Help! > Specific Engine Help
A Simple Stirling
PaulR:
I've made two hot air engines of my own design in the past (and a 'tin can stirling' as well which actually ran very well), neither of which would run so I'm probably setting myself up to fail again but here goes...
I came across this image on the net showing an engine which appears to be quite simple to make. I don't have an easy way to make the heat sinks around the cold end and cylinder (I wouldn't fancy using a parting tool to that depth n number of times!) but I thought I could use a stack of 'washers' and separators (aka smaller washers!), with the end ones soldered in place (or use longitudinal studs to make them removable if that was judged necessary).
I'd prefer to use metal that I have on hand (brass, steel and maybe some alum*) rather than graphite, CI and bronze but I wouldn't mind trying a ball race for the main bearing - how is the shaft usually secured in this case, a press fit or Loctite? For the displacer I'd maybe try the usually-suggested marker pen tube and glue or preferably turn down some thin walled brass tube very carefully and make a thin bush to screw in the rod.
I know the detailed calculations for these engines are beyond my abilities (and interest), do the proportions for the displacer/displacer cylinder and the power cylinder/stroke look about right in this drawing and is there a 'right size' for the flywheel?
I recall seeing some hot air engines on Youtube years ago which were made entirely of steel (I can't seem to find them now) which seems to suggest it's possible to use common metals, or maybe the maker was just exceptionally skillful :shrug:
I've got a few plans for Stirling engines but none of them appeal (due to materials, design or machining called for). If I'm setting off on a fool's errand again, please let me know :Lol:
Dave Otto:
You really don't need any fins on the power cylinder, you could also substitute the fins on the cold end of the displacer cylinder with a water jacket.
Dave
PaulR:
--- Quote from: Dave Otto on August 01, 2025, 02:18:11 PM ---...you could also substitute the fins on the cold end of the displacer cylinder with a water jacket.
--- End quote ---
Yes, I did that on one of those that failed. I think the volume of water was too small as it warmed up very quickly although I don't think that was the reason it didn't run, I think I just made the parts too much like typical steam engine parts. :facepalm:
vtsteam:
Hot air engines aren't as complicated as people think.
You don't need to do complex calculations for what you want to do at all. Rule of thumb for most simple hot air engines that use say a small lamp type burner is a little greater displacer volume than power cylinder volume. Say 1 to 1 up to 2 to 1 -- not critical, say 1.5 to 1.
When you think about it, the cooling really only determines how long the engine runs after starting. Ideally there is enough for continuous running, but you should be able to start an engine with less efficient cooling.
More important than cooling efficiency is the requirement to thermally isolate the heated end of the displacer cylinder from the cold end, as much as possible. This means that thin walls for the displacer cylinder are important. Ideally the walls are stainless steel, or other low heat conducting material. If you can't make a thin continuous wall, at least a waisted wall helps.
Next in importance is that you have good compression without any air leakage through gaskets and connections.
Then it is important that friction be reduced to a minimum. The greatest source of friction is going to be the power cylinder/piston fit. That has to be as perfect and polished as you can manage. The metals chosen should be good types for running together.
The weight of the power piston is also important in a horizontal engine so lighten where possible.
Compression should be noticeable when spinning the crankshaft.
Those are the basic requirements, IMO.
vtsteam:
Suggestions I have re. your drawing -- the bush for the displacer guide looks too short for a horizontal engine, unless your displacer clearance is excessive in the displacer cylinder (to avoid rubbing the walls). It would be less of a problem in a vertical. The big problem in a horizontal hot air engine is keeping the displacer clear of the cylinder wall with say .030"diametral clearance on a small engine, and wear in that bush creating both friction, and air leakage.
Placing the flywheel between the two cylinders separates them further than they would need to be if on a single crank. Therefore your connecting passages are longer than necessary increasing deadspace.
Your displacer is spherical on the end, but the displacer cylinder end is shown as an eased flat. This means more deadspace. I guess this is because of your choice for a pre-shaped displacer, and the difficulty of a spherical shaped cylinder head?
I think it might be easier to just make a square ended displacer from aluminum tubing, carefully thinned over a wooden mandrel, and then capped at both ends. Turn a shoulder on the raw cap material to fit the tubing. Pass the pushrod through the length of the displacer and screwed into the head end. The inner section of pushrod in the displacer should be thinner (shouldered) than the end connected to the crankshaft. The transition point is at the lower cap and buts up against it as a shoulder when the rod is screwed in place. Seal all joints with a high temp auto engine sealing compound or muffler sealant.
Check for leaks by dunking displacer/rod assembly in a glass of hot water. Bubbles will show the leaks.
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