Stirlingengineforum.com

So there is no such thing as a vacuum and those space walks can be done without a space suit! <<< Rhetorical point so it doesn't need a question mark. Not intended to be answered.

It is common practice to speak of Differential Pressure as positive and negative.

http://avstop.com/ac/flighttrainghandbo ... tmospheric.)

(Negative pressure is any pressure less than atmospheric, and positive pressure is pressure greater than atmospheric.)

Sorry I tire of you two.

Technically still pressure, just a negative direction from ambient. One thing to note: If an engine depends on the area of negative pressure to run, the maximum pressure difference is small and it will have very low power to size and weight. Vacuum and atmospheric engine's both are very low power producers, unless very large. If you desire high power to weight or size ratios, forget negative pressure engines. If you want table top toys, that may fill that nich.

I know it sucks but! We live in an ambient environment. If it gets too low we die! To me that is a big negative.

airpower wrote: ↑Wed Mar 16, 2022 12:37 am

Tom Booth wrote: ↑Tue Mar 15, 2022 6:18 am ............
Likewise, as with "negative" and "positive" terminals on a battery or electrical supply "negative" does not literally mean an absence of electricity.

"Negative" is a common English word, the general meaning of which is "less than": ".Of or relating to a quantity, number, angle, velocity, or direction in a sense opposite to another of the same magnitude indicated or understood to be positive." https://www.thefreedictionary.com/negative

This is accepted usage in the English language, especially in scientific/engineering terms

........

When you have more Electrons than Protons you have a negative number.


Less than zero


Modern Science is doing space walks at pressures of very very roughly about
0.00000000000000000000088725345303 Pa
(still not negative pressure)

I'm sorry if you are unfamiliar with common English words or engineering terminology. I don't know if you are under educated or "over educated" but "negative pressure" is a common and generally well understood phrase. Pressure gauges are manufactured with negative readings on them by equipment manufacturers all over the world.

Most people have a "negative pressure" machine in their own homes (called a vacuum cleaner). Your lungs create negative pressure when you breath.

The term “negative pressure” is often used in engineering to refer to a situation in which an enclosed volume has lower pressure than its surroundings.

https://chemistry.stackexchange.com/que ... e-pressure

Negative pressure may refer to:

Negative value of a pressure variable

https://en.m.wikipedia.org/wiki/Negative_pressure
The computer generated PV diagram YOU presented here of a Stirling Heat pump/Cryocooler shows negative pressure readings.
Unfortunately this has caused confusion for you apparently. Refrigeration systems often operate at a vacuum, systems have to be purged, so vacuum pumps have gauges with negative (below atmosphere) pressure readings.

So is there some relevance to this derail or are you just on some campaign to re-educate the world, recall all the compound pressure gauges in manufature as "wrong", rewrite all the textbooks, rewrite computer programs redefine definitions in language etc

Addendum: Nice to see you talking some sense for a change "nobody"

But a Stirling engine does not "depend on" negative pressure for it's work output. Heat/expansion does most of the work.

Unlike IC engines however, completing the cycle in a Stirling engine does not require stored energy in a flywheel.

A Stirling engine is not limited in power as a result of this obvious advantage.

Nobody wrote: ↑Wed Mar 16, 2022 6:17 am . One thing to note: If an engine depends on the area of negative pressure to run, the maximum pressure difference is small and it will have very low power to size and weight. Vacuum and atmospheric engine's both are very low power producers, unless very large. If you desire high power to weight or size ratios, forget negative pressure engines. If you want table top toys, that may fill that nich.

Actually, the energy available from "negative pressure" via an "atmospheric engine" is no joke either:


https://youtu.be/JsoE4F2Pb20

https://youtu.be/Uy-SN5j1ogk


I'm not sure why so many people come to this forum who spend most of their time ridiculing, belittling and generally spreading discouragement about the technology.


https://youtu.be/Zz95_VvTxZM

This is quite interesting and seems to relate to what I have been attempting to relate.

https://www.quora.com/Why-is-heat-trans ... ic-process


I was lead into researching what exactly constitutes a "polytropic process" due to reading a paper on Stirling engine mathematical (computer) modeling, where the article stated that a mathematical model based on "polytropic processes" came closest as far as being able to accurately predict actual physical engine performance.

So what is "negative heat transfer" in a polytropic process?

Apparently, it is exactly what I have been observing and attempting to describe or relate, but using the term "adiabatic".

Adiabatic expansion (and cooling) alone, however, does not include cooling due to WORK output, or the conversion of heat into work, which was the actual basis of my argument: Adiabatic expansion/cooling PLUS the conversion of heat into work could have the effect of COOLING the working fluid BELOW ambient/atmospheric temperature, resulting in a pressure drop below atmospheric pressure, so that the piston is pushed back during the compression stroke without the aid of any energy stored in a flywheel.

So NEGATIVE heat transfer in a polytropic process is, apparently where, although heat is added to the system, the net result is cooling, due to work output.

In other words, heat is added, the gas expands and does work and the gas cools. But how can there be MORE work output than the heat added?

According to the response in the Quora question:

the work that is obtained is not only due to the heat supplied by us but also from the internal energy of the system that's why we are getting more work than heat supplied.

In other words, the "working fluid" contained, or sealed inside the engine is already at something like 300° Kelvin before we even begin to add any additional heat. So there is actually all of that "internal energy" available.

Remember, a 100% efficient engine would result in a cold side temperature at absolute zero. A "perfect" heat engine would be able to utilize ALL the heat.

Wait a minute, hold on, that's impossible... Right?

Well yes, it would be, likely impossible to use ALL the heat, including all the "internal" latent heat and end up at absolute zero, but, apparently utilizing SOME of that stored internal energy is NOT impossible

Think of all that "latent" or pre-existing "internal" heat (kinetic energy) as energy ALREADY stored in a flywheel.

We give the flywheel a little extra push, then slow it down by extracting a little MORE energy than we put into it. That is certainly possible, RIGHT? The flywheel, after all, already had an enormous amount of stored energy before we started, but, of course, if we kept this up, the stored energy would eventually be used up and the flywheel would slow down, slower and slower, until it stopped.

But what if there were some outside mechanism maintaining the flywheel speed?

If we take a little more energy out than we put in, then the difference would or could be made up by this "outside mechanism".

In a Stirling engine, IMO, or based on my observations and analysis, that "outside mechanism" is the ambient/atmospheric heat and pressure. (or kinetic energy in the ambient air).

When a little "extra" work is extracted from the working fluid "flywheel", it is restored by the ambient surroundings which define the baseline energy state of the system.

It is something like dropping a buoyant object into water. It doesn't just hit the surface of the water and stop, it goes deeper than the surface, then bob's back up, and oscillates up and down for a while until all the energy is dissipated, but the water level defines a kind of "baseline" that will always be returned to.

In a similar way, when "extra" energy is extracted, and the heat/pressure level in the engine dips below outside atmospheric temperature/pressure (kinetic energy level) It is immediately restored to correct the imbalance, but that results in an overcorrection, which sets up an oscillation.

The remarkable thing about extracting energy (heat/kinetic motion) from a gas that makes the process a little different from gravity or buoyancy is that in an expanding gas, taking out WORK is equivalent to taking out HEAT and results in a drop in temperature, so... The more work we can extract the lower the resulting temperature and the more heat/energy can then be recovered from the surrounding ambient to make up the difference.

In effect, after heat is added to expand the gas to push the piston to do work, and that heat is entirely converted into work in one stroke, the engine is then put in a state where it is acting like a "flame licker" or vacuum engine powered by atmospheric heat/pressure kinetic-solar energy.

In other words a Stirling heat engine is 1/2 powered by added heat that we supply and 1/2 powered by solar energy. So in a sense, as far as the small amount of heat we.need to add to set the engine in motion, that heat is being utilized at 200%

That is, the heat we add is recovered as work, which results in COOLING, but then the heat supplied by the atmosphere to restore balance is also recovered as work.

So the heat we add, is utilized in such a way as to produce cooling, and the resulting cold makes it possible to momentarily draw upon some of the ambient energy store.

But wait a minute, isn't that supposed to be impossible?

Personally I'm not all that concerned about what was considered possible or impossible 200 years ago when it was first asserted without any real evidence that a heat engine cannot utilize the heat of the ambient surroundings. I've just been trying to understand and discover how these engines actually operate and it seems abundantly clear to me that the piston is returned by atmospheric pressure, not the added heat.

However, this is not "perpetual motion". In order to temporarily utilize that surrounding ambient energy, SOME energy must be added each cycle, or every 1/2 cycle to produce the cooling that makes that momentary ambient heat extraction possible.

Anyway, this so-called "negative heat transfer" during a polytropic process, where the WORK output is greater than the heat input, due to drawing on "internal energy", is apparently a thing.

This is the same, or very simar to how gases are liquified in an expansion engine or expansion turbine.

The gas is first compressed and the heat of compression removed. (That is, the compressed gas is allowed to cool). The gas is then released adiabatically through an expansion engine or turbine. It should be noted that an "expansion" engine or turbine is fundamentally no different than a normal compressed air driven reciprocating engine or turbine. They are powered by the compressed gas expanding through them, except that these engines or turbines are kept cold and insulated so that the expansion is adiabatic. That is, the gas expands, doing work to drive the turbine, but cannot take in any heat, so the turbine or engine is being driven by the "internal" or latent heat in the compressed gas, so that as a result of this work output, (heat loss, as in a gas heat and work are equivalent) the gas looses it's internal energy and cools rapidly to the point where it condenses into a liquid.

Everything on this planet has been pre-heated by the sun to some 300°kelvin (+ or - a few degrees) so there is an enormous amount of latent "internal" energy in the air around and about, long before we add a relatively small amount to our hot air engine, and there is a long way to go, that is, there is a lot of internal/latent heat in any quantity of air or gas, that would need to be utilized before we could ever use "all" of the heat, down to absolute zero.

This is the thread to post it on so I will post here instead:

1. If you heat a gas at constant volume, you get a change in pressure.
2. If you heat a gas at constant pressure, you get a change in volume.
3. The opposite of 1 and 2 are also true

Hot air engines work on this basis:

At bottom dead center, the internal gas is heated, causing a change in pressure. Only when the change in pressure overcomes static friction, the piston begins to move, allowing the pressure to drop while the volume changes.

Remember points 1 and 2, at top dead center, the pressure is lower than bdc, the volume of contained air is higher than tdc and the temperature of the air, in an ideal system, is whatever is needed to be at that pressure and volume for that mass of air considering atmospheric pressure.

The energy needed to reach TDC can be defined by the amount of energy required to reach that temperature plus the amount of energy converted to mechanical energy.

The efficiency of reaching TDC is mechanical energy ÷ total heat input × 100%.

You can never escape the fact that changing the pressure and/or volume of a constant mass of air requires a change in the temperature of the mass of air.

Now to reach BDC, the mass of air must be cooled as compressing it without cooling it would make it work like an air spring.

When it cools, the internal pressure decreases until it overcome friction, then atmosphere returns the piston to BDC.

So a hot air engine actually pulls power from the heated air twice in a single rotation of the crankshaft.

It draws power from internal pressure on the upstroke and power from depressurization on the downstroke.

But in one way or another: the gas must change temperature because it is of constant mass.

Now, if you could recover the energy from the cooling process to help the the next cycle heat, then okay, you could reach 100% efficiency, but that's the million dollar trick

That's my trick. I have 100% thermal efficiency, I know how to reach absolute ~0°K and get energy back doing it.

But you will never accomplish it with a heat engine all by itself, you need a system that captures and cycles heat.

All hot air engines operate on a heat cycle.

If you do not cycle the heat, you do not operate.

I'm not sharing my system, I haven't made money with it yet and I'm feeling greedy.

But I gave you enough hints to work on your own ideas. You need a system, that recapture and uses the expelled heat for the next heat cycle in such a way that it does not interfere with the cooling cycle in a negative way. This is what regenerator is trying to accomplish but can never succeed at.

Daemon wrote: ↑Fri Nov 04, 2022 2:50 pm . . Only when the change in pressure overcomes static friction, the piston begins to move, allowing the pressure to drop while the volume changes.

(...)

You can never escape the fact that changing the pressure and/or volume of a constant mass of air requires a change in the temperature of the mass of air.

Now to reach BDC, the mass of air must be cooled...

When it cools, the internal pressure decreases until it overcome friction, then atmosphere returns the piston to BDC.

I more or less thought the same thing 12+ years ago. But could not really explain how an engine could run at say, 3 or 4 thousand RPM via such an inherently slow heating and cooling process.

Heat up the gas, wait for it to expand, then cool it down and wait until it gets cold enough to contract.

Logically, with such a gradual heating and cooling process, would increasing the temperature of the hot side, (for exame, with MAP gas, as in the following video) not require additional time for cooling each cycle?

https://youtu.be/dtuvNYMS84I

The cognitive dissonance between theory and observation over the years led me to consider the possibility that there was something more going on than simple heating/expansion followed by cooling/contraction.

Tom Booth wrote: ↑Mon Nov 07, 2022 9:09 am.
Logically, with such a gradual heating and cooling process, would increasing the temperature of the hot side, (for exame, with MAP gas, as in the following video) not require additional time for cooling each cycle?.

No, the reason is because the rate at which heat energy will transfer from one material to another material has an exponential relationship with the difference in temperature between the materials.

For example, ice water will cool a mass to 10° C much faster than water that is 10° C.

When temperatures are almost equal, the rate of heat transfer is really slow.

It's one of the reasons temperature is so hard to control exactly in industry, that and temperature control is laggy.

So when you increase the hot side temperature you increase the rate of heating and the rate of expansion exponentially on the hot side, but now the air is hotter, so the rate of cooling on the cold side also increases.

This phenomenon is precisely why a hot air engine becomes more thermally efficient with larger temperature differentials.

You can do an experiment at home to confirm what I am telling you.

Go to the hardware store and buy an infrared thermometer gun, then buy a large chunk of metal.

Heat the metal to a fairly high temperature.

Take temperature measurements at predefined time intervals.

Plot the cooling curve.

You will see that the cooler the metal gets, the longer it takes to cool the same amount of degrees as when it was hotter.

In fact, you will find that over 2/3rds of the time to cool to ambient temperature will be at less than half of maximum temperature.

Daemon wrote: ↑Wed Nov 09, 2022 12:54 am
(...)

So when you increase the hot side temperature you increase the rate of heating and the rate of expansion exponentially on the hot side, but now the air is hotter, so the rate of cooling on the cold side also increases.

(...)

You will see that the cooler the metal gets, the longer it takes to cool the same amount of degrees as when it was hotter.

This is the issue I have with the "standard" thermodynamic model (or "Ideal" Carnot/Stirling cycle)

To quote a random but typical explanation:

https://homework.study.com/explanation/ ... engin.html

Do you see the problem? You should, as you have already stated it in a nutshell.

We are supposed to imagine heat transfer taking place, both heating and cooling "isothermally".

Even if given an "infinite" amount of time, it is clearly impossible for there to be heat transfer between a gas and a source and/or sink at the same temperature, along an isotherm.

Well, so I'm told by the experts in this supposed "science" there is a little compression, which results in an infinitesimal rise in temperature, so then a tiny heat transfer can take place, and then a tiny bit more compression and another infinitesimal rise in temperature, etc. etc.

So given an infinite amount of time it is then supposed to be possible to carry out these impossible isothermal processes.

https://youtu.be/s3N_QJVucF8

So, is the gas REALLY "hotter" with or after adiabatic expansion?

At this point, I think there is a better word to describe what is actually going on; "polytropic", maybe?

At any rate, whatever the technical term might be, there is no such thing as a "quasistatic" expansion that takes "infinite time" to transfer heat from a hot "reservoir" to an already hot gas, in any real engine running at any speed whatsoever.

What we actually have is rapid, near adiabatic expansion WITH WORK OUTPUT. That is, the transfer of the molecular kinetic energy of the gas to effect the kinetic motion of the engine itself or "WORK"

Heat and work are equivalent, so when the gas does work on the piston the temperature of the gas drops. The temperature does not drop due to some infinitely slow process of transferring heat from a cold gas to a cold "reservoir" during isothermal compression.

At least, certainly, not in an engine running at 3000 RPM.

Or explain to me how we can have two "quasistatic" or inherently "infinitely slow" processes taking place in approximately 1/50th of a second. Or an "isothermal" expansion with heat transfer from a hot source into an equally hot compressed gas in 1/100th of a second, or heat "rejection" from a cold expanded gas to an equally cold "sink" in the same time frame of 1/100th of a second.

Tom Booth wrote: ↑Wed Nov 09, 2022 4:46 am
This is the issue I have with the "standard" thermodynamic model (or "Ideal" Carnot/Stirling cycle)

To quote a random but typical explanation:

Resize_20221109_051935_5021.jpg


https://homework.study.com/explanation/ ... engin.html

Do you see the problem? You should, as you have already stated it in a nutshell.

Yeah, so the "experts" are wrong about the temperature and pressure changes in the chamber.

Who cares, the temperature and pressure are absolutely cycling, guaranteed in the engine.

Every time I improve my heat sinks and better insulate my hot side from my cold side, my engine runs faster and harder.

The faster I heat the gas and cool it down the faster the engine runs.

The expansion is not adiabatic, it's incredibly uneven, fast and with rapid pressure changes.

Hell, my thermoacoustic steam engine even has explosions in the chamber on the upstroke. (When I get home I'll post the slow mo, literal explosions)

Daemon wrote: ↑Fri Nov 11, 2022 8:20 am

Tom Booth wrote: ↑Wed Nov 09, 2022 4:46 am
This is the issue I have with the "standard" thermodynamic model (or "Ideal" Carnot/Stirling cycle)

To quote a random but typical explanation:

Resize_20221109_051935_5021.jpg


https://homework.study.com/explanation/ ... engin.html

Do you see the problem? You should, as you have already stated it in a nutshell.

Yeah, so the "experts" are wrong about the temperature and pressure changes in the chamber.

Who cares, the temperature and pressure are absolutely cycling, guaranteed in the engine.

Every time I improve my heat sinks and better insulate my hot side from my cold side, my engine runs faster and harder.

The faster I heat the gas and cool it down the faster the engine runs.

The expansion is not adiabatic, it's incredibly uneven, fast and with rapid pressure changes.

Hell, my thermoacoustic steam engine even has explosions in the chamber on the upstroke. (When I get home I'll post the slow mo, literal explosions)

I certainly agree, 100% that to "better insulate my hot side from my cold side" will result in better performance and more power output, absolutely, because this reduces conductive heat transfer through the rigid engine body, which non-effective heat transfer, reduces effective, work producing transfer of heat into the working fluid.

And yes, temperature and pressure are most definitely cycling. Bouncing rapidly back and forth between hot/high pressure and cold/low pressure. No question about that, if the engine is running at all.

But a "fast" transition from hot/expansion to cold/contraction tends to rule out heat being transfered in or out of the working fluid by ordinary conduction or HEAT transfer, alone, or even primarily.

Yes, heat does conduct more quickly given a greater temperature difference, just, IMO, not enough more quickly to account for the apparent instantaneous drop in temperature seen in a running engine. Heat conduction, especially between a gas and a solid is inherently very slow, like the very gradual expansion seen before the engine actually runs. The piston very gradually creeps further and further out as heat is applied and the air in the engine gradually expands. That is heat being conducted from the heat source into the gas at top speed.

What tends to be ignored or overlooked is the heating and cooling produced as a result of "WORK". Or the conversion of heat into work, and work into heat.

It is absolutely possible for a compressed hot gas to expand and do WORK and cool back down instantaneously. That is how many gases are liquefied on an industrial scale.

Obviously, however, doing something like putting ice on a Stirling engine makes it run faster. The question though is why?

The traditional, established, standard explanation is that the ice "sinks" or absorbs more heat more quickly.

However, I think we can probably agree that increasing the heat on the hot side will have the same effect.

But if I heat up a "free piston" thermal lag type engine with no flywheel, the piston does not fly out of the cylinder due to increased expansion. It just runs faster, which makes cooling by a slow process of conducting heat to a sink even more unlikely.

The measured temperature of the "sink" does not necessarily increase as it should if more heat is being transfered into it. That is, there is not necessarily more heat transfer. There is however, a quite obvious and measurable increase in mechanical motion. The engine runs faster and produces more WORK output.

To illustrate how this might be possible; I think of it as something like two people pushing a cart back and forth between them, with a generator attached to the wheels.

When one person pushes the cart it moves and generates power as it rolls along on the way over to the other person. There is work output until it reaches the other person, who then can push the cart back.

Increasing the temperature difference is akin to the two people moving further apart.

With the two people further apart, the cart can then roll further and there is more freedom for each person to shove the cart further and the cart to fully convert all of that effort into power output, without the effort being nullified by the other person immediately shoving back.

However close or far, there is not any transfer of energy from the one person doing the shoving of the cart and the other person.

If they are very close, they are both just leaning on the cart and nothing happens.

Applying the illustration to a Stirling engine, the one "person" doing the shoving is the energy input that results from the heat being added expanding the gas.

The other person doing the shoving back is atmospheric heat and pressure.

In between, the wheels are generating power output.

I think the preponderance of evidence indicates that atmospheric pressure pushes the piston back after the gas has done enough WORK to cool down enough.

I think that if atmosphere (the actual supposed "sink") were absorbing energy and cooling the gas, taking the energy of the heating and expansion away, then the piston would only travel outward until it transfered enough energy to the outer atmosphere to reestablish a balance and then it would simply stop, just as happens when first heating the engine, before it starts running.

So, a "better sink" does not necessarily mean more heat being transfered from the hot side to the cold side and out to the surrounding atmosphere.

The apparent "better sink" may just mean that more freedom of movement has been established resulting in more opportunity for work output.

I may be wrong, I suppose, but I've more or less been forced into arriving at this conclusion as a consequence of a lot of hours spent making observations and experimenting and straining my brain to the limit trying to figure out what in the world is actually going on inside one of these engines.

Anyway, I do, most certainly look forward to your slow motion video. Thanks very much for doing that.

Tom Booth wrote: ↑Fri Nov 11, 2022 11:33 am
The traditional, established, standard explanation is that the ice "sinks" or absorbs more heat more quickly.

However, I think we can probably agree that increasing the heat on the hot side will have the same effect.

But if I heat up a "free piston" thermal lag type engine with no flywheel, the piston does not fly out of the cylinder due to increased expansion. It just runs faster, which makes cooling by a slow process of conducting heat to a sink even more unlikely.

I really question your statement about the cold side not getting hotter with increased heat input, it's the opposite of what I've experienced in my experiments with heat engines.

Have you personally taken measurement on a running model or are we talking theory? I tend to trust my own physical measurements over theory if we're talking theory.

Pretty much every free piston, thermal lag and thermoacoustic engine I've seen either had a spring, rubber or crankshaft-flywheel assembly to retain it's power piston.

As far as I understand, the power piston is much lighter than the displacement chamber, if there's nothing to hold back the power piston a bit, how does pressure build or drop to move the displacement chamber?

Daemon wrote: ↑Fri Nov 11, 2022 11:05 pm
I really question your statement about the cold side not getting hotter with increased heat input, it's the opposite of what I've experienced in my experiments with heat engines.

Yes, I should always qualify such statements. With some Stirling engines, "properly" (whatever that might mean) "load balanced". That is; not more heat applied than the engine can actually use, with the "right" throw, (connecting rod length & off center from the flywheel), timing (advance angle between power piston and displacer) compression ratio, and possibly most importantly the material the engine is actually constructed out of, that is, the heat conductivity of the engine body.

People have asked in here, "why won't my engine run", and post a picture. The whole thing is made out of copper pipe.

When I get a new engine, the first thing I do is take off all the metal parts that could possibly conduct heat through from the hot to the cold side.

Well, just now, in searching for my own video, I came across this relatively recent video from Kontax.

https://youtu.be/JGYr7kAERFg

That's a better explanation than I gave, which was no explanation, but just showing what I had done.

https://youtu.be/HQT5JviF-qk

That hand full of skinny little metal bolts may seem inconsequential, but it you think of heat as something like electricity, each of those bolts is short circuiting heat.

Conductors of electricity or heat require insulation to be effective. The air inside the engine is perhaps the worst possible conductor of heat you could find. The heat needs to be forced to travel through this "working fluid" ONLY. If there is any other better conductor available, the heat will choose that path instead and so inevitably the cold side will heat up due to receiving heat through these conductors without having done any WORK (in the thermodynamics sense) to get there. When heat is converted to work it is no longer heat, it is the absence of heat, which we perceive as COLD.

In the early days hot air engines were constructed entirely out of cast iron. Why won't an engine made of copper tubing work? The engine body conducts all the heat so virtually no heat at all will enter into the working fluid.

For any serious experiment, I would also attempt to insulated the space between the plates because the air outside the engine can transfer heat by convection just as well as the air inside:

https://youtu.be/fwWTfyoq9rk

The engine body and displacer material heat conductivity also need to be taken into consideration.

Helium and hydrogen make good working fluids because they are considerably better at conducting heat than air.

Pretty much every free piston, thermal lag and thermoacoustic engine I've seen either had a spring, rubber or crankshaft-flywheel assembly to retain it's power piston.

There are several good examples on Youtube of thermoacoustic engines running with the flywheel removed.

https://youtu.be/HUWt3YrxoB4

And, of course, being very skeptical myself, I've done experiments to verify that this is possible and not some kind of trick video editing or some such thing.

As far as I understand, the power piston is much lighter than the displacement chamber, if there's nothing to hold back the power piston a bit, how does pressure build or drop to move the displacement chamber?

That last bit, I'm not entirely sure what you are asking.

I assume, maybe, by "displacement chamber" you mean the displacer itself ?

At any rate, the only thing that could be "holding back" the piston, in an engine such as in that last video, would IMO be the pressure difference between the working fluid and the outside atmosphere.

So the question is, how can the working fluid possibly cool down and contract so quickly as to pull the piston back down the cylinder?

With the engine running so rapidly, IMO, it could not be due to (slow) conduction of heat out of the working fluid to the cylinder, out to the atmosphere. At any rate, I think that would only result in an equalization of pressure, but the pressure of the working fluid is IMO obviously dropping way below atmospheric pressure nearly instantaneously and stopping the piston cold in it's tracks and pulling it back down the cylinder, even as tremendous amounts of heat are still being applied.

So good question. How does the pressure drop so instantaneously so that the piston does not just shoot out of the cylinder like a bullet?