Engines > Your Own Design

It's Just Hot Air

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PaulR:
Time to take the plunge and try to make a Stirling/hot air engine. I made two in the past, neither of which worked, probably due to excessively heavy components, mediocre workmanship and not following any specific plan. This time I think I've got a better grasp of what's needed (thanks to vtsteam, Jason and Wilf for input in the other thread) so fingers crossed for third time lucky!

Today I made the displacer from some 0.5" seamed steel tube. It's very thin and was round enough to do the job. I can't recall what it was originally but it was coated white so had to do some paint stripping first. Sawing it was a flipping nightmare but got it done eventually and faced the ends to length. Made the end caps from steel because why not, they're both about 1mm thick with a similar sized spigot inside the tube. One has a short boss inside which is threaded, the other has just a hole so a shouldered displacer rod can form a seal and be well anchored.

I silver soldered the holey end then pulled in the other cap with some studding, removed the studding then soldered that end. It was a case of fluxing then bending thin silver solder wire into a rough circle matching the ID then leaning the part on a bit of metal so I could get at the end of each cap to encourage the solder to flow the right way. I had to repeat the process twice for the end with the boss, I don't think I got the inside of the tube quite clean enough  :facepalm2: Anyway, worked ok in the end and putting it in the lathe for a spin it seems to be running true enough.

Next I dunked it in a glass of very hot water and was pleased to see only a very occasional bubble from around the temporary studding (I thought that would be a lot worse) so I think it'll be airtight once the proper rod is in place.

http://www.youtube.com/watch?v=AatV-XQlDTA

PaulR:
Started the displacer cylinder by turning down the outside of a mild steel rod to 15.4mm first (to suit o-rings I have) rather than have to use a wooden mandrel later and find I couldn't get the OD right. Next I drilled the hole for the connecting pipe, figuring it would be easier to do on a solid metal rod than a very thin walled one. Following that, drilled then bored out the ID to 14.25mm leaving a wall thickness of about 0.6mm. I only have a 6mm boring bar and the displacer is 52mm long so I decided to do one half, flip it round then do the other as absolute concentricity and finish isn't essential.

Since the displacer is 12.7 dia I have a radial gap between it and the displacer cylinder of 0.75mm which should be ok.

Cylinder placed (somewhat off centre!) over displacer:



Displacer partially inserted in cylinder:



Next I need to make the end cap and the rectangular plate which will be soldered about 6mm in from the end which will allow in to fit in a recess in the frame while compressing the o-ring by about 20-25% which I think is about right.

PaulR:
Made the rectangular displacer cylinder collar to size and made some small holes for the screws and the centre of the cylinder. Prepared the frame, glued the two together then drilled through to make sure the holes are nicely up. Separated the parts then bored out the collar to fit. Next the diasplacer end cap then solder the bits together.

Also need to get a move on with the plan as I've only just finished faffing about with the cylinder to cylinder pipe and I'll overtake the drawing at this rate (and I haven't even considered the piston, rod, crank and flywheel!)  :Lol:

PaulR:
Today I made the displacer cylinder end cap and left it about 0.5mm wider than the tube for better solder contact area as per vtsteam's advice. I then silver soldered the cap and then the collar near the other end. You might just see there's about 1mm showing either side of the o-ring and I'm aiming for about 1mm compression so 3mm will go into the recess which is the next task. I also threaded the four blind holes that shorter versions of these screws will go into. The hole facing the camera in the second photo is for the pipe to connect to the power cylinder.

I was pretty generous with the solder to make sure the thing is airtight but it'll be a while before I can test that. Now it needs some cleaning up!

PaulR:
Time to make the connection to the frame so I drilled and threaded a 10mm through hole then bored a 3mm deep rebate none of which was much fun: the off-centre part induced horrible vibration, the outside jaws meant I had to keep stopping to apply cutting oil and it was hard to see what was happening. I had to do the boring at a lower than ideal speed which also generated some high pitched chatter. In the middle of all this I accidentally disassembled the quick change tool post and for the life of me couldn't get it to go back together such that the handle ended up in the right place! After about half an hour of cursing it finally agreed to play ball. To top things off, when I removed the part from the chuck I found I'd also been trying to thread one of the jaws - I thought the last millimetre was hard going!  :lolb: I now have a little groove in the last tooth of that jaw but hopefully it will have nothing more than a cosmetic impact.

Anyway, here's the part and happy to say it at least fits together well despite all the tribulations.

 

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