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Lamina Type Hot Air Engine Antics

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Well Andrew, I can't discount friction here. Aluminum piston running in an aluminum bore is a poor combination for that. There are other engines of this type however using materials other than graphite -- though it's ideal for a small hot air engine.

Perhaps the small size of the engine makes it more sensitive to friction. The usual size I've seen experimented with is 24 mm dia. This one is half that, meaning a 1/8 size engine in volume.

But just a sense from having the thing in front of me, I feel like it should run somewhat without going to graphite, which would undoubtedly be better. I have no real reason for that. Maybe it's just hard for me to believe that the sellers would send out a non-functioning design, since it would cause endless troubles for them on Ebay, and a rapid removal. These look to be CNC turning center products, and they went to the trouble of adding in ball bearings -- not something you'd do to make a quick buck off a a non working look-alike.

I could make a graphite piston -- I have it for just such a purpose, and did use it on my big Stirling of more than a dozen years ago. But I just want be sure I understand this engine's needs, as is, first before giving up. I guess my interest is not just to get a runner, but to understand why this one is stalled -- because that leads to a better understanding of what really happens inside.

Hermetic, one of the really obvious problems to me is that you cannot move the heat lamp. There's a recess bored in the base into which it fits, and it is about 6mm bigger in diameter than the lamp. So very restricted horizontal adjustment.

Second problem, is, the lamp is too close to the test tube -- mainly because the base is too thin and the hole that the lamp sits in is not a through hole.

With regard to the theory of how these engines work -- there's still lots of argument on the net. One thing to notice is that the working cylinder is finned so it is a cooling source. Also, while logically it would seem that the lamp should be located at the end of the test tube after the regenerator, in practice, similar engines work best with the lamp on the other side of the material -- as this one was set up by the maker.

However, it also doesn't run, so it's anybody's guess what would be best here.

I think my first step will be to add some height to the base and cut the end off so that the lamp can be moved anywhere along the tube.

The regenerator material is (and was originally I believe) stainless steel wool from a pot scrubber pad. How much, and where positioned is a big question. Since the lamp couldn't be moved, also, the regen's position was even more critical.

To add even more questions:

The engine fixings (if you could call the 2 tool handle clips that) block the fins of the working cylinder.
I have heard that some engines ran without regenerator material at all.
Some engines run without a flywheel either (so called acoustic free piston engines)

Andrew, not to say we can't try a graphite piston if other manipulations don't pan out, but I just want to see if I can get it running by adjustments first.

It's a bit out of my scale of things Steve - I tend to work on bigger stuff so I have zero personal experience !

Never the less I look forward to your further experiments - we were always told a college that Sterling engines were the future.

hermetic: shows an engine working with the heat source virtually in the position that yours is in with the piston one side, regen material other side, and heat source between, and seemingly heating the regen material slightly, so I think that you are pretty much spot on with the layout, there seems to be a bit more regen material in this one, but it starts to oscillate almost immediately, and we see the stroke increasing although we dont see the moment when the flywheel does its first full circle. Is there a seal between the test tube and the cylinder, like an O ring? it needs to be airtight. Try a drop of thin oil on the piston to increase the seal, although if it has never run, it can hardly be worn! add a bit more regen material. I have fallen down a lamina/laminar flow engine rabbit hole, I have red wine and dark chocolate, and may be gone some time! Happy new year!!

Andrew, I think the small size of this one in comparison with even the other small "test tube" engines is making it difficult. But therein lies something to figure out if it is possible to make it run eventually. If it were an easy runner, I'd learn nothing.

re. the future.....maybe those professors were right to hope for a good result, but only by changing the common approaches. If I can generalize, I'd say every hot air engine problem is a heat exchange problem.

We can say friction is a major hot air engine problem and be correct in a sense. But we can also say friction can be ignored with more power. Friction is only critical if you don't have enough force to defeat it. A lawnmower engine has plenty of internal friction, but it has magnitudes more power so it cuts grass.

Because most hot air engines I've seen amateurs build are very low power (even "Big" ones -- single digit watts and under) reducing friction is a big focus. That, and increasing burner efficiency. And pressurization to increase heat transfer in a denser fluid (compressed air).

The problem with this little engine is, not enough micro power to defeat the present micro amount of friction. Yet, a relatively huge amount of heat (the very small alcohol lamp flame....nevertheless, about 100 watts) is going into the system. Heat is not moving where and when it should. That's the real problem.

Hermetic, happy wine and chocolates! :beer:

1.) I've used light oil on the piston and all bearings. Compression is slightly improved.
2.)I've sawed off the end of the base so I can now move the lamp. I also raised the engine 1/2' on a piece of wallboard. This makes the flame pointed instead of squashed out around the test tube.
3.)I've tried the engine without any regenerator material. I tried several different flame locations for that case.
4.)I've tried the engine with about an inch plug of regenerator material. I've moved that regenerator to several positions along the length of the tube.  I tried several different flame locations for that case.
5.) I've tried the engine with about an 3 inch plug of loosely packed regenerator material. I've moved that regenerator to several positions along the length of the tube.  I tried several different flame locations for that case.
6.) I haven't tried more tightly packed regen material yet.

Best action seemed to be exactly as pictured in other engines, with regen material about half the total length of the test tube, located a little more than a quarter length away from the aluminum cylinder. And with a small space open at the far end. Flame located right about where the regen starts. When I did that and flipped the flywheel, the engine took longer to slow to a stop, and seemed to want to continue toward the end.

Yes, the glass tube is socketed into the aluminum cylinder with an O-ring.

Here are some pix of a few of the various setups:


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