The inversion temperature of gas and its role in hot air engines

[VincentG]

I don't really have a firm grasp on the definitions of the 1st and 2nd law. Just looking it up on https://www.britannica.com/science/seco ... modynamics shows the following.

The second law of thermodynamics asserts that heat cannot move from a reservoir of lower temperature to a reservoir of higher temperature in a cyclic process. Rudolf Clausius, a physicist who first formulated the law, stated that “a cyclic transformation whose only final result is to transfer heat from a body at a given temperature to a body at a higher temperature is impossible.” The law describes the amount of work that can result from a transfer of heat.

I have no contention with that at all and in fact trying to prove this wrong seems like banging your head against a brick wall. I think I've said before that my stance is more of an extreme adherence to the natural flow of heat and PV=nRT.

[Fool]

.

There are a lot of consequences involved by the second law. It confuses many people as a result. One of the consequences is the Carnot limit. Another is that a heat engine won't run from only one hot heat source, it can't be 100% thermodynamically efficient. All those difficult consequences are interrelated, but the math and logic proving that, can be a bit much for many people. Few if any highschool math programs could handle it. But at a certain college level it becomes apparent.

One other consequence of the second law is, that it doesn't allow tricking it. Lots of people, cycles, open cycles, regenerative cycles, have tried. It seems much more productive to prove why a new/old scheme fails, than building one to see if the law is wrong.

Case in point, the Brownian Ratchet.

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

I think it won't work because the pawl and machine, will be way heavier than any Brownian motion can move? But I question it, and the official proofs.

.

[Tom Booth]

.

There are a lot of consequences involved by the second law. It confuses many people as a result. One of the consequences is the Carnot limit. Another is that a heat engine won't run from only one hot heat source, it can't be 100% thermodynamically efficient. All those difficult consequences are interrelated, but the math and logic proving that, can be a bit much for many people. Few if any highschool math programs could handle it. But at a certain college level it becomes apparent.
...

What is at all complicated about Tc/Th ?

Tc is the engines cold side temperature.
Th is the engines hot side temperature.

How far can the heat "FALL" according to Carnot? Well, Carnot really had nothing to do with this. So how far can the heat "FALL" according to later morons who still clung to the caloric theory?

Well, all the way from the high temperature down to absolute zero.

So let's say the hot side is 375°Kelvin.

The heat can fall 375°

Makes sense.

But how far can the heat ACTUALLY fall?

Well 375° would be 100% of the way. What percentage of that can the heat NOT fall?

Let's apply the Carnot limit.

Tc/Th

Th is 375°K
Ambient is about 300°K

300/375 would give how far the heat CANNOT fall right?

So the heat can't go down any further than ambient, obviously. 300/375 tells us that once the heat "falls down" to Tc (300°Kelvin) from Th (375°Kelvin) it can't go down any further and therefore:

300/375 = 80%

Still 80% of the total distance left to fall before we get to zero K.

The heat can actually only "fall" 20% of the way down from 375°K to 300°K

That math is not complicated at all and pretty easy to understand and follow, so what's the problem?

The problem is, it's complete BS. Heat isn't a liquid that falls down like a waterfall.

This extremely simplistic math has absolutely nothing to do with heat utilization. It has nothing to do with any engine. Nothing whatsoever to do with any engine efficiency.

That there is something complicated about it is just a smoke screen. There is nothing at all complicated about the math. The math is just based on a complete fallacy that heat "falling down" has something to do with the work that can be derived from a heat engine in the same way the distance water falls down determines how much work can be derived from a mill wheel powered by a waterfall.

It was a good guess, but a WRONG guess nonetheless.

But "difficult"?

Hardly.

That is just a lie to save thermodynamicist from embarrassment. To cover up the stupidity and idiotic simplicity and impossibility of this "Carnot Limit" nonsense.

In reality, engine efficiency has nothing to do with how far heat can fall down the temperature scale. That's just nonsense.

[Fool]

.

What is at all complicated about Tc/Th ?

Nothing, but! The second law isn't Tc/Th. It is, 'Heat won't spontaneously flow from cold to hot, making cold colder and hot hotter.' The consequences of this expand out in many ways, including, but not limited to, heat death of the universe, isolated systems running down, entropy increasing, can't run a heat engine on hot alone, prohibition of the second kind of perpetual motion, and max n=(Th-Tc)/Th which can also be written, max n=1-Th/Tc, max COP=1/n as well as others. The confusion lies in the ability to define it any of those ways, and then derive the other consequences. So if a standard definition is searched, you will find several, the deductions will possibly include all. I've left out statistical definitions and quantum mechanical definitions, because that gets even more complicated, well beyond the scope of this web site.

In reality, engine efficiency has nothing to do with how far heat can fall down the temperature scale. That's just nonsense.

When did your opinion become an authority on "reality"?

Temperature difference, and temperatures, have a big, huge, effect on heat engines. Efficiency is just one of those effects. Power to size. Power to weight. Power to friction. Power to heating area, cooling area. The list goes on. Lower driving force mean a bigger engine is needed for the same output power.

Your wish, that it is not that way, has no effect on reality.

.

[Tom Booth]

I've done actual experiments in the real world. Experimental results are something that takes place in "reality".

Theories, mathematical models, opinions, estimates, do not always necessarily reflect reality.

In my numerous experiments, conducted in various ways, it appears clear that an external ∆T is only required to get a Stirling engine started, after running for some time it may develop its own temperature or energy imbalance due to "work" output; at that point, it appears that heat "rejection" is no longer necessary, AT ALL.

Once the engine is actively running, converting the heat into mechanical motion then heat is going in and "work" or mechanical motion, not "heat rejection" to a "cold reservoir" is the result.

It would be impossible, and a violation of conservation of energy if for every Joule of heat going into a Stirling engine a Joule of work was the result AND ALSO a Joule of "waste heat" rejected to a "cold reservoir". Yet that is what would be necessary for the current "Carnot Limit" theory to be true.

Out of respect for VincentG I will no longer respond to your off topic rants here.

Infact, I'm not likely to respond to your nonsensical posts at all from here on out. It's a waste of time.

My numerous experiments posted to YouTube speak for themselves.

The "Carnot Limit" is obvious crap. Meaningless nonsense, as has been demonstrated again and again, experiment after experiment.

The expected quantity of heat "rejected" as calculated by the so-called "Carnot Limit" equation does not stand up to testing. Not even close. Infact, rather than "rejecting" heat at the cold plate or cold side, it appears that a Stirling engine may actually take in additional heat from both the hot and cold sides similar to a VM cycle (Vuilleumier cycle) converting BOTH heat inputs into "work".

[VincentG]

Tom as long as it's in a thread like this and not one meant for experimental results, and it stays civil, I get something out of the back and forth between everyone. Often new ideas are discussed or old thoughts clarified.

[Tom Booth]

Tom as long as it's in a thread like this and not one meant for experimental results, and it stays civil, I get something out of the back and forth between everyone. Often new ideas are discussed or old thoughts clarified.

Thanks,

Regardless, it's a huge time sink debating the same theoretical issues over and over and over.

What little free time I have would be better spent in the workshop than on this forum debating the same issues over and over with "fool".

When he does some experiments of his own that demonstrate different results than what I'm getting I might be interested.

[VincentG]

The expected quantity of heat "rejected" as calculated by the so-called "Carnot Limit" equation does not stand up to testing. Not even close. Infact, rather than "rejecting" heat at the cold plate or cold side, it appears that a Stirling engine may actually take in additional heat from both the hot and cold sides converting BOTH heat inputs into "work".

If fool would find it worth his time, it would be helpful to show a block diagram that starts with the initial quantity of heat and illustrates its distribution through the engine as it gets split between useful work, atmospheric work, and finally what remains to be rejected upon compression.

If he is interested in this, please keep it ideal with no concern for friction of materials, and as basic as possible, so us uneducated folk can follow along. Using Matt's example, keep the numbers as round as possible.

For instance, an initial 100 joules of heat (from 300k to 600k) does 30 joules of mechanical work (lifting a weight for example), does 20 joules of work on the atmosphere (driving a 1 inch piston through a 3 inch stroke for example), and then must reject 50 joules to the sink to complete the cycle.

As always, showing your methods for each step is appreciated.

[matt brown]

Temperature difference, and temperatures, have a big, huge, effect on heat engines. Efficiency is just one of those effects. Power to size. Power to weight. Power to friction. Power to heating area, cooling area. The list goes on. Lower driving force mean a bigger engine is needed for the same output power.

Fool - I get what you're saying, but your wording is lacking, consider...

(1) deltaT has little effect on efficiency vs sequence of events (path)

(2) deltaT has little effect on power vs engine volume and gas mass (charge pressure)

Meanwhile, the 800 lb gorilla sitting in the corner is deltaQ which presupposes we know Qin and Qout of an engine.

I don't sweat deltaT, but focus on deltaP...where a sequence of events effect the pressure in the gas circuit via the ideal gas law. ECE cycles are much more sensitive to deltaP issues than deltaT issues, but many guys never make it this far...

[Fool]

.

Yes.delta T isn't a parameter a designer has much control over. Size begats heat flow. Sweating the efficiency is the least of design concerns. Cost per Watthour is of greater concern. That is why coal fired steam plants have been so popular in the past.

I can't run my engines with.a Qc reservoir any lower than about 100 C , yes boiling, or 20 C.river.wster, so typically assume one of those. So delta T, begats Th. Realistically.

I'm not a Plutonian Engineer, LOL

Theoretically, keeping the cold temperature constant, Delta T is describing what Th will be.

Either way it's looked at, delta T has serious effects on engine performance. Try to get the same performance out of a delta T of 1 C verses 1000 C. . Very difficult to get it to work at all, let alone two real engines that perform the same with real materials, and findable temperature here on Earth.

Changing Tc to make delta T differences.look negligible is cheating. Tc is probably one of the least adjustable parameter for an engineer.

In fact heat exchanger area and flow, are often way more.selectabe than either of Th or Tc. Heat exchangers are restricted by cost, volume, mass, etc...

.

[Fool]

.

Delta T is why expecting any significant power from an inside the home wood stove isn't very likely. Not because of efficiency, but because of amount of energy available.

Typically my wood stove runs at 300 to 400 F, and often closer to 300 F (roughly about 150 to 250 C). Room temperature is 20 C to 25.C.

Also, the more power that is extracted, the more wood will need to be burned to get the same house heating. Then more ash will need to be dragged out of the house. Think Watt hours per pound of wood.

An outside furnace might help. It could run hotter, won't need to drag wood into nor ash out of the living room. Living area won't fill up with smoke, (a very big deal). Could have a long feed shoot to get a much longer unattended burn time. A rocket stove certainly has my interests, for it's clean burning. Extra heat would go into extra power, maybe pumped out into the grid and get paid. Fuel could be logs, press to logs, pellets, sawdust, grass logs, leaf logs, shredded: prunings: brush: and yard debris. Or any crop.

One down side would be needing to go outside to tend the fire. There are ways around that.

.

[Tom Booth]

I posted this design to the forum way back in 2012:

stirling_stove.jpg (26.34 KiB) Viewed 4658 times

Using the draft to draw the cold outside air in past the engine, increasing the ∆T, while also preheating the combustion air and eliminating cold drafts in the house caused by negative pressure.

Many local building codes these days require a fresh air duct near the stove anyway.

My own experiments with this arrangement (outside fresh air intake ducted directly to the burn box) proved to me it does help maintain "positive pressure" inside the house, eliminating drafts and making the whole house generally more comfortable.

The illustration, however, lacks a specific Stirling engine design and I have yet to get around to building and testing such a Stirling engine/stove combination.

I do have a suitable wood stove that could be easily adapted though.

[Tom Booth]

Also, if the Stirling engine is in direct contact with the flame (secondary combustion) and charcoal inside the firebox temperatures can reach over 1000°F

https://erc.cals.wisc.edu/woodlandinfo/ ... 8405hr.pdf

[matt brown]

Delta T is why expecting any significant power from an inside the home wood stove isn't very likely. Not because of efficiency, but because of amount of energy available.

You missed my favorite combustion issue: HHV (High Heat Value) vs LHV (Low Heat Value). If you're gonna burn stuff for efficiency, you need this on your bucket list. Google is calling, but the nickel tour is Kcals and BTUs are unrelated to HHV and LHV. In Europe fuel sells via LHV and you get what you paid for, but in USA fuel sells via HHV and you get ripped off.

What's the difference??? 8% for gasoline, but 18% for pure hydrogen. Yep, a hydrogen ICE will loose 18% of energy content before Carnot wakes up, unless you capture the "steam" exhaust and can use it

[Fool]

.

The saga of low and high heat values is reflected by other forms of engineering data. A two hundred Watt transistor probably should be run at 50 Watts or less, etc...

.