Lamina Type Hot Air Engine Antics

[vtsteam]

Maybe ten years ago I sent away overseas for a $30 Ebay Lamina type "test tube" hot air engine. This was early on, before there were dozens to choose from all over the net. The engine in the pic looked decent, but 5 weeks later, when it arrived it turned out to be somewhat smaller than imagined.    Kinda like those offers in the back of comic books (before they became graphic novels)

Worse, it didn't run.

I messed around with it, changing out the regenerator material, repositioning it, etc, etc, but it never did run. I put several days into trying to get it to go, but finally put it on a bookshelf to gather dust.

For some reason I have just taken it down again today, and started fiddling around with it, just in case I was wrong in what I tried the first time around, and missed something. But nope. Another few hours killed today.

I mean I know I can build a working hot air engine from scratch. I have in the past. And in fact I have a much larger one in mind (and have had that one in mind ever since I started building my (and still unfinished) "new lathe". In fact that was the reason for the lathe....but that topic might best be ignored for the present)

BUT (another but), ye see, some of the questions I have about my long-planned much larger engine, might be answered by trying to figure out what I can do to get this little bit of a contraption to function as it should. I mean in fact it SHOULD work it has all the bits and pieces an engine of this type should have. Which is the reason I keep coming back to it. Because it doesn't run.

So, why, that three letter word, is the reason I'm starting this thread.

Here's the little bug:

 

[awemawson]

Does it even try to turn Steve if given a helping hand?

I’m guessing friction is the issue, that displacer looks relatively heavy.

[vtsteam]

Well, Andrew, there is technically no displacer in this type of engine, just a piston.

re Friction:

The flywheel and big end of the con rod have ball bearings. I believe the small end is just pinned. When apart, I can give the flywheel a spin by hand and it sustains for quite awhile considering its tiny size and mass, and it even stops and rocks a few times because the balance is off by a tiny amount. So that seems to be pretty low bearing friction overall.

Now for the piston itself, it's 12mm dia, x 17mm L, hollowed aluminum.
Piston + con rod + bearings :  complete assembly weighs 5 grams.

Piston cylinder fit: It takes 7 seconds for the piston and conrod to drop ~ 7mm under their own weight if I hold the cylinder vertically with my thumb over the bottom end.

Here's a pic of the bits:
 

[awemawson]

The only other one of those I’ve seen had a very light graphite piston which of course is a handy material in that not only is it very low friction but also can take the heat well.

[hermetic]

An observation! I have no knowledge of these engines, and only a nodding aquaintance with the theory by which they work (or in this case, don't) The heat from the spirit lamp heats the air in the tube, which expands, driving the piston down, and the ensuing pressure drop in the cylinder causes the air to lose some of its heat to the regeneration material? If that is the case, the regeneration material needs to be right at the end of the tube where it will stay cooler, and the heat must be just enough to start and maintain the process, too much heat and it wont work as it is operating on a very slim temperature differential. Although  the op has tried it in different positions, I would move the regen material out of the hot area, and reduce the heat, and see if it works then. What was the original regen material?
Phil

[vtsteam]

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.

[awemawson]

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]

http://www.josepino.com/projects/build-simple-lamina-engine 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!!
Phil

[vtsteam]

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.

[vtsteam]

Hermetic, happy wine and chocolates!

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:

 

 

 

 

[SwarfnStuff]

I tried one similar, Elmer Verberg's from memory. I differed in that the tube contained a few glass marbles. Other members of my club got them to run, I alas failed completely so turned it into an air engine with a piston? type valve on a cam - that worked at least.

Anyway, have you tried with your spirit lamp out at the end of the tube? I ask because that seemed to  be the position in the glass ball version. Nothing to lose by trying.
All the best for 2022,
John B

[vtsteam]

Yes John, thanks!  I have tried it at the end of the tube in all of the above mentioned configurations.

I'm thinking about the small size of the piston and small stroke, and resultant low compression ratio.

I might try increasing it by putting a piece of solid rod in the test tube end, reducing its volume.

[russ57]

You may be able to reduce friction by washing the oil out of the bearings.
I vaguely recall reading the oil introduces drag.


-russ

[vtsteam]

Russ, thanks,  I've tried it both ways so far, dry and lightly oiled (Singer sewing machine oil). Still no go.

Piston seal is better with oil and when I flip the flywheel I get more revolutions with oil before the wheel comes to rest. That says to me that at low revs, at least, oil reduces friction in this particular engine. If it actually ran, but slowly, then maybe we'd suspect that the added viscosity of oil might be limiting the top speed. But it won't even turn one complete revolution under its own power.

The dry aluminum piston in an aluminum cylinder is not a good bearing pair. Dissimilar metals would be better (with the exception of cast iron pairs), eg. graphite in a bronze or cast iron cylinder would be lower friction. An aluminum cylinder does however sink more heat at the cold end. And the low mass of an aluminum piston is considered an advantage so, well, everything is a compromise one way or another.

BTW in this engine there is no danger of carbonizing oil in the power cylinder (as there is in a conventional high performance displacer type engine) since the temps are relatively low, and the cylinder is finned.

[vtsteam]

I tried reducing the test tube volume, and thereby increasing the compression ratio, by putting a 5/16" x 2.5" steel rod inside, along with some of the stainless wool from before, but still no go.

Next possibility, I noticed that some of the runners on YT  have what looks like a nozzle restriction between the test tube and the power cylinder.

In this engine, the power cylinder is the same bore as the test tube. I might try adding a thin shim orifice behind the test tube junction.

[vtsteam]

Almost a year later.....got it running!

 

It's doing about 180 RPM here, The main changes were separating the base so the lamp could be moved, packing half the test tube tightly with regenerator wool right to the end, and using graphite as a lubricant in the cylinder. Ideally, as Andrew remarked earlier, a graphite piston would be better than aluminum, but I didn't want to do that for this small purchase, just getting it running for now and trying different regenerator and lamp configurations was all I wanted.

It's not a very awe inspiring runner, but at least it does what it is supposed to, and it helps me work out a couple of the problems with regard to the engine I want to build. Happy with that after all this time! 

[awemawson]

I've got quite a bit of random graphite as electrodes for my plunger EDM machine Steve if you'd like some to experiment with.

[vtsteam]

Thanks Andrew, I appreciate it. But I have nearly a 2 foot length of 1" graphite rod, a bit of which I used in my father's hot air engine. I made it for his 83rd birthday fifteen years ago. He and the engine, too are gone now. I just have a few photos.

 

 

I don't think I'll improve the little lamina engine. It's too small to be very gratifying as a runner. I do want to finish my larger engine that I started last year, though and it is helping to play with the poorly made little lamina.