Stirling Engine Thermodynamics

[Tom Booth]

For anyone who might be interested I have posted an Indigogo campaign to raise funds to build a prototype Stirling Engine. I've been theorizing on this subject for years and if possible would like to move beyond just theorizing an making sketches and such and do some real testing. According to Tesla, a heat engine should be able to maintain it's own "cold hole" or sink by using some of the energy produced to run a heat pump of some kind to remove excess heat.

Tesla was trying to do something which the majority of the learned scientist of his day considered an impossibility, and so it remains. Regardless, a heat engine CAN at least run on ambient heat AND evaporative cooling: https://www.youtube.com/watch?v=ARD3ctp80ac

A wet piece of paper does not provide much of a "cold hole" IMO and I think it might be possible to extend this idea in various ways.

I'm also not at all sure that concentrating ambient heat in the air with a compressor and using the heat and cold that can be produced by such means to run a Stirling Engine is actually a violation of any law of thermodynamics as there is mass in the form of air warmed by the sun moving through the system used as "fuel". This is something quite different from simply using a Stirling Engine to operate a second Stirling engine in reverse. Quite obviously this would not work in a closed loop but what is being proposed is not a closed loop system.

Any additional feedback would be appreciated. Thanks,

Tom

[Tom Booth]

At the begining of this thread, I had stated that according to my understanding of Stirling engine thermodynamics, in the process of converting heat into mechanical motion a Stirling engine produces a cooling effect. A potentially huge cooling effect. Possibly even functioning like a refrigerator during part of the cycle when the air in the engine is being expanded by the momentum of the piston.

I put forward the idea that maybe, if that were true, puting insulation around or over the cold end of the engine would help to keep it cold and improve the efficiency, since heat would not be entering the "sink" which the engine was actually refrigerating to some degree.

Well, after all these years my curiosity finally got the better of me and I went ahead and sent for some Stirling engine kits to experiment with.

Please tell me what you think. This engine ran over an hour longer with insulation covering the sink. (In this case the top, it was running on a cup of hot water). Longer, that is, than it ran previously without all that insulation.

According to conventional heat engine theory, it should not have been able to run much at all.

It was rather difficult recording this, holding the camera with one hand and trying to tape insulation over the top of the engine with the other.

https://youtu.be/fFByKkGr5bE

Am I crazy, or did it also start running just a little faster, after it settled in with the insulation covering the sink?

To me it appears the engine starts running slightly better or faster with the insulation, after it recovered from my accidently slowing it down when the insulation rubbed against the flywheel.

[Tom Booth]

With the help of a friend with a stop watch, I counted the RPM of the engine before insulation was used to cover the cold side or sink, and after.

I got:

RPM with no insulation on top: 162

RPM with the cold sink insulated: 180



That is an increase in RPM of about 55 revolutions per minute, give or take a revolution or two, but I think that is a fairly accurate count.

[Tom Booth]

Sorry, 18 rpm faster.

I miss-calculated that somehow but wasn't able to edit it in time to correct the numbers.

I got some more nylon bolts at the hardware store, so I can put another engine together and run some more tests, but do you believe after I posted the RPM numbers to a Physics forum, they locked the discussion because they said it was "perpetual motion" and/or "a violation of the second law of thermodynamics".

https://www.physicsforums.com/threads/s ... ne.991714/

So now, apparently model toy size Stirling engines have been officially deemed "perpetual motion" and a "violation of the second law of thermodynamics".

Unbelievable. what has happened to objective science or even scientific curiosity?

Good grief!

[Tom Booth]

So,.. do you think insulating the small piston cylinder might cause the engine to overheat and stop?

In this video, I've insulated everything, including most of the power cylinder, though the top 1/2 is still partly exposed.

I don't think I actually agree with your statement, but in order to rule it out, I can also more thoroughly insulate the cylinder. This is how it looks now:

IMG_20200809_020541777_resize_13.jpg (29.04 KiB) Viewed 10370 times

IMG_20200809_020528596_resize_19.jpg (31.68 KiB) Viewed 10370 times

IMG_20200809_020520633_resize_53.jpg (34.08 KiB) Viewed 10370 times

As can be seen, there is still a slight air gap.

This engine ran 25 RPM faster running on hot water after the top plate was insulated. That is almost triple the engines advertised speed of 100 RPM (+)

https://youtu.be/zEqg1TgLqXI

I timed the RPM at 295.

[Tom Booth]

I ran the experiment again, with additional insulation around the piston and cylinder, as much as possible without interfering with the mechanics of the engine.

There was no change.

The engine still ran 25 RPM faster than it did before adding any insulation to the top "cold sink" at a steady 295 RPM. According to my count, immediately after making this video:

https://youtu.be/Iq6snxiXbGg

I can agree that the piston is an energy sink of sorts in that it is the mechanism for converting heat into mechanical motion.

[Tom Booth]

Tom, when you think about the operation of your engine, you have to keep in mind that its working piston not only manipulates the working medium inside the engine, but also pumps the "cold" atmosphere around the engine, which cools the engine. Therefore, it is logical that your engine, after applying the insulation, will increase its number of revolutions by "self-regulating" in order to increase the amount of air it now needs for cooling. But from this kind of engine operation "at idle", you can't conclude anything about the power, thermodynamic or operational efficiency of your engine (although it is probably higher). With the help of insulation - you are manipulating the direction and speed at which your heat flows (from a cup of hot water). With insulation - you just manipulate the temperature difference in your machine (Heat engine, by its own work, can neither overheat nor subcool!)!
Tom, do you think that the designers, constructors and owners of these toys have not (so far) already played with such experiments? Why all this? Happy experimenting!

Why all this?

Mostly it seems like there is an awful lot of strange ideas out there that purport to explain how a heat engine actually works, which make no sense to me and completely contradict my own observations and even common sense.

The "Carnot efficiency" of my little model engines can be calculated to be AT MOST 18.9% given that it was running in that video, on boiling water in a room at 85 degrees fahrenheit..

That means, according to the so-called laws of thermodynamics, that more than 80% of the heat applied to the engine MUST ABSOLUTELY pass right through to the sink for the engine to operate at all.

If that is true, clearly then, completely smothering the "sink" of the engine with insulation should prevent it from operating. The fact that in actuality, it runs faster, indicates to me that the people making these videos have no conception of reality as far as how a heat engine REALLY operates

https://youtu.be/Qdb2hpgeu8Y

https://youtu.be/LUoUb4hGMH8

https://youtu.be/_n3Z_YBzvDQ

It seems they have become completely lost in mathematical abstractions that were arrived at more than a century ago by a French philosopher who never even laid eyes on an actual steam engine or heat engine of any kind.

If a Stirling engine is taking in heat above ambient and the temperature of the heated air inside the engine in the course of its operation, is falling below the temperature of the ambient by the time the air reaches the "sink", so that insulating the "sink" allows the engine to run faster, then common sense tells me that 80% or more of the heat is not just passing right through the engine to the other side.

Anyone actually familiar with these engines knows that, running on a cup of hot water, the only way to get the engine to run faster would be to cool the sink with an ice cube or something. It stands to reason then that the insulation is preventing heat from entering the engine, allowing it to run cooler, just as I said might happen when I started this thread ten years ago with the first post:

I had written:

What I'm wondering is just how much heat is actually being absorbed in this way i.e converted into work as opposed to the heat being absorbed by the heat sink (the cold end of the chamber at ambient temperature).

If more heat is extracted as work than what actually reaches the heat sink, then theoretically, insulating the cold end of the displacer chamber against the external ambient temperatures would improve engine efficiency

.

I think my observations at that time were correct.

Anyway I have sent away for this four channel thermometer to get some objective readings.

DIGITAL-4-CHANNELS-THERMOCOUPLE-THERMOMETER.jpg (97.94 KiB) Viewed 10348 times

https://perfectprime.com/products/tc981 ... f95c&_ss=r

This seems to be a relatively inexpensive but high quality thermometer with multiple probes that can measure the actual temperature of the room as well as the hot and cold plates of the engine simultaneously.

Common sense tells me that if I add 50 joules of heat to a heat engine to create an imbalance, the engine only needs to utilize those 50 joules of heat to bring the temperature back into equilibrium with the environment.

So what is all this nonsense that to be 100% efficient, a heat engine would have to operate at "absolute zero"???

It makes no sense.

[Bumpkin]

It's sorta like saying a water wheel would produce more power by lowering water from ten feet to zero feet elevation, than it would produce by lowering water from twenty feet to ten feet elevation. I don't live at sea-level, but fortunately that has nothing to do with efficiency.

Yes the closer a given temperature difference gets to absolute zero, the more potential is represented, but efficiency isn't a measure of energy conversion from non-existent potential.

Some of these professor types could fill up a chalkboard trying to solve one-plus-one, and still come up with the wrong answer.

Bumpkin

[Tom Booth]

Lol,

To me saying that a perfect heat engine has to cool the air down to absolute zero is kind of like saying that a perfect ladder, when it comes time to climb back down, must reach down all the way to the center of the earth.

[Tom Booth]

Tom, you mistakenly call this adiabatic expansion (4-1), from Tmax to Tmin, "cooling effect" because it does not exchange any heat, but only changes the temperature of the working medium! However, you correctly conclude that adiabatic processes must take place under insulation in order to be adiabatic (without heat exchange)!

I don't believe I've ever concluded that: "adiabatic processes must take place under insulation", because I don't believe that to be true. Usually, or often, adiabatic means that a process was too fast for heat to be conducted, but in such a circumstance energy can still be exchanged as "work".

There is not just a temperature change or cooling due to expansion, because, in a sense, the heat has more space. Rather heat leaves the system AS work, having been converted to work. Though really, at least in theory, this is just a transfer of kinetic energy.

In other words, "pressure" is the combined kinetic energy of air molecules striking the piston, driving the piston. The piston moves because of a transfer of kinetic energy from the "hot" air molecules to the piston. This loss of kinetic energy from the gas (to the piston) IS a lowering of the temperature of the gas, (which temperature is a measure of the kinetic energy of the gas.)

This heat/ kinetic energy is not conserved. It leaves the system as work. Such lost or used heat (used to do work) is replaced by the heat applied to the engine, not by re-compression of the air.

Adiabatic compression does not cause significant re-heating of the gas IMO. Such compression would be caused by stored momentum in the flywheel driving the piston back inward, compressing the air. This, apparently is not the mechanism or actual process, because Stirling engines can be observed operating without a flywheel.

Logically, a flywheel that is not there cannot store energy, rather the piston returns as a result of the adiabatic cooling, which causes the air in the cylinder to "contract". Or put another way, to loose energy (heat/pressure or kinetic energy) to a level below atmospheric heat/pressure/kinetic energy.

[Tom Booth]

The efficiency of heat conversion into work is directly proportional to the temperature difference,

That is Carnot's conclusion but personally, I think the idea is based on an erroneous concept of heat (as "caloric" or an indestructible fluid) and has nothing whatsoever to do with what most dictionaries (or people) define as efficiency.

Carnot thought a heat engine performed work as a result of the "FALL" from a high to a low level, like water falling and turning a mill wheel, without any change to the water-like substance: "heat".

In that sense, I guess there is some justification for associating "efficiency" with the "fall" in temperature, but even there, the term seems misplaced to me.

I could, for instance, make a waterwheel with paddles made from chicken wire. The "efficiency" would be very low, but the height of the "fall" has not changed.

The maximum energy available for extraction or conversion, has what to do with efficiency?

To me it's like saying I have five gallons of gas in my tank and I can get 20 miles per gallon.

So the logic is, if I put 10 more gallons in my tank, I have increased the "efficiency" of my car engine by 200 miles.

If heat is fuel for a heat engine, then increasing the heat (temperature difference) has no effect and no bearing on "efficiency", any more than putting more gas in my car increases my miles per gallon.

[Tom Booth]

Tom, you're overcomplicatedly explaining (to yourself) what a "heat engine" is and how it works. It's all much simpler. A heat engine is a device in which a “working medium” undergoes cyclical changes of its state, whereby heat is converted into useful work. In the Carnot engine, the Carnot thermodynamic cycle takes place: consisting of two "isothermal changes of the state of the working medium" and two "adiabatic changes of the state of the working medium", which alternate during the cycle: isothermal compression on T_min, adiabatic compression to T_max , isothermal expansion on T_max, adiabatic expansion to T_min, and so on in a circle.

In adiabatic compression, the heat engine takes a certain amount of kinetic energy from the flywheel, and then, in adiabatic expansion, it returns the same amount of energy to the flywheel. Therefore, adiabatic changes of state in the heat engine are performed without energy consumption. In an adiabatic state change, the working medium acts as a "spring": "how much energy" is needed to compress the spring "so much energy" that spring returns during expansion. That is why the Carnot thermodynamic cycle is called the "ideal cycle" because the adiabatic change of state takes place without loss of work and heat.

Heat engine turns work into heat and heat into work in an isothermal change of state of the working fluid! Since the amount of work required for isothermal compression at T_min is much less than the amount of work that the working medium performs during isothermal expansion at T_max, the excess work thus obtained gives us the heat engine as "useful work". And that's all!

I tried to show you this process schematically in the T-S diagram, but I don't know why I lost this picture:

So you say: "In adiabatic compression, the heat engine takes a certain amount of kinetic energy from the flywheel,..."

Now, I think I've pointed out numerous, numerous times in these various discussions that a Stirling engine seems to run just fine without any flywheel at all. If there is no flywheel, how can it be that "In adiabatic compression, the heat engine takes a certain amount of kinetic energy from the flywheel,..."

Also, how is all that, (which I think contains a few miss-statements, such as "Therefore, adiabatic changes of state in the heat engine are performed without energy consumption." Does not that contradict: "In adiabatic compression, the heat engine takes a certain amount of kinetic energy from the flywheel,...") less complicated than my statement that "heat is a fuel for a heat engine"?

https://youtu.be/bOL6QVnQQzc

[Tom Booth]

That is why the Carnot thermodynamic cycle is called the "ideal cycle" because the adiabatic change of state takes place without loss of work and heat.

I'm not exactly sure what you intend to say or mean by that statement, but it needs clarification at a minimum. Otherwise, it is wrong.

Work and heat are interchangeable.

In adiabatic expansion there is no loss of heat by conduction or radiation, but heat can be converted to work which results in a drop in temperature.

That is a rather difficult concept to wrap your head around, what we sense as heat just "disappearing" to be transformed into mechanical motion,.but that, apparently is what happens.

[Tom Booth]

This engine is doing work, powering a linear generator. Still, it runs without a flywheel.

Any engine, even an internal combustion engine needs some means of getting it going. To suggest the engine is running by "the torque of the hand" is obsurd.

Anyway, this example has no "massive crankshaft". It has no flywheel or crankshaft at all.

https://youtu.be/cAyw_dOioMU

[Tom Booth]

"And as for this nonsense that some heat engine can work without a flywheel, it’s the same as saying that a heat engine can work without a thermodynamic cycle!"

I await a response.

There are other examples to be found.

https://youtu.be/HUWt3YrxoB4

No crankshaft. No flywheel.

Probably many of the "free piston" type engines.

https://youtu.be/z0CnohQZvlQ

No crank or flywheel, generating electricity.