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The following was a response by forum members Stroller and Tom Booth regarding my question about the true nature of the conversion of heat energy inside a hot air engine.

As far as I have seen, there is no experimental evidence that suggest heat energy is converted to work, and only evidence that heat energy is dissipated to colder bodies as the gas is expanding.

So I would like this discussion to stay focused on proving that heat energy is directly converted to gravitational potential energy as the gas expands against the piston and lifts a weight. A crankshaft need not be considered for this example, just a piston lifting a weight vertically.

My proposed hypothetical ideal cycle would therefore state that 100 percent efficiency would be reached if the heat energy could be recycled from the expanded gas with no reduction to Tmax, in order to be reapplied to the heat source and used again to full effect(an obvious impossibility).

Stroller wrote: ↑Tue May 21, 2024 5:24 am

It's the action of the expanding gas "consuming" energy that I am interested in. Is internal energy really reduced when the gas expands, other than by conduction losses to the surrounding colder surfaces of the engine. And if so, how exactly?

The internal energy of the entire expanding volume isn't reduced, but the internal energy per unit volume is, because the total internal energy of the gas is spread out more, into a bigger space.

By the same token, the temperature of individual molecules of air doesn't change, but the bulk temperature of the gas does reduce, because the thermometer, or finger end, isn't getting hit so often because the molecules are more spread out or rarified in the expanded volume; the pressure falls.

This is why Charles Law holds experimentally (ignoring or minimising conduction losses with insulation)

T1/V1 = T2/V2

As the volume expands, the (bulk) temperature drops, along with the pressure, as we can see from the combined gas law

(P1V1)/T1 = (P2V2)/T2

Of course, in a real working Stirling engine, the heater is continuing to put energy into the gas while it's expanding, and this is a more complex case, because the rate of flow of energy across the wall of the cylinder will change if the temperature of the working gas inside is changing.

Tom Booth wrote: ↑Tue May 21, 2024 6:05 am

Stroller wrote: ↑Tue May 21, 2024 5:24 am

It's the action of the expanding gas "consuming" energy that I am interested in. Is internal energy really reduced when the gas expands, other than by conduction losses to the surrounding colder surfaces of the engine. And if so, how exactly?

The internal energy of the entire expanding volume isn't reduced, but the internal energy per unit volume is, because the total internal energy of the gas is spread out more, into a bigger space.
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This statement (bold) is, or would be, a violation of conservation of energy.

Your statement is basically that energy can GO OUT from the working fluid as WORK but all the energy is still in the working fluid but just spread out more.

That would double the total amount of energy.

Say 1000 joules gone out as "work" but the same 1000 joules still in the working fluid but just spread out in a greater volume.

So the original 1000 joules has split and turned into 2000, 1000 going out as "work" and 1000 remaining but just spread out in a larger volume.

Tom Booth wrote: ↑Tue May 21, 2024 6:16 am

Stroller wrote: ↑Tue May 21, 2024 5:24 am

It's the action of the expanding gas "consuming" energy that I am interested in. Is internal energy really reduced when the gas expands, other than by conduction losses to the surrounding colder surfaces of the engine. And if so, how exactly?

The internal energy of the entire expanding volume isn't reduced, but the internal energy per unit volume is, because the total internal energy of the gas is spread out more, into a bigger space.

By the same token, the temperature of individual molecules of air doesn't change, but the bulk temperature of the gas does reduce, because the thermometer, or finger end, isn't getting hit so often because the molecules are more spread out or rarified in the expanded volume; the pressure falls.
...
...

Same problem with this later statement regarding the temperature of individual molecules.

Unless, the expansion is "free expansion into a vacuum".

But we are discussing engines. Gas expanding and doing external work.

An "individual gas molecules" strikes the piston, the piston moves. Kinetic energy was transfered. The individual gas molecule looses energy, slows down, is now "colder", has, as an individual molecules LESS energy, not as you suggest above, that the temperature (energy) of the individual molecules stays the same.

That violates conservation of energy.

What you are claiming is the individual molecule transfered energy to the piston but still retains the same amount of energy, doubling the energy.

That would clearly be a violation of conservation of energy.

Unless of course you're talking about isothermal expansion, but that only means heat is added to compensate.

When a particle transfers heat to the piston and loose energy it then perhaps contacts the hot plate, gaining the energy back, but that is not energy simply "spreading out" into a larger volume.

In an adiabatic expansion, (doing work on a piston) not only is the energy "spread out" reducing the temperature, but the individual molecules cool down, move slower, loose energy as well.

https://youtu.be/AVCX5BfPn6A

I don't believe that the wire can gain and lose heat as quickly as it does

This certainly confirms that work is the result of heat, but does nothing to confirm heat is converted to work. For this instance you would probably need a thermal camera with a high frame rate, and record the wire motor under no load along side another under full load. Then the heat dissipation rate of the wire could be observed.

VincentG wrote: ↑Sun Jun 16, 2024 3:37 am This certainly confirms that work is the result of heat, but does nothing to confirm heat is converted to work. For this instance you would probably need a thermal camera with a high frame rate, and record the wire motor under no load along side another under full load. Then the heat dissipation rate of the wire could be observed.

True, the example is more suggestive, the wire IS thin and COULD theoretically cool off by dissipating heat. What I think is interesting is that unlike many Nitinol engines this demonstrates that the engine can run on a hot water bath alone. No ice bath as is quite often used. So MAYBE the wire is cooling "instantly" due to "WORK". (conversion of heat to work) rather than heat dissipation, or heat dissipation alone.

But you are right, not 100% conclusive, but potentially, with a little more data collection, could be.

I don't know if you understand the point of the prior drawing.

The nitinol wire spring is held in place by a pin,

The wire spring is "trained" to extend when heated.

There is a weight placed on top.

The wire is heated (Q in) but held by the pin.

The pin is then pulled suddenly.

The Nitinol spring propels the weight into the air doing "work" and instantly cools and contracts back to its former shape.

It is, or has been my argument that this happens with a Stirling engine "air spring" maybe 25 times per second (at 1500 RPM) "throwing" the "weight" piston/crank/flywheel off (down the cylinder) and instantly "contracting". due to work output.

Much much too fast for heat to be dissipated.

Is that "proof"?

To me personally I think it is just obvious. Almost just a matter of common sense. The expansion of the gas must be "adiabatic" (without energy transfer as heat) because heat transfer is quite slow.

Different people require different levels of "proof" before they are convinced of something.

In a Stirling engine the "pin" holding the "spring" is often the crank assembly and connecting rod.

Though heated and wanting to expand the working fluid cannot immediately because it is restrained at TDC.

Then as the crank moves past TDC it is like pulling the pin and the spring pops, does work suddenly (adiabatically) and returns (contracts).

How can it be otherwise if the heat was 100% converted to work by the expansion process? It lost all the energy given as heat to work so returns to its original "shape".

The working fluid "contracts" and the piston returns even if the power cylinder is non-heat-conducting and enveloped in insulation so heat dissipation is actually impossible.

Well, the ultimate skeptic will claim insulation doesn't work because it's really a conductor and Stirling engines, though actually very efficient are very inefficient and though heated to 3000°F by a propane flame don't actually take in any heat...

And besides, there is no such thing as "vacuum" and a gas cannot "contract".

No convincing some people in spite of heaps of evidence.

This is enough evidence to satisfy me:

https://youtu.be/LG09AXAjpio

I'm doing the experiments myself, so I can rule out fraud. Some will suppose I have a battery and little motor in there or something I suppose, or maybe I put an ice cube under the Styrofoam.

They think I'm "rejecting science". LOL.

All I can say in that circumstance is do your own experiments. Then you can make sure not to hide an ice cube under the Styrofoam.

The Nitinol spring propels the weight into the air doing "work" and instantly cools and contracts back to its former shape.

It is, or has been my argument that this happens with a Stirling engine "air spring" maybe 25 times per second (at 1500 RPM) "throwing" the "weight" piston/crank/flywheel off (down the cylinder) and instantly "contracting". due to work output.

Much much too fast for heat to be dissipated.

Is that "proof"?

No.

I think it would be helpful to heat a wire with a torch to the point of glowing and notice how fast the heat is dissipated in free air. The heat dissipation is instant, it's only rate of dissipation that seems slow, but the temperature gradient it always maintained. Otherwise the whole wire would glow.

VincentG wrote: ↑Sun Jun 16, 2024 8:18 am

The Nitinol spring propels the weight into the air doing "work" and instantly cools and contracts back to its former shape.

It is, or has been my argument that this happens with a Stirling engine "air spring" maybe 25 times per second (at 1500 RPM) "throwing" the "weight" piston/crank/flywheel off (down the cylinder) and instantly "contracting". due to work output.

Much much too fast for heat to be dissipated.

Is that "proof"?

No.
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Not proof enough for you maybe.

IMO this whole subject was settled by experiment decades ago

https://youtu.be/-X9McP2WnDg

One among dozens and dozens...

Granted, historically, much more experimental evidence that work = heat than heat = work, but it usually goes without saying that if A = B then B = A.

How much evidence constitutes "proof" is a mater of personal conviction or belief.

Devising experiments where heat is converted to work is much more challenging than the other way around, so maybe there is something of a gap to fill

I'm all in favor of more experiments, but personally I've seen enough.

Tom Booth wrote:Much much too fast for heat to be dissipated.

Why? Isn't there just as much time for heat to get in, which you seem to accept, as there is for heat to get out. Both have a half cycle, a full stroke?

It seems contradictory to expect one while denying the possibility of the other.

VincentG, to obtain experimental evidence you need to used meticulous laboratory techniques, and documentation. You want to "confirm" something you need to come up with a theory for what it is that you want confirm. Converting heat to work needs to start with definitions. There are standard definitions for thermodynamics. It would be wise to start with those. The following link may help:

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

Wikipedia wrote:The term "thermal energy" is used loosely in various contexts in physics and engineering, generally related to the kinetic energy of vibrating and colliding atoms in a substance. It can refer to several different physical concepts. These include the internal energy or enthalpy of a body of matter and radiation; heat, defined as a type of energy transfer (as is thermodynamic work);

Wikipedia wrote:Relation to heat and internal energy:

In thermodynamics, heat is energy transferred to or from a thermodynamic system by mechanisms other than thermodynamic work or transfer of matter, such as conduction, radiation, and friction.[1][2][3] Heat refers to a quantity transferred between systems, not to a property of any one system, or "contained" within it.[4] On the other hand, internal energy and enthalpy are properties of a single system. Heat and work depend on the way in which an energy transfer occurred, whereas internal energy is a property of the state of a system and can thus be understood without knowing how the energy got there.

IMHO: The colloquialism "heat converted to work" is only ascribable to an engine running a full cycle, where internal energy change, adds up to zero. That leaves the only change to the system, as a whole: heat in: heat out: work in: work out.

Once you have defined what heat is, you need a hypothesis of how heat Joules are converted to force-and-distance/work Joules.

Does the energy transfered in, start as some form other than heat, get converted to heat, transfer in as heat, get converted to something inside (internal energy, temperature), then get converted to work? Or some other testable process?

To test for quantities of heat and work, some way of measurement for each must be used.

For a heat engine running steady state on a constant load, heat per time can be garnered by measuring inside and outside the hot plate temperatures. That allows standard heat transfer rules to calculate the Watts of heat going through the wall.

Power out can be calculated from torque and RPM.

A single stroke, starting at ambient and adding 100 J, will end with V and T both being larger, and the mass being higher. The energy was converted to internal energy that has lifted and now holding the mass 100 J higher. Think of this as compressing a spring putting a mass on it and letting it get lifted up 100 J higher. The energy of compressing the spring is converted to height when released.

Putting heat in, compresses the spring, letting it expand releases that internal energy as work.

If you remove that energy, as heat, to a cold sink, the mass returns to the starting point. The same 100 J must be removed. Net overall work available for output will be zero.

It is through a different forward path than the return path, that thermal energy is converted to work, a cycle, because of saving work during compression at a lower temperature.

All that can be measured with proper laboratory practices.

Fool wrote: ↑Sun Jun 16, 2024 12:43 pm

Tom Booth wrote:Much much too fast for heat to be dissipated.

Why? Isn't there just as much time for heat to get in, which you seem to accept, as there is for heat to get out. Both have a half cycle, a full stroke?

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Not in my view

It can take several minutes for the engine to heat up enough to reach "operating temperature", which could be 500°F or more.

It (the working fluid) has to cool down from there enough for the pressure to drop enough for atmospheric pressure to push the piston back. It can't dissipate heat back out through the walls of a hot engine, the same as it went in. (I'm thinking of a thermoacoustic type engine where the working fluid is retained almost entirely in the hot chamber, no displacer or cold side except when doing work, very briefly expanding into the power cylinder which in my experiment is glass insulated glass = non-heat conducting).

Cooling all the way back down to ambient by dissipation of heat would take just as long as it took to get up to operating temperature. The engine is already red hot. The working fluid even has to cool the whole time heat is still being added.

It's illogical IMO to think heat can be conducted away while the engine is being continually heated with a blow torch. It's not equal time at all. It took several minutes to heat the engine up hot enough to where it can transfer heat IN so quickly.

So you think that the time it takes to heat up the hot plate is the same amount of time it takes for the gas to reject heat to the cold plate?

Sounds to me that the same "heating" time is required to cool off the cold plate if it is run on ice, and comes down to operating temperature.

Meanwhile modern Stirling theory seems to recognize that the engine after coming up to temperature, has two inside temperatures that the gas comes in contact with when pushed by the displacer to each of the hot or cold spaces. The gas in contact with either of those two spaces has just as much time to pick up heat in the hot space, as to release heat in the cold space.

Tom I am in no way doubting that work creates heat energy. And you have to focus on the heat energy in the gas and not the engine body.

Fool thanks for that response. The only thing I don't agree with is the following. The weight can stay elevated, and a new mass can be lifted on the next cycle, for positive net work.

"If you remove that energy, as heat, to a cold sink, the mass returns to the starting point. The same 100 J must be removed. Net overall work available for output will be zero."

VincentG wrote: ↑Sat Jun 15, 2024 11:08 am The following was a response by forum members Stroller and Tom Booth regarding my question about the true nature of the conversion of heat energy inside a hot air engine.

As far as I have seen, there is no experimental evidence that suggest heat energy is converted to work, and only evidence that heat energy is dissipated to colder bodies as the gas is expanding.

When 10 'heat' source units are exposed to a gas via a temperature differential whereby 2 heat units are input to the gas then 8 heat source units remain until 2 heat units are recovered. Note that this is purely quantitative and not qualitative (input can be Thigh vs output Tlow but each unit of 'heat' equal).

VincentG wrote: ↑Sat Jun 15, 2024 11:08 am So I would like this discussion to stay focused on proving that heat energy is directly converted to gravitational potential energy as the gas expands against the piston and lifts a weight. A crankshaft need not be considered for this example, just a piston lifting a weight vertically.

Here's some evidence of a guy generating power from lifting 'rocks' via heat...




Another interesting scheme is the Humphrey cycle

VincentG wrote: ↑Sat Jun 15, 2024 11:08 am My proposed hypothetical ideal cycle would therefore state that 100 percent efficiency would be reached if the heat energy could be recycled from the expanded gas with no reduction to Tmax, in order to be reapplied to the heat source and used again to full effect(an obvious impossibility).

Consider a fancy 10 lb rock (piston) inside a vertical cylinder suspended by a total gas force of 10 lb at 300k. This bugger will stay suspended (forever) until the temperature increases the pressure of the gas 'slightly', whereupon the rock rises 'slightly'. Simply assuming that if only we could lift the rock via ambient heat is a no-go since expanding the gas at 300k (aka isothermally) will decrease gas force <10 lb. In effect, the ambient work potential of the gas is stalled. This is the downside of PV work where the sequence of events defines a path that is crucial.

In your original proposal, Stroller immediately jumped to isobaric expansion since this is the only way to lift the rock, but this also allows something you added about recycling the heat. "...if the heat energy could be recycled from the expanded gas with no reduction to Tmax" was possible at 300k then who cares about recycling it, but same scheme would improve output when 600k or whatever where recycling input would be paramount.

Stroller's isobaric pitch is worth consideration since the increase in temperature during expansion allows recycling to the next expansion. The pitch would go something like this...isobaric expansion 300k to 600k with 1:2 volume expansion then recycle 600k 'exhaust' as input to next cycle. Note this PV





I picked a PV that lacks T distractions solely to demonstrate isobaric output vs the other guys...

VincentG wrote: ↑Sun Jun 16, 2024 2:07 pm Tom I am in no way doubting that work creates heat energy. And you have to focus on the heat energy in the gas and not the engine body.
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If you propose that the "heat energy" (internal energy of the working fluid once "heat" is transfered into it) needs to be conducted out, how do you propose its going to get out other than the way it came; through the engine body.

The engine body is HOT radiating heat into the engine.

Somehow the gas in the engine has to instantly cool back down so the piston can return and do this maybe 20 times or more per second. Send heat out through scorching hot walls that are radiating heat IN. Good luck with that.

You can't just ignore that the engine body is hot while the working fluid SOMEHOW manages to get cold inside this roasting hot environment inside a scorching hot engine body.

Common sense dictates that the heat/energy is leaving in some manner other than the way it went in.

That would be by conversion to work, which is instantaneous. Supposed "isothermal heat rejection" takes literally FOREVER. "Quasistatic".

Hot air inside a hot chamber not a wire in the open wind.

Vincent - imagine a non compression 300-600k cycle where a low tech turbine does work during 1:2 isobaric expansion. Note that Ericsson used isobaric processes for regen vs Brayton used isobaric processes for source and sink. The scheme here is no backwork due to no compression from engine. Instead, 600k 'exhaust' is funneled backwards over engine while ambient pressure supplies isobaric compression. Over simplified for sure, but I think you get the idea.

hey Vincent, have you seen this

https://en.wikipedia.org/wiki/Isobaric_process