Stirling Engine Thermodynamics

[VincentG]

Hmmm, I think we are on the same page here...let me clarify what I mean. I think the efficiency of these engines comes from the great pressure swings of the working gas with varied temperatures and relatively small heat input. So I think the point of the cold side heat sinks being key to efficiency is to quickly lower the temperature of the working gas to reduce the pressure. The working gas itself has relatively little thermal capacity, so I don't think much heat really needs to be expelled at the end of the day.

If I understand your position from various threads, you argue that the cycle of the engine itself is keeping the cold side cold. I would argue that may actually be happening, but for the wrong reasons. It may be that poor displacer timing is causing the engine to use its own power to operate as a heat pump, like in a motor driven cryogenic cooling application, but for only a partial cycle of the engine. So I theorize much more power potential from these LTD engines is possible with only changes to displacer timing. And more still to be gained from thermal optimization, including ceramic based materials as you suggest.

I'm just going on intuition here, so there's a good chance I'm wrong. But, hey, that's what this forum is for after all. I am happy to have someone as enthused as you to bounce ideas off of. -Vincent

[matt brown]

Hmmm, I think we are on the same page here...let me clarify what I mean. I think the efficiency of these engines comes from the great pressure swings of the working gas with varied temperatures and relatively small heat input.

Vincent - what you guys are struggling with is the vast chasm between the Stirling cycle and a range of Stirling engines. The stirling cycle should be quite simple to grasp, but making an engine that approaches it will remain a challenge.

Stirling Cycle :

(1) the internal energy of an IDEAL gas is directly linear absolute temperature
(2) IDEAL isothermal processes have constant temperature AND constant internal energy REGARDLESS of volume changes
(3) during isothermal expansion, volume increases while pressure decreases, but the temperature remains constant AND THE INTERNAL ENERGY REMAINS CONSTANT
(4) and, during isothermal compression, volume decreases while pressure increases, but the temperature remains constant AND THE INTERNAL ENERGY REMAINS CONSTANT
(5) thanks to the kinetic theory, work=heat
(6) therefore, ALL source heat=expansion work
(7) and, ALL sink heat=compression work
(8) at 300k (think room temperature) the source heat req'd for 1:2 expansion WILL EQUAL the sink heat req'd for 2:1 compression and Wpos during expansion=Wneg during compression. Yep, since heat=work, expansion values correspond with compression values BUT ONLY FOR THE SAME VOLUME CHANGE AT THE SAME TEMPERATURE
(9) a 600k-300k Stirling cycle has expansion at 600k and compression at 300k, but the internal energy of the gas is TWICE AS HIGH AT 600k THAN AT 300k. This means that you will need twice the source heat for a 1:2 expansion at 600k as you need for sink heat of a 2:1 compression at 300k. Get it...2 units of heat sourcing 2 units of Wpos at 600k, with 1 unit of Wneg sinking 1 unit of heat at 300k. Yep, the heat=work thing varies directly linear absolute temperature (k)...due to internal energy.

Thus, an ideal 600k-300k Stirling cycle has .50 efficiency per Carnot, but only due to the compression process. The Stirling cycle has regeneration to 'get between' the high & low temperatures (isotherms) but regen is secondary to the main Carnot buzz.

As I said in another recent post, Carnot barely escaped obscurity, but his work was kept alive until Otto revived interest in hot air engines.

[matt brown]

Stirling engine/s:

Most DIY Stirlings that attempt useful work hover around the 600k scheme (~600F) with 2:1 compression. Sure, the isothermal input is a major hurdle, but the biggest hurdle remains 'piston' phasing. An old German guy (Hubert Stierhof) used to always describe this issue as "misbehavior" in that the mechanics (so far) precluded any engine from following the cycle. One of the most credible DIY Stirling engines ever made remains the Jim Dandy #6. Check it out, and note the details, especially 2.5 HP at 220 rpm

http://www.starspin.com/stirlings/jimd6.html

Many guys have wasted their life chasing this dream, so tread lightly, cause when the dog stops barking, he's dead, and when the wife stops nagging, she's dead...

[VincentG]

Thanks for weighing in here Matt. I appreciate your formal education on the science behind all this. I may be dense but hear me out. How to produce an engine with true isothermal expansion is beyond me. We could argue ideals indefinitely but to me overall efficiency is not the main goal, just absolute power output from a wood fire. So I'm left with what we have access to, the standard piston engine. It seems to me that any appreciable compression ratio will only cause pumping losses and is of no gain to a Stirling, where we have the ability to just raise ATM.

The LTD has the lowest compression ratio of all Stirlings and yet runs on the lowest delta. It stands to reason that if made of high temperature materials, it could have high power capabilities. Note also that a low compression ratio means a larger displacer volume v. working cylinder volume, which I believe is the key to its effectiveness.

The Jim Dandy #6 is impressive. And I think its low RPM is the key to extracting power from the Stirling cycle.

[matt brown]

The whole Carnot buzz evolved from his cycle concept. Carnot published his famous "Reflections on the Motive Power of Fire" in 1824 and was the first known treatise on (1) a cycle, but more important (2) a compression cycle. Context is crucial here, as this was during the steam age, when the 'British' ruled the industrial revolution (actually, it was the Scotsman - think Scotty in Star Trek). Yes, Stirling dates to 1817, but was more of a curiosity akin other hot air engines than a credible contender against steam. Until Nikolaus Otto had his eureka moment, nearly all hot air engines lacked any compression process since this was Wneg, and would heavily tax these already low powered engines. However, when Otto was 'messing' with an Otto-Langen engine (google this) he accidentally introduced a small compression process (while adjusting it's valves) and...the rest is history. His patent dates to 1876, and this is what revived interest in 'hot air' engines, and resurrected Carnot research.

The key thing with hot air engines is not falling into a trap where heat is a catchall of convenience. Heat can mean temperature, but heat also means energy, and both share few similarities. The basic problem with both the Stirling and Ericsson cycles is that any measure of efficiency requires regeneration. I spent years trying to figure out an easy way to quantify regen heat, and my Heat Triangle post on another thread here cover the basics. The bottom line is that we can squabble about Carnot and Thigh vs Tlow forever, but few know that the amount of regen heat exceeds input heat in most Stirling cycles (on paper, let alone in reality). It's kinda like a classic steam engine where the boiler dwarfs the engine, a Stirling regenerator transfers more heat 'around' the cycle than input. So, if you worry about 'isothermal' input, worry more about regen. This is why (amongst other nasties) all things Stirling is dominated by tinkertoys and scam artists.

If you're new to this, google Rider hot air engine (the 2 cylinder 'alpha' version) vs the Rider-Ericsson ('beta' version). When I was kid (eons ago) Disneyland used a small Rider engine to heat & operate the popcorn machine on their carts.

[VincentG]

This is certainly a fascinating topic. I think the key take away for me is that I am challenging the very idea of the "regen" process. I believe it is a waste of air space and usable power. The quickest way to change air temperature is to pass it through an exchanger with the greatest delta. Having a regenerator that is only at an average internal temperature has I think just been a bandaid for current designs. In this case, I believe energy is being wasted on the regenerator, not the gas. I can only convey this by word so well....I hope you follow along with my thread. I have a few simple tests in mind to quantify my thoughts, wrong they may be.

[Tom Booth]

. This is why (amongst other nasties) all things Stirling is dominated by tinkertoys and scam artists.

Mr Browns reason for being here bleeds through on occasion. Certainly not out of enthusiasm for Stirling engines apparently.

So why do you hang around here Matt?

[Tom Booth]

This is certainly a fascinating topic. I think the key take away for me is that I am challenging the very idea of the "regen" process. I believe it is a waste of air space and usable power. The quickest way to change air temperature is to pass it through an exchanger with the greatest delta. Having a regenerator that is only at an average internal temperature has I think just been a bandaid for current designs. In this case, I believe energy is being wasted on the regenerator, not the gas. I can only convey this by word so well....I hope you follow along with my thread. I have a few simple tests in mind to quantify my thoughts, wrong they may be.

I tend to agree, in theory at least, a regenerator may be robbing power, maybe. though I'm not sure what distinction you are making between "regenerator" and "exchanger". A regenerator IS a heat exchanger, is it not?

You said: "The quickest way to change air temperature is to pass it through an exchanger with the greatest delta"

What "exchanger" are you talking about here? Some alternative to a regenerator, or are you talking about the heat input heat exchanger?

[Tom Booth]

... to me overall efficiency is not the main goal, just absolute power output from a wood fire. ...

The LTD has the lowest compression ratio of all Stirlings and yet runs on the lowest delta. It stands to reason that if made of high temperature materials, it could have high power capabilities. Note also that a low compression ratio means a larger displacer volume v. working cylinder volume, which I believe is the key to its effectiveness.

I agree with this:

It stands to reason that if made of high temperature materials, it (LTD Stirlings) could have high power capabilities

So finding high temperature displacer materials has been high priority in my research. Ceramic fiber is great, but generally too floppy for an LTD. Foamed glass or foamed ceramic seems potentially ideal.

Note also that a low compression ratio means a larger displacer volume v. working cylinder volume, which I believe is the key to its effectiveness.

That I'm not so sure about.

I think the large displacer chamber volume is very detrimental.

The goal of an LTD "pancake" type design is to have as much surface area as possible with as little air volume as possible. A huge surface area with only a very small traveling distance (air volume).

A low compression ratio is I think, an unfortunate consequence of the design rather than the "key to it's effectiveness".

[Tom Booth]

I think 100% isothermal expansion is an "ideal" only in somebody's pipe dream

It would leave the piston fully extended with nowhere to go without throwing away all the surplus heat that was added, which is simply an impossibility. Heat cannot be conducted away fast enough, even if there could be some theoretical power gain, you just don't have a functional engine

With a rapid adiabatic expansion, the heat is converted to power output and is out of the way. I don't know what could be more ideal than that.

[Tom Booth]

In an effort to rule out variations in "emissivity" or different materials appearing hotter or colder under infrared. I set up three engines with the same type of metal plate heat exchangers. Two non-operating "control" engines and the one running engine with the extended throw. One control was just sitting on a cup of room temperature water and one on a cup of hot boiled water. The running engine is on a cup of hot boiled water

The results are a little strange

The idle engine on a cup of plain water had readings of 76° F

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The idle engine on a hot cup of water read 74°F

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The running engine read 69 to 70°F

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What's a bit strange is the idle engine on a cup of hot water read slightly cooler than the idle engine on room temperature water.

Aside from that, the running engine with extended throw still read colder than either of the idle engines by nearly 5°F

[Tom Booth]

As time passes ..

The #1 control on plain water stayed the same 76°
The #2 control on hot water rose in temperature to 80°
The running engine, beginning to slow down rose in temperature to 71°

I'm taking the coldest readings from each engine. The running engine, of course also has hot spots at the piston and crank bearings around 80 to 81° which I think can be attributed to friction of the moving parts

[VincentG]

It does seem that the engine is operation as a heat pump, unintended as that may be. A thermal camera is definitely not optimal for this but that's a pretty temperature difference.

[VincentG]

A low compression ratio is I think, an unfortunate consequence of the design rather than the "key to it's effectiveness".

Consider that a high compression ratio creates a relatively large change in volume. A low compression ratio creates a relatively small change in volume, which brings us closer at least to isothermal expansion. Does it not? I'm not educated enough to understand the equations.

What "exchanger" are you talking about here? Some alternative to a regenerator, or are you talking about the heat input heat exchanger?

I hope to test this out, but for now maybe this quick drawing will help. The displacer is always made of an insulating material, for reasons I can understand. However I think this is underutilizing the surface area available to us. Note: the sawtooth edges of the drawing are just a representation of added surface area.


https://lh3.googleusercontent.com/Zfnnc ... authuser=0

Imagine the displacer is now controlled by camshaft or solenoid and no longer subject to constant motion. The displacer now has two distinct heat sinks(copper or aluminum) on either end, separated by a non porous insulator(ceramic or other). These sinks are meant to maintain high and low temperatures respectively. Now with full control of the displacer, let it rest momentarily directly in contact with its respective heat exchanger(the hot and cold ends). Not only does this reduce volume on the "off" side of the displacer to near zero, but it also heats or cools its respective sinks to become an active in the next cycle.

Of course this is just theoretical but with this design implemented, combined with proper timing, I think the regenerator is just a flow restriction and no longer needed.

[Tom Booth]

It does seem that the engine is operation as a heat pump, unintended as that may be. A thermal camera is definitely not optimal for this but that's a pretty temperature difference.

I wouldn't say a heat pump.

A heat pump just relocates heat, and requires a mechanical input to do so.

The engine is not so much just relocating as consuming the heat.


What I think is clear though, and contrary to what is generally supposed, the engine cannot be "rejecting" heat to an environment that is warmer than the engine, insulation or no insulation.

I've tried to get this across before using logic. Logically the piston could not return without stored energy in a flywheel, unless the internal temperature was dropping below ambient.

I tried demonstrating it by insulating the cold plate. If the engine does not stall due to overheating, by having the cold side insulated, then there must not be as much heat needing to be "rejected" as generally supposed.

The infrared camera seems to show what I've been trying to say for the past 12 years. What has gotten me banned from all the science and physics forums.

I've even been accused in this forum of "fooling myself" by allegedly putting ice on the engine, or putting it in the freezer or some such thing.

Well, I've been doing very simple experiments anyone could do themselves. I'm not trying to fool myself or anyone else, just trying to report my observations.