Stirlingengineforum.com

Suppose we had a "fire piston":


And what if we put a copper plug or some heat pipe type heat absorber that could carry the "heat of compression" away somewhere in the bottom.

Push the plunger down and this generates heat and the heat is absorbed by the copper or whatever.

If we then lift the plunger up, back to its original position at the top of the cylinder, the air in the cylinder will be colder than the ambient surroundings and will begin absorbing or taking in the surrounding ambient heat until the temperature of the gas in the cylinder again equalizes with the ambient.

We could then repeat the process, push the plunger down again and once again generate "heat of compression" that could be absorbed and carried away by our heat pipe at the bottom.

I think, perhaps we have something similar going on in a "thermal lag" or "laminar flow" (same thing?) type engine, particularly when running "free piston" without a flywheel.

The engine is first idle the power cylinder at thermal equilibrium with the environment.

Give the piston a push inward and some "heat of compression" is generated and this causes the piston to pop back out or up the power cylinder which converts the thermal energy into "motive force".

At some point enough thermal energy is converted to "work" that the working fluid is cold, similar to the fire piston when the plunger is pulled back out after the heat was carried off by our heat pipe, the air that has lost energy and has then been re-expanded, becomes cold.

This time however, in the laminar flow engine, the thermal energy was not "carried away" by being absorbed or conducted to a heat pipe or other "sink', it has been transformed into work or mechanical motion. That is, the motion of the piston traveling down or out of the power cylinder.

The gas at this point, with the power piston at the extremity of the power cylinder, (all the way out) and the gas expanded and cold, the working fluid is in a condition to take in heat from the surroundings. The internal pressure is low and outside atmospheric pressure pushes the piston back down the cylinder.

So, in this scenario, during "compression" the cold working fluid is absorbing heat from the power cylinder walls while also absorbing or taking in "work" energy that the atmosphere is performing in driving the piston inward, and the heat is being concentrated by being compressed into a smaller area, but also, when the working fluid expands and finally gets cold, not only does the air in the cylinder get cold but the air in the heating chamber with the hot wad of steel wool, also gets cold so that it can absorb heat from the steel wool and the walls of the heating chamber much more rapidly than is possible when the air in the heating chamber is hot.

So really, contrary to most theoretic models that assume that the working fluid is "rejecting" heat during the compression stroke, I really believe it is much more likely that the working fluid is absorbing, taking in, concentrating heat, as well as converting the atmospheric "work" into additional "heat of compression", all at the same time.

This of course results in a high concentration of heat and pressure when the piston reaches TDC and the piston is driven back out, repeating the process. At BDC the gas is again cold and ready to take in and concentrate more heat as it approaches TDC.

So, in a sense, the heat engine is the "heat sink" taking in heat/energy from multiple sources and converting that thermal energy into mechanical motion or "work".

The only time heat might leave the working fluid would be at, or near full compression when the piston is at TDC and the concentration of heat/energy has become elevated to the extent that the working fluid is hotter than the source heat heating the hot chamber. But from there, I don't think it goes far, it is partly converted back into work and then reabsorbed by the cold working fluid during the next "compression".

Tesla, like Carnot, assumed heat engines operated by an inflow of heat "flowing" towards a "sink" or "cold hole", but if the analysis presented here is accurate, that is not really the case at all.

Does this destroy Tesla's ambient heat engine scheme?

The basic premise Tesla presented, that heat enters into a heat engine and is CONVERTED to work rather than flowing through to a "cold hole" is still basically sound.

However, the idea that some extremely cold "cold hole" is necessary to motivate the heat to flow into the engine appears to be somewhat misguided.

The real "cold hole" is the working fluid itself.

However energy is taken in via multiple paths. There is the applied heat source, ambient heat and also "work" input from atmospheric pressure.

OK, but quite obviously as "everybody knows" increasing the temperature difference allows the engine to produce more power because of the steeper gradient. The heat travels "down hill" faster.


If not, then what is the reason.


I haven't had much opportunity to experiment with thermal lag engines. Actually, I haven't exactly located a specific "sink" area to which cold might be advantageously applied.

As far as an LTD type engine with displacer, I have an alternative theory why it runs better with both heat and ice applied to opposite sides.

I don't think it really has anything to do with heat running "down hill" to the cold faster or stronger.

When does the displacer move, uncovering the cold side?

When the piston is traveling out during the power/expansion stroke.

How does this help the power stroke? It seems to me this would put the breaks on. The engine is in the middle of executing the power stroke and suddenly we move the displacer well before BDC and give the working fluid a good dose of cold.

It seems to me that this can only Diminish power during THIS power stroke, in the short run.

What though does it accomplish to increase power?

Well, it makes the working fluid colder on the return stroke. What happens on the return stroke? Atmospheric pressure pushes the piston back to TDC, adds "work" energy to the gas, increases the speed of the return stroke producing more rapid compression, It allows the engine to derive more energy from the outside atmospheric pressure. It actually shoves more heat BACK towards or into the heat source. The extra energy is derived from atmospheric pressure, the heat "flow" to any "sink" is therefore, if anything SLOWER. The heat is being held back more effectively More heat is converted to work. LESS heat gets THROUGH the engine.

The "fire piston" so to speak, is being slammed harder and faster by atmospheric pressure when cold is applied. It makes the return stroke stronger, which THEN results in more heat of compression from the Work input from ambient pressure.

Nothing to do with more rapid heat flow to a "sink".

It is I think, more like a paddle ball.

When using a paddle ball, as the ball is flying out (power stroke) the paddle is pulled back. As this "expert" explains, there are two basic moves.


https://youtu.be/YSreLt-qR1k?si=mwVFnGj4hq57s5MY

Applying cold at just the right moment is much like pulling back on the paddle ball rubber band so that the next hit is stronger, or at least possible.

Pure "ideal" isothermal expansion (adding heat) all the way to BDC in my opinion is the equivalent of cutting the string on the paddle ball. There is not "pull back" before reaching BDC. Probably not ideal at all.

Tom, I think you are spot on with this perspective:

"So, in a sense, the heat engine is the "heat sink" taking in heat/energy from multiple sources and converting that thermal energy into mechanical motion or "work".

It is easy for most people to accept/understand, that air get´s hotter when compressed. But to acknowledge that compressed
air get´s (a lot !) colder, when expanded and preforming work, and is doing this without a heatsink, can be more difficult.

The heat does not need to run some where to get cooled.

Heat is converted to work, ergo less heat is left, that is getting cooler . . .

Is a compressed air engine regarded as a heat engine ?

There has been numerous accounts of various cold air and/or "perpetual" ice machines. Some witnessed and/or seemingly verified, virtually always involving compressed air being manipulated through a series of heat exchangers (pipes).

For example, about the same time Tesla was writing about his "cold hole" engine this article appeared in the Chicago Tribune and was reprinted in several other publications.

From: Albion Journal, Volume 31, Number 23, 21 December 1899 — Page 7


Scott Robertson says he has collected about 200 such articles or accounts, including some patents.

But of course, for each such inventor there is always a dozen highly educated and trained scientists who will aver that if such a machine existed it would be a violation of the laws of physics, therefore the inventor is ipso facto a charlitain.

Goofy wrote: ↑Thu Nov 30, 2023 9:26 am ... air get´s hotter when compressed. But...
air get´s (a lot !) colder, when expanded and preforming work, and... without a heatsink...

In a typical Stirling or other hot air engine, this occurs to one degree or another each cycle, with the compression stroke, followed by an expansion(with work) stroke.

I think, for that reason, at least in theory, the cold side of a Stirling engine, potentially, could "self-cool" more effectively than by, say, an application of cooling water.

If "self-cooling" or cooling by expansion+work output is taking place, the cold side of the engine cannot very easily or effectively get any colder than whatever it happens to be in contact with. If it is in contact with ambient air, then it can self-cool only as cold as the ambient air. If it is in contact with cooling water, then it really can't self-cool any colder than the water it is in direct contact with (the water being more effective at transferring heat than air). If it is cooled with ice, then it can get as cold as the ice by self-cooling to the temperature of the ice. Dry ice, liquid air, whatever. The engines ability to cool itself, by converting heat into work is limited by the temperature differential, but not for the reasons that have been assumed for the past 200 years.

It isn't because more heat is transfered from the hot to the cold side more quickly or more effectively. I think it is because there is more "room" for compression and expansion. That is, the oscillation is greater.

I suspect the benefits of making the cold side colder are actually pretty limited. There needs to be a lot more experimenting.

I think a very effective insulation would likely be as effective at increasing power output as any application of cold.

I'm curious about the possibility of a rotary Stirling because, why is cooling necessary? In a reciprocating engine cooling is needed to allow the piston to return after expansion. A rotary engine doesn't need to stop the rotor and reverse the rotation to bring it back to TDC, so why would it need cooling of any kind at all?

Well, theoretically, the only reason the heat is turning the rotor is because the heat is so desperately trying to make its way over to annihilate or combine with the cold, to get through the engine to the cold side.

Actually, what we generally call "cold" (the ambient temperature side of the engine) is already very HOT on the absolute scale of temperature. All we do is add a very small amount of additional heat. We could keep on adding small amounts of additional heat regardless of how hot the engine gets. Aside from preventing material failure, who needs cooling any more than what cooling the engine can provide by converting heat into power output, which seems to be quite substantial and quite beyond the so-called "Carnot limit".

Tom Booth wrote: ↑Thu Nov 30, 2023 10:50 pm
I'm curious about the possibility of a rotary Stirling because, why is cooling necessary? In a reciprocating engine cooling is needed to allow the piston to return after expansion. A rotary engine doesn't need to stop the rotor and reverse the rotation to bring it back to TDC, so why would it need cooling of any kind at all?

There's 2 issues at play that favor a piston scheme: (1) closed cycle, and (2) compression cycle. A non-compression open cycle surely favors a rotary scheme, but this is inherently both low power and low efficiency. Why "inherently" ??? Well, give it a try and you'll see why...

Once you abandon all non-compression cycle attempts, you might try something like Barber's 1791 open cycle scheme with piston compression, isobaric heating, and rotary expansion (via turbine).

matt brown wrote: ↑Fri Dec 01, 2023 9:19 pm

Tom Booth wrote: ↑Thu Nov 30, 2023 10:50 pm
I'm curious about the possibility of a rotary Stirling because, why is cooling necessary? In a reciprocating engine cooling is needed to allow the piston to return after expansion. A rotary engine doesn't need to stop the rotor and reverse the rotation to bring it back to TDC, so why would it need cooling of any kind at all?

There's 2 issues at play that favor a piston scheme: (1) closed cycle, and (2) compression cycle. A non-compression open cycle surely favors a rotary scheme, but this is inherently both low power and low efficiency. Why "inherently" ??? Well, give it a try and you'll see why...

Once you abandon all non-compression cycle attempts, you might try something like Barber's 1791 open cycle scheme with piston compression, isobaric heating, and rotary expansion (via turbine).

A "Stirling" is, I think, by definition closed cycle, and rotory engines generally, (closed cycles especially), often include some form of compression/expansion cycle.

I was thinking specifically about the rotary vane type engine under discussion recently on the other thread:

As Bumpkin suggested: "I think the vane pump could make an engine on it’s own without any other complication by heating one side and cooling the other..."

Explored further here:

viewtopic.php?f=1&t=5581&start=30#p20639

And here:

viewtopic.php?f=1&t=5581&start=30#p20643

Such a system working at all would depend on the (Tesla's*) concept of cooling via conversion to work being discussed here.

* "by continually transforming heat into mechanical work"

THE PROBLEM OF INCREASING HUMAN ENERGY by Nikola Tesla Century Magazine June, 1900

https://teslauniverse.com/nikola-tesla/ ... man-energy

Incidentally I was thinking with the duel sided rotor:
The vanes could be fit to a very close tolerance to reduce friction, so rather than rubbing against the housing they could follow a guide with a roller baaring.

Something along these lines:

The gas is expanded, inside a piston and cylinder, with work, when the inside volume gets larger. The gas does work on the piston.

The gas is compressed, inside a piston and cylinder, with work, when the volume decreases. The piston does work to the gas.

If the process is adiabatic, expansion will reduce internal energy. That is depicted by a temperature drop. Compression is the opposite with a temperature rise. This is not "cooling" or "heating". "Not", because adiabatic constraint enforces, "implies", zero heat transfer.

Heat will transfer whenever there is a nonadiabatic communication between temperature differences. Such as, hot cylinder walls and cold gas, or the opposite.

True for the engines and heat pumps we are discussing here.

Fool wrote: ↑Sun Dec 03, 2023 6:40 am The gas is expanded, inside a piston and cylinder, with work, when the inside volume gets larger. The gas does work on the piston.

The gas is compressed, inside a piston and cylinder, with work, when the volume decreases. The piston does work to the gas.

If the process is adiabatic, expansion will reduce internal energy. That is depicted by a temperature drop. Compression is the opposite with a temperature rise. This is not "cooling" or "heating". "Not", because adiabatic constraint enforces, "implies", zero heat transfer.

Heat will transfer whenever there is a nonadiabatic communication between temperature differences. Such as, hot cylinder walls and cold gas, or the opposite.

True for the engines and heat pumps we are discussing here.

As I mentioned recently on another thread:

I've pretty much avoided the "let's beat up Carnot" thread as the subject has been pretty much talked to death already and just seems to lead to endless unresolvable arguments that go nowhere.

Case in point, "Fool"'s remarks above.

Fool agonizes over his perceived or imagined severance between "internal energy" and internal thermal energy (or in common language; "heat").

For a gas, a drop in temperature as a result of "work" output produces the same result as a drop in temperature due to conduction of heat away to a lower temperature "sink".

Actually, "work" is all-in-all a more effective "refrigeration" method as conduction is relatively slow, it takes time to coax heat to move through some solid material into a "sink". A temperature reduction due to work transfer is INSTANTANEOUS and not limited by the "sink" temperature. This is how cryogenic cooling to near zero K (to liquefy helium for example) is possible in an ambient environment where there is no low temperature "sink" at such a low temperature.

One big problem is that the general field (cryogenic refrigeration) has various military applications, warhead cooling, night vision etc. so research and development in this area has often been classified, under military contracts and NDA's

If the process is adiabatic, expansion will reduce internal energy. That is depicted by a temperature drop. ...This is not "cooling"

"Cooling" is variously defined in English dictionaries as:

"having the effect of making something less warm or hot"

"noun: the action or process of making or becoming less hot."

A "temperature drop" results in something becoming "less hot", does it not? "Temperature drop" and "cooling" are therefore synonymous. The method or means by which something becomes cooler or less hot is not part of the definition.

So how "Fool" arrives at the conclusion that "a temperature drop" "is not 'cooling'" poses something of a mystery to me

That a compressed gas can expand, doing work to drive a piston or turn a turbine, with the result that the gas cools and "contracts", or can be "compressed" with virtually no, or a vanishingly small expenditure of energy to do the "compression" is well known among those familiar with the various methods of cryogenic cooling, air-cycle refrigeration, turbo expanders, expansion engines and so forth.

As a Marine engineer, Goofy, at least, having actually built several expansion engines has experience in this area and knows what he's talking about, and consequently what I've been talking about.

What I think is potentially quite interesting is a rotary vane turbine could be powered by the heat from a steam boiler, but without extremely high pressure as such a turbine operates, or would operate on the heat transported through the engine by the steam rather than from any pressurization of the steam.

Of course a low volume, intermittent pressure differential would be generated internally with helium gas or whatever, within the engine, but there would be no large high pressure boiler to potentially explode.

I think each chamber, between the vanes, could act very much like the fixed quantity of gas rapidly expanding and contracting inside a thermal lag engine.

A rotor could be of virtually any size with each chamber around the perimeter acting as a separate power "cylinder".

By having many vanes, in one rotor, say 12 or 24 or more, a very high torque could be developed.

I like your theories!
I do agree with the cooling by expansion part. It would take a lot of testing to get it right though. Especially for it to be a closed cycle. Because if it doesn't cool down enough you end up with continually hotter and hotter cycles. And there isn't very much separation of zones in a rotor, the cold side will heat up pretty fast as well, just from conduction.



I'm looking at a way to make this possible for my generator, but I'm running into a few mechanical problems. One would be the sealing on the wall as the vanes wear down a bit.
This could maybe be solved with springs between separate vanes. But in a rotor setup this would mean that one side would be working to seal against centrifugal forces trying to fling it outwards.
I would love to find a way to run one rotor inside another. As that would solve a few issues for my plan. So if you have any thoughts on that I'd be happy to hear/read them.

matt brown wrote: ↑Fri Dec 01, 2023 9:19 pm There's 2 issues at play that favor a piston scheme: (1) closed cycle, and (2) compression cycle. A non-compression open cycle surely favors a rotary scheme, but this is inherently both low power and low efficiency. Why "inherently" ??? Well, give it a try and you'll see why...

This is an easy way to make a statement. Can you elaborate on why a rotary system is inherently low power and efficiency? There are plenty of strong rotary motors out there working on compressed air. Also rotary vane compressors outcompete piston ones in efficiency.
So I'm wondering what insights you have that I'm missing.