Ted Warbrooke's Stirling 1: Question

[Alphax]

I have a simple question and would be interested in what answers it generates on this forum.

Ted's Stirling 1 design has no displacer and no regenerator. Those functions appear to be handled integrally within the system as a whole, rather than relying on discrete physical components within the system.

Question: To what extent is it likely that this approach only works on small models? Could it scale?

[Alphax]

Its been a day since I raised the question. Not much interest in it, it seems. Perhaps I'll rephrase it in the hope it tempts some responses.

The background to the question is that that some little 'desktop' stirling engines, such as Stirling 1 designed by Ted Warbrooke can be made to run despite their lack of physically defined displacer moving parts or discrete regenerator parts. They run with the absolute bare minimum of just 4 moving parts in total (piston, crank, crankshaft, flywheel). There are other designs that also have this same absolute minimum number of moving parts, yet appear to function as Stirling engines.

If the thorny issues of end-applications and efficiency are set to one side, then it appears to be the case that it is possible to design and build working Stirling engines with just 4 moving conventional mechanical parts (i.e. ignoring liquid piston designs for the moment). If that is true (it seems it may be) then the implication is that the tasks performed by conventional mechanical displacer components and the thermal regenerator components are being performed (probably highly inefficiently, but that concern is intentionally set to one side) by the system as a whole.

If that is true, and it seems it is, then the question is: can the design be scaled up and still be made to work as a (larger) Stirling engine (ignoring efficiency concerns)?

[airpower]

But Ted still uses a flywheel and crank
https://youtu.be/5UzO16dCl6A

At small scale for a minute or two a lot of things work. Something of use 24/7 is another matter.

[Alphax]

Airpower.... thanks for your thoughts.

Yes, as I mentioned in my last post, Ted's design does indeed use flywheel and crank.

And I agree with you that at small scale some things can work for a few minutes - in fact that is the whole point of my question! Do you think it could be made to work at a larger scale (even slightly larger)?

The key point is not that it works for a few minutes (which it plainly does), but that it works at all!

[airpower]

Yes but it's all about efficiency and it's bad.
Look at this thing with a Scotch Yoke Mechanism and the displacer free hanging only the small rod for friction. The Marble just manages to run.
https://youtu.be/3BuF3WH3bco

[Alphax]

Airpower..... yes, I understand your point about the efficiency not being good.

I'm keen to ignore the whole argument about efficiency, as I explained in my second post. Like you, though, I too think it's efficiency is poor.

However - speaking personally - I don't accept that it is all about efficiency.

In other words, just because it has poor efficiency (which I'm assuming is highly likely), it doesn't follow that that is "a bad thing". I know that sounds odd, but I can explain why it doesn't matter (to me), which is why I framed the question in the way that I did.

For example, if you look at the Fluidyne (liquid piston) Stirling engine patented by the UK Atomic Energy Authority in 1973, it also has no conventional displacer and has a minimum of moving parts (actually even fewer than Ted's Stirling 1 design) and is in some ways quite similar to Stirling 1 in that it too works using the utmost mechanical simplicity. Unsurprisingly it also has an abysmal efficiency of somewhere around 3.5% to 4% which sounds absolutely terrible until you consider that it can be installed and operated in extremely remote and dirt-poor parts of the world and can be maintained with nothing more than a beat up old screwdriver, or a wrench, by any unskilled operator. As long as it has a source of power (sun) it can work indefinitely for no cost and do basic work like pumping water for years on end. It's low efficiency is no drawback to it's usefulness and application.

So..... can I ask the same question: does anyone here think that the basic Stirling 1 design (single piston, no displacer) could still be made to work if scaled up - even if only scaled up a small amount? Set aside any concerns about what it might ever be used for, or the idea that it's efficiency would be poor - might it work if enlarged? And, of course, if you feel it would not - then why would it not?

[Tom Booth]

I think it looks like a rather standard Thermal Lag Stirling.

https://youtu.be/J6EtuYYmbRQ

https://www.camdenmin.co.uk/products/bu ... stirling-1

https://patents.google.com/patent/US5414997A/en

Is it not?

As far as I can tell anyway


https://youtu.be/ZjNDsxsGRqM

Generally, IMO, the extra long tube acts as a regenerator.

Putting a wad of wire wool in the tube provides additional surface area, allowing the tube to be shortened somewhat. Then it is often called a Laminar flow or thermal acoustic, but otherwise nearly identical.

There is a long thread here by derwood on the subject of his large thermal Lag / laminar flow engine:

viewtopic.php?f=1&t=1052&hilit=derwood

https://youtu.be/tXbXcl8yb0c

A sketch from the book appears to show a regenerator matrix of some sort:

Screenshot_20220201-174129.jpg (103.19 KiB) Viewed 7977 times

Or I don't know what else the cross hatching is supposed to represent.

[Alphax]

Thank you Tom. That all looks very useful and interesting so I will do some reading and respond.

[Alphax]

Tom, thank you so much for directing me to derwood's long thread, in which you also make a number of very relevant and insightful comments.

Having read it through just the once I can honestly say it exceeds anything I expected to find - it is a treasure trove of informative observations, potential explanations and good honest discussion.

It will take me a few days, I think, to do it all justice, but I have to say (if you can't already tell) that I'm delighted with it - particularly your own comments in your post at the top of page 2.

As for my original question - you have more than adequately answered it.

I will respond here in a little more detail in due course, but thank you again! But there is no need to wait for me to respond should you - or anyone else - like to add more comments and information, in fact I would welcome it with open arms.

[Alphax]

Some thoughts.......

I'm grateful to Tom Booth for pointing out some fascinating experimental work on single piston (Warbrooke style) engines here. I'm still finding my way round on this site.

One thing that strikes me having looked at that experimentation (derwood's thread and others) is that the terminology used by experimenters is quite varied.

I'd add that there is a large, academic literature relevant to the development, thermodynamics and physics of the general class of mechanically simple Stirling cycle engines into which these single piston (with or without flywheels) fit. I mention this academic point because (a) some of the literature is very old (well over 100 years) and (b) the bulk of it is brand new.

Above all, though, it seems that - exactly like the experimental work discussed in derwood's (and other) threads - the academic literature too has quite varied terminology and few clear-cut distinctions between "types" of mechanically simple single piston Stirling cycle engines.

This variation in terminology is significant in and of itself.

These little engines have a very wide range in terms of possible physical manifestations and plausible approaches to theoretical treatments.

I'll leave it there, but add something interesting..... at least I think it is. It is a readable paper published some 20 years ago by Los Alamos Condensed Matter and Thermal Physics Group (search "New Varieties of Thermoacoustic Engines", Backhaus and Swift, 2002 in case I fail to get the link here): https://citeseerx.ist.psu.edu/viewdoc/d ... 1&type=pdf

[Alphax]

Oh, I forgot to mention one point.

Right at the beginning I said that Ted's Stirling 1doesn't have a regenerator. But is that a true statement.

Tom Booth rightly point out that Ted Warbrooke's sketch shows what looks like regenerator material of some sort. Indeed it is the same sort of material (wire wool/stainless pot scourer) used as regenerators in other Stirling engines and Ted himself actually uses the word "regenerator" in the phrase "regenerator material".

However - in the text Ted makes no reference to a "regenerator" as either a discrete component part of the total system or as a function of the total system. Equally, he doesn't say that it isn't a regenerator.

Rather, in those parts of the text where he refers to how it all works, he lumps the material in with the tube it occupies and calls it all collectively the "heater". Equally, he also hedges his bet and calls it (as a heading for sketches, dimensions and text) "HEATER/REGENERATOR UNIT" (his capitals, not mine).

So..... my conclusion is that it didn't matter to Ted one way or the other whether or not this was any sort of "regenerator" or not.

Therefore if trying to categorise Ted Warbrooke's Stirling 1 engine as "having no regenerator" (as I did in the beginning of this thread) may or may not be correct, depending on your personal preferences only. Like Ted.... I don't think it matters.

[Tom Booth]

Tom, thank you so much for directing me to derwood's long thread, in which you also make a number of very relevant and insightful comments.

... particularly your own comments in your post at the top of page 2.

That is a controversial area, and I've received a lot of flack for my views but, I've continued experimenting along those lines with other types of Stirling engines if you're interested

I did not believe a "high heat" type Stirling could ever operate on a low or modest heat input like an LTD type engine, but I'm starting to think differently about that too.

In this Ted Warbrooke's Stirling and all the related types with a loooong nose for DISSIPATING heat so the heat is discarded, I wonder how much sense does this design really make?

Like, suppose, instead of heating the tube along it's whole length or in the middle somewhere, would it not make more sense to just heat the very end, or small section? And, instead of metal that conducts heat quickly from the hot area to the cold, probably the length of tube should be ceramic or other NON heat conducting material, except at the relatively small point of heat input.

Whatever heat conducts through the metal body of the engine does not transfer to the working gas. Doesn't such a long metal tube not only conduct heat away from the working fluid, but also radiates heat away to the surounding air? Instead it should conduct heat from a metal point or TIP only and from there into the working gas, and that gas containing space should be insulated so the gas can expand and transfer pressure to the piston without loosing energy anywhere else.

In other words, isn't such a long metal tube like a long UNINSULATED electric wire grounding out all along it's length? But, of course, grounding out heat rather than electricity?

So I took out the aluminum displacer and replaced it with a wood displacer, to eliminate a huge, and rather obvious channel for short circuiting heat before it can do its job of expanding the working gas.

As a result, the high heat Stirling that took several minutes of heating with a propane torch to get going, with a wood, rather than aluminum displacer, started up rather easily on a tea candle.

So, now I'm thinking; how this can be applied to the Warbrooke and other Stirling engines of similar type?

So blocking a lot of heat with a cement board "heat shield" showed some improvement I think, in this little engine:

https://youtu.be/R_QB5amihko

But still required a lot of heat to get going.

Replace the heat draining aluminum displacer though and...

https://youtu.be/bQ44Rm40unA

That still leaves the long metal tube, so today I happened to be shopping at the Amish store down town and found what I think could be a solution.

IMG_20220205_002821199.jpg (218.32 KiB) Viewed 7895 times

One of these thimbles should make a nice hot cap at the end of a ceramic displacer tube, so virtually NO heat will be conducted through the engine, but only into the working gas. The little dimples will even increase the surface area of the compact heat zone, without creating additional surface area for heat loss.

Anyway, I wanted to mention this possibility after watching some of derwood's videos where it seems he is needing to really blast that long tube with some really really high heat from a blow torch.

https://youtu.be/k7nwtB4KIbc

Not to be critical or imply that what he's achieved is not fantastic, but compare this with a similar long nose type engine running on a tiny tea candle? So, I think there may be additional room for improvement.

[Tom Booth]

I've read through the additional posts. As far as "thermoacoustic" in general, IMO it seems like a red herring. As a THEORY of how Stirling/heat engines in general operate, I just don't find it compelling or believable. So called "acoustic power", applied to heat engines, just seems like so much mythology.

Like, OK, when a sledge hammer hits an anvil it makes a sound, a ringing noise like a bell perhaps,but to focus on that sound and imagine it has something to do with the actual mechanics of blacksmithing and ignore the burly man wielding the sledge hammer ponding some iron into shape, so too, engines run at "frequencies" and may sometimes exhibit various "acoustic" characteristics, but the car radio playing is not what actually runs the engine.

As far as regenerators, to begin with, the invention of Robert Stirling's "economizer" was, I think, based on the complete fiction that heat could be reused over and over and over again, that the "caloric" when transferring from the regenerator to the working gas, caused the gas to expand, then when reabsorbed caused the gas to contract, and Stirling, apparently thought such a process could continue indefinitely without loss, almost

So, is this true?

Well if the heat/energy is transfered to the piston and out of the engine to power the load the engine is driving, then we are looking at a real violation of fundamental science, the conservation of energy.

So, what does a regenerator actually do, if anything?

It could retain some EXCESS heat that could not be converted to work output, increasing engine efficiency, but that is assuming excess heat input and low initial efficiency. So, maybe it is sometimes useful and sometimes not, depending on multitudinous factors.

A regenerator may be useful for increasing surface area and other things, but probably not for it's original purpose, as a reservoir of caloric.

At the moment, I'm leaning towards the idea that in an ideal engine, it could be discarded, but not many, if any, heat engines have achieved perfection, so a regenerator can possibly still be of benefit.

Like the long tube, it could be, and probably often is, a kind of internal short circuit, disipating as much or more heat than it conserves.

In reviewing Stirling's patent recently, I noticed he recommends insulating the regenerator space with masonry or whatever was available, NOT leave it in contact with a metal or other heat conducting tube directly exposed to the outside atmosphere, as is seemingly the universal modern standard for such engines.

I think we need to think of heat as being like electricity. We don't want any exposed wires anywhere.

So far, in taking that approach, and insulating the heat engine, even where it is ill-advised by every informed authority on the subject, even at the presumed, essential "heat sink", I've only seen improvements in engine performance.

A regenerator, to use another electrical analogy, is perhaps useful as a capacitor of sorts.

A typical heat engine regenerator, however, I'm afraid, is rather like a capacitor with some nails driven through it, hopelessly short circuiting all over the place, but with heat engine efficiencies so abysmal to begin with, it may yet produce some gain in performance by at least mitigating some of the unrestricted heat losses

[Alphax]

Tom

It is odd that you've had flack for expressing that idea. Heat and work are mutually interchangeable scalar quantities. The dimensions of heat are ML2T-2 and the dimensions of work are also ML2T-2. The SI unit of heat is the Joule. The SI unit of work is also the Joule.

So in a system where you are able to extract some useful work from it, the amount of heat left behind in the system will consequently be lower than it was before you took the work out of the system, unless you keep putting heat in to replace the work that you took out. Which is universally true for Stirling engines and not controversial.

[Alphax]

One thing worth taking a look at is the Rijke tube.

It contains most of the features of a Stirling engine - it is a tube to which you supply heat and in response it does work.

It is a long (compared to its diameter) open ended tube with one flat disc of metal gauze inserted roughly a quarter the way along its length. Thats all it is.

When a heat source is supplied to heat the metal gauze, the gauze transfers heat to the air within the tube (the working fluid). The heated air responds in a startling way and begins to oscillate violently within the human audible range (it gives off a high pitch loud hum). Rayleigh used this as an entertaining laboratory trick to amuse his students in 1877 - it is recorded (in his book) that it was so intensely loud that it shook the room.

The work done in oscillating the air arises from a standing wave with a wavelength of the order of twice the tube length - quite large and of significant power.

To muddy the waters here just a little, standing waves are far from the only vibration mode open to the working fluid in a tube heated near one end. Travelling waves can also be established and these can also be excited to carry significant power.

So to address one of your questions about the long noses of tubular Stirling engines (of which Ted Warbrooke's is but one), an answer is that it seems that the length itself is not incidental because the heat transfer to the working fluid requires length significantly greater than diameter in order to excite and maintain either travelling or standing waves.