Stirling Engine Thermodynamics

[Nobody]

The mass of the piston is storing work force identically to a flywheel. The springiness of the fluid is in place of the crank, and returns the piston, thus producing a cycle tuned to the natural frequency of the system.

[Tom Booth]

The mass of the piston is storing work force identically to a flywheel. The springiness of the fluid is in place of the crank, and returns the piston, thus producing a cycle tuned to the natural frequency of the system.

Well, actually, a Stirling engine running without a flywheel does not store work force in the piston "identically" to a flywheel

A flywheel and crankshaft revolve on an axis so that the direction of the force stored drives the piston back after 1/2 revolution, regardless of the fluid having completely cooled or not, which is supposed to be the whole point of having a flywheel.

Work force stored in the piston is linear, traveling in one direction. If anything, the stored energy represented by the weight and momentum of the piston itself acts to drive the piston further outward in the same direction it was traveling due to the expanding gas driving it out.

To say the fluid is springy, or acts like a spring, is not wrong, but it is incomplete.

What you call the "springiness of the fluid" involves adiabatic expansion, which results in lower pressure and lower temperature, which causes the gas to contract, or, you can say that atmospheric pressure reverses the course of the piston, due to being higher in pressure than the internal fluid which has in fact been expanded and cooled by the stored work force of the outward traveling piston "stretching" the "spring" so to speak. That stored force acts against, or in opposition to the atmospheric pressure. There is no angular momentum without a flywheel and crankshaft, the piston, by itself, does not rotate and reverse course every 1/2 turn like a flywheel.

There is no way that the work force stored in the piston itself acts to reverse the course of the piston in the same way as a flywheel.

Compared to a heavy flywheel, the force stored in a piston is rather negligible.

[MikeB]

I quite agree with Tom: the mass of a piston is directly _ counter _ to the ideal operation of _ any _ engine - a massless piston is always going to be better overall than a heavy one.

[Tom Booth]

I quite agree with Tom: the mass of a piston is directly _ counter _ to the ideal operation of _ any _ engine - a massless piston is always going to be better overall than a heavy one.

I suppose if you agree with me, I should just keep quiet.

Of course, I'm not going to do that.

I don't think I actually said anything about "better" or "ideal". You may be right, I don't know, but that wasn't the point I was trying to make.

My theory of how a Stirling engine operates without a flywheel, I think, actually requires a rather heavy piston.(absent some alternative load, weight or flywheel)

Gas heats, expands and drives the piston out. At some point the Gass is fully "stretched" or expanded to a point of equalibrium with the outside atmosphere.

At that point it is the weight/momentum/work force/energy stored in the piston that causes the piston to continue to travel past the point of equalibrium with the outside atmosphere. (Or buffer chamber in a sealed engine)

The weight of the piston causes additional expansion and cooling (in addition to cooling due to work output).

So, I wonder, if the piston were virtually weightless, or very light, would the gas expand enough to cool the gas enough to result in pressure low enough to effect a return of the piston?

TK motor's engine has a diaphragm "piston". Likewise, many other "tin can" and "ultra ltd" type engines have virtually weightless Pistons (diaphragms) but these engines DO have flywheels.

In this video "Mower of doom" states that the weight of the piston is in some way critical to the engine's ability to run without a flywheel.

https://youtu.be/DyPxNNJQo9M

The rather large glass test is, I guess, about 1/3 the weight of the engine, at about 4:00 towards the end of the video he says if the piston is "too heavy" the engine won't run at all (without a flywheel). What about Too Light? Is that possible?

I"ve seen (in videos mostly) numerous diaphragm type "free piston" engines that seem to require a heavy weight of one sort or another on or attached to the diaphragm (or flywheel) for the engine to run.

Why do these various engines have heavy weights on the diaphragm "pistons"?

https://youtu.be/Pqe1HyZRbHI

https://youtu.be/4Dq99HwAQL0

https://youtu.be/o97CIOxVxe0

Obviously a weight on top of a diaphragm piston is not helping cool the gas by "stretching" or expanding the gas it is weighing down, right?

It is causing the gas to do additional "work" though, in lifting the weight.

Work, could be in the form of a linear generator rather than a weight.

https://youtu.be/cAyw_dOioMU

So, I think, for a "weightless" piston, some external load would assist in the internal adiabatic cooling of the gas, though a heavy magnetic piston passing through a copper coil is not weightless either.

[MikeB]

"Why do these various engines have heavy weights on the diaphragm "pistons"?"

I suspect the main reason is that the engine is at atmospheric pressure inside, at rest. Once the engine has been 'fired up' the internal pressure will rise substantially. For a diaphragm-based engine in particular, the theory is that outside pressure returns the piston, i.e. ideal operation has the AVERAGE pressure in the engine be exactly the same as the outside atmospheric pressure. Since that is difficult to achieve, a weighted piston (if pushing upwards) counter-acts the 'excess' pressure inside the engine.

[Tom Booth]

"Why do these various engines have heavy weights on the diaphragm "pistons"?"

I suspect the main reason is that the engine is at atmospheric pressure inside, at rest. Once the engine has been 'fired up' the internal pressure will rise substantially. For a diaphragm-based engine in particular, the theory is that outside pressure returns the piston, i.e. ideal operation has the AVERAGE pressure in the engine be exactly the same as the outside atmospheric pressure. Since that is difficult to achieve, a weighted piston (if pushing upwards) counter-acts the 'excess' pressure inside the engine.

That makes sense.

Though I've seen some such Stirling engines operating in an inverted position.

This curious "free piston" engine, for example, appears to have an enormously weighty piston just hanging out of the cylinder.

https://youtu.be/iizQvNYKQP8

He has a bit of trouble keeping the piston in a good operating position, but it does eventually run smoothly maintaining an "average pressure", but I don't think by the weight of the piston compensating for the excessive internal pressure. The piston is being drawn upward into the cylinder every 1/2 cycle against gravity.

Also, contrary to what might be expected, the recurring nocking sound appears to be a result of the piston being drawn in (up) too far, due to an Imbalance of pressure? The low pressure being inside, apparently.

Similarly, "mower of doom" installed a "steel spring" in order to prevent his piston from "banging into the orifice" on the return stroke, to replace a silicone rubber bumper, that previously served the same purpose.

https://youtu.be/cAyw_dOioMU

With excess internal pressure, in either engine, I would think the last thing anyone would need is some such spring or bumper to prevent the piston being drawn in too much.

[Tom Booth]

One question that keeps nagging at me through all this is, if the gas is literally contracting due to becoming cold (molecular attraction), after doing work, is the gas also doing work as it contracts and "pulls" the piston inward?

If so, is it possible the gas could (also) be cooling by doing work as the piston travels inward "pulled" due to the elastic nature of the gas OR

Is the piston entirely pushed in by outside atmospheric pressure, in that case, is the air outside the piston cooling at all?

Talking about gas being "elastic" I'm reminded of this interesting video:

https://youtu.be/lfmrvxB154w

If the gas were "double cooling" in some way, both as it expands as well as when it contracts, this could explain the "banging into the orifice" phenomenon, where the gas seems to cool and contract more than what would be expected.

This seems to be most pronounced and observable when the engine is operating a linear alternator "free piston".

The magnetic piston does "work" passing through the copper coils, generating electricity IN BOTH DIRECTIONS. Rather than just converting heat to "work" by expanding and pushing out, it is also, in one way or the other doing work as it contracts in the other direction as well.

What is doing the work and cooling? The internal "contracting" gas or the outside higher pressure atmosphere (or both).

[Nobody]

"Work force"??? I meant energy. Me bad. Sorry.

Energy is stored E=1/2mv^2. Piston mass moving, or, add up the mass on flywheel rim times velocity of the rim. Both store energy the same way, by moving mass. Just pointing out there are other ways to store energy than a flywheel.

Yes your more complex description of springiness almost has it.
Gas decreasing in temperature or pressure doesn't always contract. For contraction to happen the container volume must decrease. That is done on that engine when the piston is slowed by the higher outside atmospheric pressure to a stop and reverse it. The momentum of the piston and internal air contribute to this.

This happens again at the high pressure side. The pressure building until it reverses. This reverse happens more suddenly because it happens at higher pressure and heat is added. That drives the frame away from the piston. The piston is lighter so moves more, faster.
Heat is removed at the low pressure side also making the turn around softer.

The lighter the piston the higher the natural frequency. That is why YouTubers are adding weight to their free piston engines, to increase momentum and tune it for length of cylinder.

They can run faster, might not even run, if disconnected from crank because mass is reduced. The crank also limits stroke. Cranks may allow cylinders to be shorter.

This doesn't apply to just adiabatic expansion or compression. Adding and removing heat will effect the natural frequency.

[MikeB]

Tom, I see you mention "molecular attraction" again - I'm not aware that any common gas has this property; indeed I was under the impression that a truly un-constrained gas would expand to fill whatever container it is in, regardless of temperature and pressure. Maybe I'm misunderstanding a concept that you are getting at, but surely a gas will only contract if there is an external pressure acting on it?

[Nobody]

Gas when expanded into a vacuum indeed does cool and does zero work. It is the Joule–Thomson effect. The liquefaction of gasses requires this. It is an irreversible process called throttling.

https://en.m.wikipedia.org/wiki/Joule%E ... son_effect

For the sake of the engines here, they all accomplish work regardless of getting work out or not. They use reversible processes because there is a moving piston/wall. The Stirling cryo-coolers don't rely on the Joule-Thomson Effect. Loading an engine to get more work out may not help.

It is unclear which, the loads, or, the RPMs, is the critical cooling factor. Doing a test at different power out levels and RPM's would be good.

[Nobody]

Work is Force times Distance. W=FX

Force is Pressure over Area.

Work into a machine happens when the force apposes the direction of motion. Apposes motion.

Work from a machine happens when the force is in the same direction as the motion. Complements the motion

[Tom Booth]

Tom, I see you mention "molecular attraction" again - I'm not aware that any common gas has this property; indeed I was under the impression that a truly un-constrained gas would expand to fill whatever container it is in, regardless of temperature and pressure. Maybe I'm misunderstanding a concept that you are getting at, but surely a gas will only contract if there is an external pressure acting on it?

A so-called "Ideal Gas" is not supposed to have molecular forces. Real gases however do, but I'm not at all certain on what scale, or if it comes into play.

I'm relating it to the rubber band analogy, which may be entirely inappropriate.

Stretching a rubber band generates heat by "doing work" on the rubber to stretch it out. Released, the rubber band "does work" and gets cold, due to using up some "internal energy" the rubber "wants" to pull itself back into the relaxed, unexpanded state it was in before it was stretched.

It is often stated that air is elastic, or has elastic like properties, more or less opposite to a rubber band, as stated in the above video in a non technical way.

So gas cools when it expands doing work and heats up when compressed, work being done on it.

At a certain point, when cooled sufficiently, the molecular attraction between gas molecules dominates and a change of state occurs. The gas condenses into a liquid. At what point is different for different gases.

Air is a mixture of gases, as well as water vapor.

You might say that "intuitively", I kind of suspect that the gas in a Stirling engine, expanding and doing work, experiences a rather sharp, sudden cooling that stops the piston in its tracks, causing it to reverse direction, at least in part, due to a sudden "condensation" of some constituent in the air/gas.

A sudden change in air pressure will condense water out of the air, if nothing else.

Just as an example of what I mean. A very common physics experiment is to put a little bit of water in a metal can, bring it to a boil, then put the lid on when the can fills with water vapor.

Remove the heat so the vapor cools and condenses back into a liquid. The can collapses.

Now, it if often stated that it is the outside atmospheric pressure that caused the can to collapse.

But, thinking about it, what caused there to be a vacuum inside the can in the first place? The vacuum inside the can was created, first, by the molecular attraction between the water molecules, right?

So when a gas in a Stirling engine expands, does work and cools, SOMETHING happens to reduce the pressure inside the engine that results in the outside atmospheric pressure dominating.

What causes the internal pressure to suddenly drop below the external pressure? The gas cools.

But cold gas can still be under high pressure. What reduces the pressure? What else can it be but molecular attraction? This seems to follow logically. Maybe.

Somehow at least in the.case of water vapor condensing, I tend to think that the violent implosion of the can is due to something more than simply outside atmospheric pressure. It is also the condensation of the water vapor which is a result of molecular attraction that creates the vacuum.

So, back to the rubber band, it cools as it does work as it contracts, NOT due to external pressure.

Is it not conceivable that a cooling "condensing" gas in an engine FIRST undergoes molecular attraction as it cools, which results in the vacuum condition inside the engine that only AFTERWARD results in the pre-existing outside atmospheric pressure dominating?

https://youtu.be/p3b9pK-O6cE


I guess it is more like a support being taken out from under something. The removed support does not actually do any "work" to "pull" something down after or as the support is being removed.

So the cooling contracting gas does not "pull" the piston inward, rather it just no longer serves as a support for the piston which allows the outside pressure to push the piston in.

At some point, before the end of the compression stroke, the in-rushing atmosphere/piston does some work on the internal gas.

I think I've worked out the answer to my own question.

[MikeB]

If it helps, bear in mind that some commercial Stirlings use Hydrogen or Helium as the working gas, so no mixture or water vapour effects. As far as I know, these are more efficient, but don't work substantially differently from air-based engines.

[Tom Booth]

If it helps, bear in mind that some commercial Stirlings use Hydrogen or Helium as the working gas, so no mixture or water vapour effects. As far as I know, these are more efficient, but don't work substantially differently from air-based engines.

I'm not sure that any pure gas like hydrogen or helium are necessarily "better", in terms of efficiency. (I don't really know).

Hydrogen and helium happen to behave the most like the "ideal gas" much of the generic mathematics involved in working with gases generally is tied up with.

Air, or some other "non ideal" gas, or combination of gases might be superior, (again, I have no real idea) but I think the mathematics gets exponentially more and more complex.

Anyway, that water vapor condenses both when running a Stirling engine on hot water (from the steam) and on ice, (from the air) is just a curiosity. Something to think about. I don't attach any particular significance to it

Air, as a complex mixed gas is difficult to model mathematically. It may have unusual properties that are far from "ideal" which have never been studied.

Gases pretty much all behave similarly, but at different temperatures, and pressures, have different boiling points, inverse temperatures, triple points. etc. Joule Thompson throttling for example actually causes some gases to heat up instead of cooling. It can all get very mind boggling as well as fascinating.

The more I learn, the less I know.

Things like "The ideal gas law" gives the illusion that it is all very cut and dry, but real gases can behave "strangely" and gas mixtures even moreso, especially when undergoing rapid changes in temperature and pressure, it can be hard to know what is really going on.

I inquired about the math for calculating the length of pipe used in refrigeration, to a major refrigerator manufacturer. The engineer they put me on the phone with told me that the mathematics that would be involved is too complex, that even the best super computer could only give a ballpark approximation, that would be wrong, so they don't bother.

He told me that they rely mostly on trial and error until they find something that works.

[skypupbob]

Anyway, that water vapor condenses both when running a Stirling engine on hot water (from the steam) and on ice, (from the air) is just a curiosity.

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. Water is not a good lubricant for steel and iron. The temperature in the engine is not high enough [180 degrees] to cook off the water vapor and it condenses.
After I typed that ,I wondered ,where would it go? I don't know . I do know that it is a problem and will replace the steel power piston with a diaphragm type. Trial and error!
It is not a pressurized engine ,so it is running air which has moisture in it ,depending on the humidity that day.