Stirlingengineforum.com

Just supposing what I said about a Stirling engine operating as a heat pump is true.

Past and current conventional thinking is that the greater the ∆T the more "efficient" the engine.

If the engine is acting as a heat pump, drawing at least some of its heat from the cold side, what would REALLY happen if the cold side were reduced to absolute zero?

Supposedly the engine would have 100% efficiency. Would it really though? If we've just completely taken away one of its primary sources of heat/energy?

There needs to be a temperature difference, but taking that to the logical extreme to at or anywhere near absolute zero would be going to far IMO. There needs to be two HEAT sources, one hotter than the other, but going too far in the cold direction on the less hot side is probably a mistake.

A third source of energy is atmospheric pressure. So the engine actually has three sources of power or energy INPUT, but should really only have one energy output, and that is, work output through the power piston.

I think a Stirling engine piston is actually "double acting".

The heat input expanding the air on the inside. Atmospheric pressure pushing the piston back from the other side.

So you have heat input, expansion, work output, compression by atmospheric pressure..

Cooling to allow compression by atmosphere is accomplished by work output.

VincentG wrote: ↑Thu Jun 01, 2023 5:44 pm
..., we can learn far more from just building and testing than years of theorizing. ...

While I'm all for as much trial and error building and testing, there is, for me at least, the matter of expense. Just making modifications to off-the-shelf models cost quite a lot, not only for the models but also the time and materials.

For me to get really motivated enough to spend the necessary hours and spend money on materials and tools I really need some plausible angle of approach, some theory.

I've spent more time just scratching my head than anything, looking at, for example, a thermoacoustic engine running without a flywheel, asking myself, how is THAT possible? That's not supposed to happen!!

Usually it's best to have at least two theories. If theory #1 is true then X should happen, if theory #2 is true than Y will happen. Anyway, most days, I have a lot more time to think than to build, since I can think while doing other things, like mowing the yard or washing dishes or taking out the trash etc. Then take a minute or two to post my thoughts here while the coffee is brewing.

I do wish I had more time and money for actual building and testing, but on occasion, when I do, I try to make it count for something, usually to test some theory.

By now, though, I'm finally getting around to trying to apply some of what I at least THINK I've learned, to see if it holds up under real world application, like the attempted engine conversions.

Bumpkin wrote: ↑Thu Jun 01, 2023 9:10 am If true, it logically leads to an ambient powered engine, where the only exhaust is”cold.” I’ve never thought that was impossible; and have been down some of those rabbit-holes myself. I sure wouldn’t discourage others from sifting through those same weeds, or even looking in whole new fields. It would be the height of arrogance to think we already know it all.

Bumpkin

As odd as it may seem, I'm less certain about the Ambient heat engine idea. It is certainly true that a Stirling engine will run "on ice" (ambient heat) but generally these engines seem a bit sluggish to me when running "cold".

My tinkering has indicated that these engines run by combining the supplied above ambient heat with the heat of compression from atmospheric pressure when it pushes back after expansion. In a sense the work of expanding against atmospheric pressure is partially recycled, like an air spring.

Something about that dynamic is altered when the heat source is ambient itself. You can't really combine ambient heat with atmospheric pressure, since, in actuality, they are pretty much both the same thing, using ambient heat to expand a gas against the (same) ambient "pressure".

The ambient heat engine theory depends pretty heavily on the idea that heat flows down towards cold like a waterfall, which appears to be mostly wrong, but I haven't entirely given up on the idea, it would probably depend on some mechanism other than "heat flowing down to a lower cold reservoir", if it's possible at all.

I see what you mean Tom, we all are coming at this from different angles with different goals in mind.

VincentG wrote: ↑Fri Jun 02, 2023 3:55 pm I see what you mean Tom, we all are coming at this from different angles with different goals in mind.

I'm not sure I'd really know what everybody's goals might be.

Originally I was living off grid and just wanted to understand how a Stirling engine works so I could build one to run off a wood stove or camp fire. That lead me into a quagmire of different types of engines and conflicting theories I'm still struggling to get out of.

One thing that is rather disillusioning is that in running an engine on cold (ice), I expected that the heat would rush in to fill the void with some great force and the engine would run like crazy, maybe better than running on heat or at least just as well. But I haven't really seen any evidence that heat is "compelled" (to use Tesla's expression), to flow towards cold. I get the impression it simply disperses rather gradually in all directions like a drop of ink in water. Not anything like a river rushing downward with great force into a ravine. Not even anything like Carnot's interception of a waterfall.

A heat engine running on cold, cold being the absence of energy, the absence of heat, runs weakly, as might be expected. I think that may very well be because a heat engine derives it's heat as much from the cold as from the hot side. There might very well be a similar problem if a "perfect insulation" could be found.

For a heat engine, cold is not a source of power. It is more like an empty gas tank.

Anyway, it seems all the theories were wrong. I've had to stop and stare for long hours on end scrutinizing these machines trying to get some clue about how they really operate. I'm not at all sure I'm out of the woods yet.

matt brown wrote: ↑Thu Jun 01, 2023 7:41 pm ...
...there you are guys, Tom was right, isothermal compression is not mandatory !!!

Sorry Matt, I've tried to understand your presentation, reading your posts over several times the past few days, but I can't say I really follow your proof, if that's what it is.

I may try sitting down with a pencil and paper to map this out.

Anyway, by way of clarification. I don't think that isothermal compression is "not mandatory", I don't think isothermal compression or expansion can actually exist in reality. It is "ideal" in the #2 dictionary definition of the word "Ideal": "existing only in the imagination". It requires an engine that is "quasi-static" that requires an infinite amount of time to complete any one leg of a cycle. The engine has to remain at equilibrium with the surroundings. Possibly useful as far as simplifying some mathematical calculations but is not actually descriptive of any real engine.

"not mandatory"????

I'm not sure I'd really know what everybody's goals might be.

My goal, plain and simple, is to produce continuously around 1000 watts to start. Then to go up from there.

Anyway, it seems all the theories were wrong. I've had to stop and stare for long hours on end scrutinizing these machines trying to get some clue about how they really operate. I'm not at all sure I'm out of the woods yet.

For having such a simple appearance, these machines are incredibly complex in operation. But my basic theory has become rather simple. Take a volume of gas and get it as hot as possible as fast as possible, then cool it off as much as possible as fast as possible, while extracting energy in the form of pressure swings along the way. I think being able to study a larger scale low rpm machine may be much easier than a small model.

Agreed, I dont think a hypothetical engine running between room temperature and lets say -150F would be the same as between ground temperature and boiling water or solar heat. Of course I would like to be proven wrong there.

What I mean about heat not being "compelled" to "flow" towards cold is, observedly during experiments using ice, which I did a lot of, the heat did not automatically flow through the engine to the ice but had to be coaxed.

In this "control" experiment for example;

https://youtu.be/41d6kIHLK7M

This engine had run on ice for 33 hours before the ice all melted and the engine stopped. For comparison I did a control experiment with the engine not running.

After sitting over ice for many hours, to check if the ice had all melted yet I'd occasionally give the flywheel a nudge to see if the engine would still run.

The engine was, in effect, down in a cold well. Though open on the top, theoretically "exposed to ambient" the heat in the air was not "automatically" traveling down to warm the top of the engine. The top of the engine was very very cold. The cold air above the engine "settled in" like a mist over a pond.

Even with the engine running, the displacer agitating the air inside the engine to keep the heat "flowing" a little, the engine ran at a very low RPM.

It seemed the ice was acting effectively to chill the engine and the air immediately above it but the heat in the warm air higher above the engine was not actively trying to travel down to get to the ice.

Oddly, though, the ice under the idle engine melted several hours sooner than it had melted under the running engine, though to some extent the flywheel of the running engine acted like a fan to keep the warm air circulating above the engine so the cold air did not "settle in".

This was one of many clues that the engine was actively working like a refrigerator to keep the ice cold even though the heat "flow" to the top of the engine, the warm air circulation, was greater above the running engine with the spinning flywheel.

The idle engine became so cold nestled in its blanketed "well", after a while it would no longer run at all, apparently due to having become uniformly permeated by cold.

The logic of putting a heat source UNDER a heat engine makes more sense to me now. The heat does not easily move down into the engine.

In other words, if you have a layer of cold air under a layer of warm air, the heat is not compelled to travel between the two layers to equalize. In fact, if mixed up mechanically, the cold air will settle out again. Anyone who has lived with a wood stove knows this, and so, the market for wood stove fans.

There are, of course, better conductors of heat than air. Copper for example. But there is apparently not anything like a gravitational force or magnetic attraction between a "hot reservoir" and a "cold reservoir", no "pressure" or "voltage" between a hot object and a cold object.

The heat going into and powering a heat engine does not due so as a result of the heat "trying to get to" the cold.

The idea that heat will be "compelled" to "flow" from a "hot reservoir" to a "cold reservoir" is way overblown.

Tesla was right that heat is a form of energy that can be converted into other forms of energy, but I think he was wrong in thinking that simply creating a "cold hole" would result in the surrounding ambient heat being "compelled" to flow in of its own accord.

In this experiment, after the engine had been "running on ice" for more than 30 hours, I put an aluminium electrical box on top of the engine which did seem to help draw down the heat.

https://youtu.be/lFhUkzHRbWo

Surprisingly though, the ice still melted more slowly under this running engine presumably drawing down heat faster than under the same engine when sitting idle on top of the ice.

More heat going down into the engine to expand the gas so the engine runs better and faster does not mean that more heat passes THROUGH the engine to continue down into the ice. It seems, in fact, that a better running engine just means a better running, more effective refrigerator, so the ice lasts longer.

VincentG wrote: ↑Sat Jun 03, 2023 5:39 am ...

My goal, plain and simple, is to produce continuously around 1000 watts to start. Then to go up from there.

... my basic theory has become rather simple. Take a volume of gas and get it as hot as possible as fast as possible, then cool it off as much as possible as fast as possible, while extracting energy in the form of pressure swings along the way....

I agree,

However, what I believe I have found, or what is true, is that the most effective and rapid cooling method is expansion work. That is, having the gas/air/working fluid doing work as it expands. I believe that this is in fact the only way a high RPM running Stirling engine is actually cooled so it can complete a cycle, making "active cooling" superfluous.

"Take a volume of gas and get it as hot as possible as fast as possible, " I agree with, but the cooling largely serms to take care of itself, if there is not a lot of heat getting to the cold side in other ways.

One way heat could get to the cold side, ironically, would be through the water being used to cool it. Liquid water has an enormous heat carrying capacity which circulating around the cold side of the engine would greatly limit the engines own self-(expansion work)-cooling preventing it from reaching colder, potentially cryogenic temperatures.

The heat going into and powering a heat engine does not due so as a result of the heat "trying to get to" the cold.

The idea that heat will be "compelled" to "flow" from a "hot reservoir" to a "cold reservoir" is way overblown.

I have to disagree here and state that the temperature difference between melting ice and room temperature may not be large enough to really demonstrate the flow. The higher the delta T the more entropy we can observe.

Take a look at this video again, which I think clearly demonstrates the direct flow of heat energy into the cold sink. If there was no flow into the cold sink, the internal pressure would continue to rise until the piston was pushed out of the cylinder. Instead what we see is a bias of flow depending on the ratio of cold volume to hot volume. Without active cooling here the engine would heat soak and internal pressure would never go below ambient. https://photos.app.goo.gl/nSMM47k3yAYa59TB9

I think this is what makes these engines hard to design, there are two distinct forces at play. One is the natural flow of heat energy from hot to cold, and the other is the "unnatural" effect of compression and expansion.

The main difference, is that the latter is not capable of producing a running engine alone. It surely can be used to enhance the engine, but often is likely the cause of parasitic drag on a poorly designed engine.

Interesting to me is that creating heat is always an easier process than removing heat. For example, there is a natural bias to compression and expansion where we reach a limit of vacuum rather easily. On the other hand, there is nearly no limit to the heat and pressure of compression.

For this reason I think you are right in that the work of expansion on the piston(and beyond as seen in free piston) should be used to supplement the cooling process. I just don't think that the perfect insulation is so important, for the same reason that a fire piston works without insulation. The process is so fast and so directed at the gas itself that none is needed.

Maybe I could put this another way. There are two basic phases of a Stirling engine cycle.

#1 heat addition with expansion and work output.

At the end of this #1 phase the working fluid is already cold as a result of expansion work, so why is a cold side necessary?

Well, atmospheric pressure now needs to take over and drive the piston back, so heat addition needs to be discontinued.

Shifting the working fluid over to the cold side provides a containment vessel that is equally as cold as the now cold working fluid, so that no further heat transfer into the engine takes place. The cold side may be slightly warmer or slightly colder, but it is not necessary that it perform "active cooling" by taking away heat, it just needs to be as cold as the working fluid has become due to expansion work.

The amount of expansion work possible is limited by the physical constraints imposed by the engine design.

A free-piston thermoacoustic type engine is not limited by the length of the connecting rod, but may be limited in other ways, like the size of the engine housing. Too much expansion and the piston would bang into the housing or possibly damage the flexures.

So in a sense, the cold side acts as insulation during phase #2 contraction, because there needs to be a temperature difference for heat to flow. The cold side being equal in temperature to the expanded working fluid, no further heat transfer into the engine takes place, at least momentarily, until the piston is well on its way home to begin another cycle and heat addition is wanted.

I have found, through experiment, that actual insulation seems to work just as well if not better, since often the expansion work actually cools the working fluid down colder than the cold side of the engine.

Insulating the cold side can have an effect similar to placing ice on the cold side or using "active cooling".

Active cooling then should do no harm, but can serve as a safeguard, for example; if the load on the engine drops reducing expansion work -cooling, but where active cooling can be and often is detrimental IMO, is where it is situated in a way and/or where materials are used that provide an easy "short circuit" for heat from the hot side to be lost directly to the water jacket without ever expanding the working fluid.

Theoretically, a well designed engine with a water jacket for potential cooling, if needed, should not see any rise in temperature of the "cooling" water, because there would be no heat of a higher temperature getting to it.

If not well designed however, the cooling water could easily heat up in other ways, such as the heat "shot circuiting" to the cooling water through the engine body as already mentioned, convection of hot air from the heat source circulating, ambient heat, if the cooling water is colder than ambient, etc.

Ideally, the working fluid should be able to carry out enough expansion work that active cooling is unnecessary. That is not to say that a "cold side" need not be present at all to serve as "insulation" for phase #2 of the cycle. Actual insulation could serve just as well, but could be a problem under no-load conditions where overheating could be a problem.

(Edit: I've been notified you've responded, but I don't think it alters the substance of this post so I'm leaving it as is)

VincentG wrote: ↑Sat Jun 03, 2023 8:35 am ...
Take a look at this video again, which I think clearly demonstrates the direct flow of heat energy into the cold sink. If there was no flow into the cold sink, the internal pressure would continue to rise until the piston was pushed out of the cylinder. Instead what we see is a bias of flow depending on the ratio of cold volume to hot volume. Without active cooling here the engine would heat soak and internal pressure would never go below ambient. https://photos.app.goo.gl/nSMM47k3yAYa59TB9

...

I think I see your point. I did not mean to imply, though that heat transfer cannot take place where there is an actual temperature difference.

Also, since you are doing the work of lifting the displacer with the plyers, and at that, very slowly, and the piston is disconnected from the crank and flywheel there is virtually no actual work load.

My argument is not that cooling takes place as a result of expansion of the gas by heat addition alone, but rather, the simultaneous (rapid, adiabatic) expansion along with work output.

At least driving the crank and flywheel against air resistance and mechanical friction at the bearings and such would be some "work", but you've eliminated that, or at least reduced it to a bear minimum.

What you are demonstrating is an approximation of isothermal expansion and contraction. By cooling through "work expansion" I'm talking about rapid adiabatic expansion along with work output.

For this reason I think you are right in that the work of expansion on the piston (and beyond as seen in free piston) should be used to supplement the cooling process. I just don't think that the perfect insulation is so important, for the same reason that a fire piston works without insulation. The process is so fast and so directed at the gas itself that none is needed.

I can agree with this, but then is not the same true in regard to "active cooling".?

The fire piston demonstrates ADIABATIC compression. (Not isothermal).

I can agree with this, but then is not the same true in regard to "active cooling".?

The fire piston demonstrates ADIABATIC compression. (Not isothermal).

Exactly. Were going for a adiabatic expansion AFTER the air cools to "ambient" while the piston has recovered all of the available energy.

I agree that ideally we could have perfect insulation AND ensure enough working volume to use all of the heat expansion AND go further into expansion to super cool the gas such that no cold sink is needed.

I just put that in the same space as isothermal compression. Maybe one day someone will figure it out and I certainly don't intend to discourage that. But in the mean time there are many tangible improvements available to us.

Consider the Rider "beta" hot air engine happily chugging away doing real work all across the country in the 1800's. Surely we can do better than that right now.