"Thermoacoustic" Stirling - theory of operation

[Tom Booth]

... Heat is absorbed by the working gas at the high pressure end and removed at the low pressure end

Heat only goes into and out of these engines through a delta T.

Delta T between the hot plate and inside gas.

Delta T between the inside gas and the cold plate.

.... room would have to be colder than the cold plate.

My point is, it would still be isothermal with those extra delta It's.

As I'm sure by now everyone is aware I have other ideas about where the heat ends up.

"Heat only goes ....out of these engines through a ....cold plate. ... room would have to be colder than the cold plate. "

I think it should at least be acknowledged that at least SOME heat is transformed into mechanical action or "work" or the engine would not be running at all.

That removing a load causes overheating by, what was it?

"when a load is taken off and the engine tends to overheat is isn't from lack of work output. It's from lack of heat input. The hot plate isn't getting cooled by imputing heat into the engine, however, heat into the hotplate continues. Hence the hot plate melts."

Based on:

"for all our engines-here, 250-500 rpm is no where near that limit. So, all expansions in the engines-here is 'with work'."

A load (shaft work) is externally applied. Extra work above and beyond what the engine needs to do to operate. Internal expansion work without a load doesn't really go anywhere, in terms of heat (or thermal energy) removal. It largely goes towards producing friction, so back into heat.

There is as you say, a lack of heat input, but that is due to a lack of internal cooling by conversion of heat to work, since that is the only variable that changed, other than an increase in speed, which would cause any cooling system to work better.

[Tom Booth]

I think they run on the train-chain theory. The nozzle forces the train to speed up promoting an inertial over evacuation until reversal and that then provides an inertial over pressure. Heat is absorbed by the working gas at the high pressure end and removed at the low pressure end

Heat only goes into and out of these engines through a delta T.

Delta T between the hot plate and inside gas.

Delta T between the inside gas and the cold plate.

If the delta T stays constant during the expansion or compression it would be isothermal.

Isothermal but colder than the hot plate.

Hotter than the cold plate.

The faster the expansion the larger the delta T would be. The flame temperature would also need to be hotter than the hot plate. And room would have to be colder than the cold plate.

My point is, it would still be isothermal with those extra delta It's.

I think I can agree with the bolded portion or the passage. The rest is addressing a point about "isothermal" I see as irrelevant on the basis that in a REAL engine isothermal expansion and contraction is mostly just plain impossible because isothermal heat transfer is a SLOW process and REAL engines just run too fast. But, setting aside the Isothermal question, I want to return to the "train-chain" concept.

First of all, I don't think this can be taken literally because, well, are individual gas molecules linked together? Personally I might argue that they are, by forces of mutual attraction and repulsion. The Ideal gas law does not recognize or take into account such forces and neither does the kinetic theory of heat in a gas, but science generally does recognize that these forces of molecular attraction and repulsion do infact exist and do, under "extreme" conditions, have an influence. Though, unfortunately, what constitutes "extreme" is not well defined. Conditions of high pressure and low temperature are often mentioned, but again, rather vague. What it seems to come down to though is ANY PRESSURE above 1atm and ANY temperature below ambient will show deviations from the ideal gas law.

But, in reality, the forces of attraction and repulsion between gas molecules always exists and always has an influence, theoretically, but setting that aside, let's assume or visualize this "train-chain" of gas molecules passing back and forth through the orifice in a (I'm just going to call it Lamina Flow) Lamina Flow engine.

To reiterate:

The nozzle forces the train to speed up promoting an inertial over evacuation until reversal and that then provides an inertial over pressure.

Trying to imagine this...

Well, for a while there, I thought it made sense, but now that I try to actually imagine it I just end up with most of the links of the chain piled up in the bottom of the power cylinder.

The "links" would need to be more like rubber bands.

If the piston moves out a lot of molecules are drawn through the orifice at high speed.

As the piston returns the chain continues to "pull" more molecules through the orifice into the power cylinder.

With the heating chamber continually receiving heat, it is difficult to imagine the "chain" withdrawing back through the orifice and back into the heating chamber. The "rubber" links still want to pile up in the bottom of the power cylinder.

You wrote: "...over evacuation until reversal and that then provides an inertial over pressure".

This "over pressure" I would then equate with the pile-up I'm imagining at the bottom of the power cylinder.

What I imagine ending up with is something like this:

Resize_20231227_123350_0802.jpg (90.22 KiB) Viewed 7956 times

The next statement is:"Heat is absorbed by the working gas at the high pressure end and removed at the low pressure end".

I have some issues there.

If the situation is something like what I've illustrated, the "low pressure end" is to the far right (blue dots) and the "high pressure end" is the pile-up in the far left of the power cylinder (red dots).

I'm not at all sure if this is anything like what you had in mind.

Assuming any of this is at all representative of reality, the main problem I have is with these assumptions:

1. Heat is absorbed by the working gas at the high pressure end

2. Heat is ... removed at the low pressure end

What I see is more a heat pump.

The working fluid is expanded and cooled (right side-blue) so it is able to take in heat from the surroundings.

The working fluid is "superheated" where the gas is at a high pressure (to the left-red) so would tend to give off heat to its surroundings.

Of course, I think we all are aware that pressure tends to remain uniform throughout an enclosed container at all times. But, I think, the cumulative effect of several cycles could kinda, sorta result in some such situation.

Trying to accurately visualize what might be going on in an engine cylinder at say 1000 RPM is admittedly exceedingly difficult.

What I do see is that the piston tends to oscillate from a central location in the cylinder, with compression happening to the left and expansion to the right.

I'll post an image or video to illustrate what I mean.

[Tom Booth]

This video of mine is about as good as any other I can find and provides a good close up view of the piston oscillation side to side (or up and down) from a central point


https://youtu.be/RODm1LCqwuk?si=Bl728NClavTCnFKb


The piston is moving so fast it is just a blur, but there is a dark area of overlap in the center between full compression and full expansion.

When heat is added, the oscillation increases and there is less overlap so the gray area appears thinner, and if the heat input is great enough, disappears entirely. (In this engine the cylinder is not long enough to show this so the piston just hits the ends of the cylinder, but is something I have seen)

When heat input is discontinued, the oscillation gradually decreased and the dark overlap area gets thicker and thicker, until the oscillation stops and the piston becomes motionless over that exact central point.

[Tom Booth]

There is an interesting passage in the paper "UNDERSTANDING STIRLING ENGINES" by William Beale (sometime prior to 1984)

You might now wonder how the movements of the piston and the displacer are accomplished, since they clearly cannot move on their own. The answer is that there are at least two ways to make the two components of the simple Stirling engine move as
we wish: (1) we can attach them to cranks through connecting rods, as is commonly done in automobile engines; or (2) we can use gas forces in a carefully designed way so that they bounce on gas springs, with the displacer always ahead of the piston in its in-and-out oscillation. Of the two methods, the use of cranks called the crank-drive, or kinematic Stirling, is the more easily understood method. The second method, which uses oscillating motions of the piston and displacer on springs, is called the free-piston Stirling. The crank-drive Stirling is easier to understand yet harder. to make, while the free-piston Stirling' is harder to understand yet easier to make in at least some of its forms.

I would argue that ALL types of Stirling engines "bounce on gas springs". Evidenced by what happens when the crankshaft and flywheel of an otherwise apparent "kinematic Stirling" are removed.


https://youtu.be/HUWt3YrxoB4?si=dRsGOkAUWshFaDMW


Generally speaking, with or without a crankshaft and flywheel, the engine will continue to run in the same way.

Now of course, there are numerous factors involved, such as the weight of the piston, size of the flywheel, cylinder bore, quantity of heat applied, load on the engine, etc. etc. that come into play so that some engines may run differently with a flywheel than without, or may not make the transition as easily as the engine in the above video, but I think, in principle, even an LTD with a comparatively large displacer works in such a way that the displacer lifts to provide heat and expansion just as the piston approaches TDC so as to achieve a "bounce" on this expanding "air spring" at just the right moment.

[Tom Booth]

In a flash there I thought maybe that forced expansion could be used in stead of water cooling. Whether you're powering a pump or using that work to "overstretch" the fluid to cool it further. I wonder which would be more effective and efficient.

Thats the ticket Jack. We are lucky to live in a world that is very close to zero pressure. There is no realistic limit to high pressure, but there is a realistic floor that can be used to our advantage(cooling). Once positive pressure(power) is high enough over atmospheric, it becomes beneficial to use the over expansion(low pressure) for cooling. Pressure and temperature can continue to rise to incredible numbers during compression(think fire piston, lightning, nuclear, etc.), while during expansion we are already close to 0, and so trading some power for cooling should become not only practical but greatly beneficial to net power.

I had a thought regarding this idea of "forced cooling" which I've previously experimented with, with some apparent success, by extending the throw of the piston more than normal on a stock LTD engine.

How is, or how might that be possible?

What came to mind just recently is; think about the traffic on a highway

All the cars and trucks are moving along at the same steady pace.

Let's say this is kind of like gas molecules "driving" down a cylinder.

But instead of a clear highway, there is a big box truck in the road.

The truck is moving slower than the smaller cars.

What happens when a small car rear ends the truck?

If the two vehicles, car and truck are both traveling along in the same direction and the car is going 75 mph and the truck is only doing 60, it would be the same as if the car ran into the truck at 15 mph and the truck was at a standstill

If gas molecules are, on average, traveling at the speed of sound, I'd say that no matter how fast the piston might move it is never going to outrun all the hot gas molecules and no matter how FAR the piston travels down the cylinder the expanding gas will ALWYS, for all practical purposes, still transfer energy to the much slower moving piston no matter how cold the working fluid gets.

That is, by extending the throw of the piston, by whatever means, increasing it's weight so it has more momentum or attaching the crank further out from the center of the flywheel so it travels further down the cylinder, eventually the temperature of the working fluid will drop below the ambient temperature.

The question is, would this be of any real benefit?

Obviously it would take additional heat and work to establish this "refrigerating" effect.

Ambient heat would move in and destroy the acquired increase in ∆T. The additional work input would be wasted.

But if the "sink" or cold side is insulated, so ambient heat can't get in to undo the gains, maybe there could be some advantage.

But if a hot air engine is an oscillator between TWO sources of energy input, going extremely cold may not have the desired effect.

I think the whole idea of heat "falling" through a temperature differential is completely and utterly wrong.

[Tom Booth]

I mean, think about it,

While I don't think kinetic theory of heat is 100% correct for various reasons, like it doesn't account for attraction, repulsion or volume of gas molecules, not to mention quantum physics and who knows what's really going on, lets just say if it is mostly right and gas molecules are basically like billiard balls randomly exchanging kinetic energy by localized.collissions (rather than some force or compulsion to "flow" from hot to cold), do "hot" billiard balls collide, or actively seek out collisions with. "colder" billiard balls? Or does the relative temperature of billiard balls have anything whatsoever with what other billiard balls they collide with or what direction they go in?

OK I've been told that most people don't take the idea of the "flow" of heat from hot to cold literally.

On the contrary, in my experience most people do, and some, especially in thermodynamics consider it gospel. To think otherwise is a kind of heresy. The "second LAW" is not just a theory, it's akin to the word of God. A LAW of the universe! Complete with superstitions regarding the fate of those who dare attempt to violate it.


Anyway, I thought this was interesting:

https://youtu.be/djyA_8Qv7pU?si=oDKt_jjVj3fWWYxp

[Tom Booth]

In this video, starting at 4:00 in .25x speed, it's pretty easy to see the movement of the hot bulb as it slips on the O-rings. The high pressure pushing the bulb out corresponds to the piston approaching TDC, and the low pressure pulling the bulb back corresponds to the piston moving away from TDC. In other words, the opposite of what you would want in a power producing engine.

So it could be that these engines are nothing more than a self-sustaining air spring, or they need many variables tuned to really come alive.

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

You are incredibly observant. I did not notice that movement of the test tube. I'm not sure I agree that it indicates "the opposite of what you would want in a power producing engine".

I mean obviously, you probably don't want an engine that is loose at the seams, but personally I think what you do want is a build up of pressure towards TDC.

I think part of the apparent "pull back" during expansion might just be due to the rubber O rings kind of rolling and stretching from the high pressure at TDC then, because the rubber itself was stretched, it tends to pull the test tube back when the pressure diminishes during expansion.

I was thinking about the "dancing styrofoam balls" in the video mentioned by Matt Brown early in the thread. While we are doing slow motion.

I figure in a hot test tube styrofoam is out of the question, but a heat proof near equivalent might be some fine Perlite particles.

Smoke is difficult to see clearly and tends to dissipate and I have not been able to get it into a running engine where it can be seen clearly. That one video I did get of some smoke was due to the wooden "venturi" burning up and the smoke and resins from the wood gummed up the cylinder and seized up the piston, then while trying to see what was wrong I noticed the streams of smoke when I manually moved the piston, but perhaps that is not definitive as it was not a running engine.

Perlite also, will not dissipate, so should be much easier to see. It might jam up the piston if it gets into the power cylinder, but maybe not. If it does though, it might be possible to put a screen in the orifice, if that doesn't interrupt the air flow too much.

[Tom Booth]

I think it is very valuable, in terms of understanding how the Lamina Flow engine works and how Stirling engines in general work, to study the Vuilleumier patent.

https://patents.google.com/patent/US1275507A/en

This is an illustration from the Vuilleumier patent presented to illustrate the principles of the heat driven heat pump in the simplest possible way:

Resize_20240104_010138_8318.jpg (40.33 KiB) Viewed 7333 times

Is the resemblance to a "thermoacoustic' (lamina flow/thermal lag) engine merely superficial?

It is unequivocally stated here:


https://www.stirlingengines.org.uk/thermo/lamina.html

The schematic drawing, below, taken from a US patent granted in 1984 is for a heat pump...

Resize_20240104_011917_7610.jpg (54.88 KiB) Viewed 7333 times

After a lot of experimentation I found the solution to turning this into a motor was simply a matter of applying heat to the end of the regenerator stack nearest the piston.

Really, to speculate that a Thermal Lag, and perhaps by extension, external combustion hot air engines in general MIGHT possibly display some slight heat pumping or refrigerating effect under certain circumstances is "pseudoscience"?

I think it would be derelict to not at least take a moment to consider the possibility.

I've gone a little further than just theorizing. I've done a few amateur level experiments that indicate a potential for some heat pump - like performance from these engines, in my own estimation.

That so many are openly hostile towards or overtly dismissive of or actually rage against with a barrage of ad hominem against a person merely researching the possibility of there being a connection that may be more than a superficial resemblance puzzles me.

I don't think I'm exaggerating when I say that the attitude of staunch advocates of the Second Law of thermodynamics, resembles that of religious zealots as well as a cult of personality.

[MikeB]

Smoke is difficult to see clearly...

Have you thought about using a coloured gas, such as NO2 ? Bit toxic apparently, but not a problem in a sealed engine.

[Tom Booth]

Smoke is difficult to see clearly...

Have you thought about using a coloured gas, such as NO2 ? Bit toxic apparently, but not a problem in a sealed engine.

I think I might have some iodine in the medicine cabinet.


https://youtu.be/RMwhrrYLzBU?si=oQMHKtzt1codM676