Stirling Engine Thermodynamics

[Tom Booth]

Most of my bought model engines have graphite Pistons, which don't seem to do well when wet with condensation either.

I've thought about ways of possibly making use of this though, with an engine running on ice or cold in particular.

The heat released by condensation doesn't really contribute much to the running of the engine, since the condensed water sits there and likely cools the engine by re-evaporating.

But if the top of the engine were dome shaped, the condensation would run off.

So, what about a cold running engine powering a water fountain that distills water from the air. The condensation contributes heat at the top hot side, then runs down and evaporates and assists in cooling the cold side while simultaneously supplying the fountain with clean distilled water.

[MikeB]

Far from being a curiosity, in my solar engine, the water vapor condenses in the power cylinder and will slow it down and then stop it.

Have you tried using silica gel (or other dessicant) to dry your air? It doesn't need to be pressurised to do this, you just need a decent seal to ensure that the dry air doesn't get replaced with damp outside air over time.

[Nobody]

In the Burnt Pancake thread Tom Booth quoted Tesla:

Tesla scratched his head a little bit and said what if instead of using heat (above ambient), you make a "cold hole". Then you can use the heat of the ambient as it flows in naturally and you will never run out so you don't have to put it back. It is no longer a closed loop. It is a linear system.

Sunshine Hits the earth >>>> Hot Ambient > Heat > Heat converted to Pressure in a Heat engine > Motive Force (work) > Electricity Generation > Eventual heat dissipation into Outer Space >>>>>

A compressed air engine is such linear system. Until you need to recompress the air. A heat engine starting with a hot working fluid, expanding so that it produces maximum work allowing the pressure and temperature to equalize with deep space, will work for one cycle. You will have to either launch it fully assembled and charged into space, or expend a lot of work creating cold evacuated space/sink in a warm atmosphere. That work, will be more than the amount gained back by the maximum single cycle you will get. However, it will work once.

Tesla's concept misses the effects of gravity and the concept of repetition or cycles. It can't be linear if it needs to cycle. All heat engines are cyclic, even if not obvious. Thanks.

[Nobody]

Insulating material discussion should be in the Burnt pancake displacer (carbon foam) thread. Or perhaps a new thread on mythical-ideal-perfect insulators and reflectors.

Gold is one of the best reflectors of infrared photons. Carbon black is one of the better absorbers of heat photons/infrared.

NASA uses the principal of shading reflection and deep space blackbody absorption to cool things such as the space station. Perhaps also their Stirling Engine's cold sink. I would.

Thermodynamics, Tesla, and Carnot theories should be here in this thread.

In the burnt pancake thread you said:

In other words, maybe what a heat engine needs to run is not hot and cold but heat and NO HEAT.

Heat on one side and a vacuum on the other.

However, a vacuum can still radiate heat, so radiant heat would have to be blocked also, with something like that "Starlite" material.

You clearly misuse the meaning of heat, hot, and vacuum.

Hot is a state of a system or part and is quantified by temperature. Kelvin if you'd like. It could be looked at as voltage, pressure, or potential/(height-gravity).

Heat is measured in Joules. Specifically transfered energy. Example: How much heat transfered in to raise a cc of water one degree K. It depends on the heat capacity of the material.

A vacuum is the absence of mass. It is transparent to photons. Infrared photons ("transporting heat") pass right through it. Vacuums do not irradiate, for there is nothing to contain heat. Transmit, pass through, transparent,... Yes. But not contain. Since there is no mater in a vacuum the two best forms of heat/energy transfer, conduction and convection, are zero. Only radiation is left. Thermos bottles are made from reflecting materials to help in that area.

Hot: Think of a high temperature as high voltage or pressure. If you want higher electric currents, turn up the voltage. If you want faster heat flow heat up the source to a hotel/higher temperature. There are no negative temperatures as there are negative voltages, so don't even think of comparing efficiencies.

Faster heat flow can also be increased by larger surface areas. That is similar to reducing the resistance in an electrical circuit, like larger wires.

It is like wanting some mass to move faster, hit it with a bigger faster-moving hammer, i.e., a bigger force.

What drives the heat transfer in a heat engine is a temperature difference. Heat is the energy that is driven. The wider the difference the faster the heat flow potential.

Energy in an electrical circuit is a product of voltage and current and time and is measured in Joules. Energy flow is called power and is a product of just voltage and current.

To get a heat engine to work, requires energy-flow/heat. To get heat, requires a temperature difference/hot-cold. There can be no heat without a temperature difference. If the engine can't shed heat to the cold space it will be capable of one cycle. Hot directly to work out. Like a compressed air engine. When the pressure is out, it is done. No amount of heating the empty cylinder will make it run. Thanks

[Tom Booth]

... If the engine can't shed heat to the cold space it will be capable of one cycle. ...

That is also what I thought for years. But then I read the many, many text, as quoted near the start or this thread that say a hot gas can be or is cooled, not only by loosing heat to a sink, but also by doing work. The gas, in doing work uses up energy, so that the kinetic energy of the gas is converted to the mechanical work output of the engine.

Carnot said ALL the heat passes through the engine.

Tesla stated that because heat is energy ALL of the heat could be converted to work so that very little, or even no heat at all needed to reach the sink.

In my study of the subject, I encountered these many different completely irreconcilable assertions regarding what exactly happens to the heat that goes into running a heat engine.

Finally, to settle the matter in my own mind, ten years after first stumbling across the conundrum, I went ahead and purchased several Stirling engines.

In several experiments I made every effort I could think of to ensure that the engine could not shed any heat to the cold space.

I thought if Tesla was right, the engine might be able to continue running, but if Carnot and nearly EVERYONE ELSE was right, and only a small fraction of the heat could be converted, the rest having to pass through to the sink or cold space to complete a cycle, or even All the heat needing to pass through to the sink, then the engine should slow down and stop.

I fully expected that a Stirling engine with the cold side blocked with insulation so that it could not shed heat would immediately slow down and stop or not run at all.

So far I've tried every kind of insulation I could find on hand or buy from the hardware store. Nothing works. The engine not only keeps running in apparent complete defiance of Carnot and most accepted theory, but it runs better, faster, more vigorously when not allowed to shed heat to the cold space.

So what is going wrong here?

The insulation is not insulating? The insulation is actually conducting MORE heat?

Seems far fetched to me, but maybe.

So I try making the cold side of the engine out of Acrylic, which is hundreds of times less heat conductive than the original aluminum cold plate

What is the result? The engine continued to run. With both an acrylic "sink" and covered by additional insulation, the engine did not stop or fail to run.

What else can we do?

Well, even a complete vacuum allows heat to radiate through, this carbon foam is supposed to block heat and radiation. Maybe a vacuum plus carbon foam.

I'm not suggesting actually using outer space or creating a vacuum inside the engine, or allowing the gas in the engine to expand into an actual vacuum. Just to block the heat from being shed to the cold space by insulating it with some combination of a vacuum, such as a Dewar or thermos type surrounding and some kind of carbon foam.

I think your misrepresentations and allegations that I (and/or other researchers) am lying, "cherry picking", practicing pseudoscience etc.are not at all justified

There are thousands of topics in this forum for people to post to if they choose, and you and anyone are free to start new threads, new discussions. I've started just half a dozen or so threads over the past ten years, maybe a few more I've forgotten. Your accusation that I'm monopolizing the forum by posting my "pseudoscience" to "every thread" is ridiculous. You have singled me out for attack, pretending that you wish to be helpful and educate us all, but from your various comments and posts you appear to have, not even a rudimentary understanding of how a Stirling engine actually is supposed to work, never mind any advanced thermodynamics.

I welcome any constructive criticism, but so far, you have offered no suggestions as far as how any experiment might be improved, you only offer insults, ridicule, accusations and attempts at character assassination.

Unless you have something in some way constructive or coherent to put forward I will not be engaging or responding to your posts any further. Strain as I might, I have some real difficulty trying to make sense out of them anyway. Where you get the idea I'm trying to create the vacuum of outer space INSIDE the engine and the like.

Sorry if my meaning was not always clear. I'm more than happy to try and clear up any misunderstanding, but given your barrage of personal attacks, I don't think understanding is what you are looking for.

[Nobody]

Try this simple experiment. Ltd Stirling running on hot water, say it turns clockwise when heated on the bottom, atmosphere on top.

Run it now with atmosphere on the bottom and ice on the top. Does it turn the same direction? Clockwise?

If so does that mean it doesn't matter the absolute temperature, only the difference? And direction? Does this also mean it is important for the cold side to shed heat?

Now, take a normalized to room temperature engine, insulate the cold side. Put ice on the insulation. Does it run immediately? How about if it sits longer? How long does it take for the cold to soak through and start running?

Does it behave differently from expected? If so, how?

Ltd engines transpire very little heat and work. It might take three weeks to heat up insulation enough to quit running, or not at all if, the outflow of heat, from the cold side to the atmosphere, is greater than the work generated.

Try your insulation experiments on a large powerful high temperature differential Stirling Engine. It should have the potential to heat up the insulation more quickly. Run it with different loads.

[Nobody]

The kinetic energy of the gas (heat) is slowed down by bouncing off the moving wall (piston), in such that the returning molecular speed is less. That momentum is transfered to the piston. It, in layman's terms, is heat converting to work. In reality both are momentum. 1/2mv^2 no conversion needed.

To get all the heat converted to useful mechanical movement, would require expansion until the gas reaches zero velocity, spin, and vibration. That requires an infinite length adiabatic cylinder and zero friction. Tesla's comment doesn't violate Carnot's rule, it just is an incomplete viewpoint. Once you complete it with the ignored science it is obvious why it is myopic.

[Tom Booth]

....

To get all the heat converted to useful mechanical movement, would require expansion until the gas reaches zero velocity, spin, and vibration. That requires an infinite length adiabatic cylinder...

Sorry, but I've heard this nonsensical argument dozens of times over the years and it is simply wrong. Completely irrational, without foundation.

Converting "all the heat" down to absolute zero is, of course, a ridiculous proposition. Why it is continually put forward, again and again and again as an argument, by otherwise seemingly educated, intelligent people is beyond me

If someone heats up a cup of water from ambient, say 70°F to boiling it does not require the removal of "all the heat" down to absolute zero to bring the water back into equilibrium with the 70°F Ambient environment.

Nor is it required that the gas molecules in a Stirling engine, first heated a few degrees above the surrounding ambient be reduced to "zero velocity, spin and vibration" for those molecules to be brought back into thermal equalibrium with the ambient environment.

The gas in the engine did not start out at absolute zero before being heated, therefore there is no necessity to cool it to absolute zero for it to return to the state where it started.

I must admit, however, that such ridiculous nonsense is the current standard of education on the subject at the university level. How such a ridiculously absurd notion manages to perpetuate from professor to student, year after year, from one generation to the next without anyone ever giving it a second thought, I'll never understand.

[Nobody]

You said: "Tesla stated that because heat is energy ALL of the heat could be converted to work so that very little, or even no heat at all needed to reach the sink."

The delta heat you speak if is not "ALL of the heat", you claim of Tesla. The delta heat is subjected to the Carnot efficiency rule. If you build or find an engine that brakes that law, please put it forth for testing.

[Tom Booth]

You said: "Tesla stated that because heat is energy ALL of the heat could be converted to work so that very little, or even no heat at all needed to reach the sink."

The delta heat you speak if is not "ALL of the heat", you claim of Tesla. The delta heat is subjected to the Carnot efficiency rule. If you build or find an engine that brakes that law, please put it forth for testing.

"Heat" as I'm pretty sure you are aware, since you recently went to some lengths to define it, as distinguished from "hot", by definition, is the transfer of kinetic energy.

Once equilibrium has been reached, transfer of energy (heat) no longer exists or comes to a halt. So to talk about the engine needing to remove or utilize "all" the heat supplied to it, from its own internal working fluid, means simply to remove or utilize whatever energy was supplied to heat it up above the starting ambient baseline.

To interpret "all the heat" to mean all the heat in the universe down to absolute zero is to make an absurd and unwarranted leap of imagination into the realm of the rediculous.

It is a straw man argument.

[Tom Booth]

It might be worthwhile to try and forget or put aside all the "Carnot limit" nonsense, (and IMO it is just that, utter nonsense) and consider what is really going on in a Stirling heat engine.

BTW, just as an historic factoid I found somewhat interesting. Stirling invented his engine (patented 1816) before Carnot published his theories.(1824). Carnot in his writings, having heard rumors of hot air engines, expressed doubt that they actually existed.

Also, the so-called Carnot efficiency formula often erroneously attributed to Carnot did not originate with him. It was worked out much later, perhaps by Kelvin, but the idea of an absolute zero did not exist at the time. Any zero temperature refered to by Carnot had to be that of the Celsius scale.

Anyway, 100% conversion of heat into work is not considered impossible In fact, the process of heating a gas in a cylinder to drive a piston to do work, as a single process, is considered a complete conversion of heat into work, neglecting loses to friction.

It is only when making reference to a cyclic process that the so-called 2nd law comes into play. The supposed problem is, how to get the piston back to the starting point.

To site one reference:

All standard heat engines (steam, gasoline, diesel) work by supplying heat to a gas, the gas then expands in a cylinder and pushes a piston to do its work. So it’s easy to see how to turn heat into work, but that’s a one shot deal. We need it to keep repeating to have a useful engine. The heat and/or the gas must therefore be dumped out of the cylinder before the next cycle begins, otherwise all the work the gas delivered on expanding will be used up compressing it back!

https://galileo.phys.virginia.edu/class ... Engine.htm

The rationale here, however, is not only illogical, it is a violation of the first law of thermodynamics.

If in pushing the piston out, all of the added heat was converted to work, then how can any of the heat that was added still be left in the cylinder to require removal?

If heat were a fluid, like water, then filling the cylinder to push out the piston would result in a cylinder full of water/fluid, and the water/fluid/heat would have to be removed. But heat is not a fluid, it is energy, and conservation of energy dictates that heat cannot be converted to work and still remain to be transfered to a sink. It's a quite blatant and obvious contradiction, so how people continue to write nonsense like the above quoted paragraph in this day and age, when it is well established that heat is not a fluid, I can't understand.

Either the heat is energy and was converted to work (really the kinetic energy was transfered), or heat is a fluid and entered the cylinder under some unknown form of hydraulic pressure associated with differences in temperature, or some such thing (entropy?)

Both ideas are compelling in their own way, but both cannot be right, I don't think, unless we are dealing with some weird quantum partical/wave duality phenomenon.

Anyway, let's imagine this:

A little heat is introduced to the cylinder by raising the displacer to expose the surface of the hot heat exchanger. The gas "expands" a little, that is, some molecules gain some kinetic energy. Soon those hot molecules bouncing around at a higher rate of speed strike the piston, the energy is transfered to the piston and the piston moves a tiny bit, the molecules that hit the piston lose energy and cool off, but, the piston, having been set in motion tends to stay in motion. If no more heat were added at this point, and if the piston and cylinder were perfectly frictionless, the fact that the volume of the gas had expanded would mean that the gas pressure inside the engine would be lower than it started, lower than outside, because the piston was driven out, but the gas did work and cooled back down, having transfered it's energy to the piston.

So, at this point the outside atmosphere, the energetic air molecules on the other side of the piston, would push the piston back to re-establish equilibrium, but...

At this point, the displacer moves a tiny bit more, and so more air becomes hot, and bouncing around, ends up striking the piston, adding a bit more to the pistons already established momentum.

Again those molecules cool down, having transfered their energy to the piston, and so the outside atmosphere would now push the piston back (with a bit more vigor) to re-establish equalibrium, but ..

The displacer moves a bit more, introducing a little more heat to energize a few more molecules that work to keep the piston moving and building up momentum.

This process continues until the displacer has gone through it's motion and introduced as much heat as it is going to, but, by this time, the piston has built up quite a bit of momentum and so keeps traveling, though the gas that propelled it has long since transfered all of it's kinetic energy gained from the added heat to the piston.

The gas, because the piston keeps moving, is now being mechanically expanded due to the momentum of the piston and so becomes colder colder even than what it was before.heat was added. The displacer is now, once again, covering the heat source, so no new heat is being added, but the gas continues to expand and cool resulting in a sharp drop in internal pressure.

At this point, the outside atmospheric pressure, having been for a long time, delayed, now drives the piston back inward with great force and without any resistance from internal pressure.

At the point when the piston is nearly driven all the way back down the cylinder by atmospheric pressure, the internal pressure and temperature being re-established, the piston, having again picked up momentum, but in the opposite inward direction, starts compressing the gas, transferring energy back to the gas while the displacer again rises, again, introducing a blast of heat, the result is a sudden intense rise in temperature and pressure again driving the piston outward... A bit more vigorously this time

As this oscillation continues, the engine picks up speed and strength

Notice, there is no "left over WASTE HEAT" in this cyclic process, no violation of the 1st law of thermodynamics, no contradiction of heat having been converted entirely to work but still requiring removal,

As far as I can see, the "Carnot limit" is nothing but a fairy tale It is completely bogus

Nobody has to look very far to find an engine that violates the "Carnot limit". Any off the shelf, common model Stirling engine on the market will do that.

"Carnot efficiency" is pure mythology. It should have been scraped as any kind of theory 100 years ago, or more like 200 years ago.

[Tom Booth]

Before a Stirling engine is given a nudge to get it started, the displacer is covering the heat source keeping the gas inside the engine insulated / isolated from the heat.

The entire engine then is at thermal equalibrium with the environment, except for the hot heat exchanger, but that part of the engine is blocked off.

So the engine requires a little push, which exposes the source of heat, which sets the above described cyclical process of intermittent heating and conversion of heat to work in motion.

What brings the temperature of the working gas in the engine back down into equalibrium with the environment to complete the cycle is not " rejection " of heat to that environment, but rather the conversion of the heat, or microscopic kinetic motion of the gas molecules to the macroscopic kinetic motion of the engine.

There is no necessity that any of the added heat be "rejected" or transfered to the sink.

[Nobody]

The gas, because the piston keeps moving, is now being mechanically expanded due to the momentum of the piston and so becomes colder colder even than what it was before.heat was added. The displacer is now, once again, covering the heat source, so no new heat is being added, but the gas continues to expand and cool resulting in a sharp drop in internal pressure.

At this point, the outside atmospheric pressure, having been for a long time, delayed, now drives the piston back inward with great force and without any resistance from internal pressure.

At the point when the piston is nearly driven all the way back down the cylinder by atmospheric pressure, the internal pressure and temperature being re-established, the piston, having again picked up momentum, but in the opposite inward direction, starts compressing the gas, transferring energy back to the gas while the displacer again rises, again, introducing a blast of heat, the result is a sudden intense rise in temperature and pressure again driving the piston outward... A bit more vigorously this time

You have left out the work direction crossover point and descriptions for those parts of the strokes.

The piston is acted on by two important forces. One from the working fluid pushing outwardly. And another from the atmosphere pushing inwardly. They are in opposition and always present. Never delayed.

The atmospheric force is constant at approximately 15 psi times the area of the piston. The internal or working fluid pressure changes as the cycles and strokes progress. Neither goes away.

Work flow is in the direction of acceleration. When the piston is accelerated, work goes into it. When it is decelerated, work comes out of it. When a fluid is compressed, pushing against the pressure, work goes into the fluid and it warms. When a fluid is expanded, work comes out and it cools.

When the internal working fluid pressure is greater than atmospheric, the piston is accelerated outwardly. Kinetic heat energy converted to momentum. The piston accelerates. Work goes into the piston. This is the first half of your depicted stroke. The atmosphere has work done on it, it absorbs heat.

When the two pressures become equal (call this position 1), the work into the piston stops. There is zero net force on the piston. As momentum carries the piston further, the inside pressure continues to do work on the piston. The working fluid pressure becomes lower than atmospheric and cooler. That temperature difference drives more heat into the working fluid from the cold side.

Also, for the entire stroke, the piston has work being done on it by the atmosphere. After position 1 that work becomes larger than the work supplied by the inside working fluid, and in opposition. So the piston slows. The inside fluid cooling with heat added from the cold sink. At the same time, heat is added to atmosphere. Energy being lost by the piston and working fluid goes into the atmosphere through the motion of the piston.

When the momentum is exhausted, the piston stops. The outside pressure being greater, the piston is accelerated inwardly. The piston, again absorbs heat energy and stores it as momentum from the atmosphere. Inwardly on this stroke. It absorbs energy from the atmosphere while in opposition to the working fluid pressure.

At the same time it is also converting work into heat and pressure by compressing the inside working fluid. It is doing work to the inside working fluid. The inside working fluid, in opposition to the motion, is having work done on it for the entire stroke. It responds, for the entire stroke, by getting hotter.

When the two pressures are equal, (Call this position 2), the force on the piston again is zero. Momentum carries it further. The piston is now slowing from the higher force inside. The piston's momentum is being converted to heat and pressure in the working fluid. The atmosphere is also working on the piston. That work is less than what the working fluid is receiving, depicted by the piston slowing.

Once position 2 is reached the temperature goes above the temperature of the cold side. Shedding of waste heat is powered by that temperature difference.

That shedding of heat continues until the momentum is exhausted. Hence what Carnot observed and calculated in all heat cycle experiments of his day. Since then it has been reaffirmed with the kinetic theory of thermodynamics, and modern improvements since then. You can not win fighting Carnot, men way smarter and better than I and maybe you too, have tried. Your experimental reports here haven't been conclusive enough to challenge that. But, I like your experiments.

An interesting note I thought of while typing this: Position 2, above, will be further towards the outside of a cylinder than Position 1. The heat absorbed from the cold side will make the working fluid more expanded at the equal pressure position.

Thanks.

[Nobody]

Imagine a double ended sealed glass cylinder with a graphite piston centered inside. Air can't get out or in. There is an equal amount of air, same mass, on either side of the piston.

The piston would be centered, pressure and temperature on either side are equal. Would it take an outside force to move the piston? Once moved and suddenly released, wouldn't the piston oscillate until the momentum energy is absorbed by friction and heat loss??

If the piston is pushed, and held/latched, to the left, the gas in the left side, will heat up and heat will go into the cooler walls. The right side gas will cool and heat will transfer from the walls to the fluid. Both heating and cooling are accomplished by adding mechanical work. Minus friction.

It is takes work input from outside to produce both this cooling and heating.

Now we are depicting why the Stirling hot side adds heat, and the cold side removes heat.

Change things on the above. Move slowly enough so some heat of compression on the left side gets out. Heat the right side. Pressure from the heated right side pushes the piston left heating the left fluid above ambient, the walls cooling down it down towards but not to ambient.

Flip flop. Take all the heat from the right side and turn off the heater let ambient cool the right side. Move the heat to the left side. Insulate from ambient turn on left heater. This is accomplished by a displacer and regenerator.

The piston is now driven to the right.< First 1/2 | Second 1/2> At some point the pressure in the left equals the pressure in the right and the piston begins slowing. This is where it goes from work into piston to work out of piston. The gas on the right is being compressed while shedding heat to ambient during this half stroke.

[Tom Booth]

Most of your analysis (a few posts back) I agree with.

Of particular interest are these statements or conclusions:

As momentum carries the piston further, the inside pressure continues to do work on the piston. The working fluid pressure becomes lower than atmospheric and cooler. That temperature difference drives more heat into the working fluid from the cold side.

Also, for the entire stroke, the piston has work being done on it by the atmosphere. After position 1 that work becomes larger than the work supplied by the inside working fluid, and in opposition. So the piston slows. The inside fluid cooling with heat added from the cold sink. At the same time, heat is added to atmosphere.

We seem to be in agreement that some heat is taken in from the sink. This seems to contradict what is generally supposed, but a careful analysis appears to make the conclusion rather inescapable.

The main point of contention would be with your conclusion:

Once position 2 is reached the temperature goes above the temperature of the cold side. Shedding of waste heat is powered by that temperature difference.

That shedding of heat continues until the momentum is exhausted.

In a thermal Lag / laminar flow or thermo-acoustic type Stirling, the gas is being crammed down into the hot end of the cylinder towards, and into the source of heat. So, although it is true that the internal temperature climbs higher than the sink, the gas has left contact with the sink or cold end of the cylinder so could not shed heat to the sink at that instant.

In a displacer type engine, the displacer moves 90° (more or less) ahead of the piston, so that by the time the gas has been fully compressed and heated, the working gas has been shunted to bring it into contact with the hot heat exchanger. So again, the working fluid has been removed from contact with the sink.

Granted, in an engine with sinusoidal motion there is some mixing and overlap, but I don't think it can be truthfully said that: "shedding of heat" (to the sink) "continues until the momentum is exhausted" as at the peak of the compression stroke, the working gas is in full contact with the heat input side of the engine and fully out of contact with the sink, so shedding heat from the working fluid to the sink at that juncture is physically impossible.