The Right Theory of the Real Stirling Engine.

[danalinscott]

Thank you for posting this PDF.
It is especially timely in my case.

It is a quick read and not as technical as I expected.
If possible can you post a version with working links to the animations.
If you cannot do this but have a version with working links I am willing to help do so.

Thanks again!
Dana

[Tom Booth]

"ENERGY that a mass can potentially exchange with the environment is called ENTROPY. "

That is an interesting definition of entropy I haven't heard before.

I'm interested in how that might apply to a cold running engine. (A Stirling engine running on ice).

[Tom Booth]

"THERMODYNAMIC THEORY OF HEAT ENGINES: CARNOT, STIRLING

"Sadi Carnot (1796 - 1832) first realized that heat is a form of kinetic energy that can be transformed into useful work but only under the condition that this energy flows. He immediately found a comparison with an aquatic turbine that converts the kinetic (potential) energy of the water into useful work. Namely, if the flow of water through the turbine is stopped, the energy conversion will also stop. In order for the water turbine to give "useful work" it is necessary that the water flow through it. We can never turn all the energy of water into
work. Complete analogy is valid for every heat engine. This is a natural fact (natural law) discovered by Carnot."

Carnot originally based his theories on the literal conception that heat is a material fluid (caloric). A fluid that, like water, flows from a "high" (Hot) to a "low" (Cold) level.

In some previously unpublished notes, it seems he changed his mind about all this. Heat is not a "fluid" and does not "flow". Today "temperature" is defined scientifically as "A measure of the average kinetic energy of the particles in a sample of matter, expressed in terms of units or degrees designated on a standard scale."

Heat transfers into a Stirling engine. Expansion of heated air takes place to drive the power piston. The heat is converted to mechanical motion. The so-called "caloric" does not "flow through" the engine and out the other side like water flows through a turbine.

[Tom Booth]

I'm sorry to say that after an examination of that PDF I have no actual concept of how these supposed hydraulic (?) Stirling's are supposed to work.

I'm not at all sure that the author has any real idea either, though I'm open to some explanation.

I may be wrong but, my impression is that the idea of heat as a "fluid" has been taken literally so there is some idea that this should apply to hydraulics.

At this point I'm not sold on the idea.

[Tom Booth]

"And in the case of "Heat engine on ice", this case is also possible if you connect "hot expansion" at ice temperature Tmax = 273°K adiabatically with "cold compression" at say Tmin = 200°K..."

I"m a bit confused.

A Stirling "running on ice" would have a cold side at 273°K (or 32°F)

The hot side (at ambient, say 295°K "room temperature" or about 70°F) would be hotter not colder, ambient heat being the actual power or energy source.

e=(Tmax-Tmin)/Tmax=(295-273)/295=0.0745 (7.5%)

Is that about right?

Anyway, how I tend to think about that, though the numbers seem rather unimpressive, is that atmospheric heat is effectively "infinite". So 7.5% of infinity is what?

According to your formula though, 92.5% of the ambient heat entering the engine will pass through the engine and go towards melting the ice. The equivalent of being "re-emitted into the environment" under these circumstances. In that case, the ice would presumably melt very quickly.

I'm not at all sure that this holds true under actual experimental observation.

7.5% of 295 (or .075×295) = 23.305

Would that not represent a temperature drop of 23.305 degrees which would result in an "output" temperature of 272°K. That is, approximately the same temperature as the ice itself.

In that case, if otherwise entirely protected or insulated from the surrounding ambient heat (the ice in a Dewar chamber or thermos) then the ice would not ever melt at all and the engine could run indefinitely.

My reasoning is, if heat flows from hot to cold and the output temperature is identical to the cold side of the engine (T min. the temperature of the ice) then the resulting heat exchange with the ice would be zero.

I would appreciate some explaination of the flaw in my reasoning.

Certainly there cannot be both 92.5% of ambient heat flowing through the engine into the ice and also at the same time 0%.

[Tom Booth]

My understanding is that the 92.5% of "un-utilized" heat represents the heat from ice temperature down to 0°K absolute zero.

That heat below the cold temperature side of the engine, colder than ice, down to 0°K cannot be converted to "work" by the engine. But does that mean all that unused heat must of necessity pass through the engine and into the ice?

I would argue that this would be impossible as heat cannot flow between two identical temperatures and from the math it appears that the output temperature and T min (the temperature of the ice) are identical.

[Tom Booth]

"The exact same analogy applies to any device (water turbine or windmill) that converts kinetic energy (EK) into useful work and vice versa"

IMO this is not really true at all. A mass flow, such as water down a waterfall or wind across the plain is not anything like the random molecular motion of gas particles.

How the energy of one (mass flow) and the other (random molecular motion) is harvested is entirely different.

Heating a gas in a Stirling engine to increase internal pressure to drive a piston does not depend on anything at all flowing through the engine like water or wind through a turbine.

A Stirling engine is not like a hydraulic cylinder filling up with fluid to push a piston, which fluid must be let back out again to allow the piston to return. Heat is not, as Carnot theorized, a fluid.

[Tom Booth]

To reference myself;

"My reasoning is, if heat flows from hot to cold and the output temperature is identical to the cold side of the engine (T min. the temperature of the ice) then the resulting heat exchange with the ice would be zero."

That, in reference to a Stirling engine running on ice.

That is my understanding of the mathematics behind Carnot efficiency. It is impossible to do any better than Carnot efficiency because, fundamentally, heat flows from hot to cold so there can be no greater extraction of energy than to bringing the heat of the gas down to the temperature of the cold side of the engine.

In theory, a Stirling engine could match Carnot efficiency.

So, in theory, it looks to me like a 'perfect' Stirling engine running on ice, could, theoretically, continue to run until the parts wore out, without the ice ever melting, if the ice could otherwise be perfectly insulated.

This seems remarkable to me in that such an engine would be producing power which could be used for cooling.

[Tom Booth]

Dear Tom,
Why are you intrigued by the nature of heat and how does one imagine what heat is and how it turns into mechanical work and vice versa?

One way of looking at a laminar flow Stirling (running without a flywheel) is that air is elastic, like a rubber band.

Think of a paddle ball:

https://youtu.be/OViXznx9KKo

With kinetic energy, where is there any need for a "sink"?

It is just a matter of adding more energy at just the right time to compensate for loses.

The air expands and pushes the piston, like a rubber band on a paddle ball, the elastic stretches, then pulls the ball/piston back and the process repeats.

You don't have any energy flowing through anything to any "sink".

If a heat engine does not HAVE TO send heat through to a sink as it's necessary mode of operation, like water through a turbine, then we are at liberty to design an engine without unnecessary accoutrements, like a displacer to shunt heat to a cold end for removal, introducing unnecessary dead air space.

It makes a difference IMO. A big difference.

[Sockmonkey]

Part of the lack of development in Stirling engines is that back when they were first invented, metallurgy wasn't quite up to handling the temperatures needed to make them work really well. Likewise it's a bit tricky to make something flexible and airtight that can withstand high temperatures.

[Yorky]

Therefore, I urge all lovers of technical thermodynamics to refrain from "inventing": "new theories", "cylinders", "displacers", "pistons", "crankshafts", as well as to stop spending their precious resources on "useless toys", but to finally start designing and building their first real useful Stirling engines!

Sorry but my toys are far from "useless". They gave me a great deal of pleasure building them and they continue to give me pleasure each time tinker with them or run them. Why would I want to invest time and money into building a "Real" Stirling engine that could not be any more usefull?

The answer is probably in the first line of your quote in so much as I am not a lover of thermodynamics, I just like building stuff.

Toodlepip

[Bumpkin]

Bravo Yorky. Another perspective is that even though I have done some physical modeling, I am a "lover of technical thermodynamics" and probably enjoy theoretical tinkering as much as you enjoy your physical models. But it would seem silly in my own search for "useful" to limit myself to only one direction.

Bumpkin

[Yorky]

Dear boys,
You seem to want to tell me: that you INTENTIONALLY build bad "heat engines", and that you INTENTIONALLY make an even worse copy out of a bad copy, and that you INTENTIONALLY misrepresent your "heat engines", and that you ENJOY it? Are you members of some technical-masochistic sect I haven't heard of yet?

Yorki, you express your selfish view that building your "heat engine" has given you a lot of pleasure. And what about us, to whom your announcement to start building the "Stirling engine" was an equally great pleasure, and with even greater pleasure we wanted to "participate" in your planning, design and construction. But you did not accept any criticism or advice! So why the announcement? What is the purpose of the forum? That is why for me, who followed your work, every decision you made and every move you made was always a new and new disappointment. You made mistake after mistake! So couldn't it be better?

So, girls and boys, first a book in hand, a theory in your head, then try to solve the 200-year-old "STIRLING'S PROBLEM": How to achieve that hot and cold piston in engine move synchronously and at maximum working volume (Vmax - hot-cold regeneration ) and at the minimum working volume of the working medium (Vmin - cold-hot regeneration) and how the cold piston "waits" while the hot piston performs the expansion from Vmin to Vmax, and how the hot piston "waits" while the cold piston performs compression from Vmax to Vmin! If you solve this problem, you have constructed a real Stirling engine that will give you much, much more TRUE PLEASURE!

Let all that I have so far presented about the Stirling cycle and the Stirling engine serve you for learning and thinking, as well as the following diagram:

Sadi,

Yes, you are correct. I did fail in my intended documentation and proposed interfacing with other members when building my model. I did apologise for that and explained life threw a couple of spanners into the works that I was not expecting.

I was drawn to this site by the very first thread in this site which pertains to be about making a Stirling Engine out of tin cans. This was interesting and captured my imagination as I am sure that it did with so many of the responders.

I could understand that thread and should have stuck with it and left you to your quest.

I will leave it at that, I am out of here.

Toodlepip

[Bumpkin]

Sadi, you might have something to contribute to the forum so I'll set aside your childish insults: "Dear boys" and cowardly straw-man blustering: "You seem to want to tell me: that you INTENTIONALLY build bad "heat engines", and that you INTENTIONALLY make an even worse copy out of a bad copy, and that you INTENTIONALLY misrepresent your "heat engines", and that you ENJOY it?" - and I'll momentarily assume that you have the scientific courage to accept honest forum protocol and not purposely misrepresent other views.
I don't know that we have much to disagree about regarding heat engines, except perhaps the title "Stirling engines." I think of that as basically meaning a closed-loop regenerating heat engine, but if we disagree I can happily debate the point and accept if I'm wrong. It seems a silly thing to worry about, but in the name of scientific precision it could be important. (On that same note, I still cringe a bit at the name "lamina" as applied to thermal lag engines, but it seems to be too entrenched to do anything about.)
I personally have limited interest in heat engines that can't produce reasonably efficient power from a moderately low grade heat source, for home woodstove cogeneration. That seems to me to be one of the very few remaining power-producing applications that haven't been made irrelevant by other technologies, so I applaud those who add art and amusement to an otherwise very limited technology.
On the other hand when power is the goal, like you, I find it frustrating to see people repeatedly disregarding basics that should be well-known, or trying to compete with technology that has long-since passed us by no matter how efficient an engine we produce. I've mentioned somewhere before that it might be useful to have a plain-language "heat engine basics" post to stick near the top of the forum, with keywords linked to more complete information for those interested. The notion didn't go anywhere and it would certainly be presumptuous to offer one up without some feed-back about what actually is understandable plain-language.
Anyway, I look forward to co-operative ways to further individual goals. That's what we're here for.
Bumpkin

[Tom Booth]

Your topic is: "The Right Theory..."

You suggest there must be something wrong with the theory of how Stirling engines operate or there would be more development.

I agree with the basic premise. But then you present a paper that states, among other things: "The power
of the engine is directly and proportionally dependent on the flow of heat through the engine.".

IMO, that is the WRONG theory that has permeated heat engine theory and design from Carnot forward. It is now well known that heat is not a fluid, and even Carnot himself retracted his theory in his notes, which unfortunately wern't published until many years after his passing.

The power of a heat engine is not dependent on heat "flowing through" but the exact opposite.

Heat that flows through the engine is wasted heat. Heat that does not flow through is heat converted and utilized to do some work.

But clearly, you have your own ideas, still rooted in Carnot's century old, obsolete caloric theory and have no real concept of how a heat engine actually works.

Nevertheless, you have no real interest in hearing anything from anyone who might actually know a thing or two, you just want to promote your own muddled and confused theories which make no sense and have no basis in reality.

Good luck with your "Real" Stirling hydraulic or whatever engine,

You invite discussion, but it seems you can only criticise others who at least have some practical experience with actual working heat engines.

IMO it is Carnot's nonsensical water wheel theory of heat that has infected heat engine theory for more than a century that has actively stifled progress, which you continue to actively promote as if it were gospel.