LTD model "Stirling" uses Lenoir Cycle

[matt brown]

Carnot's main focus was on isothermal compression, which he conceived as "transporting" Caloric through the engine to the "cold reservoir". A completely fictitious idea, bordering on delusion. A kind of mental psychosis that still, unfortunately, infects people's thinking regarding heat engine efficiency, or just basic operation.

You think Carnot "nailed it" LOL

I believe Carnot's ridiculous declarations need to be completely eradicated. The sooner the better, for the future benefit this could bring to all of mankind.

The so-called "Carnot Limit" has had a stranglehold on people's thinking about heat engines long enough.

Indeed, lots of Carnot's stuff is delusional; however, he nailed adiabatic compression. If you look at any of the common 4 process cycles as PV plots, notice that when following a cycle from its lowest energy point to its highest energy point, that the 2 successive process plots become steeper. This increases the work area, but says nothing about thermal eff. Carnot had to use isothermal-adiabatic-isothermal-adiabatic sequence since adiabatic-isothermal-adiabatic-isothermal was a no-go. It appears you've be eating the forbidden fruit (reading Reflections), so, reread Carnot's spin on adiabatic compression and note Carnot used massive backwork of adiabatic compression to maximize 'jump' between high & low energy states which was lunacy in his day.

Look at that Lenoir PV again and consider similar Atkinson cycle overlaid within any given Lenoir cycle. Yep, simply add an adiabatic compression process anywhere you like and...viola...you have an Atkinson. Now, let's compare Lenoir vs Atkinson. It's self-evident that the Atkinson will have less Wnet than Lenoir PER CYCLE, but it's not self-evident which cycle is more eff (Wnet/thermal input unit) since this will require 'numerous' process calculations. However, if one did the calculations, one would discover that despite Atkinson having less Wnet than Lenoir per cycle, Atkinson is more eff (per thermal input). And, as Atkinson compression adiabat approaches expansion adiabat, Atkinson increases thermal eff towards 100% despite less Wnet per cycle. The secret sauce of adiabatic compression is nothing more than all work can be transformed into heat (per process) but all heat cannot be transformed into work (per cycle). The 'magic' of our modern heat engines is adiabatic compression, not internal combustion.

I agree that there's a lot of bogus Carnot conclusions, but they can all be nixed when heat engines are approached as a physicist vs an engineer. When approached as energy, all this PVT stuff has preconditions that are often ignored, and reconciling these preconditions requires careful study. It's a long road and there's no short cuts...

[Nobody]

Matt, I stand corrected on what Carnot was and was thinking. You are correct I have never read Refections. Only think I've heard of him.

I still think that this whole mess could easily be cleared up by a long lecture on very simple Calculus methods for deriving the Carnot Cycle and the efficiency rule. Not sure if I have the patience necessary to enter it using a cell phone.

Start with the Papin Cycle, also known as the water pumping cycle. From the book "Evolution of the Heat Engine" by Ivo Kolin. Then move to the Carnot Cycle. Look at both the PV and TS diagrams to understand.

[Nobody]

Tom

Try any "off-the-shelf" model Stirling engine.

How is an engine that gets way less than one percent efficiency going to support your comments here?

An engine getting less than the Carnot limit still supports... Well... Carnot.

Doesn't it!

You are still not getting reviewed by any peers, except those that disagree.

[matt brown]

I still think that this whole mess could easily be cleared up by a long lecture on very simple Calculus methods for deriving the Carnot Cycle and the efficiency rule.

Yes, it's the missing calcs that allow for imaginary conclusions, but anyone serious about thermo should learn them on their own (no shortage of info). Meanwhile, just imagine how tough thermo would be without PV plots as visual aids.

I learned thermo without formal edu, so, I get most of the common misunderstandings. However, it's kinda funny here at times, since (as you know) there's waaaay more to heat engines than just this PVT stuff. Most guys consider thermo a tough subject, but I think that it's just a tad too abstract for many due to ICE dominance. It's hard to 'unlearn' stuff.

[matt brown]

An engine getting less than the Carnot limit still supports... Well... Carnot.

KUDOS, I've been waiting for someone to point this out !!! Furthermore, I'm clueless why guys are wasting time on LTD when they're little more than an academic distraction like a drinking bird or Crookes radiometer. I'm want some useful output, more along the lines of Symanski's Jim Dandy #6...

jim dady 6.jpg (264.51 KiB) Viewed 6260 times

[Tom Booth]

Tom

Try any "off-the-shelf" model Stirling engine.

How is an engine that gets way less than one percent efficiency going to support your comments here?

An engine getting less than the Carnot limit still supports... Well... Carnot.

Doesn't it!

What proof exactly do you have, or what evidence that ANY Stirling engine is "less than one percent" or "less than the Carnot limit" ?

Have you ever actually measured the ratio of heat input to "waste heat" output of any engine whatsoever?

Can you provide any reference whatsoever to any such actual measurements having ever been made in the past 200 years? I've looked, I've asked many times.

Using Carnot's bogus ∆T to allegedly calculate efficiency proves absolutely nothing about the actual efficiency, but as far as I can find, that is all anyone has ever done for the past 200 years.

I've done numerous tests/experiments/measurements. It seems, often, virtually no heat whatsoever is passing through the engine(s). And not because no heat is being applied or taken in.

You can say all you want that that is "less than the Carnot limit", on what basis?

The actual reality of the supposed "Carnot engine" and/or "Carnot Limit" has never been established or verified experimentally.

[Tom Booth]

To put it susinctly, quoting the Encyclopedia Britannica, Carnot efficiency is:

The ratio of the net work output to the heat input is equal to the ratio of the difference between the temperatures of the heat source and the heat sink divided by the temperature of the heat source

This is entirely theoretical, originally based on the Caloric theory. Point out, if you can, when and where this nonsense has ever been experimentally verified.

Anybody?

[matt brown]

To put it susinctly, quoting the Encyclopedia Britannica, Carnot efficiency is:

The ratio of the net work output to the heat input is equal to the ratio of the difference between the temperatures of the heat source and the heat sink divided by the temperature of the heat source

This is entirely theoretical, originally based on the Caloric theory. Point out, if you can, when and where this nonsense has ever been experimentally verified.

Anybody?

Stirling_PV_1.png (26.04 KiB) Viewed 6251 times

Tom - if all we had to go on was above PV, then it would be impossible to confirm much of anything...

[matt brown]

However, once we know a lot about thermo details, each process can be 'reduced' to some minor calculus like the mumbo-jumbo at left here. Yep, those little equations contain the missing heat and/or work input and output per process which can often be further reduced into simple conclusions. We can all look at a good PV plot and see stuff, but few can look at a good PV plot and see all the heat & work, input & output, per process. However, once understands the mumbo-jumbo is correct, then it follows that various conclusions are correct. What anyone can correctly deduce from a good PV plot will depend upon the particular cycle and the thermo experience one has.

[matt brown]

Stirling_PV_3.png (15.17 KiB) Viewed 6247 times

OP...
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Consider attached PV a sq cycle where V2 would be reduced slightly such that P at point 2 equals P at point 4. This returns us to another sq cycle, and many comm'l SE hover around sq cycles, tho different widely within this 'class'. So, consider this another sq cycle (4 legged flavor) wherein Pr=Vr=Tr=2, such that previous 'heat triangle' was the 'upper half' of this PV plot. Let's recycle the same values, such that (I'm going to use the mismatched points per this plot) pt 4=300K and pt 1=600K. We already know from the previous heat triangle that if input Q=5 during isochoric process 4-1 (here) then Q=2.77 during isothermal expansion process 1-2. And we know (or should know) that since there's no change in internal energy for an isothermal process (dU=0) then ideal regen process 2-3 will (also) have Q=5. Now, if we fudge that isothermal input Q=2.77 just a tad to Q=2.8, we'd find...

heat input 2.8
heat output 1.4
work output 1.4
ideal regen 5

OK, are you guys onboard with how you game this stuff ??? Note that for THIS cycle with one fall from ideal grace where everything remains ideal except for regen, regen=.8 would 'tax' this system 1 unit of add'l heat vs only Wnet=1.4 units !

Yep, regen cycles are not for the faint of heart, and the devil is in the details. No, alphax, lots of guys know this, they just keep it secret (would nix their grant schmooze). I'll leave it as extra credit (lol) for those who wonder about same cycle but monatomic (merely sub in mono Cv & Cp) and discover how same .8 regen gets a break (mono vs dia). I'm big on GA & RA...

Again, don't get me wrong, I'm still in the chase, but to quote Twain "I never let my schoolin' interfere with my education."
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Tom - remember this ??? The whole post was walking everyone thru a common Stirling Cycle and filling in some blanks, namely regen heat, and this post hinges on knowing various things that I can build on to explain other stuff. I started with a 300k-600k cycle, then I filled in each process heat and work. No, I didn't conjure up those process values from staring in a crystal ball !!!...no, they come from the calculus in previous post, and this mumbo-jumbo was derived via hard core experimental data like Cv & Cp heats. Not surprisingly, this cycle has .50 eff per Carnot buzz !!! You have a common thermo problem where PVT values are easy to grasp like gauge values, but Q & W require calcs. It's best to be able to do these calcs, but it's not mandatory (for laymen) since we have online calculators. All you have to do is run a short string of variables thru each process until you see a pattern emerge among other variables, all thanks to PV=nRT.

[Tom Booth]

There are a few principles, rather well established.

1. For heat to travel, there needs to be a hot body and a cold body. there needs to be a temperature difference.

If heat goes into an engine on the hot side, to get back out of the engine on the cold side, the cold side temperature must rise above the temperature of the ambient surroundings.

If an engine is running, it is taking in heat. Therefore;

If an engine is running by being supplied with heat, then the cold side MUST be, according to the Carnot Theorem, some measurable degree in temperature above the ambient surroundings.

If there is no measurable elevation in temperature of the cold side of the engine, then Q out can only be zero or less.

Then work out is equal to Q in.

This is not "mumbo-jumbo" it's simple mathematics and common sense, based on established principles. It also completely demolishes the "Carnot limit".

I've had engines running and running with no detectable rise in temperature at the supposed "sink" whatsoever.

Therefore taking Work net = Qin - Qout efficiency is 100%

These kind of experiments are very easy to carry out. Measuring temperatures is no problem at all.

Every experiment I've done demonstrates that efficiency = 1-Tc/Th is wrong.

1-Tc/Th may for example indicate a "Carnot efficiency" of AT BEST 19%

Actual experimental measurements however show an efficiency near 100% simply by virtue of the fact that the temperature of the cold side of the engine remains cold, at ambient temperature, therefore no heat can be leaving the engine. Not directly as heat anyway.

That is why I say, take any off-the-shelf, import toy Stirling engine and do some experiments Carnot efficiency is very easily broken

[matt brown]

Let me see if I got you right....if small LTD is less than .01 eff, then Qout is more than .99 Qin (via Carnot buzz) and this would mean that cold plate should be nearly the same temperature as hot plate, and you (obviously) observe that this is not the case. Therefore, since cold plate is effectively ambient temperature, while hot plate is substantially above ambient temperature, then more work is being produced. Is this the jest ?

[airpower]

An engine getting less than the Carnot limit still supports... Well... Carnot.

KUDOS, I've been waiting for someone to point this out !!! Furthermore, I'm clueless why guys are wasting time on LTD when they're little more than an academic distraction like a drinking bird or Crookes radiometer. I'm want some useful output, more along the lines of Symanski's Jim Dandy #6...

...

Not LTD is the distraction the Stirling Engine itself is the distraction, with it you will be stuck in ~40% efficacy forever.
Still a lot of work to be done, like automatism to get more data but 40% is yesteryear's dream.
First thing first but by look of things Covid hocus-pocus is done with and May 2 is get go time.

https://en.demotor.net/stirling-engine/stirling-cycle

[Tom Booth]

Let me see if I got you right....if small LTD is less than .01 eff, then Qout is more than .99 Qin (via Carnot buzz) and this would mean that cold plate should be nearly the same temperature as hot plate, and you (obviously) observe that this is not the case. Therefore, since cold plate is effectively ambient temperature, while hot plate is substantially above ambient temperature, then more work is being produced. Is this the jest ?

Well, no, not exactly.

To start with, your conflating percentage of heat with quantity of heat.

You say "More work is being produced" more than what?

1% Carnot efficiency would represent virtually no temperature difference at all. Maybe 1 or 2 degrees.

Most model LTD engines would not even run on such a small ∆T.

A more realistic example might be an LTD running on a cup of boiling water, and putting some actual quantity of heat in joules entering and leaving the engine on that would be helpful

You give an example where the temperature difference itself would be barely detectable.

[matt brown]

I was taking a guess at LTD running on handheld heat. OK, what dT are you talking, something like 30k ? As dT increases, I'd think that overheating (and stall) would become a major issue when LTD volume is so small.