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If you look at the standard "Carnot cycle" it consists of isothermal, followed by adiabatic expansion.

If you read Carnot's explanation in his book it reads:

Thus rarefied, the temperature will fall spontaneously, as occurs with all elastic fluids; admit that the rarefaction may be continued to the point where the temperature becomes precisely that of the body B.

I can agree with this, but I also ask, why should it stop there? "Body B" is the "cold reservoir". The working fluid cools by expansion to become just as cold (if not colder).

But at that time Carnot had no concept whatsoever of heat being converted to "work".

Going on he wrote:

(3) To condense the steam by putting it in contact with the body B, and at the same time ​exerting on it a constant pressure until it is entirely liquefied.

This makes sense if we are talking about a slow isothermal process. As the gas is gradually compressed the heat that develops can be carried off, but in an engine turning at 3000 RPM, the gas already cold, (baring heat transfer to an equally cold "sink") you are talking compression at a rate much faster than a fire piston. The heat of compression doesn't have time to go anywhere. So the "heat out" during isothermal compression, I don't think can actually happen.

IMO rather than loosing heat during compression, while the gas is expanded and cold is when the working fluid has an opportunity to take in heat (on or near the hot side).

Maybe the heat is quantum tunneling out of the engine or some such thing, but the conventional explanations don't hold water IMO.

There is certainly SOME near isothermal transfer of heat when the engine is running at a very low speed, such as when starting up, or when under a heavy load perhaps, but as the RPM climbs, the ratio of adiabatic to isothermal, I should think, would tend more and more towards adiabatic.

The only time heat could be very quickly CONDUCTED out of the engine (from the working fluid) would be near TDC when the working fluid is compressed and very hot. When the ∆T is greater, then heat transfer is faster, but at that time, particularly in a thermal lag or thermal-acoustic type engine, all the working fluid is compressed and confined to the hot end. Likewise, though usually imperfect, in a Gamma, the displacer has working fluid, during compression, shifted over to the hot side, the cold side being covered/insulated from the working fluid by the displacer.

I don't see any real opportunity for "isothermal heat rejection". in an actual engine that is running up to operating speed.

Anyway, I think the saving grace of those old cast iron engines is that they ran fast enough that there was some adiabatic compression so they did not loose all the heat to cold metal.

BTW regarding "perfect insulation" or insulation generally.

My original purpose in "insulating the sink" was as an experiment, not necessarily a suggestion intended as a practical application.

My reasoning was that if the cold side were effectively insulated so heat could physically not be "rejected to the cold reservoir" because that path was blocked, if such "heat rejection" was actually necessary than the engine should rather quickly overheat and be unable to run, the build up of excess "waste heat" destroying the temperature differential.

I've run the same or similar experiments many times with many different engines and different types of insulation as well as engines using different non-heat conductive materials.

The engines not only ran with the sink insulated, often they would run better than without the cold side insulated.

What someone might do with that information, as far as engine design who knows? I'm not suggesting that finding a "perfect insulation" is necessarily "the answer", just a point of fact someone might like to take into consideration, or do further experiment to figure out what's going on.

Personally I'm convinced a cold "sink" is not necessary and designing an engine without it might lead to improved performance, so that's the direction I'm going in until time proves otherwise.

Tom Booth wrote: ↑Sat Jun 03, 2023 2:28 pm
Personally I'm convinced a cold "sink" is not necessary and designing an engine without it might lead to improved performance, so that's the direction I'm going in until time proves otherwise.

I concur, just sketch this out...

I updated my cel timeline and added color to distinguish Dis A half of cycle from Dis B half. I'm still messing with my phone camera and may half to resort to scanner (but haven't used it since upgrading to win10 a few yrs ago).

Anyways, Tom, if you take up the challenge and sketch this out, you'll see a few interesting things besides no isothermal compression/cooling. I worked this basic scheme out this morning where I transposed this via single-acting pistons and everything remains valid. The trick here is that the piston is exhausting 500cc at 300k to displacer cold space while the displacer moves 1000cc from 600k hot space thru regen into 500cc of its 300k cold space via 2:1 isobaric compression (thanks to 2:1 thermal ratio).

The normal problem with schemes is that they're half-baked. I like the term scheme, since it warns us (up front) that any conclusion might be bogus. With heat engines, I think a basic scheme requires (1) valid thermo (2) valid sequence (3) valid mech. Oddly, with most ECE, finding a valid mech that coincides valid thermo and sequence remains the major challenge.

Here's an out of the box example that few realize...

Consider 2 engines, A and B, where both have a similar 1:2 isothermal expansion from the same volume:

Eng A: 1 bar pressure at 300k is heated to 600k at constant volume into 2 bar pressure, then expanded 1:2 via isothermal heating whereby pressure drops to 1 bar but still 600k.

Eng B: 2 bar pressure at 300k is 'heated' at 300k during 1:2 isothermal expansion whereby pressure drops to 1 bar but still 300k.

Interesting, both A & B expansions produce the same work (same MEP) from the same 'quantity' of heat, but each from a drastically different 'quality' of heat. I suspect this was what Tesla tried to game.

VincentG wrote: ↑Sat Jun 03, 2023 8:35 am

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.
...

I'm not sure if maybe I misunderstood or jumped to conclusions about what you were trying to say here, but to maybe clarify my point about Tesla's Ambient heat engine ambition.

Tesla stated in his article, or implied that all that would be required is to carve out a "cold hole" in the midst of the ambient environment and the heat would "be compelled" to flow into that cold space in the same way water would flow into a tank submerged in a lake.

I suppose what I'm overlooking here is getting the water to flow from the lake into the tank would require some kind of conduit for the water to flow through.

A pipe.

I had my engine surrounded by insulation, but overlooked the fact that the air above the engine is also an excellent insulator.

My disappointment in the performance of the engine stemmed from my expectation that the top of the engine completely exposed to "ambient heat" would result in a virtual "waterfall" of heat into the engine causing it to run quite vigorously. Instead it ran, barely, as if submerged in molasses.

Compared with an engine running on hot water, the video of the engine running on ice looks like it was filmed in slow motion.

I experimented with putting a big aluminum box on top of the engine and even putting an electric fan nearby to keep the air circulating which helped a little. The speed of the engine could be increased by hitting the top of the engine with a hair dryer but, I did not get the impression that there was any real pressure, so to speak "compelling" the heat in the room to flow down into my artificial "cold hole" passing through and powering the engine in the process. The engine just sat in a pool of cold and could barely be coaxed into running at all.

Maybe what was needed is a better "pipe" an effective heat pipe that could transfer the heat to the engine, or perhaps putting the ice ON TOP of the engine would work better.

Actually, at one time I did try this. I insulated the engine on top, leaving a compartment for an ice cube so the ice could be placed under the insulation on top of the engine. The bottom of the engine was left exposed to the ambient heat. This actually worked so well I got rather excited about it and carried the engine upstairs to show my wife.

As I tried to explain what was going on the engine which had been running rather vigorously very suddenly seized.

My wife, looking on as I muttered WTF said "maybe it froze up again".

I carried the engine back downstairs to investigate but can't really confirm what caused the piston to suddenly jam up but it was somewhat reminiscent of an earlier apparent freeze up.

This engine in the below video apparently froze up just sitting on the ice as a control not running at all.


https://youtu.be/fnxC8hymFLU


The engine I was showing my wife with the insulation and ice on top seemed to be "stuck" in the same way but unfortunately I was just fooling around at the time and did not get it on film.

By the time I got downstairs and removed the insulation the engine had come unstuck and the ice was starting to melt. The engine did not seem to want to run until I dried it off and let it warm up for a while, I did eventually get it running again, (on hot water).

I can't be sure if this is actual freezing or just the aluminum cylinder contracting from the cold and locking up the piston. Either way, not enough heat going in through the bottom of the engine remained to prevent the piston from "freezing up" from the cold at the cylinder which is rather surprising as the ice was a distance away, separated from the cylinder by about 1/2 inch of insulation. Also the ice WAS melting. The (apparently extreme) cold seemed to be confined to the area of the power piston. Or maybe it had to do with contraction of the aluminum cylinder, contracting more than the graphite piston. Anyway I thought further experiment was pointless, unless I could figure out what was causing this "freez-up" and devised some means to prevent it.

Anyway, I think heat dispersal is random in all directions. As a result the hot space or object is cooled down as much as the cold space is warmed as a gradual equalization takes place.

In a way you could just as well say that the cold "flows" towards the heat.

That heat actually makes a beeline for the cold creating a direct "flow" that can be intercepted to extract energy from like a paddlewheel in a river is I think, generally false. The heat disperses like billiard balls in a break, randomly in all directions.

In a "fluid" (gas or liquid) cooled air molecules travel back towards the heat source just as readily as hot molecules travel towards the cold. There is no directional, one sided "flow" of heat towards cold.

Maybe if the heat were confined to traveling through a heat conductor in some way but like a copper electric wire, the conductor would need to be insulated.

Controlling and directing the heat flow is not just a matter of letting it happen because the heat is in some way "compelled" to move towards cold.

It is more like the hot and the cold mutually dilute or flow into each other to become warm, unless, of course, one or the other is really overwhelmingly hot or cold, or the quantity of one is much greater than the other, but then the cold could just as well "overpower" the heat as the other way around.

In your video, what it looks like to me is that there isn't much movement of heat (or cold) one way or the other unless you make it move by lifting or lowering the displacer.

Put another way, I think (though I have not yet done the experiment, but may)...

If I put a copper wire between an ice cube and a cup of hot water then took an infrared video, what would the infrared imagery show?

If this was a tube siphoning water out of a full glass to an empty glass, you would see a definite one way flow.

I think however, with the hot water and the ice and copper wire, the infrared would show the cold creeping up the wire from the ice while the heat traveled down the wire and they would meet somewhere in the middle and start blending together. It would not be a strictly directional one way flow just like water flowing one way through a tube.

I guess you could say it appears the cold is traveling up the wire because the heat is draining out and going down into the ice, but, well...

I don't think the heat would "flow" continuously to the cold, they would just kind of neutralize each other in the middle until they both equalized with the room temperature.

Thats an interesting theory that I'll have to consider a way to test, though it seems contradictory to what you have said in the past about cold being a lack of energy.

But could cold be a force of its own? Google says no. But I have to question how if you remove heat from water it phase changes to ice, yet can then produce a tremendous amount of energy from expansion. I suspect it's some phase change magic unique to water but still intriguing.

Regardless, to me the effects on gas pressure remain the same. Although if you consider cold an energy of its own then I'd argue active cooling is even more significant.

Sorry to respond late to a post way back-thread; It’s my busy time. Anyway, for Tom and your response to my supposition that if you don’t need to remove heat externally because it’s removed internally as energy conversion, then ambient power is possible:
All heat engines are heat-difference engines; we just abbreviate. Kind of like the thread has run lately to talk of cold; of course there is no such thing as cold, just like there’s no such thing as vacuum, but the words are still useful. Anyway, if a heat-difference engine could make its own cold side below the temperature of the hot side, then why would it matter if the hot-side temperature was only ambient? Power removed would make the cold side colder — thus it could run on ambient temp with the “hot-end” heated externally with plain air or water. I’m not arguing for or against that end-result possibility, but it seems to me that logically; that’s where saying you don’t need external cooling goes.

Bumpkin

Eastern philosophy posits the world we live in is no different than the world's we visit in dreams, both being equally just projections of our own thoughts.

There has been a lot of "Quantum Woo" and "New Physics" leaning in the same, or a similar direction; Like some old Star Trek episode, we live in a simulation, where anything we imagine manifests as reality.

https://youtu.be/b2HAgQYSv5w

Sometimes I really wonder.

As far as cold being a lack of energy or alternatively a "frigoric" substance, like the "caloric", I doubt any of our interpretations come very close to the actual underlying reality, whatever that may be.

All I know is what I've observed in my experiments and what I've observed in life generally. Taking a swim in a lake on a sunny day for example. On the surface the water can be quite warm, a few feet down it can be quite cold. The heat in the upper warm layer seems to make no great effort to transfer its energy to the colder, denser layer.

If I gently, pour cold water dyed blue with food coloring, into hot water dyed red, or vice versa, the two fluids seem quite content to seperate into layers, the cold blue water immediately sinking to the bottom, the hot water floating on top, the hot water seems to be in no apparent immediate rush to transfer its energy to the cold layer, infact, it serms the layers are content to remain separate more or less indefinitely unless agitated by some outside force.

This is the sort of problem I've encountered in trying to engineer an "ambient heat engine". The supposed "FLOW" between hot and cold is not always, or automatically present for us to extract energy from.

This has its positive and it's negative aspects.

On the negative side, extracting energy from a temperature difference is not as simple as just putting a turbine between the hot upper layer and cold lower layer of water in a pond to intercept the "flow".

On the positive side, it opens up the possibility of domesticating heat in the same way we have tamed electricity, by using conductors and insulators.

The electrical force stored in a battery does not disperse into the surroundings as a thermal battery might, we don't see a current until we call for it by throwing a switch to make a circuit. I think we just have to treat heat in much the same way.

Most heat engine designs are so hopelessly short circuited with "leaks" everywhere it's a wonder they can run at all. Often they only do so by so overloading the "circuit" with heat at least some still gets through to power the engine.

Heating a Stirling engine on top seems like it would be a good idea, as it would help keep the hot and cold layers seperated, but in practice, I've gotten the impression that this works TOO well. Like I said, the engine will just sit in the "cold hole" until it freezes up unless some extra method is used to bring or conduct the heat down into tbe engine. It doesn't just easily flow down on its own. The hot air rises and stays there. The cold sinks and stays there.

In a way I think that is good news. The "cold hole" is easier to maintain if the heat is so indifferent about moving into it. We have more control as far as introducing a measured quantity of heat to expand a quantity of air to power an engine without the heat running amuck and neutralizing the ∆T without our permission.