Nasa "free piston" displacer in combination with crankshaft?

[Jagang]

Could a "Nasa" type tin can stirling engine, drive a conrod/crankshaft mechanism as well?
Or isn't that displacer mechanism not suitable for driving a conrod/crankshaft?

[Tom Booth]

If it did, it would no longer be free piston.

A displacer is driven. It doesn't drive anything. (Other than air)

But why?

[Jagang]

If it did, it would no longer be free piston.

To be more specific: Would it be possible to make an engine with a free displacer?
A bit like a inverted ringbom, so to speak, which the "NASA" tin can version basically is, if I'm correct.

A displacer is driven. It doesn't drive anything. (Other than air){/quote]

But why?

Because of compactness, building simplicity (one crank, short crankshaft, single conrod), and the ability to turn in both directions, if possible to begin with.

[Tom Booth]

If I understand your question, my interpretation of which is (paraphrasing): Could a Ringbom type, free displacer engine have a crankshaft on the piston, but not on the displacer?

Taking a look at the original 1907 Ossian Ringbom hot air engine patent, I think, most likely, it could probably work.

Screenshot_20210426-083919.jpg (109.92 KiB) Viewed 7931 times

Thanks for the question, as I had not done much research on this type of heat engine before.

Previously I had assumed that "Ringbom" was a reference to the way the displacer is kind of ring shaped and goes on a "bom" or boom (as in arm-like lift) or something.

[Jagang]

If I understand your question, my interpretation of which is (paraphrasing): Could a Ringbom type, free displacer engine have a crankshaft on the piston, but not on the displacer?

Yeah, well. That would be any ringbom engine.
The similarity between the regular ringbom engine, and the "NASA" tin can version, is that in the first, the displacer is actuated by the difference in pressure inside, and outside the displacer cylinder, while in the "NaSA" engine, the displacer is actuated by the pressure difference inside and oitside the displacer.

But in a way, it's the same principle.

https://www.youtube.com/watch?v=dOSGpzGhrJQ

So basically my question would be like: If I replace the weight on top of this free piston engine, by a conrod and a crankshaft, would it still operate?

[Tom Booth]

So basically my question would be like: If I replace the weight on top of this free piston engine, by a conrod and a crankshaft, would it still operate?

Someone else may have a better idea, but in my limited experience, most likely it would work, but at a lower RPM. The reason I say this is because I've seen several examples of just the opposite; engines designed to run with a crankshaft, but than would run just as well or better "free piston".

Free piston, the engine runs at a frequency that seems to be dictated by the geometry of the engine. With a crank, the RPM is limited by the inertia of the extra hardware so it runs slower.

Just for example, I wanted to see if one of these inexpensive engines could run free piston, (and also to test if hot expanding air leaked around the piston as the engine heated up).

It was difficult to film and hold a torch with an unsteady hand, (actually, I had to hold the torch between my knees) but clearly the engine ran without a crank, and of course, naturally the reverse is true, but without a flywheel the engine ran at a much higher RPM/frequency and otherwise required no modification.

https://youtu.be/iOs3BADFeKI

Of course, the only way to be 100% sure is to try it.

I believe I've also seen examples of similar engines to the one in the video you posted driving a crankshaft (with and without a flywheel as well).

May I ask what purpose you have in mind for adding a crank?

Edit: PS, as far as to "replace" the weight. That may be possible, but may also not be necessary. Probably attaching a crank to the weight would work, or taking away SOME of the weight.

From my research on "free piston" engines, some load, such as a generator, (or a heavy weight, in this case), is needed to dissipate energy, or the engine may tend to overheat and stop running. Probably a flywheel of approximately the same weight as the weight on the free piston engine would provide the right balance, but the load provided by a generator can apparently serve the same purpose.

In other words, it seems there needs to be a balance between the heat input and the "work" output of the engine. Otherwise additional cooling may be needed to dissipate excess unconverted heat.

[Jagang]

May I ask what purpose you have in mind for adding a crank?

Yup. :-)

A vertical and bilateral-symmetrical "marine" stirling engine for a model boat.
With a conrod from the top end of the cylinder, where the working membrane would be, all the way to the bottom, where I would install the crankshaft and flywheel for a low centre of gravity.

The candle/burner would sit between the crankshaft, and the cylinder, while the crank would not be situated directly under the cylinder, but "in front of" of "behind" the cylinder. Seems like a pretty clean design to me. If it works, ofcourse. Allthough I'm still thinking about what linkage to use, since this might not be the best one.

So.. A bit like this:

(Sorry, this quick small pencil sketch looks horrible when scanned, but you get the idea..)

img021.jpg (48.97 KiB) Viewed 7896 times

[Tom Booth]

Cool!

Something tells me, though, that all that indirect linkage might have too much resistance or mechanical stress.

I had a thought that perhaps a hard top with a pipe leading to a piston cylinder or diaphragm to make a more direct pneumatic connection with the prop could eliminate some of the mechanical stress inherent in the linkage joints and levers. Something like this; modifying your drawing a little:

img021.jpg (95.01 KiB) Viewed 7879 times

I've been working on restoring an old player piano. This video shows one similar. All the complicated mechanical apperatus is powered by air. All those tubes running through. It has given me an appreciation for the simplicity and versatility of this type of pneumatic power transmission.

https://youtu.be/weFbSJ_yA80

[Jagang]

Cool!

Something tells me, though, that all that indirect linkage might have too much resistance or mechanical stress.

I don't know. We have walking beam engines allready, right?
They run well with all the indirect linkages.

But....:

I had a thought that perhaps a hard top with a pipe leading to a piston cylinder or diaphragm to make a more direct pneumatic connection with the prop could eliminate some of the mechanical stress inherent in the linkage joints and levers. Something like this; modifying your drawing a little:

You got a point there, allthough in that case I would mount the working diaphram further away from the flame (probably higher up), since balloons do not react well to heat. :-) Hmm, I might try some variants here. But low crankshaft and flywheel are important, as is a still inverted cylinder for easy low-tech heating.

I've seen a few boats with crankshafts on top, but they looked pretty unstable to me.
And they used a belt drive to spin the propshaft, which causes friction.
I'd prefer a propshaft directly driven by crankshaft drive instead, to make the most out of the limited power a Stirling has to offer.

Hmm, got something to think about.

[Tom Booth]

You got a point there, although in that case I would mount the working diaphragm further away from the flame (probably higher up), since balloons do not react well to heat. :-) Hmm, I might try some variants here. But low crankshaft and flywheel are important

What I was thinking yesterday was something more like:

img021_20210430091339516.jpg (62.89 KiB) Viewed 7847 times

Sorry for the awful doodle. I'm on my phone. But the point of the Player Piano reference was that the air line pipe could stretch practically anywhere.

[Jagang]

Sorry for the awful doodle. I'm on my phone. But the point of the Player Piano reference was that the air line pipe could stretch practically anywhere.

No problem. But I think long pipes may give too much of a phase delay.
And the whole idea was to have an engine in a relatively small package.

I might start tinkering tomorrow.
Too tired now. And so far never succesful anyway.

But if something positive happens, I'll keep you posted.

[Jagang]

And I'd prefer a conventional engine layout in relation to the boat anyway.
I like when it clatters a bit, like a small diesel engine. :-)

That's purely an aesthetic thing for me.

[Jagang]

Well, cylinder and displacer are ready and the silicone glue that keeps the displacer in place, is drying. Hopefully, tomorrow I can assemble them together, and perform some "kick" tests.

[Jagang]

Well, I got the cylinder/displacer assembly finished now, but I can't test the "kick" of it.
Seems like I have to build the rest first, in order to discover it doesn't work.

[Tom Booth]

I think I mistook your flywheel to be the boats propeller.

Anyway, looking forward to some photos of this build.