Isothermal Heat Transfer

[Stroller]

It is easy to demonstrate at slow speed with a cold sink.

I've been looking for a good video which shows the Ts and Ps as the cycle progresses, but haven't found one yet. Do you have a linky?

I agree that piston blow-by isn't the whole story about why the working fluid drops below ambient air pressure. It's just one of the contributing factors I mentioned in my description, which included the cooling induced by the displacer moving working fluid to the cold side, rarifaction caused by forced expansion due to the momentum of the piston/flywheel towards BDC, and energy removed by work done acelerating the piston/flywheel.

[Stroller]

What I did find was this animation of the Inresol AB Gamma along with a simulation of piston positions vs forces on the main bearing in the X and Y planes.
https://youtu.be/m-tAWEfUeeg?t=150
It's quite interesting to consider the amplitudes and timings of those forces, and what the implications are for where in the cycle pressure is being applied to the power piston, and comparing those amplitudes with the acceleration/deceleration forces on the displacer piston.

My takeaway is the lighter you can make the reciprocating parts, and thus the rotating flywheel, the better balance and performance you're going to get.

[Tom Booth]

But maybe gas can "stretch" like a rubber band. IDK.

But I can see that the piston in a Stirling engine does infact return. Without a flywheel. Without a displacer. With no apparent "sink" or viable heat outlet. I've done the experiments myself, so I can't really just dismiss what I see happening.

The piston is leaky, air expanded by heat inside the stirling engine will leak past it while it is pushing it to BDC. As it starts to cool and contract due to work extraction, increase in volume (due to the piston's continued motion towards BDC), and also due to the fact that the displacer is 90 degrees ahead and already starting to move working fluid to the cold side, the piston and flywheel's momentum does indeed "stretch the gas like a rubber band". i.e. they are preventing the contraction that would otherwise be starting to take place by this part of the cycle. Just past BDC, the working fluid's gas pressure is now below atmospheric, due to the earlier piston blow-by and the ongoing cooling induced by displacer motion forcing more of it to the cold side.

Hence the 'forceful return stroke' as ambient atmospheric pressure shoves the piston downwards during the first half of its motion towards TDC, and before the displacer starts to transfer working fluid back to the hot side. Because the piston is leaky, and the flywheel and piston have inertia as well as momentum, some ambient air will be entering the engine, this will restore the full-cycle average pressure at running equilibrium.

I've also tested for this "leaky piston" "blow by" theory.

The "open" end of the power cylinder on this engine is completely sealed by a balloon and rubber bands around the cylinder and connecting rod.

Any leaks or "blow by" should have some visible influence on the balloon. But even with the extremely high heat of a propane torch there is nothing.

This actually surprised even me. I was expecting at least some "leakage" during the warmup.

https://youtu.be/iOs3BADFeKI?si=qfV4s0cOQ5dVN9AM

But no, after the piston .moved out a little to compensate for, or in response to the initial expansion that was it. The balloon did not inflate at all.

[Stroller]

It certainly looks like the piston is well sealed, and not being constrained by a mechanical linkage, the working fluid won't develop a high enough pressure to push past. Just moves it further out as you say.

[Fool]

Tom, why would the balloon inflate? The gas heats up, expands, pushes the piston outward causing some inflation. The engine then runs. Any leakage by the piston would make the piston run closer to the closed end. The gas going by the piston would cool and return to it's original volume. Hence it would take up the same volume total as before it was heated or leaked by.

[VincentG]

I can vouch that there is some leakage past the piston once pressure increases. There is more leakage past the displacer rod in the small models. But its a separate issue from the return power stroke.

[Fool]

"Why is rubber "elastic"?"

Because it has tensile strength. Fluids, liquids and gasses, don't. As professor Baker said, "First rule of physics, F=MA and you can't push on a rope.". I would like to add that you can't push on an unconfined fluid, and can't pull on a gas ever. Surface tension is the only pull I know of in a liquid.

Guitar strings stretch, and contract when released, because they have tensile strength. Fluids, Newtonian, have zero shear forces at zero velocity. Meaning they will keep moving regardless how small the force. Guitar strings, solids, have shear forces proportional to applied tensile stress. The stress strain relationship is modeled by Mohr's Circle.

Also rubber heats when stretched and cools when contracting, as does all solids, I think? Opposite of a gas.

"In the video the instructor is using his muscles to pull the plunger so the gas can expand. So it seems as though he is doing all the work."

I would like to add, the instructor is inputting work. Overall the battle of the gas pushing out, is less than the atmosphere pushing in. This difference requires work input to the engine. That work input can come from the instructor, or from the saved MV inertial motion of the engine, even if the piston is the only motion. A light weight piston will have to move faster to store the same energy.

The gas doesn't know this, but the piston will react by being pushed out and by being pushed in. It my not be traveling in the direction of total/net push, so it will tend to slow, or speed up.

"In reality the gas temperature could go down or, non-intuitively up, depending on the gas and it's "inversion temperature", or maybe that is only at atmospheric pressure, if NOT expanding into a vacuum."

Basically true. I think the overall debate as to why there is Joule-Thompson cooling is not completely hammered out.

The Wikipedia link talks intermolecular attraction, and repulsive effect.

Gas molecules attract by the gravitational force (negligible) and electrostatics (intermolecular {outside the molecules} forces). They produce a pressure because of kinetic energy, MV, and collisions, not from repulsive forces. If you read carefully, Wikipedia is subtle but correct. There are no molecular repulsive forces.

"Maybe Carnot was right all along and heat really is a fluid. I don't know."

I don't think a model has to be right to have correct answers. We have left a lot of old ideas behind as "extra entities" calorics aren't wrong, just an extra entity.

Stroller and VincetG, thanks for your inputs. I think Tom's free piston moving inward while being heated by a propane torch can only be explained by the gas slowly leaking by the piston seals. As to why the return stroke appears stronger, may just be, because the gas inside is getting more rarified, and has less resistance than the atmospheric pressure because it is a cooler stroke. Slamming the head pushes more gas out. Like bouncing a ball with a slow leak.

Bouncing a hot ball. LOL

[Tom Booth]

Tom, why would the balloon inflate? The gas heats up, expands, pushes the piston outward causing some inflation. The engine then runs. Any leakage by the piston would make the piston run closer to the closed end. The gas going by the piston would cool and return to it's original volume. Hence it would take up the same volume total as before it was heated or leaked by.

You've got some imagination.

[Fool]

Just reporting what you saw. Heated by a torch. Free piston getting closer to top/head, i.e., less gas cushion. Balloon not inflating because the same amount of gas is in the cylinder trapped by the balloon, but now on cold side of piston.

What is there to imagine?

[Stroller]

I can vouch that there is some leakage past the piston once pressure increases. There is more leakage past the displacer rod in the small models. But its a separate issue from the return power stroke.

If it's the contributing factor that makes the difference between internal pressure falling from above ambient to below near the end of the expansion power stroke, then it's not a separate issue, it becomes the primary cause of the ambient air pressure giving the piston a shove on the return stroke.

I hesitate to call it a 'power stroke', because the engine will have lost as much or more momentum on the expansion power stroke than it gains on the return stroke.

An ICE engine with worn piston rings and low compression has less power than a freshly re-ringed engine. I suspect the same is true of Stirling engines.

[Tom Booth]

Just reporting what you saw. Heated by a torch. Free piston getting closer to top/head, i.e., less gas cushion. Balloon not inflating because the same amount of gas is in the cylinder trapped by the balloon, but now on cold side of piston.

What is there to imagine?

You just wrote above:

The piston is leaky, air expanded by heat inside the stirling engine will leak past it while it is pushing it to BDC. ...
...
....Just past BDC, the working fluid's gas pressure is now below atmospheric, due to the earlier piston blow-by...

... Because the piston is leaky, ... some ambient air will be entering the engine,....
...

You seem to think there is air blowing past the piston in both directions enough to have some significant influence on the operation of the engine, but somehow this "blow by" just gets cold and shrinks away, then gets sucked back into the engine past an air tight piston, 10 times per second so, somehow, miraculously, it has no visible influence on the balloon. Yet:

Just past BDC, the working fluid's gas pressure is now below atmospheric, due to the earlier piston blow-by...

Big influence on the engine inside but no influence on the balloon outside as the air blows by the piston.

What's to imagine?

Oh, I don't know.

That hot gas with enough pressure to blast it's way past a perfectly honed piston/cylinder seal somehow instantly cools down loosing all force and power by the time it reached a flimsy balloon about a sixteenth of an inch past said "blow by" point.

But the same gas that looses all this energy instantly once outside the engine couldn't possibly loose energy due to work INSIDE the engine.

[Fool]

You are mixing Stroller and my posts. Yes the gas leaks by the piston in both directions. It maintains an average pressure balance of approximately the atmosphere. The gas in the hot space is less dense because it is hotter and at the same pressure. When the piston returns "more forcefully" and slams the head, the pressure peaks, making more leak by. Overall more gas leaks out, than into, the hot zone. Average density stays about the same because the average pressure is still atmospheric. The volume inside gets smaller, so the piston gets closer and closer. It is approximately a one for one volume change with the other side of the piston for zero total cylinder gas volume change. The gas after moving out of the hot zone cools, if needed, to the atmosphere temperature. So the balloon doesn't expand. Why, have you got different data?

[VincentG]

If it's the contributing factor that makes the difference between internal pressure falling from above ambient to below near the end of the expansion power stroke, then it's not a separate issue, it becomes the primary cause of the ambient air pressure giving the piston a shove on the return stroke.

I hesitate to call it a 'power stroke', because the engine will have lost as much or more momentum on the expansion power stroke than it gains on the return stroke.

An ICE engine with worn piston rings and low compression has less power than a freshly re-ringed engine. I suspect the same is true of Stirling engines.

Two reasons it's a separate issue.

1: These small model engines are just a representation of the real thing. The leakage v. displacement is far greater than a big engine would have. When I get around to filming a demonstration for you, it will be with a 60cc Essex hot air engine with a much better piston seal, where the air leakage is not a contributing factor.

2: What leaks going out, leaks going in. In practice, I've found that running Stirling engines find a natural balance of internal v. external pressure.

Rather than anyone trying to explain this, I encourage you to get the biggest and best Stirling engine you can get your hands on (your wooden LTD should work great here) and disconnect the displacer linkage so you can operate it by hand. Move the displacer and see for yourself.

[Stroller]

Thanks Vincent, I'm loooking forward to seeing the video of your Essex engine. I just watched some of the 'Myford Boy' series on making a replica Essex engine, which was really interesting. Around 15W is just what I need for keeping a battery topped up to keep my diesel heater and exhaust heat recovery water pump running during a powercut.
https://www.youtube.com/watch?v=HH8Xkw4 ... 7F6D848CB2

The materials have arrived to make a better hot cylinder wall and displacer for my LTD engine, so I'll start posting some progress reports on that soon.

[Tom Booth]

You are mixing Stroller and my posts.

No, Stroller & I apparently cross posted. I never even saw his post at all before posting.

Yes the gas leaks by the piston in both directions.

Nonsense. Now I can comment on Strollers post. Maybe if your piston and cylinder is so badly worn it can no longer hold air, or a model with mismatched parts. but at best that would result in your engine running poorly, not improve it or somehow be a factor in allowing it to run.

It maintains an average pressure balance of approximately the atmosphere. The gas in the hot space is less dense because it is hotter and at the same pressure. When the piston returns "more forcefully" and slams the head, the pressure peaks, making more leak by. Overall more gas leaks out, than into, the hot zone. Average density stays about the same because the average pressure is still atmospheric. The volume inside gets smaller, so the piston gets closer and closer. It is approximately a one for one volume change with the other side of the piston for zero total cylinder gas volume change. The gas after moving out of the hot zone cools, if needed, to the atmosphere temperature. So the balloon doesn't expand.

LOL,

So much silliness not worth my time and energy to try and find something in all that that might make any sense at all that might be worth commenting on.

There is no significant "leaky piston".

There is no "blow by"

Your whole analysis is based on something springing from your imagination.

Why, have you got different data?

Non sequitur. No idea what data you're talking about "different" or not.

Edit: reading VincentG's posts just now, if that's what you mean,

He appears to be basing observations on an engine where the crank is removed and he moves the displacer by hand.

Most Stirling engines use dry lubricant and have to account for thermal expansion, so the piston is designed to be slightly loose until it heats up and expands due to friction while running to make a perfect seal.

Operating by hand, probably his engine shows some leakage. How he's testing for that, I have no idea. I have no idea what engine, the condition of the engine, what lubricant, nothing. Don't care. It's insignificant.

All engines, including IC engines need to allow for thermal expansion. The allowance goes away soon after the engine starts running.

Most engines have rings to maintain a continuous seal while allowing some room for piston expansion and contraction

Quite often I build engines, check carefully for leaks, make custom pistons I fit or hone down myself and use a non flammable liquid lubricant when possible.

Aside from that many Stirling engines have diaphragm power pistons, or rolling sock pistons or leak proof magnetic actuators or otherwise completely eliminate any possibility of leakage. They run better for it. You imply a Stirling engine requires a leaky piston to operate.

That's absolutely ridiculous.