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Fool wrote: ↑Sun Jun 16, 2024 2:04 pm So you think that the time it takes to heat up the hot plate is the same amount of time it takes for the gas to reject heat to the cold plate?

Sounds to me that the same "heating" time is required to cool off the cold plate if it is run on ice, and comes down to operating temperature.

Meanwhile modern Stirling theory seems to recognize that the engine after coming up to temperature, has two inside temperatures that the gas comes in contact with when pushed by the displacer to each of the hot or cold spaces. The gas in contact with either of those two spaces has just as much time to pick up heat in the hot space, as to release heat in the cold space.

The title of the thread is "Experimentally confirming the conversion of heat energy to work"

I was talking about my experiments where as far as possible, the cold plate, sink or cold space was eliminated or removed or blocked by insulation.

Maybe the two engines running on the same ice cube with the ice sandwiched between them will answer VincentG's question.

At least it should mostly take out the "no such thing as perfect insulation" factor.

The ambient heat won't have to pass through any imperfect insulation to get to the ice. The heat would mostly all have to go through one engine or the other.

If the heat is REALLY and LITERALLY being converted into the engines combined mechanical motion and the heat is actually "disappearing" inside the engines then at a minimum, the ice should take a really really long long time to melt.

Just as a reminder, in previous experiments ice consistently took significantly longer to melt while "powering" a running Stirling engine vs. the engine not running.

The question there was, did the ice eventually melt anyway from heat passing through the engine or from heat going through the less than perfect insulation on all the other sides of the ice.

Using two engines with the ice between them takes the "imperfect" insulation out of the picture. To get to the ice the heat has to go through one engine or the other.

I think it's a good experiment, even if only suggested on the science/physics forum as a joke.

If you propose that the "heat energy" (internal energy of the working fluid once "heat" is transfered into it) needs to be conducted out, how do you propose its going to get out other than the way it came; through the engine body.

The engine body is HOT radiating heat into the engine.

Somehow the gas in the engine has to instantly cool back down so the piston can return and do this maybe 20 times or more per second. Send heat out through scorching hot walls that are radiating heat IN. Good luck with that.

You can't just ignore that the engine body is hot while the working fluid SOMEHOW manages to get cold inside this roasting hot environment inside a scorching hot engine body.

Common sense dictates that the heat/energy is leaving in some manner other than the way it went in.

That would be by conversion to work, which is instantaneous. Supposed "isothermal heat rejection" takes literally FOREVER. "Quasistatic".

Hot air inside a hot chamber not a wire in the open wind.

Negative. What I mean is you have to think of a perfectly ideal engine body, like my epoxy chamber attempts to copy. A body that efficiently allows heat to pass in, through the gas and then out, all while maintaining distinct processes.

Talking experimentally, if my driven displacer/free piston video doesn't show that isothermal heating and rejection can be extremely rapid, I'm not sure what would.

Tom Booth wrote:Somehow the gas in the engine has to instantly cool back down so the piston can return and do this maybe 20 times or more per second. Send heat out through scorching hot walls that are radiating heat IN. Good luck with that.

How about the idea of expanding the gas in a scorching hot cylinder so that it stays hotter, closer to Th than it would in a cold space. Hmmm call this isothermal expansion.

Then, push the hot gas through a device that absorbs and retains the thermal energy in a counter flow direction to capture all the MCv∆T energy going from, say Th to Tc. The energy is returned to the gas on the return stroke going from Tc to Th, in an opposing counter flow direction. On the hot to cold stroke, the gas can come out in a cold space already at Tc. It potentially could be called an economizer or maybe a regenerator.

Then compress the gas in the freezing cold cylinder. This will make the gas stay colder, closer to Tc, than it would if compressed in a hot space. This is called isothermal compression. That way the cold space only has to reject the tiny heat of compression. If a small amount of compression, small piston and stroke, a very very small amount of energy rejection, miniscule.

The two spaces, hot and cold, could be combined into opposing ends of a single cylinder, and the gas pusher could incorporate the counter flow energy saver. It is probably worth calling this double temperature cylinder, a "displacer cylinder".

Doh! Robert and James Stirling already invented it.

The beauty of the regenerator is that, from forced convection, the gas loses or gains it's energy just as fast as it can be pushed through.

That is why a good regenerator will have poor longitudinal conductivity and excellent lateral conductivity. And why James and Robert's first regenerators were a stack of very closely spaced plates with longitudinal holes for the gas to flow freely straight through while conducting and saving heat laterally.

The beauty of the combined system is that the energy of going from Tc to Th is saved from going from Th to Tc. The only energy needed to be supplied is the energy of expansion. And the only energy needed to be rejected is from the compression. This lets us design engines with a wide temperature difference to capitalize on a higher efficiency.

Better materials are where we should be concentrating our efforts. There are many good configurations to choose from, they all have specific areas where they are good.

Isothermal is only quasi static if you want the "perfect" efficiency of having the gas be the same temperature as the plates. If some efficiency loss is acceptable, and it is a compromise, the inside temperature can be lower than Th and higher than Tc and the heat transfer will be much much faster. It can be closer to isothermal but lower in temperature differential.

matt brown wrote: ↑Sun Jun 16, 2024 3:25 pm Vincent - imagine a non compression 300-600k cycle where a low tech turbine does work during 1:2 isobaric expansion. Note that Ericsson used isobaric processes for regen vs Brayton used isobaric processes for source and sink. The scheme here is no backwork due to no compression from engine. Instead, 600k 'exhaust' is funneled backwards over engine while ambient pressure supplies isobaric compression. Over simplified for sure, but I think you get the idea.

I get what you are saying. "... imagine a non compression..." And "... while ambient pressure supplies isobaric compression." Expansion? LOL guffaw!

The question I have is, to my knowledge, isn't isobaric less efficient than isothermal? Would you not be replacing back work with larger heats of expansion? Wouldn't that lead to the same or worse Carnot limit?

I loved your chart showing the four ideal paths. Each path keeps one parameter constant and the rest slop all over the place.

The adiabatically path should be of interest to Tom. It is a zero heat transfer path. It is also the path of adiabatic work with a load. The only energy transfer is through work and loaded. A direct work temperature relationship. Also, it only works if the temperature of the mass surrounding the gas is somehow magically kept at the same temperature as the gas. It won't work if the gas is inside a "scorching hot cylinder".

That is the reason I poopoo adiabatic anything. When the temperature of the gas changes, the gas will lose or gain energy from the mass around it, even if that mass is considered insulation.

VincentG wrote:Talking experimentally, if my driven displacer/free piston video doesn't show that isothermal heating and rejection can be extremely rapid, I'm not sure what would.

That's one of those facts that should convince people. Good point.

VincentG wrote: ↑Mon Jun 17, 2024 4:54 am

If you propose that the "heat energy" (internal energy of the working fluid once "heat" is transfered into it) needs to be conducted out, how do you propose its going to get out other than the way it came; through the engine body.

The engine body is HOT radiating heat into the engine.

Somehow the gas in the engine has to instantly cool back down so the piston can return and do this maybe 20 times or more per second. Send heat out through scorching hot walls that are radiating heat IN. Good luck with that.

You can't just ignore that the engine body is hot while the working fluid SOMEHOW manages to get cold inside this roasting hot environment inside a scorching hot engine body.

Common sense dictates that the heat/energy is leaving in some manner other than the way it went in.

That would be by conversion to work, which is instantaneous. Supposed "isothermal heat rejection" takes literally FOREVER. "Quasistatic".

Hot air inside a hot chamber not a wire in the open wind.

Negative. What I mean is you have to think of a perfectly ideal engine body, like my epoxy chamber attempts to copy. A body that efficiently allows heat to pass in, through the gas and then out, all while maintaining distinct processes.

Epoxy is almost completely non-heat conducting. I guess I don't really know what you're talking about here. How does a non-heat conducting body allow heat to pass in or out period?

Epoxy blocks heat, so how does that work? I must be missing something.

Talking experimentally, if my driven displacer/free piston video doesn't show that isothermal heating and rejection can be extremely rapid, I'm not sure what would.

What video is that?

Were you "driving" the displacer using your hand with a pair of plyers? Please repost the link to the video so I know which video you're talking about, there were quite a few.


I think I understand your statement "I am in no way doubting that work creates heat energy" but one doesn't necessarily convert into the other.

Like an army marching over a bridge "creates" vibration in the bridge But marching itself is not converted into vibration. The marching soldiers do not "disappear" and become or turn into vibration in the bridge.

So, reversing that, heat "creating" work. (Rather than work creating heat).

Heat goes in, is absorbed by the working fluid and the working fluid expands doing work.

Work was done but the heat was not affected and the gas remains expanded and full of heat even after the work is done?

OR

Heat goes in, is absorbed by the working fluid and the working fluid expands doing work.

Work was done but the "heat" IS affected and the gas does not remain expanded and "full of heat" after the work is done but collapses or implodes because when the heat is converted to work output, the heat is no longer there to keep the gas expanded.

This is talking about heat as if it were a thing. Some substance that the working fluid absorbs and swells up with, which then either remains or "disappears".

But if heat is energy then when work is done the energy is simply transfered. It goes in in a form we call "heat" and goes out in a form we call "work".but the so-called "heat" and "work" are just names or labels for the self same units of energy.

Generally IMO if the process is relatively slow and open to heat exchange (not insulated) then as heat is absorbed and the gas expands, as work is done the heat converted to work can be replaced by additional heat so the gas remains expanded. That's "isothermal" expansion.

If the process is relatively rapid then as the heat causes the gas to expand suddenly and do work, there is not time for the heat to be replaced so the gas, upon expanding and doing work
immediately collapsed and does not remain expanded. That's "adiabatic" expansion.

How fast is "relatively fast" is not entirely clear in the literature.

Whet is the threshold where a process transitions from "relatively slow" (isothermal) to "relatively fast" (adiabatic)?

That depends on so many different variables it's impossible to pin down exactly as a generalization. It depends on the circumstances.

But insulating stuff tips the scale toward adiabatic. Making the process rapid tips the scale towards adiabatic.

A slow process (relatively) with no insulation tends toward isothermal.

We are talking though about speeds that are in the realm of molecules zipping around. All pretty fast from our big lumbering human perspective.

But an engine running at over 500 RPM made of non-conducting (insulating) materials, adiabatic processes tend to dominate.

How insulating it has to be and at what RPM is not easy to pin down.

Of course it can be a combination of both.

"Extremely rapid" means what exactly?

What we see as "extremely rapid" could be slow as molasses from the point of view of hot gas molecules, and with heat transfer is also contingent on conductivity of materials, heat capacity, insulation, reflectivity, rate of absorption, direction of flow etc.

An adiabatic process CAN be very slow IF you have perfect or nearly perfect insulation or perfectly non-conducting materials.

So, the two engines sandwiched together with ice in between arranges things so that the "insulation" is about a perfect as it can be, since it isn't needed.

If the heat/energy goes into the mechanical motion of the engines or "work" then the ice is not receiving that heat, so shouldn't melt.

Basically the two heat engines together would be keeping the ice "refrigerated" by converting the ambient heat into work.

https://youtube.com/shorts/AvZ_GvFu8xE? ... Q_Q_tJuxF2

The displacer is being driven between 1500rpm and 2000rpm with a cordless dremel tool.

Only the structure of the chamber is epoxy. The hot and cold plates are aluminum and I did my best to thermally decouple everything.

The bottom of this page details the construction.

viewtopic.php?t=5570&start=60

So, reversing that, heat "creating" work. (Rather than work creating heat).

Heat goes in, is absorbed by the working fluid and the working fluid expands doing work.

Work was done but the heat was not affected and the gas remains expanded and full of heat even after the work is done?

This. Except I don't mean heat "creates work", just that work is often a consequence of heat.

Isothermal is only quasi static if you want the "perfect" efficiency of having the gas be the same temperature as the plates. If some efficiency loss is acceptable, and it is a compromise, the inside temperature can be lower than Th and higher than Tc and the heat transfer will be much much faster. It can be closer to isothermal but lower in temperature differential.

This exactly. Great post overall too, my only objection is that a perfect regenerator is a whole other development ordeal in its own right(and likely not possible) , so for the point of this discussion I am purposely leaving it out.

VincentG wrote: ↑Mon Jun 17, 2024 9:22 am https://youtube.com/shorts/AvZ_GvFu8xE? ... Q_Q_tJuxF2

The displacer is being driven between 1500rpm and 2000rpm with a cordless dremel tool.

Only the structure of the chamber is epoxy. The hot and cold plates are aluminum and I did my best to thermally decouple everything.

The bottom of this page details the construction.

viewtopic.php?t=5570&start=60
...

I'm still not sure what this is supposed to be demonstrating

Is the engine running on ice? (With ice on the bottom)

In slow motion there seems to be a lag in the response, but in a direction opposite to what I'd expect if there was heat on the bottom (when the displacer goes up the piston goes down indicating the gas was cooled when driven to the bottom?)

Maybe you could break it down. I'm not sure what it is I'm supposed to be seeing there or the significance.

VincentG wrote: ↑Mon Jun 17, 2024 9:26 am

Isothermal is only quasi static if you want the "perfect" efficiency of having the gas be the same temperature as the plates. If some efficiency loss is acceptable, and it is a compromise, the inside temperature can be lower than Th and higher than Tc and the heat transfer will be much much faster. It can be closer to isothermal but lower in temperature differential.

This exactly. Great post overall too, my only objection is that a perfect regenerator is a whole other development ordeal in its own right(and likely not possible) , so for the point of this discussion I am purposely leaving it out.

IMO "isothermal" only really applies in situations involving phase change. Like boiling water or steam engines.

Not really possible in a Stirling engine.

"Fool" simply making a statement or claim doesn't make it so. Neither does your agreeing with him.

Ice water on the bottom plate and ambient heat on the top plate. It is demonstrating the response time of the displacer chamber with nearly zero work load, piston "compression", or "expansion". The piston is just following the gas pressure being affected by the nearly isothermal temperature change.

You can see that the piston stays still when the displacer is resting on the hot or cold plates. So the chamber is not yet at the limit of thermal transfer rate.

So ideally the rpm would max out at the point where the free piston just barely "rests" at TDC and BDC. But that is not the subject of this thread.

Similar performance was observed with a higher temperature delta but at that point the mass of the piston begins to affect its movement.

IMO "isothermal" only really applies in situations involving phase change. Like boiling water or steam engines.

Not really possible in a Stirling engine.

"Fool" simply making a statement or claim doesn't make it so. Neither does your agreeing with him.

In most every example of a Stirling engine I've seen I might agree. But that has been the whole subject of my work, and I believe it's "nearly" possible.

VincentG wrote: ↑Mon Jun 17, 2024 10:30 am Ice water on the bottom plate and ambient heat on the top plate. It is demonstrating the response time of the displacer chamber with nearly zero work load, piston "compression", or "expansion". The piston is just following the gas pressure being affected by the nearly isothermal temperature change.

Isothermal means there is no temperature change. So saying: "The piston is just following the gas pressure being affected by the nearly isothermal temperature change" makes no sense.

The working fluid is driven into the cold plate and cools down. How is that a "nearly isothermal temperature change"?

It's a complete contradiction in terms.

A temperature change that is not a temperature change?

Are you sure you know what the term isothermal means?

You can see that the piston stays still when the displacer is resting on the hot or cold plates. So the chamber is not yet at the limit of thermal transfer rate.

So ideally the rpm would max out at the point where the free piston just barely "rests" at TDC and BDC. But that is not the subject of this thread.

Similar performance was observed with a higher temperature delta but at that point the mass of the piston begins to affect its movement.

I am implying that the temperature remains constant after the gas has moved from the hot space to the cold space and vice-versa.