Isothermal Heat Transfer

[Fool]

You just type 5 paragraphs explaining that pistons leak

How is a piston that stays cold, atmospheric temperature, going to heat up and seal?

Yes leakage is tiny. If you set a piston into a vertical cylinder with a sealed bottom, the head, how long does it take to settle to the bottom/head? Isn't that a report of leakage? Or do you have alternative data in this test?

[Tom Booth]

You just type 5 paragraphs explaining that pistons leak

How is a piston that stays cold, atmospheric temperature, going to heat up and seal?

That's one reason I often make my own pistons if necessary, especially if running an engine on ice.

Otherwise, as I've already said:

"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."

I don't consider heat generated by "friction" associated with Carnot "heat rejection" from a "perfect" frictionless engine, since the heat is already converted into work (mechanical motion) before friction can be generated.

Friction can also be virtually eliminated;

NASA zero clearance no contact pistons made of exotic alloys like Invar. so not an entirely unavoidable LAW of the universe. Ferrofluid frictionless seals are also possible, magnetic bearings, air bearings etc. lots of ways to bring friction loss to near zero

Yes leakage is tiny. If you set a piston into a vertical cylinder with a sealed bottom, the head, how long does it take to settle to the bottom/head? Isn't that a report of leakage? Or do you have alternative data in this test?

Are you talking about the video showing that an LTD can take high heat? Different topic.

The only reason the engine was upside down in a vertical cylinder was so I could blast the bottom with the propane torch without having to hold it up in the air with my hand. It has nothing to do with testing for leaks.

Your whole argument that a piston can return due to "leaks" rather than cooling (temperature decrease) from work (energy) output, is at best, uninformed and just plain silly.

[Stroller]

The piston on my LTD stirling isn't as leaky as I thought it would be. I've just had the engine apart to tinker further with the temporary displacer, so I checked the iron piston clearance by setting it partway down the iron bore and pouring some 95% alcohol in to see how fast it would leak past the piston.

It didn't.

The displacer rod is leaky though. How should I set about making a low friction seal for that I wonder? Vaselined leather?

[Tom Booth]

The piston on my LTD stirling isn't as leaky as I thought it would be. I've just had the engine apart to tinker further with the temporary displacer, so I checked the iron piston clearance by setting it partway down the iron bore and pouring some 95% alcohol in to see how fast it would leak past the piston.

It didn't.

The displacer rod is leaky though. How should I set about making a low friction seal for that I wonder? Vaselined leather?

I don't know your situation exactly, but I've sometimes used a loose fitting rubber washer or O ring, or leather, tightened down with a pressure plate.

Something like this:

Compress_20240427_165554_4225.jpg (10.72 KiB) Viewed 4054 times

The rubber or whatever seal is loose, but fairly close fitting so that tightening down the plate causes the O ring or whatever to bulge.

The tension on the screws can be adjusted to get a nearly perfect, nearly frictionless seals, with a drop of lube of some sort.

If it begins to wear, you can just tighten the screws slightly to close the gap again.

I used that method going back ten years or more. It's what I used in this experiment, many years ago, when mpeg video was still the standard video format on the web:

Not very easy to see, but the plate around the displacer push rod in this video is made that way.

I like it because it is easily adjustable until you find a balance between a tight seal and low friction

https://youtu.be/S8MBj6_sdSw?si=opFXxGpbIS0fMgqN

[Stroller]

Thanks Tom, food for thought. I might well be better off cutting a thread on the top of the displacer rod guide and putting in a squishy O-ring with a screw down cap over it I can adjust easily, rather than an internal seal.

Apologies for the thread drift, carry on as you were.

[Tom Booth]

...cutting a thread on the top of the displacer rod guide and putting in a squishy O-ring with a screw down cap over it I can adjust easily, ...

Sounds like a more elegant design that does the job. Great idea.

[Fool]

I've seen it called a rolling sock seal.

I would think it needs to be initially a toroid or donut shape, with a circular crossection. Then, when cured, pressed into service into the oval crossection that is commonly shown.

There is another seal I've seen, that is called a sock seal. It is commonly used to provide the expansion seal for air shock absorbers. The best, IMHO, is the one used for Gabriel Air Shocks. It is similar but has an open internal/pressurized end, and longer.

Think of putting a piston in a sock. Take the cuff of the sock and fold it back over itself. Attach the cuff to a thin walled cylinder. Pressure inside the cuff keeps it inflated away from the piston. The sock rolls and unrolls on the piston as it moves in and out of the cylinder. I've never seen one on an actual Stirling. It does require a minimum pressure.

A flat diaphragm could be made by spreading some RTV onto a thin cloth using a putty or drywall, knife. The sock seal could be made the same way using a cloth cylinder. The cloths would strengthen the rubber.

The fatigue life of different compounds and cloths would be interesting to measure.

He's talking about Gabriel Hijackers.

https://gabriel.com/product-search/ligh ... tnum=49235

[VincentG]

Sorry for the delay. I just cobbled this video together tonight.

I had freshly oiled the piston and displacer rod here, which makes this Essex very air tight so there is no significant effect from losing or gaining air mass.

I could take it apart and attach the pp rod to a weight to show the force better, but take my word, it has a strong cold stroke.

Stroller I hope this somewhat answers your question. If not, let me know what may help.

https://youtube.com/shorts/6vWLqvtjqu8? ... SpsQqHklvh

[Fool]

I can't make heads or tails out of that. I don't even know what question that is supposed to answer.

Strokes of both directions were observed. Both seemed about the same. One made a different sound than the other. The sounds were about as loud. Not sure what pressures were involved. The pressures appear to be alternating below and above ambient. I don't even know what position it was in before, during and after heating.

Guessing the piston was centered at internal pressure one atmosphere. I don't know how that was assured or exact.

[Stroller]

Sorry for the delay. I just cobbled this video together tonight.

I had freshly oiled the piston and displacer rod here, which makes this Essex very air tight so there is no significant effect from losing or gaining air mass.

I could take it apart and attach the pp rod to a weight to show the force better, but take my word, it has a strong cold stroke.

Stroller I hope this somewhat answers your question. If not, let me know what may help.

https://youtube.com/shorts/6vWLqvtjqu8? ... SpsQqHklvh

Thanks, Vincent, for taking the time to make the video. I take it that the engine was cold when you filmed it?

Moving the displacer doesn't change the overall volume of air in the cylinder. The intriguing thing is that the piston doesn't follow the displacer, but moves the opposite way! So what's happening?

Could it be that as you pull the displacer outwards, the displaced air is forced through the narrow gap between the displacer and cylinder wall, and attains quite a high velocity in the direction of the power piston due to that narrowness, which causes a pressure wave that pushes the piston outwards? Then when you push the displacer back towards the centre of the engine the opposite occurs: The high velocity rarifies the air between piston and displacer, and that sucks the power piston back towards the centre of the engine?

[VincentG]

Fool, yes the engine was equalized at the middle of the pp stroke, but it doesn't really matter, it will find its own balance.

The noise is due to the power piston hitting the displacer at TDC.

Stroller, the Essex has a "cold cap" at the displacer end and a hot middle section. This is why the power piston moves opposite. It's also why the Essex is so effective.

The volumetric expansion of the Essex is excellent. It can sweep the full 60cc of power piston displacement when the displacer is moved back and forth.

I had heated the middle section with a torch before filming.

[Stroller]

I had heated the middle section with a torch before filming.

Ah, OK, that changes my understanding of what's happening. You're certainly getting a good amount of momentum built up in the flywheel and power piston/connecting rod on that return stroke!

[Fool]

Isn't that an alpha machine?

[VincentG]

No, it is definitely a Beta, operated from the hot side. The hot expanding gas is not forced through the cold side like the modern Beta design.

The design is so effective that the displacer lags the power piston, instead of the normal arrangement of the displacer leading the power piston.

[Stroller]

The design is so effective that the displacer lags the power piston, instead of the normal arrangement of the displacer leading the power piston.

This drawing from the Myford Boy website shows the displacer leading the power piston by the usual 90 degrees doesn't it? The power piston is halfway to the outer end of the cylinder but the displacer already got to the outer end and is about to start moving back towards the middle.