Stirlingengineforum.com

Well, I still don't think I get it.

Put in another way, heat transfer can be near 100% efficient while passing through a metal heat sink, but this does not take into account the large internal force generated from the metal expanding, which goes wasted, mostly because it is hard to utilize and the heating is not cyclical. Here we are using the heat energy and not the molecular force from temperature.

Gas on the other hand experiences the same molecular result from kinetic energy, but we instead use only the force(pressure) and not the heat energy.

Jack wrote: ↑Tue Nov 12, 2024 10:59 pm

VincentG wrote: ↑Tue Nov 12, 2024 11:16 am
I believe the work will come from the kinetic energy of the gas at elevated temperature. Normally the kinetic energy is wasted, as pressure only increases linear to temperature, while molecular energy does not, as evident by the emission of visible light above a certain temperature, and phase changes.

I haven't read the whole thread here, just loosely following. But here you touch on something that I've been thinking about and basing my own project around. This might go a little off topic.

My reasoning was that if heat going into a fluid changes the kinetic energy of the molecules, then why aren't we focusing on that?
Pressure is a second order effect. Kinetic energy has to be equalized into pressure.
So in my mind any piston engine is going lack efficiency just because of it using pressure to push the piston. A lot of the kinetic energy in the molecules is wasted by bouncing into the walls and ceiling of the cylinder and wherever else. Heat loss..
If we were to point all the molecules into one direction and let them bounce off a turbine, that would already improve a lot.
And if we use a turbine that doesn't rely on the molecules bouncing off it (because that's one bounce and only half its energy transferred after that the molecules is off playing somewhere else) but in stead relies on the molecules wanting to adhere to it we'd be even further along.

Just my .02

I was thinking along the same lines up until fairly recently.

What is "pressure" exactly?

Kinetic "THEORY" says pressure is caused by the gas molecules bouncing off the walls of the container, but I'm not entirely convinced that theory is literally true or actually makes sense.

How is kinetic energy "wasted by bouncing into the walls and ceiling of the cylinder"?

I can, for example, pump up the tires on my truck. I bought a set of new tires and had them put on about 2 years ago and they have been holding 35 psi ever since.

Same with my shop compressor. Once the tank is fully pressurized it can sit there indefinitely and the pressure is never "wasted" as a result of the gas molecules loosing energy by bouncing off the inside walls of the air tank.

It has puzzled me for years now that if you fill an air tank all the work put into compressing the air is immediately lost as heat, apparently 100% but you still end up with a tank full of compressed air ready to do work. That air does not need to be heated back up or have that lost energy returned to it, so where is the energy to re-expand that gas and do work powering pneumatic shop tools, like impact wrenches actually coming from?

Well, as the gas re-expands it gets cold. Very cold:

https://youtu.be/2hYQtB4QkEY

To my mind, this is the same as evaporative cooling. When molecules of a substance "expand" from a liquid to a gas there is a drop in temperature, we say because of the "phase change" but that doesn't really explain what's actually going on.

You get a cooling effect similar to evaporation when compressed air leaves an air tank but without involving phase change from liquid to gas.

You get a similar effect when compressing a spring

Compress a spring and it temporarily heats up, then equalizes in temperature with the surroundings. All the heat generated from the work involved in compressing the spring is dissipated and lost. The spring itself, however, is now ready to do work. The spring does not "loose energy" while being held under tension. Likewise, compressed air, like a compressed spring does not loose energy while being compressed. Yes, it gives off or loses heat in the process of being compressed, but does not loose energy to the walls of the container holding it under compression.

A spring that was held under compression will likewise cool down when released and allowed to expand.

What all that boils down to is that the individual molecules of a gas that is compressed is not "bouncing off the walls of the container" any more than a compressed spring is "bouncing off the walls". The spring tension is a property of the spring itself.; internal molecular forces that are causing the material the spring is composed of to want to assume a certain shape.

Likewise a volume of compressed air wants to expand due to intermolecular forces within the gas molecules themselves wanting to maintain a certain volume or distance from one another.

Like two magnets when the like poles are pushed close together, they repel each other, but not because of "kinetic energy", not because they are "bouncing off each other". The repulsive forces is sustained and continuous and is a result of the internal molecular makeup of the magnets themselves not any "collision" or kinetic motion or impact between them.

Sound travels through air at about 767 miles per hour.

Sound does not travel in a vacuum but by air molecules knocking into each other like a Newton's Cradle.

There are about 4.4 sextillion air molecules per cubic inch.

So imagine a Newton's Cradle with something like 160 septillion separate steel balls all lined up.

How closely spaced would the 160 septillion gas molecules have to be to effectively transmit sound 1,125 feet per second for several miles?

A Newton's Cradle doesn't work without the steel balls touching without any.breaks. It would seem to me the same would be true of air molecules transmitting sound. They need to be very densely packed.

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It seems to me that gasses behave more like pool balls than Newton's Cradle.

You will need to incorporate how gasses are 1000 times, or so less dense than liquids or solids. And how the volume of a gas depends on the container size, not temperature, as for solids and liquids.

You will also need to incorporate how solids and liquids have a lot higher sound speed.

https://en.m.wikipedia.org/wiki/Speed_of_sound

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Fool wrote: ↑Wed Nov 13, 2024 11:16 pm .

It seems to me that gasses behave more like pool balls than Newton's Cradle.

You will need to incorporate how gasses are 1000 times, or so less dense than liquids or solids. And how the volume of a gas depends on the container size, not temperature, as for solids and liquids.

You will also need to incorporate how solids and liquids have a lot higher sound speed.

https://en.m.wikipedia.org/wiki/Speed_of_sound

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So molecules in a solid are MORE densely packed than gas at 1 atmosphere, so what?

Personally I don't think air molecules would transmit sound a centimeter if all the molecules were zipping around in a thousand different random directions and different speeds without interacting (kinetic theory of gases). It would be impossible to transmit a coherent sound wave any distance at all.

Sound waves in the air can be distorted and deflected by wind. So how could they propagate through a total chaos of particles moving in random directions?

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Your personal thoughts have no scientific bases.

Gasses are a 1000 times or so, less dense. If the intermolecular forces are only valid for a stretch of a couple of radiuses, how are they valid at densities 1000 time less? Can you get an increase in distance less than 10 for such an expansion? How about twice that 2000? Forget those two questions, what's the mean free path for nitrogen at STP?

What is the distance between N2 molecules at STP? 3.34 × 10 − 9 m.

Diameter of N2 is 0.363 × 10 − 9 m.

That's about 18 radiuses out. Well beyond 1% probably well beyond 0.01% of the force strength. Negligible compered to the momentum. Well above the speed of boiling. That is why it is a gas and always pushes.

Sound propagates just fine through a cloud of bouncing molecules that always push.

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You think 1000 times the distance of one billionth the width of a human hair is a lot?

It's all relative.

Gravity, a probable aggregate of molecular attractions, keeps Pluto orbiting the Sun 3.7 billion miles away.

Magnets attract and repel across a table, likewise static electricity attracts bits of paper, All large scale, visible forces.

The distance between air molecules is a million times smaller than "too small to see".

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VincentG, Temperature and pressure are, both combined, a manifestation of kinetic energy. You can only have purely temperature energy in a solid or a liquid. For gasses, constant volume, take one away, the other drops. T lowers, P lowers. Temperature and pressure are intertwined.

Energy can be pulled out by temperature difference or expansion.

Temperature is a direct measure of internal thermal energy, U, it is not a measurement of heat. Pressure is a direct measurement of potential energy, kind of sort of. Like g in Mgh. M and h are like volume, sort of. Thermal energy is like v in 1/2Mv^2.

So thermal energy is like energy stored in the velocity and mass of a flywheel. And pressure energy is like energy stored in a compressed spring.

A spring will stay compressed forever, a rock will stay on a hill forever, and pressure will stay forever.

A flywheel will slow down, bleeding energy off as heat, and other avenues. Thermal energy will slowly bleed off until reaching temperature equilibrium with its surroundings.

All the bouncing against the piston is because of all the bouncing against the other walls (floor and ceiling). Once the piston moves outward there will be less bouncing on the other walls and piston. The gas molecules will continue to bounce against all walls, while pushing the piston out right up until they all condense and pressure is zero, at zero K.

The other walls, return the gas molecules at the same speed, without absorbing any energy, if their temperatures are the same. If the gas is colder, the walls will speed the gas up, on average. Very difficult to have adiabatic walls during an expansion, because the gas temperature is changing.

Remember, the wall molecules are moving too. They are bonded into solid position, but vibrating relative to their temperatures.

In other words, the walls don't waste energy, they preserve it. If hotter they are additive during an expansion. During a compression, additive if they are colder. Yes rejecting heat is adding work output, kind of, sort of. It's more like saving previous work/energy.

If the molecules were to all suddenly travel in unison toward the piston, imparting all their momentum to the piston, suddenly stop moving in a big lump and drop to the floor, it would be the proverbial expand to zero Kelvin scheme again. A Harry Potter, or, Star Trek "Q", magic trick, don't you think. But it wouldn't violate any laws of physics, just be very unlikely.

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Tom Booth wrote: ↑Thu Nov 14, 2024 12:46 am You think 1000 times the distance of one billionth the width of a human hair is a lot?

It's all relative.

Gravity, a probable aggregate of molecular attractions, keeps Pluto orbiting the Sun 3.7 billion miles away.

Magnets attract and repel across a table, likewise static electricity attracts bits of paper, All large scale, visible forces.

The distance between air molecules is a million times smaller than "too small to see".

Because Tom can't see them, he can't understand them. You empiricists are all alike. Of course, that is why science is mathematics, tied in occasionally with observation.

The molecular forces drop off in strength at 1/r^12, and, 1/r^6. Within a few radiuses they are negligibly small. Do the math. Oh yeah, that's beyond your ability. I guess you'll need to take my word for it, or ask some professor somewhere. Don't ask just one.

In other words, they behave differently from gravity and magnetism.

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Tom Booth wrote: ↑Wed Nov 13, 2024 6:56 am

Jack wrote: ↑Tue Nov 12, 2024 10:59 pm

VincentG wrote: ↑Tue Nov 12, 2024 11:16 am
I believe the work will come from the kinetic energy of the gas at elevated temperature. Normally the kinetic energy is wasted, as pressure only increases linear to temperature, while molecular energy does not, as evident by the emission of visible light above a certain temperature, and phase changes.

I haven't read the whole thread here, just loosely following. But here you touch on something that I've been thinking about and basing my own project around. This might go a little off topic.

My reasoning was that if heat going into a fluid changes the kinetic energy of the molecules, then why aren't we focusing on that?
Pressure is a second order effect. Kinetic energy has to be equalized into pressure.
So in my mind any piston engine is going lack efficiency just because of it using pressure to push the piston. A lot of the kinetic energy in the molecules is wasted by bouncing into the walls and ceiling of the cylinder and wherever else. Heat loss..
If we were to point all the molecules into one direction and let them bounce off a turbine, that would already improve a lot.
And if we use a turbine that doesn't rely on the molecules bouncing off it (because that's one bounce and only half its energy transferred after that the molecules is off playing somewhere else) but in stead relies on the molecules wanting to adhere to it we'd be even further along.

Just my .02

I was thinking along the same lines up until fairly recently.

What is "pressure" exactly?

Kinetic "THEORY" says pressure is caused by the gas molecules bouncing off the walls of the container, but I'm not entirely convinced that theory is literally true or actually makes sense.

How is kinetic energy "wasted by bouncing into the walls and ceiling of the cylinder"?

I can, for example, pump up the tires on my truck. I bought a set of new tires and had them put on about 2 years ago and they have been holding 35 psi ever since.

Same with my shop compressor. Once the tank is fully pressurized it can sit there indefinitely and the pressure is never "wasted" as a result of the gas molecules loosing energy by bouncing off the inside walls of the air tank.

I was talking about pressure by heating, not by adding molecules.

Jack wrote: ↑Thu Nov 14, 2024 4:12 am
I was talking about pressure by heating, not by adding molecules.

So am I

A Stirling engine is a sealed chamber. You don't add or take out any molecules.

What I'm thinking, just by analogy, imagine a room full wall to wall and floor to ceiling with beach balls. The space between them is a vacuum.

If you heat up one wall of the room, the beach balls in direct contact with that one wall will get hot and expand in size.

Those expanding beach balls will put pressure on all the other beach balls and all the walls equally throughout the entire room. The heated beach balls on one side would not be flying off the hot wall all the way across the room to "bounce" off the other wall. All the other beach balls are preventing that, packed together

To me, from what I've seen experimentally, and facts, such as the mean-free-path of air molecules being infinitesimally small, like a millionth the width of a human hair, the Lennard-Jones potential, how sound travels trough air in waves etc. all together,... the kinetic theory of gases that gas molecules cause pressure by bouncing off the walls like ricocheting bullets, just doesn't make any sense and IMO could not be a true picture of reality.

So really, I think my earlier criticism of my NASA/INFINIA engine design is probably baseless. I thought the hot air molecules could not transfer pressure to the piston effectively because the molecules would have to pass through the cold regenerator and cold water cooling system through all the other cold gas molecules.

If the hot gas molecules are only transferring pressure indirectly, not having to actually travel between the heater head and the piston then the intervening cold gas, cold regenerator and cold walls are no impediment to the transfer of pressure.

Probably the NASA guys actually knew what they were doing.

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Tom, you are beginning to see some of the facts. NASA engineers, obviously know what they are doing. Duh! Yes, the molecule's kinetic energy produces pressure. Yes they don't bounce off from one wall and then the other wall. Yes NASA used classical theory.

It is a chain reaction of bouncing off the wall, then traveling the mean free path, then bouncing off another set of molecules. They then travel another mean free path and bounce off from another set of molecules and so on. Over and over again, slowly mixing, and resisting mixing. Eventually, way down the chain a set of molecules bounces off to the piston.

Like a table full of pool balls, not touching each other. Hitting one transfers through a series of balls until another ball hits the other side of the table.

Molecules don't get larger from increased energy/Temperature, in solid, liquid or, gas. The Lennard-Jones potential describes that. It also shows they never touch, and lose any significant attraction when above boiling temperature energy levels. It's right in front of your face. Even NASA appears to know this.

Beach balls touching and enlarging from increased temperature is totally misleading, wrong and misinformed. The molecules never touch each other in any solid, liquid, or gas. Their electron clouds prevent that. In a gas they bounce off from each other when their electron clouds get close.

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Well I guess the electron cloud could be seen as a beach ball.

I still don't see what you, Tom, think heat itself is then, in your theory.
How does the beach ball get larger? What does heat do to it to make it larger?

This topic has drifted wildly, but I want to make sure the conclusion is not that energy transfer is equal to the power piston be it hot or cold. Matt has proved and it should be obvious by now that there is more power potential in a hot connected piston.

Yes, pressure transfers through cold just fine. However when the pressure is the result of heat energy in the first place, the cold space is nothing but a black hole for the energy that was introduced to the system.

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Jack wrote:Well I guess the electron cloud could be seen as a beach ball.

Nice try, but no. Electron clouds 'touching' never happens. They get close but bounce before touching. The electron clouds increase in size very little with heat, compared with the mean free path observed for a gas, that increases a lot during, expansion with cooling. Think about just the forces of the electron clouds being the shell of a ping pong or pool ball. They don't come into play until bouncing. The closer they get the stronger, but the electron clouds don't quite touch. Mean free path is why a gas is many times less dense than a solid or liquid. Temperature increases speed. Increased volume increases mean free path, lower density.

Solids and liquids have bonds between the electron clouds. Solid bonds are stronger than liquid bonds. Those bonds are described using the Lenard-Jones Potential Theory. Gasses by the kinetic theory, and fluid dynamics continuity equations.

I agree with you that there is no cohesive logic to the beach ball analogy. How does it account for phase changes, solid, liquid, gas? It doesn't.

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