Stirling Engine Thermodynamics

[Nobody]

https://en.m.wikipedia.org/wiki/File:Te ... _angle.png

That link is is for a graph of adiabatic temperatures for the heater red, and blue cooler/sink working fluid, verses crank angle for a theoretical Stirling Engine. The straight lines are the theoretical isotherms associated with the hot and cold plates.

If you look at it you will see heating and cooling of both thermal plates. The heating of the cold plate/sink is at a greater temperature and for a longer interval than for the cooling period. This means that the sink will receive more heat than it rejects.

[Tom Booth]

https://en.m.wikipedia.org/wiki/File:Te ... _angle.png

That link is is for a graph of adiabatic temperatures for the heater red, and blue cooler/sink working fluid, verses crank angle for a theoretical Stirling Engine. The straight lines are the theoretical isotherms associated with the hot and cold plates.

If you look at it you will see heating and cooling of both thermal plates. The heating of the cold plate/sink is at a greater temperature and for a longer interval than for the cooling period. This means that the sink will receive more heat than it rejects.

The image is public domain, so I think it is fine to reproduce it here for convenience.

Temperature_vs_angle.jpg (61.48 KiB) Viewed 6655 times

The text by the author says this is an actual (not theoretical) heat exchanger (singular) so I'm not entirely sure if it applies to the regenerator or the hot and cold sides of the engine.

There is much missing/unknown information. What type of Stirling engine is this? Heat exchanger material? Thickness?

Certainly an aluminum heat exchanger would respond differently than my non heat conducting acrylic "cold" plate.

The chart does not reveal ACTUAL heat rejection.

Your conclusion, nevertheless states:

"This means that the sink will receive more heat than it rejects."

I'm not sure what you mean by that. "Receives" from where? The internal working fluid or the external ambient?

Anyway, the graph is interesting in that it seems to possibly confirm my contention that the heat during compression exceeds the temperature of the heat source itself, and the cold of the internal working gas during expansion becomes colder than the sink. (The straight lines, representing simply the temperatures of the heat exchanger(s) [not "isotherms"?])

You again, I think, neglect to take into consideration the critical element of the positions of the piston and displacer relative to the working gas.

In many Stirling engines, when the internal working fluid is hotter than the sink making it thermally possible for the heat to be "rejected", the sink is nevertheless out of contact with that working fluid. That is; the cold heat exchanger is covered by the displacer or the gas has been otherwise evacuated so as to be out of physical contact with the cold plate or ambient sink.

Again, it seems rather disingenuous to complain that I have posted my "pseudoscience" to "every thread" and then constantly bump my threads to the top with what appear to be attempts to refute my observations and/or experimental results. I welcome the challenges, though, as my only objective is to get to the actual truth, but if you want to refute my, what you call "claims", l think you'll need to come up with something more substantial than this chart, which in actuality appears to only confirm what I've been trying to say all along, if you don't follow in the footsteps of the "snake oil salesman" as you put it, and leave out relevant information.

Significantly, I think, it looks like the internal temperature will be below that of the cold "sink" through about 160° to 180° when the cold plate is exposed. Which logically would seem to indicate that my diagram indicating as much (that the engine acts as a cooler drawing heat from the sink, represented by cold blue ice crystals/snowflakes) during that portion of the cycle (#5 in the diagram) is substantially correct.

This, IMO is why a Stirling engine will immediately respond to having ice placed on the cold plate. Not because the ice is drawing away more heat, but rather because the plate is delivering less heat "backwards" through the sink into the engine, allowing the engine to reach a greater temperature difference (through adiabatic expansion / internal self-cooling / conversion of heat into work). The same effect seems to result from simply insulating the sink from the relatively "hot" ambient so-called "sink".

Stirling_engine-cooler_by_Tom_Booth_CC.png (55.61 KiB) Viewed 6652 times

A non- sinusoidal type displacer mechanism, such as the following, will greatly increase the effectiveness of the displacer as a barrier to unwanted/parasitic heat loss, dissipation or infiltration. That is, the cold and/or hot plates are fully covered through nearly 180° each.

Note the rod with the stops that act to quickly flip the displacer between fully covering the hot plate to fully covering the cold plate.


https://youtu.be/P0JWvEQPUJs


Further improvement in heat regulation might be possible using a cam.

[Tom Booth]

.... The heating of the cold plate/sink is at a greater temperature and for a longer interval than for the cooling period. This means that the sink will receive more heat than it rejects.

The Wikipedia article graph you linked to, looks like, to me, that the working gas on the cold side is warmer than the sink plate, (if that's what is actually represented, the cold heat exchanger) through exactly 180° of the cycle, which is 1/2 not a "longer interval", if "longer" certainly not by much, a degree or two, maybe.

As said before, at the 150° - 160° mark or about TDC (top dead center) or peak of the compression stroke, the sink is isolated from the working fluid, (between about 3 to 4 in my diagram, when the cold (ambient) heat plate is covered by the displacer.

The secondary heat generated on the thin, almost non-existent sliver of air on the cold side of the displacer, due to the increase in pressure, could produce little if any effect, nearly all, if not all the working gas having been shunted to the hot side.

It would also, as mentioned, be better if the cold heat exchanger could be fully covered by the insulating displacer through the entire 160° interval when the internal temperature is higher than the cold plate or cold sink, as in the above video. (I personally find this type of engine impressive, in that for a quite large, bulky engine, is runs on a very small temperature difference).

Your conclusion: "This means that the sink will receive more heat than it rejects." Remains a puzzle.

By "sink" do you mean the outside ambient air or the cold heat exchanger/plate. (Or something else?), And to say that "it" will "receive more heat than it rejects" is rather confusing.

I assume you mean the cold side plate/heat exchanger will receive more heat from the working fluid than it passes over to the outside ambient, which is what "reject" would normally refer to. Heat rejection to the sink.

You will have to unpack that statement because I can only guess at what point you are trying to get across.

All I'm fairly sure about is that you have vowed to continue pointing out my errors and misleading "pseudoscience", so whatever you're trying to say I assume it is intended to prove me wrong somehow.

I think that so far, you have not succeeded in that respect .

[airpower]

Completely weight balanced Stirling engine with double displacement and how it came about.
https://youtu.be/elp3eOojvTI?t=438

Inefficient solar-powered stirling engines
https://www.youtube.com/watch?v=STn3rcLIW1I
https://youtu.be/av2aKSG1aRA
https://youtu.be/8QE-CmKxz40

[Tom Booth]

Completely weight balanced Stirling engine with double displacement and how it came about.

https://youtu.be/elp3eOojvTI?t=438

Inefficient solar-powered stirling engines
https://www.youtube.com/watch?v=STn3rcLIW1I
https://youtu.be/av2aKSG1aRA
https://youtu.be/8QE-CmKxz40

The link provided has a time stamp, resulting in the, quite long video, starting very near to the end, I was a bit confused by the "...and how it came about" because it looked like there was no information on the subject (that is all at the begining of the video)

I'm reposting the video with the time stamp removed, just for my own and others convenience, who might be interested in watching the whole thing.


https://youtu.be/elp3eOojvTI


Unfortunately (for me anyway) the video text is not in English.

What strikes me though, is that this is very similar to an engine concept that I was working on a while back:

viewtopic.php?f=1&t=2525

The MIT double displacer engine in your video though, apparently is intended for a different purpose (just to block air flow around the displacer?) than what I had in mind, (to make a combined engine/heat pump) and so the timing and relationship between the two displacers is not the same as what I was working on, which was closer to the Vuilleumier heat pump -with an added power piston.

The MIT engine seems to just have what looks like a split displacer, that shifts horizontally from side to side, the two halves otherwise moving together, almost as one vertically.

The various contraptions I had been working on were put on the back burner as I was not able to get anything working and/or did not have the time to finish them and/or got distracted or involved with other concerns.

I haven't entirely given up on the idea though.

IMG_20200110_133944100_crop_11_resize_84.jpg (72.17 KiB) Viewed 6628 times

IMG_20200530_010901596_crop_91_resize_99.jpg (55.47 KiB) Viewed 6628 times

viewtopic.php?f=1&t=1029&p=12516&hilit= ... ier#p12516

The only "completed" engine had multiple mechanical issues to overcome, binding and bending of the too thin inner displacer connecting rod primarily, which I hoped to solve with some telescoping marshmello roasting fork handles I came across, but never completed that engine.

[airpower]

^^^^ The reason for the double displacer to have a smooth running engine.
Some google translation
Disadvantage of a motor with oscillating displacer movement: Mass must be accelerated and braked.
Disadvantage of a motor with rotary displacement: The working gas is talented in the housing rather than moved.
Advantage of a motor with oscillating displacement movement: good thermal efficiency
Advantage of a motor with rotary displacement: smoothness
How can both types of motions combine and their benefits are used? The solution: The displacer is hung on two eccentric and is moved in this way as the dome rod of a locomotive.
Model 1 Air still mostly whisked/beaten and not moved. Model 2 sides chafe

[Tom Booth]

Kontax guy tries to explain...

https://youtu.be/0ZzPWfLDAOI

[Tom Booth]

Kontax guy tries to explain...

https://youtu.be/0ZzPWfLDAOI

This video is interesting to me for a number of reasons.

I've spent some time swapping out metal plates for acrylic and have to agree that although there is a vast difference in heat conductivity between aluminum vs. plastic, as far as the cold sink side is concerned anyway, it seems to make no difference in the engine's ability to operate, but I've assigned different reasons.

I tested the infrared transmittance of the acrylic on my engines, which are not Kontax, so the acrylic may not be the same, I don't know, but they seemed to be entirely non-infrated transmitting, at least to the specific wavelength used by my infrared thermometer gun.

Researching the subject further it seems Acrylic is able to transmit infrared in certain bands but not others. Acrylic becomes entirely opaque to infrared if thicker than about 1/5 of an inch or so. The acrylic on my LTD engines are thinner than that.

There is "the greenhouse effect" in that acrylic transmits the visible light spectrum from sunshine which then, hitting the displacer, "bounces off" or is re- emitted as infrared which is not transmitted as well, or not at all, and so builds up inside, or so I thought.

"Bottlenecks" are mentioned several times in the video, but aren't always clearly identified or explained, I don't think.

There is some room for additional experimenting.

Perhaps the acrylic top could be coated with aluminum, like one of those "space blankets" to fully block BOTH the conductive heat AND any potential infrared radiation out of the engine, though I've already tried acrylic and insulation. Maybe just glue some aluminum foil onto the acrylic? Inside or outside? Maybe both?

Perhaps the ever resourceful heat went through the acrylic by infrared then switched to being conducted through the insulation. Not impossible I guess, but the heat has to actually be there.

My attempts at taking temperature readings of the sink plate so far seem to indicate that the heat is not always getting there. Neither the acrylic nor the insulation covering showing any elevation in temperature, to either touch or infrared thermometer.

The one important factor not taken into account in the Kontax video is the conversion of heat into work.

Heat goes in, by whatever means, but does not need to be provided with a path out because the heat energy that goes in is converted to mechanical motion.

My experiments even seem to indicate that, for whatever reason, the engine even runs better with the path out, (apparently) blocked.

So, ...

I have gotten together a bunch of stuff from the hardware and grocery stores, Perlite, faux super insulating "Starlite" ingredients (extra fine flour, powdered sugar, baking powder & soda, corn starch) wood for making pyrolyzed charcoal, refractory cement...

I even sent away for some sheets of Aerogel.

Hopefully I'll find some time to do some additional experiments with all this stuff soon. As well as put together some model engines based on new (or rather old forgotten) designs, like with the P 19 type linkage with "dwell" and so forth and so on.

My long range goal with all this is to work out an answer to all the questions in my mind regarding how Stirling engines actually operate, using off the shelf and modified cheap, small model engines, that don't cost much to fool around with, then hopefully apply that to some scaled up, power producing engine(s)

Personally I think heat engines are still a long way from reaching their full potential.

Though I never set out to challenge or disprove generally accepted ideas about how Stirling engines operate, a roadblock to even researching the question has arisen in the shape of Sadi Carnot wielding the sword of the second law ready to foil my efforts to construct a more efficient heat engine at every turn

Personally, I think that the so called "Carnot limitation" is not only wrong as a general philosophy ( The philosophy of: "It is IMPOSSIBLE!!!"), but somewhere along the line it has been misinterpreted so as to make things seem even MORE impossible than they actually are.

If, as I suspect, Stirling engines are actually Self cooling internally, then the temperature of the cold "sink" is certainly a kind of limit, in an UNINSULATED engine.

The simple act of putting an insulating disk of aluminized styrofoam over the cold sink of a toy Stirling engine, has been sufficient reason to get me banned along with any discussion of the subject from several science and physics forums.

How odd.

I thought that aluminized styrofoam alone should have been enough to block both convection as well as radiation heat loss, creating a real bottleneck that would slow the engine down.


https://youtu.be/fFByKkGr5bE


Instead of slowing, it ran faster and for a record amount of time, compared with previous runs using the same engine without insulation.

I don't really care about proving or disproving Carnot efficiency or the Second Law, or ruffling any feathers so much as just finding out how these engines really work and what might allow them to work better.

[Nobody]

I should have included this link,

https://en.m.wikipedia.org/wiki/Stirling_cycle

The Wikipedia webpage has the following description for the graph.

Temperature versus crank angle from Wikipedia.

Temperature_vs_angle.png (5.1 KiB) Viewed 6592 times

Figure 5 illustrates the adiabatic properties of a real heat exchanger. The straight lines represent the temperatures of the solid portion of the heat exchanger, and the curves are the gas temperatures of the respective spaces. The gas temperature fluctuations are caused by the effects of compression and expansion in the engine, together with non-ideal heat exchangers which have a limited rate of heat transfer. When the gas temperature deviates above and below the heat exchanger temperature, it causes thermodynamic losses known as "heat transfer losses" or "hysteresis losses". However, the heat exchangers still work well enough to allow the real cycle to be effective, even if the actual thermal efficiency of the overall system is only about half of the theoretical limit.

Heat goes from higher to lower temperature. The working fluid is hotter than the cooling space or sink, for a longer time, and at a greater delta-T than for when the working fluid is cooler than the sink. That means the sink will tend to heat up from the working fluid, not cool down. No length of stroke will change that. That was described in my previous description several of my posts back.

[Nobody]

The graph, if taken as a whole, refutes your conclusions, and your theory, and your experiments. So what gives?

Carnot's theories?
Your experiments?
Your theories?

Or our understanding of you experiments and Carnot's theories, or more scientifically modern kinetic theories? <<<< Ding ding ding. (Okay, that was rude!LOL) (So was that.) Our understanding being wrong here leads to our goal here, to learn.

The modern kinetic theory of thermodynamics and computer modeling all seem to be helpful in this learning endeavor.

I've chased your discourse around the threads here, and you've proposed several "new" thoughts that don't seem so helpful. I will list some of them here, with humor added:

> The baseball bat bashing back and forth theory.
> The homunculus-hill-car and pushing while getting tired and quitting theory.
> The child trampoline crankshaft theory.
> The disappearing heat into work theory.
> The magical engine growing stronger when load is added theory.
> The theory that the above all seem to be based on, however you claim it to be wrong, yet you erroneously clam modern, and ancient theories, are wrong because they are all based on it, (They are and are not.), Caloric Theory.

The reason Caloric Theory disappeared is that the Calorics needed to somehow disappear and become work/energy. Also friction, work input, made it necessary for Calorics to spontaneously appear, because Calorics didn't get depleted no matter how many times a cannon was bored. They weren't simply knocked loose by friction. They dismissed Caloric theory because Calorics weren't conserved. Instead there was a conservation of energy.

At the time, the other competing theory already conserved energy and didn't need an extra entity and predicted everything observed. So using, Occam's razor to shave off the extra entities of Calorics, baseball bats, homunculus, children, growing engines...etc..., We are left with the Kinetic Theory of Thermodynamics.

The Kinetic Theory is very helpful here. Please try to stick with it for any descriptions.

I think the insulation experiments you've been doing are explainable using only that theory. The extra experiments I suggested may help. If they comeback with bizarre and unpredictable results, it will strengthen your previous experiments. If they come back as expected, it will lead to a better understanding of engines, insulation and materials for us.

Here is to experimenting and learning! Thanks.

[Tom Booth]

...
Heat goes from higher to lower temperature. The working fluid is hotter than the cooling space or sink, for a longer time, and at a greater delta-T than for when the working fluid is cooler than the sink. That means the sink will tend to heat up from the working fluid, not cool down. No length of stroke will change that. That was described in my previous description several of my posts back.

Yes, well I read the Wikipedia article. It does not clarify if this "heat exchanger" (singular) refers to the regenerator, in which case it is all internal (heat exchange with the regenerator not the sink) and irrelevant.

But supposing the article really should say "heat exchangers" (plural) referring to the hot and cold plates, as I pointed out already, when the working gas is hot, logically, that is due to heat input and compression, or when the engine is at TDC - power stroke.

IMG_20211120_112027633.jpg (132.93 KiB) Viewed 6588 times

Which is the crank position when the displacer is in position to expose the hot plate and insulate the cold plate, isolating the cold heat exchanger from the hot-at-that-moment working gas.

Loosing heat to the cold heat exchanger at that crucial part of the cycle would rob the engine of power. Stirling engines are generally designed to avoid that and in general, most do, though perhaps not as effectively as they should.

Even if it were 100% true and accurate that in general, the sink plate tends to heat up from the hot air, due to the secondary heat of compression, in spite of the gas and plate being out of contact with each other making this physically impossible in many cases, that is no proof that such heat loss is necessary as a matter of thermodynamic "Law", or unavoidable.

This is just an example of how pushing that idea tends to stifle progress and innovation. Which has been holding back the progress and development.of Stirling engines for 200 years already.

[Nobody]

I got from the graph and article, the red lines are the heated space. The red curved line is the temperature of the working fluid in the heated space.

The blue lines are in the cooled space. The blue curved line is the temperature of the working fluid in the cooled space.

The ideal motion of the displacer is to be in contact with the hot heater plate for the entire time the piston compresses and in contact with the cold cooler plate for the entire time the piston expands. This is only diminished, not abolished, when harmonic motion is used. The indicator diagrams show that.

From Wikipedia Thermodynamic Cycle.

PV_real1.png (51.6 KiB) Viewed 6585 times

https://en.m.wikipedia.org/wiki/Thermodynamic_cycle

[Tom Booth]

....

The ideal motion of the displacer is to be in contact with the hot heater plate for the entire time the piston compresses and in contact with the cold cooler plate for the entire time the piston expands.

Hardly. Your opinion regarding the "ideal motion" of the displacer, observably, does not reflect how Stirling engines actually operate in the real world. Feel free, however, to provide some reference from a competent source.

Assuming that maximum compression corresponds with maximum heat, and the displacer is set 90° ahead of the power piston, measuring back on the graph from the crank angle representing maximum heat/compression puts the point at which the displacer moves to cover the cold side very near the point where the cold side gas temperature curve goes above the temperature of the cold side heat exchanger.

[Nobody]

That graph is for an Alpha.

Furthermore there is always working fluid in contact with both heater and cooler walls. Regardless of engine or positions. It is very small at times, some is always there.

When you produce an engine with no cold heat exchanger, your theory will gain credibility.

[Tom Booth]

That graph is for an Alpha.

Furthermore there is always working fluid in contact with both heater and cooler walls. Regardless of engine or positions. It is very small at times, some is always there.

A matter of design, not any "Law" and certainly debatable.

If I empty one 55 gallon drum into another, there may, infact be some residual, but for all practical purposes that "very small" amount is inconsequential, not the 80% or 99.9% of the "waste heat" called for according to the so-called "Carnot limit".