Senft Pancake Style LTD Stirling

[CarlW]

Years ago I organized a group build of 6 Senft LTD engines from his book "An Introduction to Low Temperature Differential Stirling Engines." Five of the six ran well and I ended up with the poor runner. I had to put boiling water on top and ice water on the bottom to get it to run at all. I played with it for a awhile and then lost interest.

One of the runners arrived in my shop last week and I determined to see if I could get both engines running. The new one was stored in an open box subject to dust and dirt and needs cleaning. Any suggestions on cleaning the Airpot dashpot cylinder? I'm thinking isopropyl alcohol in ultrasonic cleaner for the cylinder. Will ultrasonics damage the graphic piston?

There is a vent screw in the top displacer cylinder cover. I no longer have the book so I cannot easily find out how to use the screw. If someone would retrieve this bit of info for me, I would be most appreciative. Now that I think about it, I need to check the phase angle of the two cranks, so add that to the information to be retrieved. IIRC it is 90 deg but does it matter which leads and which follows? And that raises the question of direction of rotation.
TIA
Carl W

[CarlW]

I'll close this thread with what I learned today. I got the non-runner out of storage and confirmed that the direction of rotation is clockwise looking at the power cylinder. The angle between the cranks is indeed 90 deg and when the counter-balance weight is vertical, the big end of the power connecting rod is to the right.

I first vacuumed the dust out of the cylinder as best as possible and then cleaned the cylinder walls with cotton swabs soaked with isopropyl aIcohol and removed the piston. I decided to not use the ultrasonic cleaner and instead used a non-linting paper towel soaked with alcohol.

I recalled that the problem with the non-runner was excessive friction in the Airpot cylinder. While cleaning that piston I noticed that some of the graphite was marking the paper towel. That got me to thinking and I made a cork jam chuck on the lathe and very carefully used the wet paper towel as an abrasive, checking the fit repeatedly. The piston had to be dried with compressed air before checking the fit because any alcohol or lint affected the fit.

Both engines are now running nicely. At startup, the vent screw is removed while the lower displacer piston plate warms up and the flywheel is spun. The screw is then replaced. Spin the flywheel and the engine should run. My longest run so far is 1 1/2 hours. I bought a big coffee mug and am making a foam insulator for it. I'll take time and temperature measurements just for giggles and grins.

[Tom Booth]

I'll close this thread...
... I bought a big coffee mug and am making a foam insulator for it. I'll take time and temperature measurements just for giggles and grins.

I hope you'll come back and let us know the results!

I should get a copy of Senft's book, but it looks rather expensive. Is it worth it?

[Tom Booth]

I'm in the process of reading Senft's book: "Mechanical Efficiency of Heat Engines" which covers quite a bit about the LTD (P-19) "pancake" type engine.

The first thing that strikes me is the departure from classical Thermal efficiency to Mechanical. The actual physical / mechanical characteristics of an engine that make it efficient (or not).

Phew!

That is a relief!

Now we are getting somewhere!

What "temperature difference" (classical thermodynamics idea of efficiency) ever really had to do with actual efficiency, I'll never know.

The second thing that strikes me is the introduction of a diagram, the likes of which I have never seen before:

Screenshot_20211119-022932.jpg (21.88 KiB) Viewed 4050 times

All I can say is Of Course!

When it comes to the conversion of heat into work in a Stirling engine, there is not only the work of expansion, or work out to consider, but also the work of compression or work in!

How satisfying! I'm already enjoying this book, having only barely cracked it open. Only on page 3 of the first chapter: "Energy Transfers in Cyclic Heat Engines".

My thought is, the "work in" according to Senft, is subtracted from work out to get the "net cyclic work" represented by W = We - Wc because some of the work of expansion must first be stored in the flywheel to be used later to carry out the work of compression.

But what about a Thermal Lag or similar engine operating without a flywheel? Then the compression work is carried out by atmospheric pressure. Would the total work output then be additive? W = We + Wc ?

I'm curious to see if Senft addresses this no-flywheel case, so shall read on, but it is quite refreshing just to see this chart and analysis where the expansion work and compression work are both recognized and represented.

[Tom Booth]

Yes!

Senft puts the study of heat engine efficiency on a whole new level.

He introduces a modification of the old PV (pressure / volume) diagram. Again, something I've never seen before. He includes the "buffer" or atmospheric pressure (designated by the horizontal line Pb = buffer pressure) and makes a distinction between "effective work" which translates into useful shaft work, or mechanical work output of the engine and "forced work" which is really negative work that has to be subtracted from the effective work to get the net work output of the engine.

The PV curve of a Flame Licker type engine is interesting.

Probably the most revolutionary result of this kind of mechanical efficiency analysis is this PV graph comparing the efficiency of a Carnot engine with the efficiency of a Stirling engine.

Screenshot_20211119-102348.jpg (48.32 KiB) Viewed 4035 times

Demonstrating that a Stirling engine has greater mechanical efficiency and greater overall efficiency, theoretical thermal efficiency being equal.

The Carnot engine requires more "forced work" (shaded areas) which must be subtracted from overall efficiency.

He also introduces the concept, and possibility of a heat engine with constant mechanical effectiveness, "having constant effectiveness throughout its range of operation".

Such an engine produces a PV diagram with no "forced work" meaning that internal pressure is greater than external or "buffer" pressure during expansion and atmospheric (buffer) pressure is greater during compression,

Though not specifically mentioned by Senft in this book, I think this is descriptive of a Thermal Lag or Thermo-acoustic "free piston" type Stirling able to run and output positive work throughout the entire cycle, both during compression and expansion. and therefore capable of operating without a flywheel

[Tom Booth]

A very interesting fact about the P-19 Ultra LTD:

The P-19 has also been operated on the evaporative cooling effect of water. Set up in the lab with cloth pads on the top plate kept moist by wick feed from two cups of water, the engine once ran nonstop for 16 days. ... The engine was finally stopped because it was needed by a student for a seminar demonstration.

Has anyone here ever replicated this engine?

I think I'll add that to my list of projects to work on when I finish setting up a workshop. I'm especially interested in the "lost motion" dwell mechanism with adjustable counterbalance.

I haven't seen any "Ultra LTD" engines that fully replicate the P-19. Here are a few I found on YouTube that come close, though apparently lacking the dwell and counterbalance linkages. The second one comes closest, having a kind of counterbalance, but Still, I think, lacking "dwell", It's hard to tell.

The first is shown operating on evaporative cooling from a wet piece of paper:

https://youtu.be/ARD3ctp80ac



https://youtu.be/V5Wi81Ycs0I

[Tom Booth]

A closer, magnified look at the "lost motion" mechanism of the P-19 Ultra LTD from James Senft's book, Mechanical Efficiency of Heat Engines.

2021-11-22 10-48-34.jpg (121.87 KiB) Viewed 3961 times

It does appear to be just a simple loop.

Along with the adjustable counter balance weight, I think the dwell could be adjusted to favor either the up or the down position of the displacer depending on if the engine is running on heat or ice.

The P-19 also incorporates regenerators in the displacer, the same way as seen in this video:


https://youtu.be/CdX7hwjhgA4

Screenshot_20211122-111646.jpg (87.43 KiB) Viewed 3961 times

[Nobody]

The Senft diagram you've provided clearly shows the curve of a real engine contained within the heat source and cold sink. Providing neither heat to the hot side, nor cooling the cold side.

It does clearly show the potential for heat to go into the working fluid from the hot source, and the potential for heat to be rejected into the cold Sink.

Both are probably true. If so, Carnot's rule is verified.

[Nobody]

Work is Force times Distance.
It is also pressure times volume.

Wo = We - Wc - W friction

We = W fluid e - W ambient c
Wc = W ambient e - W fluid c

W/V = P

We/V = P fluid e - P ambient c
Wc/V = P ambient e - P fluid c

Whether there is a crankshaft or free piston. Doesn't change.

Even if it is an acoustic engine with no piston, open ended with a regenerator stack.

Senft's equation doesn't change. You may add my details to it. The end result is the same because Work ambient compression is the same value as Work ambient expansion. They will cancel.

Work fluid compression is smaller than Work fluid Expansion because the temperature for compression is lower, because heat is rejected to the cold sink, lowering the temperature.

Without that rejection the two works would equal and you would get zero work out. Actually worse, minus friction.

[Nobody]

Here are two more books I've found interesting.

The Evolution of the Heat Engine by Ivo Kolin

Another James R. Senft book:

An introduction to Stirling Engines

Enjoy.

[Tom Booth]

The Senft diagram you've provided clearly shows the curve of a real engine ...

Sorry, my mistake. That was the wrong diagram comparing the Stirling with an "arbitrary" engine.

This is the one comparing the Stirling vs. Carnot engines.

IMG_20211127_152748660_MP.jpg (123.88 KiB) Viewed 3868 times

Thanks.

[Tom Booth]

...
Work fluid compression is smaller than Work fluid Expansion because the temperature for compression is lower, because heat is rejected to the cold sink, lowering the temperature.

Without that rejection the two works would equal and you would get zero work out. Actually worse, minus friction.

So, by that reconning, it would seem that you completely discount ANY cooling or temperature drop whatsoever due to the conversion of heat into work?

I would think that in any circumstance, SOME heat of the internal working gas is converted to work during the power stroke (expansion, driving the piston out) resulting in a lower temperature (adiabatic cooling with transfer of kinetic energy to the piston/crank/external load) so the compression is subsequently easier.

You seem to be saying that there is no such thing as any lowering of the temperature of the working fluid due to the working fluid doing work output to an external load. No conversion of heat into work whatsoever. Is that right?

At any rate, you make no explicit mention of factoring in any cooling due to the conversion of heat into work.

Just to be clear, so I understand your position. If so, you are certainly not alone. Take this article for example which states about the working fluid:

Gas
There's a volume of gas permanently sealed inside the machine in a closed cylinder. It can be ordinary air, hydrogen, helium, or some other readily available substance that remains a gas as it's heated and cooled through the engine's complete cycle (the repeated series of operations it goes through). Its only purpose is to move heat energy from the heat source to the heat sink, powering the piston that drives the machine, and then to go back again to pick up some more. The gas that moves heat is sometimes called the working fluid.

https://www.explainthatstuff.com/how-st ... -work.html

(Emphasis added)

Or this, going back to the authority of Carnot himself:

The production of motive power is therefore due in steam engines not to actual consumption of caloric but to its transportation from a warm body to a cold body

[Nobody]

Tom said "So, by that reconning, it would seem that you completely discount ANY cooling or temperature drop whatsoever due to the conversion of heat into work?"

Good catch. What I wrote is wrong. Let me rewrite it please.

Work fluid Compression is smaller than Work fluid Expansion because the temperature during compression is lower. It is kept at a lower temperature because heat is rejected to the cold sink during compression.

The temperature is lowered by heat being absorbed into the regenerator.

The temperature of the working fluid doesn't change during expansion because heat is added isothermally from the hot source. This makes up for the expansion work output. No temperature change.

Without that absorption into the regenerator and rejection to the cold sink, the two works would equal and you would get zero work out. Actually worse, minus friction. The compression would follow B>B'>A and the heat gain from compression would instead be rejected into the hot source.

Zero total work out, would result in zero cooling from work.

The compression rejects heat to the isothermal cold sink until reaching point D. At point D, again the displacer flops adding back to the working fluid the stored heat from the regenerator, raising the temperature back to T-hot, arriving back at point A again.

[Tom Booth]

Tom said "So, by that reconning, it would seem that you completely discount ANY cooling or temperature drop whatsoever due to the conversion of heat into work?"

Good catch. What I wrote is wrong. Let me rewrite it please.

Work fluid Compression is smaller than Work fluid Expansion because the temperature during compression is lower. It is kept at a lower temperature because heat is rejected to the cold sink during compression.

The temperature is lowered by heat being absorbed into the regenerator.

The temperature of the working fluid doesn't change during expansion because heat is added isothermally from the hot source. This makes up for the expansion work output. No temperature change.

Without that absorption into the regenerator and rejection to the cold sink, the two works would equal and you would get zero work out. Actually worse, minus friction. The compression would follow B>B'>A and the heat gain from compression would instead be rejected into the hot source.

Zero total work out, would result in zero cooling from work.

The compression rejects heat to the isothermal cold sink until reaching point D. At point D, again the displacer flops adding back to the working fluid the stored heat from the regenerator, raising the temperature back to T-hot, arriving back at point A again.

I guess we can agree to disagree.

You still claim "zero cooling from work." which IMO is absurd. You claim Isothermal expansion and contraction. IMO Isothermal anything is negligible, next to non-existent, especially in an engine running at any speed, with a glass non-heat conducting cylinder, and/or a non-conducting acrylic non-heat exchanging plate.

If there was really "zero cooling from work" there would be no work, no engine movement whatsoever. The "working fluid" does thermodynamic work any time the gas is expanding and driving the piston.

Isothermal expansion is a "Carnot Cycle" idealization that postulates an "infinitely slow" running engine, to allow time for heat absorption during expansion so that there is no cooling effect.

No REAL engine runs "infinitely slow". Running at any speed at all there is mostly adiabatic expansion.

Idealized abstractions just don't represent the way real engines operate in the real world.

If a "free piston" engine with no flywheel, crank or displacer, were to actually run isothermally, with more and more heat added the whole while it is expanding with no flywheel or anything else to stop it it would fly out the cylinder like a bullet, because there would be no cooling and contraction to pull it back.

Anyway, this is getting off topic for this thread. It you want to debate this or some other issue, There is a button at the top of the main forum page for "New Topic".

[Nobody]

It is unfortunate that the first P-T diagram did not include Adiabatic expansion and compression curves. That would have been a very telling piece of data.

The comparison is between an ideal Stirling and an indicator diagram of assumed a real engine. The description for the image is insufficient, and it lacks numbers.

It does show lack of cold plate cooling, and lack of hot plate warming. The entire curve is inside the temperature lines.

The adiabatic lines would have shown that the curve wasn't Adiabatic either.