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 

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 

[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]

...you could also substitute the fins on the cold end of the displacer cylinder with a water jacket.

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. 

[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.

[Sanjay F]

ooooo......this is going to be interesting, I know nothing about the design of these machines, but I'll be following along 

[tvoght]

I can understand not wanting to cut fins in the lathe. My particular set of skills do not seem to allow it...


It might not be a good solution for you, but one possibility is to clamp the cooler to a rotary table and using a slitting saw in the mill spindle, cut the fin gaps by rotating the cooler through the saw.
I have used this basic technique on a CNC mill and having a program do the sawing, but it's the same idea. Here is some documentation of that process for a Ringbom Stirling I built.
One advantage of the technique over using a lathe is that the gaps don't have to be cut all the way around. That is the case on my Ringbom.


--Tim

[PaulR]

I can understand not wanting to cut fins in the lathe. My particular set of skills do not seem to allow it...


It might not be a good solution for you, but one possibility is to clamp the cooler to a rotary table and using a slitting saw in the mill spindle, cut the fin gaps by rotating the cooler through the saw.
I have used this basic technique on a CNC mill and having a program do the sawing, but it's the same idea. Here is some documentation of that process for a Ringbom Stirling I built.
One advantage of the technique over using a lathe is that the gaps don't have to be cut all the way around. That is the case on my Ringbom.

Thanks Tim, I've already taken a quick look through your build thread... nice! Unfortunately I don't have a mill so some creative solution is needed.

[vtsteam]

Honestly, I don't think fins make a big difference, unless there's a fan or vehicle motion involved, per IC practice.

Water cooling does make a difference, and is far more effective in a hot air engine.

I have a hot air engine with no fins, but a lot of solid aluminum mass in the form of heavy connected mounting plates, and these have enough surface area to dissipate heat for continuous running. In any case, if you do use a built-up cooler section, aluminum is preferable to steel. I think it's possible that a solid aluminum cooler would do more than a built up soldered set of steel washers just to create fins, but you never know 'til you try.

Finally, I do wonder if using a parting tool in aluminum is possible for you to try? Then you'd have the style you want, and the best material for it. 

[Minh Thanh]

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...

...

Hi
Stirling Engine
You need to solve 2 main problems: 1- Airtight, 2 - Friction
Airtight and reduce friction to a minimum for the engine to run, because the power of a small stirling engine is very small.

<a href="https://www.youtube.com/watch?v=NbRfY4YzQnw" target="_blank">http://www.youtube.com/watch?v=NbRfY4YzQnw</a>

If you can do it well, it can make for some interesting projects.

<a href="https://www.youtube.com/watch?v=O-hfsQfuBlI" target="_blank">http://www.youtube.com/watch?v=O-hfsQfuBlI</a>

[PaulR]

You need to solve 2 main problems: 1- Airtight, 2 - Friction
Airtight and reduce friction to a minimum for the engine to run, because the power of a small stirling engine is very small.

Thanks for the interesting videos 

I wonder how engineers of the first half of the 19th century building working hot air engines addressed these issues given the machinery and materials of the day?

[Minh Thanh]

I wonder how engineers of the first half of the 19th century building working hot air engines addressed these issues given the machinery and materials of the day?

 I don't know, but if I had the machines of that time and with common materials like iron, aluminum, cast iron, brass... then I could make many types of engines.

[PaulR]

I don't know, but if I had the machines of that time and with common materials like iron, aluminum, cast iron, brass... then I could make many types of engines.

Aluminium didn't come along until the second half of the 1800s 

[vtsteam]

I wonder how engineers of the first half of the 19th century building working hot air engines addressed these issues given the machinery and materials of the day?

They built them large (for the available power output), ran at low RPM, and used a lot of cast iron as a material, and often relied on packing for seals and temperature isolation. They adapted the configuration of their engines to the materials available, which varied from easier to scale down or more efficient configurations possible with more specialized materials.

Cast Iron has advantages of low friction for power piston and cylinder (even without lubrication) and can be used to mold complex shapes. Where needed it has good heat transfer qualities. Its drawbacks are that it is heavy (compared to aluminum) and where unwanted it has nevertheless good heat transfer qualities, compared to stainless steel.

[Alyn Foundry]

The hot air engine is a path well trodden in today’s times. I do remember chatting to a gentleman whose grandfather had bankrupted the family’s foundry business ( Butterworth of Manchester ) trying to design their own version. The Robinson patent engine was such a simple design that many hundreds were being used daily by the late 1890’s.  It is interesting to note that only the half man power and above engines were water cooled, air cooling was more than adequate for the smaller powers.

The most important tip for ensuring a working motor is making sure that a decent temperature differential is maintained during running.

   Graham.
<a href="https://www.youtube.com/watch?v=_3qiYZRrE6g" target="_blank">http://www.youtube.com/watch?v=_3qiYZRrE6g</a>