Stirlingengineforum.com

Nobody wrote: ↑Fri Mar 04, 2022 8:28 am ...

P.S., It is blatantly obvious from Carnot's theory that heat flows in the hot side, and out both the cold side and out as power, proving Caloric Theory wrong.

That was not Carnot's theory.

Carnot's theory was based entirely on Caloric theory.

Using his water wheel illustration: water flows over a water wheel and out the other side without one drop being "lost" to work output.

Though obviously a water wheel does produce work output.

That the compression stroke is "easier" if heat is thrown away is just wrong headed and silly.

The build up of heat and pressure during compression is what gives the engine it's torque and power.

The atmospheric pressure that drives the piston inward to begin with becomes velocity and momentum. The piston had been set in motion

That motion/momentum/velocity is then converted back into heat and pressure to drive the piston back out.

Taking away heat at that point can do nothing but reduce subsequent power output. It is rather like intercepting the trajectory of a rubber ball with a net just before it hits a wall. The ball will bounce back with much less force, or no force at all.

That there is some benefit to be gained by removing the heat from a heat engine at any point in the cycle is counterproductive

Effort would be better directed towards finding ways to more effectively convert that heat into work output.

Concentrating heat and pressure in a compression stroke does not rob power from the engine. It just concentrates the energy, transforming the momentum and velocity back into heat and pressure, or a form of energy that can then be utilized for power output.

Nope.

Nobody wrote: ↑Fri Mar 04, 2022 8:28 am
P.S., It is blatantly obvious from Carnot's theory that heat flows in the hot side, and out both the cold side and out as power, proving Caloric Theory wrong.

Yikes, this is very poorly worded vs Tom's response which is right on.

Tom Booth wrote: ↑Fri Mar 04, 2022 10:04 am That the compression stroke is "easier" if heat is thrown away is just wrong headed and silly.

As I said previously, growing up in an ICE world clouds thinking, and the misdirection can be amusing at times. Tom, reread your statement again and consider PVT values for adiabatic compression in common ICE vs isothermal compression in SE. It should be...blatantly obvious...that compression is easier when the heat is rejected as during isothermal compression where heat out = work in.

Tom Booth wrote: ↑Fri Mar 04, 2022 10:04 am
That there is some benefit to be gained by removing the heat from a heat engine at any point in the cycle is counterproductive

A cycle operates between a high energy state and a low energy state. The only benefit to removing heat during the cycle is allowing processes to combine into a cycle !!! otherwise, yeah, it's 'counterproductive'.

Tom Booth wrote: ↑Fri Mar 04, 2022 10:04 am
Effort would be better directed towards finding ways to more effectively convert that heat into work output.

So far, regen appears the only magic, hence the endless SE interest.

Tom Booth wrote: ↑Fri Mar 04, 2022 10:04 am
Concentrating heat and pressure in a compression stroke does not rob power from the engine. It just concentrates the energy, transforming the momentum and velocity back into heat and pressure, or a form of energy that can then be utilized for power output.

Consider 4 stroke ICE with fixed input temp. Dismissing potential auto ignition issues, a high compression ratio is more eff than a low compression ratio (yeah, due Carnot) but the high compression ratio is less powerful per cycle (aka rpm). As compression temp approaches input temp (via increasing compression ratio) this 'engine' approaches a gas spring.

matt brown wrote: ↑Fri Mar 04, 2022 3:15 pm.... It should be...blatantly obvious...that compression is easier when the heat is rejected as during isothermal compression where heat out = work in.

Isothermal compression?

Is that a real thing?

There is this chart posted previously by 'nobody', supposed to be actual temperature readings from a Stirling engine.

Could you point out which section of the graph represents isothermal compression?

matt brown wrote: ↑Fri Mar 04, 2022 3:15 pm

Nobody wrote: ↑Fri Mar 04, 2022 8:28 am
P.S., It is blatantly obvious from Carnot's theory that heat flows in the hot side, and out both the cold side and out as power, proving Caloric Theory wrong.

Yikes, this is very poorly worded vs Tom's response which is right on.

Really!

Tom Booth wrote: ↑Fri Mar 04, 2022 8:54 am That was not Carnot's theory.

Carnot's theory was based entirely on Caloric theory.

Using his water wheel illustration: water flows over a water wheel and out the other side without one drop being "lost" to work output.

Though obviously a water wheel does produce work output.

Let's go with that. Carnot was a mathematician so his theory is based on mathematics, specifically calculus.

Yes the predominant heat engine theory at the time was Caloric. Wow! Can you blame him for using analogies that were commonly understood at the time! No. Are analogies imperfect? Yes. He probably knew it.

So the theory is not based on his analogy. His theory is based on solid calculus and observation. His analogy is based on peer ignorance. Kinetic theory came later, and Carnot's theories were put to the test. Some, Caloric, we're discarded.

Think about what the n=(Th-Tc)/Th shows. It says that the exact same engine operated at two different potentials gets two different percentages of work out for the same heat in. The efficiency changes just by rasing the temperature or potential.

Compare that with a water wheel. Does the same exact water wheel get a higher efficiency when the pipe feeding it is raised higher? No. It's the same mgh sized water wheel. Same work out for higher energy in. Thus, the analogy falls apart. The efficiency gets worse when potential, or head, increases. Opposite of a heat engine.

Okay, analogies are imperfect. Let's look at what the math tells us. For n to change with rising temperature, less heat must go in per work out. The ratio of heat in minus heat out verses work out must get lower. Heat in can be kept constant, so heat out must be less. That violates the conservation of Caloric and the two become mutually exclusive. Both can't happen.

Combine that with heat rejection being at a lower temperature, and it clearly shows that calorics won't be conserved, meaning the new kinetic theory is better. Carnot probably would have pointed that out had he lived long enough.

Maybe poorly worded but the only thing correct from Tom was his comment that work can be used from a water wheel. " a water wheel does produce work output." And that my comment was Carnot's theory, agreed, I said from Carnot's theory. Note that the word "from", means a staring point.

I probably should sit down to make some graphs and charts and kinematic drawings, so the things we talk of can be visualized. There is a great quantity of ground to cover. I'm not sure it would be understood.

I tire of hearing falsehoods that science is wrong, with zero proof of it. Build something that violates an accepted science and we'll talk. Until then i can use science even if it is wrong. F=ma is wrong, Einstein proved that.

Tom,

Displayed in the graph are a hot section and cold section. The temperature of the fluid in the two sections vary no more than about 100 degrees from the isotherms. Considering that the difference between hot and cold is 400 degrees and the dynamics inside an engine, calling that an isotherm isn't too much a stretch.

Remember, there needs to be a temperature difference for there to be heat transfer. I wouldn't exactly call that adiabatic as heat needs to go in to keep the temperature from changing as little as it appears to. The cooler battles the heating caused by a work absorbing compression. Similar for the heater and work producing expansion.

Nobody wrote: ↑Sun Mar 06, 2022 9:20 am Tom,

Displayed in the graph are a hot section and cold section. The temperature of the fluid in the two sections vary no more than about 100 degrees from the isotherms. Considering that the difference between hot and cold is 400 degrees and the dynamics inside an engine, calling that an isotherm isn't too much a stretch.

Remember, there needs to be a temperature difference for there to be heat transfer. I wouldn't exactly call that adiabatic as heat needs to go in to keep the temperature from changing as little as it appears to. The cooler battles the heating caused by a work absorbing compression. Similar for the heater and work producing expansion.

Isotherms? What isotherms?

The red and blue straight lines represent the temperatures of the hot and cold heat exchangers.

Yes, I think it is a stretch and I can otherwise make no sense of your further conjectural ramblings

What is clear from the chart is that the combination of heat from compression + input heat causes the temperature of the working gas to exceed the input temperature .

The question is, is that helpful or harmful?

Likewise on the cold end, expansion decreases the temperature of the working fluid on the cold side below the temperature of the sink?

Is that helpful or harmful? Does it add or subtract from the potential work output?

Personally I think it is helpful because it actually increases the effective ∆T above and beyond what is provided by the heat source and sink naturally or passively

The "dynamics inside the engine" act like a "Maxwell's daemon" accumulating heat at the hot end and in effect refrigerating the cold end .

It is this higher than supplied ∆T that creates a potential for > efficiency

Tom Booth wrote: ↑Sun Mar 06, 2022 1:39 pm
Isotherms? What isotherms?

The red and blue straight lines represent the temperatures of the hot and cold heat exchangers.

Yes, I think it is a stretch...

Tom, it's an approximation.

Tom Booth wrote: ↑Sun Mar 06, 2022 1:39 pm
Is that helpful or harmful? Does it add or subtract from the potential work output?

It is this higher than supplied ∆T that creates a potential for > efficiency

Normally, 'crossing the line' will tax system in both lost work and add'l heat.

What I find interesting about this graph is that both 'humps' coincide and around 150 degs.

Nobody wrote: ↑Fri Mar 04, 2022 8:28 am
Carnot was a mathematician so his theory is based on mathematics, specifically calculus.

His theory is based on solid calculus and observation. His analogy is based on peer ignorance. Kinetic theory came later, and Carnot's theories were put to the test. Some, Caloric, we're discarded.

Think about what the n=(Th-Tc)/Th shows. It says that the exact same engine operated at two different potentials gets two different percentages of work out for the same heat in. The efficiency changes just by raising the temperature or potential.

I said from Carnot's theory. Note that the word "from", means a staring point.

@Nobody

Heads up, class is in session...

I spent most of yesterday (re)reading Carnot's Reflections and pondering Carnot buzz. First, there's no calculus in Reflections. Second, all the common Carnot theorem...principle...equation (whatever) stuff we hear (so often) was merely attributed to Carnot many years AFTER the kinetic theory was accepted. However, in Reflections, he does make several theorems. It appears you never read Reflections, and I don't recommend it to anyone, since it'll only confuse laymen while driving thermo guys nuts. The nickel tour is that Carnot nailed the cycle concept, but was merely using it as a means to ferret out the mechanical equivalent of work from limited experimental data, namely Cv & Cp heats. He got lucky with isothermal input/output concept, but totally choked when it came to adiabatic work since he was trying to determine what we now call gamma. OK, he got some things right, but some things wrong, but most of the wrong was due to lacking data.

Nevertheless, my spin on carefully reading Reflections is that Carnot was trying to determine if work output varied from same heat input depending upon input temperature. IOW, to borrow that lame waterfall analogy, does the work for a given distance vary depending where in the waterfall work is removed ? But steam at that time was severely limited by low pressure limits, so he took to air modeling, and despite period limitations, he did an xlnt job making steam/air parallels. To keep this short, Carnot was chasing...consider 3 cycles A, B, C, and let's use n=(Th-Tc)/Th to expose his chase:

A (600-300)/600 = .50
B (1000-300)/1000 = .70
C (1000-700)/1000 = .???

These days, both A & B are nobrainers, but C slips under the radar; yet, A vs C is what stalled the kinetic theory and had Thompson (aka Kelvin) a disbeliever for years. There's a big difference between standing on the shoulders of giants vs retracing their footprints...

@Nobody

This is from wiki article on Kelvin and note his line near bottom:

"the conversion of heat into mechanical effect is probably impossible, certainly undiscovered"

These guys would have paid dearly for some 'blatantly obvious' insight...

matt brown wrote: ↑Sun Mar 06, 2022 3:04 pm
Normally, 'crossing the line' will tax system in both lost work and add'l heat.

What I find interesting about this graph is that both 'humps' coincide and around 150 degs.

What I find interesting is that for approximately 1/2 cycle the temperature of the working gas on the cold side of the engine is colder than the sink, to which heat is supposedly being "rejected" to.

In addition, when the cold side temperature is hotter than the sink, due to "secondary heating" during compression, the working gas has been largely isolated from the sink (cold side heat exchanger) due to having been shifted over or "displaced" to the hot side.

Logically, this would seem to indicate that the "sink" in a running Stirling engine, does not function as a dumping ground for excess heat, but instead acts as a secondary heat/energy source.

The problem with this is that due to the limitations imposed by a sinusoidal motion, some of this heat is introduced during expansion, working against further potential reduction in temperature and increase in the ∆T.

This "theory" arose in my mind back in 2010 when I speculated that insulating the "sink" might allow a Stirling engine to self-cool it's cold side below ambient which, by increasing the ∆T should allow the engine to run more efficiently

viewtopic.php?f=1&t=478

It was not until fairly recently I was able to test this theory experimentally, and found that a Stirling engine does indeed run quite a bit better when the "sink" is very thoroughly insulated.


https://youtu.be/Iq6snxiXbGg

By insulating the sink, engine RPM increased from 270 to 295.


https://youtu.be/zEqg1TgLqXI

This experiment was repeated in several different ways, with different types of insulation. Here, the engine can be seen running, and the RPM visibly increases when insulation is used to cover the "sink", no other changes or modifications to the engine being made.


https://youtu.be/fFByKkGr5bE

These and other experiments appear to contradict Carnot's theorem, or the general modern interpretation in the form of the "efficiency equation", that in this case would result in a calculation that some 80% to 90% of the heat entering through the bottom of the engine would have to pass straight through and out the top, being "rejected" to the sink.

I was actually quite surprised the first time I ran this experiment, thinking that "everybody" was no doubt right, and the insulation would create a bottleneck, the engine would quickly overheat and come to a screeching halt

But that did not happen. The engine did not even slow down. It ran quite a bit faster, and visibly and audibly more energetically. As I had speculated might happen, back in 2010.

It is strange that posting these experiments on various science and physics forums only results in my being banned and any discussion of it being locked.

Tom Booth wrote: ↑Sun Mar 06, 2022 7:51 pm
...or the general modern interpretation in the form of the "efficiency equation"...

Xlnt way to describe it !!! Most guys simply take that eff. eq. at 'face value' and dismiss (or don't know) that there's some preconditions. Again, I have no issue with most of the stuff 'derived' from Carnot except (1) the preconditions and (2) noting it's not by Carnot. So, as long as the kiddies play nice and stay in their room, there's no problem.

Interesting test on insulating top plate. I remember seeing this somewhere on site, but it had slipped my mind. Well, the proof is in the pudding, and I'm curious what Senft would say. However, I notice that this LTD is 'open cycle' with 'slots' around lower plate (like most LTD). I suspect something is going on here, and suspect that this would never run (with or without insulation) if 'closed cycle' (no slots). That would be an interesting followup test; have you tried ?

Strange that no one cared, since this is the type of thing that usually draws tons of interest. When you get down to the small values that LTD run, it's often hard to chase down what's really going on.

matt brown wrote: ↑Sun Mar 06, 2022 8:46 pm ..., I notice that this LTD is 'open cycle' with 'slots' around lower plate (like most LTD). I suspect something is going on here, and suspect that this would never run (with or without insulation) if 'closed cycle' (no slots). That would be an interesting followup test; have you tried ?

I don't know what you mean by 'open cycle" or "slots" so can't answer your question.

Normally, I would think "open cycle" would mean air is entering and leaving the engine.

If so, perhaps it appears there are open 'slots', but that is not the case.

The displacer chamber is clear acrylic. There are no slots.

Here is a video showing what the kit consisted of and another showing removal of the insulation.

https://youtu.be/p5XoocPoyac

https://youtu.be/fwWTfyoq9rk

The engine is not "open cycle". No working gas enters or leaves the inside of the engine.