lots of ways to skin a cat.

[PaulR]

You can use either silver or brass braze for the displacer cylinder end if it is of steel. Stainless would have required silver.

I usually braze the end on before final turning/thinning of the displacer cylinder. I leave a small step at the very end so the end piece maintains a reasonable joint width.

If you are making the cylinder from solid, you can leave the end closed, and not have to braze one on. Boring to a thin end can be tricky, but of course do-able. Or you can bore to an end on long stock, then carefully cut it off close -- with careful measurement -- lots of ways to skin a cat.

Thanks Steve, I'll probably go with soldering the end cap on. Currently thinking about the best way to attach it to the bulkhead/frame and whether or not to use a connecting pipe to the power cylinder or a drilled passage. My drawing currently looks like the attached but I quite like the appearance of the Carette toy engines that use a pipe.

I've rejigged the strokes as well to make sure the crank pins don't overlap with the crankshaft. The ratio is now coming out at 1.69 which should be fine. Also, I'm thinking of using 3mm drill rod for the displacer rod but I don't current have tap/die to turn the internal portion down so as to form a sealing shoulder so I might have to rely on engine sealant at the top (maybe slip a tiny washer over it to help sealing).

[vtsteam]

Actually you don't need a particular tap and die size, could be metric, could be Imperial, as long as it's smaller than the rod stock you are going to use. Just turn it down for the portion inside the displacer to the major diameter of the size you have.

Only the very end of the displacer rod actually needs to be threaded -- the rest of the interior portion can be smooth (the major diameter of your thread). If you then widen it to your stock size where it will butt against your lower cap on the outside, you can screw it into place from outside the displacer, and there will be relatively good sealing at both ends and a secure attachment. A wipe of a suitable compound (maybe muffler patch cement) before screwing tight will complete the job