Stirlingengineforum.com

I think the questions or controversies brought up in connection with this video probably warrant further discussion.


https://youtu.be/SHyke4hUNOs


I'm starting this thread in response to Matt Brown's post here: viewtopic.php?f=1&t=5481&start=30#p18456

matt brown wrote: ↑Tue Dec 20, 2022 1:38 am Hall video is very interesting, but his engine running so well without flywheel indicates piston & crank mass are effective 'flywheel'

This point was already addressed in the other thread, but just to recap briefly, there are examples of Stirling engines running with no flywheel OR crankshaft.

and engine has very low power per stroke.

Don't know how.you derived the power of the engine but seems irrelevant to the point being made: that there is a drop in internal pressure allowing outside atmospheric pressure to effect the return of the piston.

I imagine all the pickups on engine are to gather data points for PV plot, but kinda strange we don't have temp and wattage values.

That might be interesting, especially temperature, but presents some difficulty because of the presumed rapid fluctuations in temperature and the slow response time of thermometers.

Basic scheme exposes an unusual thing about adiabatic processes: adiabatic compression & expansion are only equal when work is done on and by the gas, but a 'workless' adiabatic expansion is slightly different (way beyond the context here).

The only thing approaching a "workless" expansion of a gas would be release of the gas into a vacuum. Not sure what the relevance is if any.

Hall video has xlnt PV plot, but also exposes how limited PV plots can be.

How so?

However, we know that after adiabatic expansion, ambient pressure would tend to retrace adiabatic expansion curve with adiabatic compression until pressure equalizes on both sides of piston. The only way to get a different 'trace' is via heat flow.

Not true. You can also jump to a different temperature due to work input or output. Though it is true, heat and work are "equivalent" I would not consider the transformation of heat into work "heat flow".

So, I'm guessing he's approximating a 3 legged cycle with isothermal compression, isochoric heating, and adiabatic expansion. This is similar the 3 legged Lenoir with isothermal compression in lieu of isobaric compression.

Relevance?

I'll further guess that Hall engine has temp above ambient at end of expansion...

I assume by "engine" you mean the working fluid in the engine. That the temperature of the working fluid would be above ambient seems unlikely; (unless it is a pressurized system, but then "ambient" temperature would be largely irrelevant. "Buffer" temp. maybe)

1) the working fluid "contracts" indicating a low temperature 2) Atmospheric pressure drives the piston inward indicating atmospheric air is at a higher energy level at that point. 3) temperature is a measure of energy level 4) Supposed actual measurement of another Stirling engine previously posted in the forum shows a temperature of the working fluid lower than the cold "heat exchanger" which presumably would be near ambient.

Admittedly however, what type of Stirling was measured and what is represented is not entirely clear.

which means very light gas mass enclosed.

Not sure how that follows, or the relevance, at any rate the conclusion seems based on prior assumptions which are questionable. The gas, presumably would be at a low temperature after expansion, resulting in contraction immediately following.

I think it's kinda sneaky to suggest (by omission) that temp at end of expansion is sub ambient like pressure.

Are you accusing Hall of being "sneaky", intentionally misleading, or what exactly?

So, my bottom line is Hall has a fancy flame licker...

Not sure what you mean by that. Figuratively?

At any rate, no. It is as stated, a "Stirling engine". Hall is, or was secretary of the Stirling Engine Society. Presumably he would know a Stirling from a "flame licker", as would pretty much anyone.

Most model SE run a very slight Carnot cycle, just barely wider than a thin adiabatic PV plot. This is the only way to have a point plot for T high and T low while being able to allow for various values within the same mech, and even then, mech requires careful construction (narrow performance envelope). Interestingly, while most ICE have a narrow mechanical performance envelope (fuel, ignition, idle, whatever) their thermal envelope is monsterous...Otto, Diesel, Brayton, and Atkinson are not heat 'sensitive' cycles.

Relevance?

Here's an interesting PV plot that I came across which simplifies stuff. Consider P4 ambient pressure, and note that green & red processes are adiabats. Lenoir cycle would end pt 4 and require isobaric compression to blue isochor with cooling but 'no' Wneg. Meanwhile, IF we imagine a trace from 4-2 an isotherm, this compression would require cooling and 'some' Wneg. Now, an adiabatic compression 4-3 would simply retrace 3-4 expansion with no cooling but Wneg = Wpos (gas spring). However, per shown PV plot, adding isochoric cooling 4-1 allows subsequent adiabatic compression 1-2 with 'minimal' Wneg since P1-P4 region is sub ambient. Obviously (OK, nearly so) P4 temp is above ambient.

Yes, it's a low end ECE Otto commonly called a flame licker...

And for some extra fantasy...similar the previously mentioned imaginary isotherm from 4-2, imagine an isotherm from pt 1 to an extension of blue isochor where all isothermal compression requires cooling but no Wneg from system due to sub ambient pressure. There's a lot of ways you can game this stuff, but severe limitations due to related PVT values.

Slight error previous post...pt 4 temp is above ambient, not P4...

matt brown wrote: ↑Sat Dec 24, 2022 2:05 pm
Yes, it's a low end ECE Otto commonly called a flame licker...

In what universe?

"The only way to get a different 'trace' is via heat flow."

Regarding the question of "heat FLOW" vs. heat CONVERSION to mechanical motion or "work" I just finally ran an experiment, insulating the "sink" with what is almost the best insulating material money can buy (purportedly): silica Aerogel, that I sent for some months ago.

The experiment was unplanned and unrehearsed. I just happened to find a steam "face sauna" in a box of stuff I had picked up at a yard sale.

I found the steamer could run an LTD Stirling.

Well not entirely unplanned as I had been thinking about doing some such experiment for a long time, which is why I had sent for the Aerogel in the first place, but finding the steamer in the box of junk (mostly brass doorknobs we're in the box) was out of the blue.

https://youtu.be/WveuREfKwVg?list=PLpx2 ... Q9pQZzY7Eu

The video would have been longer, but my phone reached its recording limit so was cut short.

I used a "solar" engine made of clear acrylic as Acrylic is already more of a thermal barrier or insulator than a conductor. Add on the Aerogel blanket and the glass globe to eliminate any drafts that might cool the top of the engine and I'm not sure what more could be done to block the "flow" of heat through or out of the engine.

This type of magnetic LTD by the way, is normally rather difficult to get running. A hot cup of coffee provides just enough heat to get it running, with several false starts, so that difficult starting of the cold engine is not out of the ordinary.

A few flaws in the experiment:

There was steam leaking through where I had cut a circle out of the Aerogel blanket.

The infrared camera could not "see" through the Coleman lantern globe, so gave a potentially false impression of it being cool inside the glass. I think it actually was, relatively speaking, but the camera did not offer genuine proof or conclusive evidence. The Aerogel felt cool to the touch though.

The plastic ring I had cut from a soda bottle shriveled up from the heat, allowing steam to escape so the steamer eventually ran dry. Regardless, the engine continued running on dry heat.

Also, the Aerogel blanket rubbed on the flywheel a little.

Overall though, IMO, none of these flaws in the experiment invalidate the probable "fact" that there could hardly have been much "flow" of heat through or out of the engine so as to bring the internal temperature and pressure down each and every revolution.

According to the standard efficiency calculations, some 80% or more of the heat flowing into the engine each revolution, would need to be rejected or flow out from the "sink" or cold side to atmosphere each and every revolution for the engine to run at all.

Seems like a valid experiment. I would have been happier to see actual simultaneous temperature measurements using thermocouples against the hot/cold surfaces rather than trying use an IR imager. I have one of those cameras and the parallax issue between the visible and IR imagers makes the point readings useless, even if there's nothing between the camera and the target. Plus the fact that different surface types will read differently at the same temperature.

These meters are cheap and plenty accurate enough. The probes included included are good enough for the temps in use here, but better ones are very inexpensive and available. This is the one I have:

https://www.amazon.com/dp/B00SH4F0LK/

Agreed.

I got the IR camera from harbor freight, just because I was there and it was convenient to buy locally, but not too impressed.

I did start another thread on a physics forum and got the same suggestion (to hook up temperature probes).

I have a four probe thermometer like yours (perfect prime) I bought, maybe a year ago, but either it needs recalibration or maybe a fresh battery. The readings between the four probes are all over the place. There is also a considerable lag as far as updating the digital display when the temperature changes. Similar to a blinking curser on an old computer, on top of the actual thermal lag. The probes take time to heat up and cool down.

So not ideal for measuring rapidly changing air temperatures inside an engine.

The main thing though is the cold side temperature.

I had an instruction booklet for calibrating the thermometer but where it might be now, who knows. I actually got online now to see if I can find a copy on the website. I'll try changing the battery first as it seemed accurate out of the box when I first got it.

Another suggestion from the physics forum (which I've heard before) is that Aerogel is no better an insulator than air. So the theory there is the heat is conducted through the Aerogel to the surrounding air just the same.

If I measure the temperature of the "sink" (cold side) and it is the same temperature as the surrounding ambient air, someone says: "of course! That's what it's supposed to do! The cold side heat exchanger is just doing it's job!"

I find such responses jaw dropping.

Another one is "those toy engines are horribly inefficient, so only a tiny amount of heat is entering the engine, so the heat leaving the engine would be undetectable anyway"

Another video. Just testing a new setup.

I found something to help channel the steam to the bottom of the engine and avoid having it getting under the Aerogel blanket or fogging up the glass.

In the process got some pretty good temperature readings with the IR camera.

After about 15 minutes the temperature of the top of the insulation did go up.a few degrees.

The kitchen is a bit cold about 67°F and has lots of windows (cool drafts), so, the glass globe is intended to help block any cool drafts from directly cooling the top of the engine (or Aerogel insulation).

Of course, the top of the engine is already very non-heat conducting, made of acrylic. The bottom plate though is still metal of some kind.

These videos are unusually long compared to others, partly because of waiting as long as possible to see how long the engine can run before overheating, loosing the temperature differential and screeching to a halt.

But so far, that hasn't happened. Perhaps Aerogel has a very high heat capacity ?

Well I've previously tried various types of styrofoam as well but seems nothing can prevent the 80% + left over "waste heat" that is supposed to be "rejected" from escaping to the "cold reservoir".

https://youtu.be/l2XcnN6QdfA

Why do I bring these experiments up here? This thread is "Andrew Hall demonstration" not Tom Booth demonstration.

Well, I think the experiments confirm one another and prove Stirling engines do not actually operate anything like an "IDEAL" (Read that as imaginary, fantasy world) "Carnot cycle" engine. By "rejecting" a very large percentage of the supplied heat to some imagined "cold reservoir" at an imaginary "lower level" of imagined pools of the imagined liquid-like "caloric".

The little toy engine here has a magnet attached to the bottom of the piston that functions as the lifter for the displacer. Mechanically, I don't think this little engine could operate without a flywheel as in the Andrew Hall demonstration, or as other Stirling engines have been demonstrated to work without a flywheel but both experiments demonstrate the same thing; Stirling engines operate in a way other than as previously assumed, theorized and taught for the past two centuries.

The weight and momentum of this long long history of the "Carnot limit" so called absolute "LAW" is seemingly as unstoppable as the indestructible "Caloric fluid" itself was imagined to be, able to pass through any material substance with impunity.

A transfer of motion is not a THING, not a substance. The mechanical action of a Stirling heat engine is a manifestation of the transfer of motion from gaseous molecules to the solid molecules that comprise the piston. Such motion in a gaseous state can be measured by a thermometer as heat.

Transfered to a solid, that motion or "heat transfer" is no longer measurable with a thermometer in the solid piston, but it's absence from the gas, the drop in temperature that results in the gas as a consequence of this transfer is still apparent and measurable. Or should be.

Andrew Hall measured the drop in pressure. Such a drop in pressure can only be a consequence of a drop in temperature.

That drop in temperature has always been interpreted as a consequence of heat "rejection" or heat transfer to a sink rather than heat CONVERSION to an entirely different form of motion, or different manifestation of energy. One measurable with a thermometer, the other only measurable with some other kind of instrumentation such as a tachometer. The invisible heat energy in the invisible gas becomes visible, manifest as the RPM of the engine, the velocity, speed, momentum, "torque" etc. of the engine. The clatter and vibration and friction are all a result of heat transformation not heat "rejection".