Ted Warbrooke's Stirling 1: Question

[Alphax]

Tom,

You make some very good points. I agree that one of their observations is that less heat rejection may result in more power (output).

What puzzles me about that, though, is the practicalities of how less heat rejection can be achieved. For example, putting big cooling fins (or cooling water jacket) on to the piston/cold end would have precisely the opposite effect - it would enable even more heat rejection. So the authors must mean that it is best to stop more heat from ever getting to the cold end in the first place. But.... how, exactly? One way could be the thermal barrier between hot and cold ends. Such thermal barriers are to be seen in many of the home made thermal lag engines shown on Youtube (these barriers often also have a pulse-tube-like restriction).

Anyway, have you seen Calcoen and Vandermeersch's Masters Thesis? It is a damn good read with lots of useful information (and photographs) of their engine build. Thankfully it is in English. It is here:-

https://libstore.ugent.be/fulltxt/RUG01 ... 001_AC.pdf

[Alphax]

Nobody,

Wow! That got long! LOL!

Yes but it is all good stuff! Thanks.

Yes, the name isn't the best, but I think we are stuck with it.

It seems you may favour the idea that they perhaps would scale up OK, and that you maybe prefer the pulse tube explanation of how they work (first suggested by Allan Organ).

You say:

There should always be a insulated thermal break between hot and cold. Conducting from hot directly to cold is always a efficiency killer.

I think that must be correct, especially in Thermal Lag engines. I also think it is the only practical way of achieving what Tom was highlighting - reducing the heat rejected at the cold end.

And, yes, I agree completely about the position of the heat regenerator - there are some weird and wonderful positions of not only the regenerator, but where the heater is positioned (in Youtube examples). It is common to see the regenerator at the extreme hot end, but with the heat source (flame) directly under the end of the regenerator nearest to the cold end thermal break! So the temperature gradient in the regenerator is reversed (very hot towards the cold end where the flame is, cooler near the extremity usually referred to as the hot end). This is odd, but evidently works!

[Alphax]

A bit more about thermal barrier and pulse tube configurations in Thermal Lag engines.

All the ones I have seen on Youtube appear to me to have either a thermal barrier or a pulse tube (more of a restrictor piece than a log pulse tube). Many appear to have both a thermal barrier and a pulse tube/restrictor (usually as a joining piece to connect the piston cylinder to the hot end cylinder). All the Youtube examples are long engines (Tailer had a "pancake" piston cold cylinder with a long hot end tube)


Here is what Calcoen & Vandermeersch (the designers and builders of the experimental Thermal Lag engine that was adaptable to test Organ's pulse tube concept and tailer's lag concept) have to say about thermal barriers and pulse tubes:-

[Tom Booth]

I mostly agree with the statement in the attachment, but I'm not clear on what is meant by "between the hot and the cold section"

IMO, the "thermal gradient" is entirely across the regenerator and the tubes containing it.

But, in all of these engines any "thermal barrier" is between the regenerator containing hot cylinder and the power cylinder.

In 99% of all commercially available versions of this general type of engine, thermal lag, acoustic, laminar flow, whatever, the tube containing the stainless (usually) steel wool is largely non heat conducting glass or stainless steel to the point of being more of an insulating material than a heat conducting material.

This puzzled me greatly for a long time.

I thought: wouldn't a good heat conductor work much better? Like copper pipe?

But in practice, use copper tubing, and likely, the engine just will not run at all.

So, in other words, the power cylinder is not the cold side. So a thermal break between the heating tube with the regenerator and the power piston is not actually a thermal break.

Maybe the paper makes clear elsewhere what is meant by hot and cold sides, But at the start there is a statement that the piston itself acts as a displacer!!

I think that is completely wrong , but would imply that the power piston or cylinder is in some way in association with "the cold side"

I've often thought that the heating tube of these engines should be constructed in sections. Perhaps a small shrouded copper section for heat input, with the rest of the tube being much less conducting, or even highly insulating.

But joining two dissimilar materials in one cylinder could be challenging.

But the shrouded copper section could conduct heat into the interior working fluid and regenerator. Once inside the heat should only transfer between the regenerator and working fluid, so the rest of the tube could be glass, or stainless steel, or better yet ceramic, or maybe titanium, now that I've become aware of it's remarkable heat retaining and non conducting properties. Titanium might be a great choice for the entire tube. It can be heated to a white hot condition that will radiate heat into the engine but not conduct heat to the rest of the engine body. Maybe put a titanium tube within an insulating ceramic sleeve with just an opening in the bottom of the sleeve for direct heat input.

The huge cooling fins often seen between the heating tube and power cylinder, IMO serve no real purpose.

(That is why I get so steamed up about,... [Don't say it...]

But this whole idea that heat engines work by "heat transfer" from a hot side to a cold side. It's a complete train wreck. The opposite of the truth.

A heat engine works because of NOT transferring heat from the hot to the cold side which forces the heat to take the only path available: into the working fluid, resulting in power production.

Yes, that is the idea of a thermal break, but the hot and cold "sides" need to be correctly identified. The power piston is not the cold side. The power cylinder is NOT the cold side. If anything the unheated end of the regenerator is the cold side, but in actuality, IMO, no explicit "cold side" is required. There are typically quite enough hidden and/or unavoidable loses in an engine without intentionally introducing more.

The whole concept that heat needs to be removed from a heat engine to a "cold reservoir" as quick and as thoroughly as possible, with as gigantic of a heat sink as possible, that heat has to be "transfered through" a heat engine to the "cold side" at all is entirely counter productive, completely detrimental to the efficiency of the engine.

Oh my god, the engine took in some heat, how can it be disposed of?

Huge heat sinks, water jackets, fans, cold, cold and more cold.

"But a heat engine doesn't run on heat, it runs on a temperature difference!"

No it doesn't. It runs on kinetic energy. Cold is the absence of kinetic energy, the lack of or reduction in kinetic energy.

I tend to get adamant about how completely wrong Carnot was about everything, simply because he was wrong about everything. He was the origin of this whole idea that heat engines work by "transporting" heat from point A to point B. That heat has to be let in at the "source" and let out at the "sink", that a heat engine operates because of a "fall" in temperature and "not only heat, but cold is needed as well" - Carnot

No it isn't.

A heat engine needs heat input, not heat output.

There does not need to be a to thermal barrier between a hot and a cold side.if there is no cold side.

That may seem like an outlandish statement, but I don't think a heat engine needs a cold side. It needs a heat input side and a power output.

[Alphax]

Tom,

I can agree with a lot of what you say. I'll be specific. The terminology "hot side" and "cold side" is potentially a source of a lot of confusion, especially in discussing how the Stirling cycle actually works in real world engines, as distinct from diagrams.

It is easy to fall into the trap of thinking that it is obvious where the heat is going when one end is designated "hot" and the other "cold". But it is far more difficult in practice to be sure what the real distribution of heat within the internal spaces and the materials forming those spaces of a working engine really is and how that distribution changes as a function of time during an actual (as distinct from theoretical) cycle. And that is before you even consider the PVT of any particular region (and there are several regions of interest at any one time in the cycle) within a real engine.

That some bits of a real engine are "hot" and other bits are, by comparison, "cold" is undeniable. But the real distributions (plural) of heat in a real Stirling engine are smeared out to variable degrees in the metalwork and ceramic work as well as in the working fluid itself as a function of cycle time. It doesn't have clean, crisp "ends" in reality. Small wonder, then, that it is a difficult subject to discuss in the "soundbites" that forums and the modern age demand of us.

There is, in other words, a rather large gap between the abstract understanding that we convey with a diagram and any real understanding that we can glean from interacting with a real, working, instrumented engine.

The concept of a "thermal barrier" is similarly worrisome - in a well designed, efficient engine there would be (I imagine) no need to introduce such a concept. Yet I think we all "get" what it means in a loose, abstract sense.

As for your statement:-

A heat engine works because of NOT transferring heat from the hot to the cold side which forces the heat to take the only path available: into the working fluid, resulting in power production.

...... I would put it very slightly differently. I would say:-

A heat engine works because of transferring a portion of heat from the hot side to the working fluid which is able to convert that portion of heat into work, the remaining heat going to waste"

Like you, I would say the better the engine, the bigger the portion (of input heat) is that goes to the working fluid to convert to work and the smaller the portion that goes to waste.

I think going beyond that is not necessarily helpful, and I agree that busting a gut to maintain a "cold end" isn't necessarily the smartest idea when confronted with the real world task of actually designing and building a working engine. That, of course, is potentially a little bit of a risky way of expressing oneself because paying unduly small (or no) attention to the "cold side" draws the ire of some armchair theoreticians who will suck you into a confrontation if you aren't careful. That is always a battle that nobody wins.

The objective is to build a better engine. Like you, it is clear to me that minimising waste heat (there is always some, it always leaks out and is lost) and maximising heat transfer to the working fluid and getting it to convert as much of that as possible into work are the three primary goals of a Stirling engine developer.

[Tom Booth]

Let me say first of all, OUR intent, presumably, on this thread, is to determine if, and then possibly how, a "Thermal Lag" engine might be scaled up.

Is that correct?

Naturally questions regarding "efficiency" are intertwined in association with any potential engine modification.

There are a few statements in your post, that IMO are assumptions.

...the real distributions (plural) of heat in a real Stirling engine are smeared out to variable degrees in the metalwork and ceramic work as well as in the working fluid itself as a function of cycle time. It doesn't have clean, crisp "ends" in reality.

I can agree that USUALLY this is true, but is it NECESSARY? Do we have to just accept it? Is it a "LAW"?

A heat engine works because of transferring a portion of heat from the hot side to the working fluid which is able to convert that portion of heat into work, the remaining heat going to waste

Again, USUALLY true, but again, is that an absolute?

...waste heat (there is always some, it always leaks out and is lost)

Really?

Are people so complacent when it comes to the efficiency of an electric motor? Oh,.. electricity, that pesky stuff, it always leaks out, short circuits here there everywhere, no point in trying to do anything about that, just the way it is.

No, the designers, developers, electrical engineers, were not constrained by any "LAW" of electrical inefficiency or "limit".

IMO a thermal lag engine that can run without a flywheel is already very near to, if not already exceeding 100% efficiency, but watching such an engine running, people indoctrinated by this "efficiency limit" mindset, just can't see what is right in front of them.

The remarkable thing is that there is yet room for further improvements.

Only using up ALL, or 100%, of the heat supplied by raising the temperature above the ambient environment is still not utilizing all of the potential heat that might be made available, in some way.

Or, is the purpose of this thread to marshal all the "proof" to the effect that a "Thermal Lag" type engine could never be scaled up to any real power producing size?

Because, for, perhaps the majority of the posts, that seems to be the dominant narrative.

I couldn't care less about the armchair theoreticians touting their equations and charts supposedly detailing that paltry "maximum" efficiency of any given engine, which cannot exceed some fictitious engine that has never been built and would have zero efficiency if it ever were.

[Alphax]

Tom,

You ask:-

Let me say first of all, OUR intent, presumably, on this thread, is to determine if, and then possibly how, a "Thermal Lag" engine might be scaled up.

Is that correct?

My answer is - yes. That is correct.


Then you ask:-

Or, is the purpose of this thread to marshal all the "proof" to the effect that a "Thermal Lag" type engine could never be scaled up to any real power producing size?

My answer is - no. That is not correct.


I have no interest in efficiency. I simply want to scale up a working Thermal Lag engine and this thread is my way of seeking opinions on that matter. That is all.

[Alphax]

Tom,

You ask (my bold emphasis):-

A heat engine works because of transferring a portion of heat from the hot side to the working fluid which is able to convert that portion of heat into work, the remaining heat going to waste

Again, USUALLY true, but again, is that an absolute?


I have no interest in whether or not it is considered to be an absolute as it has no bearing on scaling up a working Thermal engine.



...waste heat (there is always some, it always leaks out and is lost

Really?

Yes, really - all Stirling engines that exist or have ever existed appear to be entirely consistent with the idea that there is always some waste heat. I think that is simply a statement of fact - if anyone happens to know of a real engine that has been competently documented to run with no waste heat then that might make an interesting topic on a different thread. But as I say, I know of no such (real) engine. And it isn't my intention to try to make one. Or discuss one!

[Tom Booth]

Best wishes from Denmark, Tom . . .

Thank you,

Best to you as well.

Just FYI to whomever, There we're a number of "off topic" questions or statements directed towards me, and or, about me earlier in the thread, I don't wish to simply leave hanging, so I will respond on this other thread.


viewtopic.php?f=1&t=5410

[matt brown]

Tom's recent 2 lines caught my eye...

(1) But a heat engine doesn't run on heat, it runs on a temperature difference!

(2) No it doesn't. It runs on kinetic energy. Cold is the absence of kinetic energy, the lack of or reduction in kinetic energy.

The word heat usually conveys temperature, but in thermodynamics is also means energy. Most heat engines run on a pressure differential driven by a temperature differential which coincides with an energy cycle. Simply creating a mech with dP and dT does not make an engine ! Nope, a working engine_also_requires an energy cycle with dU. The value of PV plots are that they're a good visual for what's happening in a cycle (or supposed to be), whether a simple compressionless open cycle Lenoir or a crazy closed cycle Brayton with multi reheat, multi intercoolers, and regen.

However, a nasty subtlety that sinks many schemes is mass 'flow', and I don't mean pistons pushing a gas thru an eng. Nope, I mean that the gas mass seeks a pressure equalization, whereby cool gas 'pools'. The Schmidt analysis for Stirling's assumes pressure is equal thruout eng at all times, but that pressures changes thruout cycle. This might seem like no big deal, but consider typical beta with fancy regen, and try to visualize exactly where most of the gas is from simple PVT values. Those cute animations with blue & red volumes are the sucker bait for countless grant schemes. As Urieli told us years ago, most of the gas remains in the regenerator (in 'high end' designs).

Alphax - Nobody hit it right on with thermal lag scalar issue...it's that increasing eng size will have volume increasing much faster than surface area. Here's a little example of how scale can effect stuff, tho waaaay off topic. Consider you have a sailboat design that you want to enlarge, but unsure as how to precede. To keep this simple, let's say that you want to double the 'size', so

2^1 is delta length
2^2 is delta sail area
2^3 is delta displacement
2^4 is delta heeling moment (tendency to heel)
2^5 is delta righting moment (resistance to heeling)
2^6 is directional stability

Yep, probably looks like a bunch of BS, but this explains why a larger sailboat can be narrower, yet stiffer, and (my favorite here) have a smaller rudder. This is what engineers are all about...knowing how & why.

[Alphax]

Hi Matt

You and Nobody have made some consistently good points regarding potential pitfalls in scaling up Thermal Lag engines (including the Warbrooke and subsequent Youtube home-brew examples of the type).

The objective of this thread is to draw out thoughts people have about what practical problems might be anticipated in undertaking a scaling up on such an apparently simple engine.

To me, that means drawing opinions from anyone who is willing to express them, especially those who have actually built and operated any type of Thermal Lag engine, or indeed, any kind of Stirling engine, with their own hands. This can be hard to do on forums.....

I would very much like to hear more from you, and from Nobody as you have both provided thoughtful comments.

Can I just address something you said though, Matt?

You ended your comment with what may be construed as a bit of a barb by some on this thread, even though that may not have been your intent:-

This is what engineers are all about...knowing how & why.

I'd rather like it if we could collectively try to avoid saying that sort of thing if possible (I understand it isn't always easy, though). I have learned the hard way during decades of engineering research working in big company, big buck projects that engineers aren't always the people with the best insight and PhDs aren't either. I say that humbly as a qualified engineer with a Phd myself!

[Tom Booth]

... And it isn't my intention to try to make one. Or discuss one!

Sorry, but earlier you said that a particular post if mine on another (derwood's) thread was something you found especially helpful in this context.

That post, you said, you liked, at the top of page #2 of that tread mentioned Carnot, the 2nd law, and how I thought this type of engine had higher than Carnot efficiency, and not much else.

[Nobody]

This caught my eye. From Tom Booth:

Are people so complacent when it comes to the efficiency of an electric motor? Oh,.. electricity, that pesky stuff, it always leaks out, short circuits here there everywhere, no point in trying to do anything about that, just the way it is.

No, the designers, developers, electrical engineers, were not constrained by any "LAW" of electrical inefficiency or "limit".

The power loss law for electrical motors is governed by the following equation:

Power lost = I^2R

Commonly referred to as I squared R losses. Pronounced: eye squared arrr losses. It is Amperes squared times Resistance in Ohms.

It is lost by the waste heat route, and is why motors get warm or hot.

I wouldn't call people complacent about it, as engineers are ever striving to reduce it.

And yes it leaks out as short circuits as well. Think corona discharges from high voltage transmission lines. Insulation is not perfect, neither are wires perfect conductors. Most motors are in the 80% to 90% efficiency levels.

Now on to heat engines:
I'd like to point out that a heat engine has a temperature difference, always.

The heated side (inside "gas"), and the cooler everywhere else side (outside/ambient).

Or

The cooled side (inside "gas"), and the heated everywhere else side (outside/ambient)

Perhaps we can all agree on the necessity of having a pressure difference, and temperature difference is the heat engines method of obtaining such pressure difference?

However a thermocouple works on a Delta T, with or without a Delta P. Another absolute claim, dispelled by a scientific fact. Arrrg!

Back on topic:
What about a double ended pulse engine? With cold section in the middle and two hot ends and two pulse tubes. Having one piston, magnets and generator in the middle. It could be highly pressurized.

[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.

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.

So... ???

[Alphax]

The last 3 posts came in this sequence - Tom, Nobody, Tom.

So I'll respond first with Tom's 2 points, then consider Nobody's if that is OK with folk.


Tom,

- regarding the first in sequence of your 2 posts. Derwood's thread is especially interesting (to me) as it is the best documented non-academic published account of someone actually making a lager (than usual) Thermal Lag type of engine that actually works, that I am aware of. So thank you (again) for making me aware of it.

Derwood's is an impressive achievement that stands on its own merits.

He actually describes it as a Lamina Flow engine, as many do and the two terms are often used interchangeably (rightly or wrongly - it doesn't matter).

The discussions on efficiency that I have seen are interesting also. However, I did express an opinion quite early on to the effect that small working desk-top models are difficult to instrument, making actual measurements practically impossible. This is what I actually said :-

It seems to me that all small models (toys, really) are always going to be thermally very inefficient. Worse.... because they are small models and not laboratory instruments, we have no way of actually measuring the inefficiencies that we know are there. So we must guess that they are there and that they are "large". On that point we must agree, I think.

Interestingly, you disagreed emphatically (your word) with that statement, which is fine as it helped me to take a view. Which is that I think that there is little point in discussing efficiency on a thread which is focussed primarily on scaling up Thermal Lag engines. Others may, of course, take a different view which, naturally, I'd have no objection to in principle. But I'm unlikely to engage further in debate about efficiency unless the relevance to scaling Thermal Lag engines is made clearer to me than has so far been achieved. That is a general remark, by the way, not directed at any one individual contributor.

If you look at the second post to be made in this thread you will see that I tried to make it plain from the outset that I am inclined to ignore efficiency in considering scaling up (for the purposes of this thread). Fundamentally I'm saying that I've not been persuaded otherwise, so far.

I note that you have started a thread on efficiency, and that is probably a good move.