Thermodynamic work vs. real work

[Fool]

Oops, yes, zero bar, zero Y, the X-axis, not zero Kelvin. Thanks.

[VincentG]

I seem to remember a lecture by Edward Bradley, Mathematician TA WSU Pullman Wa., where the inside of the natural log function was finite, despite going to Infinity. He also said a student sarcastically hollered out "Fine! So build one! LOL". Sorry, seemed relevant. Finite area means finite heat, not endless.

Ok, I can understand that the line approaches infinite but is definite at its end. This is really what Matt has been gaming lately. We really just have to build an engine with a significantly larger expansion ratio than anything out there. That's not hard. But it limits output on an already output limited cycle.

The ideal Stirling cycle is so ideal that it can't even be drawn with real values.

The green cycle is a much better fit realistically.

[Tom Booth]

Indicator diagrams were originally purely utilitarian instruments used by engineers for diagnostic purposes for giving a steam engine a "tune up". Serving the same basic purposes that a pressure gauge and timing light serve in a mechanics repair shop today, to keep the engine running in top notch condition.

Until some natural philosophers wanting to play at pretending they were engineers got their paws all over it. Having no real engine and no real understanding what it was for they just made up ridiculous nonsense in an effort to impress their fellow philosophers in an effort to appear knowledgeable.

Modern PV diagrams have no real remaining value or purpose other than to perpetuate the mythologies that were developed by these wannabe engineers who never really understood their real purpose.

[Fool]

Give me a little time, I'm hoping to add some more to your last thoughts. Numbers, maybe.

[VincentG]

Looking forward to it.

[matt brown]

Ok, I can understand that the line approaches infinite but is definite at its end. This is really what Matt has been gaming lately.

Yep, I'm trying to nix some of that "all the way down to zero K" backwork. In effect, I'm trying to create a crazy "virtual vacuum" for as much of the compression as possible.

We really just have to build an engine with a significantly larger expansion ratio than anything out there. That's not hard. But it limits output on an already output limited cycle.

A larger expansion ratio requires some out-of-the-box thinking vs typical out-of-phase stuff, but dwell, etc. can achieve this. However, you do hit on the 'obscure' nasty that only becomes 'obvious' when scheming larger expansion ratios where the pressure swing brings a host of issues that the herd avoids. Overall, heat engines are pressure engines, so I always game pressure before sweating heat issues. Remember, the work we seek is PV work, not heat work (which is total BS).

The ideal Stirling cycle is so ideal that it can't even be drawn with real values.

An alpha PV comes close, but common gamma and beta PV are bogus.

The green cycle is a much better fit realistically.

Indeed, the green cycle avoids many Stirling issues such as regen and isothermal heating. It compares well with low end Otto and Lenoir cycles, but better than either. The only bugger is how to get the heat in...

[Tom Booth]

(...)
Remember, the work we seek is PV work, not heat work (which is total BS).
...

So...

Both pressure and volume in a heat engine are a result of. What?

Wishful thinking, perhaps?

[Fool]

So...

Both pressure and volume in a heat engine are a result of. What?

Wishful thinking, perhaps?

Energy. Usually called internal energy. It can be transfered in by any one or more of the following:

Chemical.
Friction, often called work.
Work, often called adiabatic work.
Electrical, often called resistance Wattage.
Nuclear.
Light, often called photonic.
Radio waves, sometimes call photonic.
Microwaves, another form of radio waves.

Oh yeah, by the way, heat too. If you know the difference, now, between heat, and internal energy, you are on the road to learning some real thermodynamics. Best of luck.

PS, that is off topic. Sorry.

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[Fool]

Overall, heat engines are pressure engines, so I always game pressure before sweating heat issues. Remember, the work we seek is PV work, not heat work (which is total BS).

I find that very important. Heat is only one of many ways for how the internal energy gets into an engine, or volume of gas. Internal energy is responsible for gas temperature and pressure, at a set volume.

Putting energy into a liquid is what converts it from liquid to gas. That internal energy manifests into temperature and pressure of the gas volume. No heat necessary.

Good point. Sorry, off topic, but important.

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[Fool]

Indeed, the green cycle avoids many Stirling issues such as regen and isothermal heating. It compares well with low end Otto and Lenoir cycles, but better than either. The only bugger is how to get the heat in...

Yea, and the heat of compression out. Same back work, smaller forward work. Lower efficiency. Shorter amount of time for heat conduction.

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[Fool]

Numbers :

Stirling cycle.

stirling-cycle-diagram.png (36.15 KiB) Viewed 1587 times

Labeling the corner points :

Starting at upper left, beginning of process #1. Call them clockwise, hah digital watch, 1,2,3,4.

Yellow line is atmospheric, call it simply 15 psi.

Values, simply call them :

Vmin 1 m^3 top dead center left.
Vmax 2 m^3 bottom dead center right Vmax.

Pmax 30 psi
Pmin 7.5 psi

Th 600
Tc 300

Labeling :
Point #1 is at 30 psi, 600 K, 1 m^3

Point #2 is at 15 psi, 600 K, 2 m^3

Point #3 is at 7.5 psi, 300 K, 2 m^3

Point #4 is at 15 psi, 300 K, 1 m^3

Ideally.

Sorry my photo editor changes the background color 'white: to 'black'. That limits my ability to produce meaningful chart edits.

Maybe someone can use an on line calculator to add heat and work values? Internal energy's? For each process.

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[Fool]

Thermal couples, Peltier Devices, seem to work from heat flow. From hot to cold, heat pulls electrons with the flow. Producing electricity of course, not work. Some but not all heat is converted to electrical Joules, having a heat/energy/cooling effect.

Hmm, thought... I guess that could also be analogous to heat flowing through an engine pushing work out while some of the heat is converted to work, but not all.

No flow out, no flow in, no work out. Work is just a hitchhiker and takes an equal amount of heat with it, but not all of it. Colder heat must flow out to make room for more heat to come in. Hmmm...

Analogously that is.

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[VincentG]

Hmm, thought... I guess that could also be analogous to heat flowing through an engine pushing work out while some of the heat is converted to work, but not all.

No flow out, no flow in, no work out. Work is just a hitchhiker and takes an equal amount of heat with it, but not all of it. Colder heat must flow out to make room for more heat to come in. Hmmm...

Well said me thinks. I'm not exactly arguing for caloric theory, but the "pressure" of heat to move to less heat is what drives these engines. Higher delta, higher pressure. Whether or not heat flows like a fluid may be irrelevant, but it is this movement of energy that work is derived from. Allowing the heat to move as freely as possible, while forcing gas pressure to swing as much as possible. In contrast is wanting to "convert" all heat to work. The former is to align with nature and the latter, to fight it.

As for the diagram, what I had in mind was more ideal values, say one degree Kelvin to 300k. The aim in going that low is to understand, graphically, what it means to be closer to Carnot efficiency.

[Tom Booth]

So...

Both pressure and volume in a heat engine are a result of. What?

Wishful thinking, perhaps?

Energy. Usually called internal energy. It can be transfered in by any one or more of the following:

Chemical.
Friction, often called work.

I don't think friction is "often called work". Can you cite a reference for that?

Work can produce friction, but friction does not power a heat engine.

It could be, by friction generating heat though.

Work, often called adiabatic work.

Adiabatic work By the engine is a result of heating the gas (or "working fluid). A consequence not a cause.

Adiabatic compression can generate heat. Which I've argued, can be "reclaimed" and used in the following cycle.

Electrical, often called resistance Wattage.

Resistance heat

Nuclear.

Heat

Light, often called photonic.

Heat

Radio waves, sometimes call photonic.
Microwaves, another form of radio waves.

Infrared.

Now your just getting down to what heat actually is. energy. But can you directly expand air in a heat engine with microwaves?

The air inside of my microwave oven doesn't seem to ever get hot.

Maybe we could do an experiment. Put a balloon in a microwave. See if it expands.

Oh yeah, by the way, heat too. If you know the difference, now, between heat, and internal energy, you are on the road to learning some real thermodynamics. Best of luck.

PS, that is off topic. Sorry.

The beauty of a "heat engine" is that it runs on heat from ANY source and is therefore multi-fuel functional by default.

It can run on HEAT from any of the sources you mentioned.

Probably not microwaves though:

https://youtu.be/zXyUaaT5jPQ

I think the balloon probably popped eventually due to the glass which with no food or liquid in the oven gets extremely hot, as I know from experience with the microwave kiln. A microwave kiln will not heat up well with the glass plate in the bottom, which heats up instead.

Personally, it didn't look like the balloon expanded any.

[Fool]

As for the diagram, what I had in mind was more ideal values, say one degree Kelvin to 300k. The aim in going that low is to understand, graphically, what it means to be closer to Carnot efficiency.

Kind of extreme, but here is how that will go. Use a factor of Th/Tc=300/1 :

Starting at point #2, 15 psi, 300 K, and 2 m^3.

Cooling off to point #3, 1 K, 2 m^3, 15x1/300=0.05.

Compressing to point #4, 15 psi, 1 K, 2x1/300=0.00666666 m^3.

Heating to point # 1, 300 K, 0.00666666 m^3, 15x300/1=4500 psi.

Point #1 : 0.00666666 m^3, 4500 psi, 300 K.

Point #2 : 2 m^3, 15 psi, 300 K.

Point #3 : 2 m^3, 0.005 psi, 1 K.

Point #4 : 0.00666666 m^3, 15 psi, 1 K.

Maximum Efficiency will be (300-1)/300=0.99666666 or about 99.7%. of course, run backwards it will have a COP of 1.0033 and a cooling quotient of 0.0033 or there abouts, maximum.

Of course not even helium will be gaseous at 1 K. So again 'ideally'.

You may be able to use helium down to 4.5 K, but I would have a safety factor, make it say 15 K. For a maximum efficiency of 95%. Use a factor of 300/15= 20.


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