Thermodynamic work vs. real work

[Tom Booth]

And to be perfectly honest Tom, after being baselessly accused of forming an alter ego, Jack, my tolerance for your "nonsense" is less than others.

Sorry about that, I was later able to confirm Jack was who he said he is.

But when the owner/moderator goes out of his way to PM me and tells me that he has evidence of some members here pretending to be my "friends" while "stabbing me in the back", (his words) in PM's to him, using spoofed IP addresses and multiple fake sock puppet personalities I may have over-reacted when you posted a response to a post originally addressed to Jack.

Of all the people here you seem to be the most "real' appearing on your own YouTube videos etc. actually doing experiments and making a contribution.

"Fool" IMO doesn't care about any of that. He is just here to post nonsense and spread confusion and lies, not even caring if his own posts make sense or not or have any continuity with the actual conversation.

Anyway, if you are going to tolerate "fools" attacks on my character you will also have to tolerate my responses. Sorry about that, but as you say, it takes two, but he is the one who starts it, as above. I'm not going to stand for being lied about and having my work and motives ridiculed and misrepresented by a muttering "fool".

[Fool]

I don't see anything wrong with those diagrams. It depicts why no one is trying to build a Carnot engine over a Stirling. Power to size..

[Tom Booth]

Just so I'm not accused of being"paranoid":

One of the owners PM's on the topic:

Resize_20240904_140457_7262.jpg (164.53 KiB) Viewed 2034 times

What he was going to "ignore" was some threat of some legal action against the forum.

[Fool]

It isn't against me. So quit guessing.

[VincentG]

I don't see anything wrong with those diagrams. It depicts why no one is trying to build a Carnot engine over a Stirling. Power to size..

You said the PV diagram would need to go outside a Carnot cycle to be more efficient. Maybe I took that too literally.

Up to this point I assumed the Stirling cycle fit inside the Carnot cycle.

[Fool]

I see what you are thinking. An ideal Stirling cycle is outside of a Carnot cycle, why is it the same, and not greater, efficiency? Good point, good question. The two cycles share the same isothermal expansion and compression temperatures. Neither are outside those isotherms.

I guess I should have said, no engine inside the ideal PV cycle will be more efficient. They can equal the Carnot only if they share the same isotherm temperatures, and all their processes are reversible.

A process that takes a cycle outside an isotherm, will pass heat between the gas and sink, hot or cold. Say expansion. The gas gets colder than Tc. Energy moves from the Tc cold plate into the gas during the expansion in the form of heat. At the maximum volume the gas is now somewhat warmer than it would be if isentropic (adiabatic with work). This means that the compression will get hotter more quickly and require more work input. In fact it will probably take more work input to expand the gas that extra far, too. It is, perhaps, a lose lose effect, and irreversible. It is called hysteresis.

It can result in the cooling and heating of different parts of a machine. A hand operated bicycle tire pump heats up at the bottom, and stays cool at the top. I imagine a sealed cylinder being pumped up and down would get hot on the compression ends, and cold in the expansion middle. Right?

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[matt brown]

I don't see anything wrong with those diagrams. It depicts why no one is trying to build a Carnot engine over a Stirling. Power to size..

You said the PV diagram would need to go outside a Carnot cycle to be more efficient. Maybe I took that too literally.

Up to this point I assumed the Stirling cycle fit inside the Carnot cycle.

Stirling vs Carnot.png (136.97 KiB) Viewed 1471 times

Here's the common comparison of Stirling vs Carnot where Carnot is "circumscribed" inside Stirling for "similarity". The beauty of this comparison is that this clearly shows maximum output of Stirling vs minimum output of Carnot. It's OK to refer to both by Thigh vs Tlow casually (hence Carnot buzz) but a more accurate reference is via Uhigh vs Ulow. IOW the difference between thinking temperature vs (internal) energy can effect scheming.

[Fool]

There is no reason why Carnot and Stirling Cycles can't have the same isothermal expansion. It just takes a longer additional stroke, and larger Carnot Engine. It's why modern engineers don't pursue the Carnot. No benefits over Stirling. Right?

Nevertheless, it is possible to equal the Stirling. The main problem is the cross over point, and how to switch from isothermal to isentropic. Conducting cylinder to adiabatic insulating cylinder. Quasi static to high-speed.

Thermo acoustic engines might be closer to a Carnot than a Stirling if someone could figure out how to measure a PV indicator diagram for one, we would find out. It seems very tough to measure equivalent volume changes, on a non moving tube. LOL

.

[matt brown]

The major difference between Carnot and Stirling is that Carnot has no regen which most consider a tradeoff between output vs simplicity.

[Fool]

Two different ways of saving reusable energy MCv∆T.

[VincentG]

Thanks to all for the help.

[Fool]

It's the least we could do. You are very welcome. Thanks for the open mind. And good nature. They come from high intelligence and excellent experience. Best of wishes.

[Tom Booth]

This diagram can be used to indicate the operation of an ideal engine operating at the zero point, with the yellow line being 1 bar. In thermo talk, the gas does work on the piston in the area above the yellow line, and the piston does work on the gas in the area below the yellow line.

But in reality, work is also extracted from the area below the yellow line, all the way back to 1 bar. Realistically, there is no back work here and shaft power would be nearly continuous.

Can any interested party add to this diagram to distinguish the difference between the thermodynamic work and the actual work available to be extracted for lifting a weight.
...

Did you receive a satisfactory answer to your question?

If so, could you give a brief recap, or give the reply # above that you consider a good answer. Or what is the actual conclusion?

[Tom Booth]

It seems to me the essence of your question is: "...in reality, work is also extracted from the area below the yellow line, all the way back to 1 bar. Realistically, there is no back work here and shaft power would be nearly continuous."

Is a vacuum engine, or "flame licker" engine not doing "real work" when it does all of its work on the "return stroke" by atmospheric pressure, exclusively?

Even if you say, in a normal Stirling, the expanding gas does work that in some way "stores" energy in a flywheel or buffer "air spring", how is this stored energy that does work on the return stroke not actually doing "real work"?

As you pointed out, this work does produce "shaft power" or "real" useable mechanical work output.

As Senft called it: "constant mechanical effectiveness".

In reading Senft's little treatise, I got the impression he was intentionally trying to avoid the "Carnot efficiency" problem by coining his own terminology.

[Tom Booth]

Personally, I think there is a big difference between converting heat into work 100% and 100% "efficiency".

"Efficiency" is a slippery term with several different possible meanings and even more possible interpretations.

Pushing a piston out with 100% conversion of heat into work and having it return by atmospheric pressure while doing no "shaft work" or external "useful work" has zero "efficiency" in terms of useful work output.

What about conversion?

OK, so, heating and expanding a gas and "doing work" converts heat into work 100% and then atmosphere turns some of that work back into heat., so, we can say the heat created on the return stroke needs to be deducted from the 100%

But, a Stirling engine has a few trucks up its sleeve.

It has a regenerator that can "store" both heat and cold.

Imagine two regenerators, one for hot and one for cold.

On expansion, at the 1/2 way mark the gas falls below ambient pressure and temperature. At that point to BDC we switch to the cold regenerator. This "stored" cold helps the piston return with greater force.

Again at the 1/2 way mark, the pressure and temperature climb above ambient temperature and pressure. But at the 1/2 way mark we switch over to the hot regenerator.

This is basically like hot and cold "air springs".

It could also actually as a "heat pump".

Let's say the cold heat exchanger when not "in use" helping to cool the working fluid is allowed to pick up some ambient heat. With the next expansion the working fluid after passing the 1/2 way mark will absorb some heat from the cold regenerator.

With compression, this extra heat will be added to the next expansion.

For something like that to work, though, the piston and/or flywheel has to have enough weight and momentum so that heat is converted into motion/velocity and back into heat.

A displacer basically accomplishes the "trick" of switching between hot and cold "regenerators". And of course an actual regenerator is an actual regenerator.

Well, what about all the excess heat?

If the engine is continually pumping heat over into the hot regenerator, the excess heat will have to be removed.

Well, yes. It does.

It is removed each expansion when the heat is converted into work with 100% efficiency..

On expansion switch to cold to "pull" the piston back with added force.

On compression switch to hot to drive the piston out with added force. Which added force can continually drive a load.

There is no heat being transfered from the hot regenerator to the cold. Actually, quite the opposite. Heat is being pulled out of the cold regenerator and pumped into the hot, and then from there converted to work.

You can add heat to the hot regenerator to compensate for heat input that is lost to external work output as needed.

Now, the "useful work" output could vary greatly but in any event there is no loss of heat traveling or flowing from the hot regenerator to the cold.

There is no heat being "rejected" to the cold regenerator.

You may just as well call these regenerators "reservoirs" or hot and cold plate, or hot and cold heat exchangers.