LTD model "Stirling" uses Lenoir Cycle

[matt brown]

Thanks for the last 2 links. OK, definitely not open cycle, so I'm clueless what's going on here. Xlnt closeups in second link, but I can't see if chamber is ring or shallow 'dish' (wondering if the lower chamber contacts lower plate directly). Very interesting, to say the least, and that's some hefty insulation you used !!!

[Tom Booth]

... I can't see if chamber is ring or shallow 'dish' (wondering if the lower chamber contacts lower plate directly)

I'm again, not quite sure what you might be referring to.

There is basically just one chamber.

The displacer moves up and down within that chamber, with room for air to flow around it.

The air within the engine is in direct contact with both the top and bottom plate.

[matt brown]

I couldn't tell in most of the videos if the chamber is simply a ring or a ring with a flat 'bottom' (think shallow dish)

[matt brown]

Tom, I got it, I watched your video again where you unbox LTD and towards end I can see chamber has metal bottom and acrylic 'ring' only. I figured this was design, but unsure. Really doesn't matter, since still closed cycle and still begs the question how it runs when so heavily insulated. Hmmm, will give me something to think about at work tomorrow, gotta go...

[Tom Booth]

While browsing around YouTube, looking for some video or other that might make things clearer, I came across this.

The same type of engine, but, this is intended to be a solar powered engine.


https://youtu.be/YAZsc5zZnuE

What I find interesting about these ALL ACRYLIC engines is that acrylic is a very very poor conductor.

This is ok for getting solar heat in, as the light from the sun can pass through, but then, how does the 80% or 90% of the supposedly unused "waste heat" get out to the sink?

Presumably it must pass back out through the acrylic, but the heat is no longer light, it is hot air. I'm sure your aware of the greenhouse effect.

Also, this guy runs the engine on a cup of hot water, not realizing it is a solar type engine.

He complains about the difficulty of getting the heat into the engine through the plastic, It takes a few minutes.

So again, how does the supposed "waste heat" that took a few minutes to get INTO the engine, not take just as long to get back out, how can the engine run at such a high speed if it takes a few minutes for it to heat up and then cool down, let heat in then back out again, if 80% or more is supposed to be just passing through each cycle.

These, and dozens and dozens of other observations lead me to question the idea of the equation that supposedly determines the heat utilization of the engine based only on the temperature difference between the hot and cold plates.

I can find no evidence in support of it's validity either historically or experimentally or casual observations such as this.

[matt brown]

I need to spend some time on youtube studying more kits. Offhand, I think that the cold plate should conduct heat at least as good as the hot plate, and preferably better than the hot plate, so as reduce stall potential (maybe more like copper cold plate, but more $$$). Remember, that 80-90% (or whatever) would be 80-90% of a very tiny heat quantity, but still, good question.

[Tom Booth]

... .Remember, that 80-90% (or whatever) would be 80-90% of a very tiny heat quantity, but still, good question.

Not to be disagreeable, but that is conjectural.

I've had engines using scalding hot boiling water, the bottom plate too hot to touch, without raising a blister, and the top plate remains completely cool, no mater how long the engine runs.

There is air being repeatedly forced at a rather high velocity, into a scorching hot metal plate. What reason is there to believe that this can only transfer a miniscule or insignificant quantity of heat?

I've also run these high temperature engines with much the same result, heating the hot cap to white hot, and still the rest of the engine stays cool to the touch. Granted it has cooling fins, but the cooling fins themselves don't get hot.


https://youtu.be/R_QB5amihko?t=53

What reason is there to imagine that just because there doesn't seem to be any heat build up in an engine that there must not be any appreciable heat going in?

It is the purpose of a heat engine to transform heat energy into mechanical motion. Admittedly, this is a rather hard idea to grasp, as we are used to thinking about heat as a "thing in itself" rather than just a transfer of motion. Heat can burn a finger, but a cool running engine won't burn a finger, so we think of the molecular motion in hot metal or hot air, and the mechanical motion of the engine as two entirely different things, though in reality, they are just two different forms of energy. So when the engine starts moving, the "heat" that was put into the engine vanishes. That is, the molecular motion of the "working fluid" is transformed and becomes the mechanical motion of the engine itself. No longer "heat" but now RPM's

Conclusion; Stirling engines, generally, are far more efficient at converting heat into mechanical motion than what they are given credit for, based on the common interpretation of "Carnot efficiency" using the "Carnot efficiency equation": 1- Tc/Th

The temperature difference really tells us nothing whatsoever about the efficiency of a heat engine. It doesn't even really indicate how much heat is available as temperature is no indication of quantity or transfer rate.

Temperature difference is just temperature difference, how it ever came to be the singular indicator of efficiency can only really be found in the imagination of Sadi Carnot who wrote: "The motive power of a waterfall depends on its height... the motive power of heat depends also on ... what may be termed, on what in fact we will call, the height of its fall." (I e. the temperature difference)

To be fair, Carnot also mentioned quantity (...), but everybody seems to have forgotten about that and there is no indication of quantity in the formula 1- Tc/Th

[matt brown]

Nevertheless, my spin on carefully reading Reflections is that Carnot was trying to determine if work output varied from same heat input depending upon input temperature. IOW, to borrow that lame waterfall analogy, does the work for a given distance vary depending where in the waterfall work is removed ? But steam at that time was severely limited by low pressure limits, so he took to air modeling, and despite period limitations, he did an xlnt job making steam/air parallels. To keep this short, Carnot was chasing...consider 3 cycles A, B, C, and let's use n=(Th-Tc)/Th to expose his chase:

A (600-300)/600 = .50
B (1000-300)/1000 = .70
C (1000-700)/1000 = .???

These days, both A & B are nobrainers, but C slips under the radar; yet, A vs C is what stalled the kinetic theory and had Thomson (aka Kelvin) a disbeliever for years. There's a big difference between standing on the shoulders of giants vs retracing their footprints...

Tom - your reference to common bogus Carnot buzz explains above post. Note than when we list eng C eff we have:

A (600-300)/600 = .50
B (1000-300)/1000 = .70
C (1000-700)/1000 = .30

Assuming heat input is linear T, notice how 300 units of heat went into A @ .50 eff vs 300 units of heat went into C @ .30 eff. I have no problem with these figures for Carnot 'engines', but understand their (often ignored) preconditions. However, Carnot was studying steam, and if A and C were actual steam engs, they both would have effectively the same output from the same input, despite drastically different boiler (input) and condenser (output) temperatures. This is all Carnot was after, but the limited temperature and pressure range of his time forced him on a wild goose chase: (1) he calculates that all gases have...various...nearly equal heats (2) then he chooses air to model steam since he's already determined not much difference between gases (3) then he invents cycle to model input & output (4) then he falls in love with air vs steam (5) while he conjures work from lame attempt at mechanical equivalent of heat (6) yet admits often limited (and questionable) data values.

So, the eff variation for A,B,C cycles (above) is nothing more than backwork during cycle (and steam ain't got much).

But don't get me wrong, I still think Carnot did a great job. Heck, as I get to be an old duffer, he reminds me of myself when I was his age (I used to drive some of the oldtimers nuts). Yep, Carnot is another childhood hero (and it's a very short list) but I also come from the steam camp, so I know exactly what he was really chasing (waaay beyond the scope of this site). So, this whole Carnot 'thing' has been a mess from day one, and it's continued for two centuries.

Tom - will follow up on your last post tomorrow (gotta turn in)

[Tom Booth]

...
Interesting test on insulating top plate. I remember seeing this somewhere on site, ...
.... Well, the proof is in the pudding, and I'm curious what Senft would say. ...

I gained a great deal of insight, reading Senft's "Mechanical Efficiency of Heat Engines" recently, (probably not as thoroughly as I should have, somewhat skimming through to various interesting sections, and now beginning to re-read it again more thoroughly). But...

From the start, Senft makes this unquestioned, unexamined assumption, without any qualification, debate or hesitancy, apparently taking it as a given, an absolute:

From the top of page 4 Cambridge 2010 paperback edition.

In a reciprocating or cyclic working engine, the expansion and compression processes do not take place simultaneously but rather sequentially. Thus to realize a self-acting reciprocating engine, means must be provided to divert and store some of the absolute expansion work and redirect it to the engine working fluid when it needs to carry out it's absolute compression work.

That means of temporary storage has always traditionally been a flywheel, infact, that was one of the first lessons I learned back in tenth grade in High school, when I took a two year trade school course in small engine repair. An engine needs a flywheel to carry it through a complete cycle.

Resize_20220307_102403_3705.jpg (113.14 KiB) Viewed 3707 times

So, I was very surprised and intrigued the first time I saw a heat engine running perfectly well after the connecting rod was accidently removed or left off, which happened to be this video.

https://youtu.be/DyPxNNJQo9M

Apparently Senft had never considered such a possibility, or had never seen any such "free piston" type engine with no apparent means of temporarily storing the expansion work for later use in compression.

He does however propose, later in the book, a hypothetical heat engine, which he considered yet unrealized at the time of writing, with what he called "constant mechanical effectiveness"

He illustrates this possibility with various pv diagrams similar to this on, showing the amount of necessary "forced work" in gray:

Resize_20220307_103435_5028.jpg (214.04 KiB) Viewed 3707 times

Illustrating that at a certain "sweet spot" of temperature and pressure, negative work disappears. That is, during expansion, the internal working gas does all the work, but with the return "compression" stroke, outside atmospheric pressure does all the work

The closest approximation to that would be (d) in the above illustration

Resize_20220307_105016_6085.jpg (245.98 KiB) Viewed 3707 times

[Tom Booth]

For this "ideal" cycle with "constant mechanical effectiveness" to take place, the outside atmospheric pressure (represented by the horizontal line Pb) or buffer pressure needs to be approximately half way between the lowest pressure during expansion and the highest pressure during compression.

It is also beneficial to have a period of "dwell" time, which extends the vertical portions of the PV loop where volume is relatively constant but pressure is rising or falling. This makes the "target area" easier to hit.

IMO, this "target area" is found Naturally, by a "free piston" type engine. Infact, it could not operate otherwise as there is no "stored expansion work" in any attached flywheel available to complete the cycle.

[Nobody]

The cycle, you point to with a blue arrow, shows rejected heat through the right side vertical line, and heat added on the left hand vertical line.

To make your cycle have 100% thermo efficiency that must be done with a 100% regeneration process.

I don't think that is possible for the fluid dyne or the laminar flow Stirlings.

Perhaps the free piston type would have trouble with that as well, I haven't looked into them very deeply. They all seem to be pressurized and have springs to store work for the compression stroke. So???

In addition the cycle fails to show any mechanical and or fluid dynamic work losses. All thermodynamics cycles fail to show them. Carnot was, and all those that have followed him were, optimistic.

But, an impressive thought Sent has shown.

One additional thought: The cycle would be running at plus or minus 1/2 atmospheric pressure so power density will be small by nature. Of course that could be increased by putting the entire engine into a pressure chamber.

[Nobody]

Oops! I just thought: Heat will be rejected in the horizontal bottom line, and absorb in the top line. Look at the T-S diagram which was conveniently left out of the discussion.

Meaning the cycle will conform to Carnot's Rule once again.

[Tom Booth]

The cycle, you point to with a blue arrow, shows rejected heat through the right side vertical line, and heat added on the left hand vertical line.

To make your cycle have 100% thermo efficiency that must be done with a 100% regeneration process.

I don't think that is possible for the fluid dyne or the laminar flow Stirlings.

Perhaps the free piston type would have trouble with that as well, I haven't looked into them very deeply. They all seem to be pressurized and have springs to store work for the compression stroke. So???

In addition the cycle fails to show any mechanical and or fluid dynamic work losses. All thermodynamics cycles fail to show them. Carnot was, and all those that have followed him were, optimistic.

But, an impressive thought Sent has shown.

One additional thought: The cycle would be running at plus or minus 1/2 atmospheric pressure so power density will be small by nature. Of course that could be increased by putting the entire engine into a pressure chamber.

Senft does show heating and cooling in those graphs which at the far right, cannot possibly be due to adiabatic expansion (heat converted to work) so, I agree. Don't confuse Senft with Tom Booth.

For this to be relevant to my own idea the right-hand side of the tracing would have to come to a point where it intersects buffer pressure. More like the "three legged" Lenoir cycle Matt Brown posted at the begining of the thread.(but not exactly)

On the left, obviously heat converted to work must be replaced, so no real issue with a straight up addition of heat there in any case. No "regeneration" is necessarily involved. Nobody (not "nobody") is advocating for perpetual motion or running a heat engine without supplying heat

The Pb line indicates buffer pressure, which, naturally includes the buffer pressure in a pressurized engine.

The point I think Senft was making, or trying to make, is that negative "forced work" could, theoretically be eliminated meaning only that when the atmospheric or buffer pressure is causing the return of the piston (compression or contraction) it is not being opposed.

Obviously these blocky graphs are highly idealized and do not represent real engines, nor does it address issues regarding frictional or other loses.

The main point is that in this scenario, expansion work does not need to be stored to be used later in the cycle for compression.

Senft explains in the begining of the book that his calculations are based on a large buffer space. Too large to be considered as a significant storage medium or "air spring".

Interesting that Senft served as a consultant at NASA.

I should, I suppose, at some point, work up a PV graph to go along with this chart:

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I don't, however, think it would be significantly different from an actual PV tracing, or actual measurements taken of an LTD type engine.

ndicator-diagram-measured-for-a-model-similar-to-that-in-figure-4-courtesy-Hiroko.png (71.61 KiB) Viewed 3681 times

Interestingly, the right hand "point" of the cycle is right at about 101.325 kPa

That is, if we add in Senft's horizontal Pb line at standard atmospheric pressure, it exactly intersects the right hand "point" of the cycle, satisfying Senft's requirement for "constant mechanical effectiveness", at least on the expansion side

I think it is also important to note that atmospheric pressure would be unopposed, all the way back to about 44.35 cubic centimeters in volume.

By that time the atmospheric pressure would have largely been converted to velocity. Pressure into Velocity/momentum conversions are not represented in a typical PV diagram, but IMO it is certainly a factor. That is, actual compression could be a result of converting velocity (created by atmospheric pressure) back into pressure/heat. So that this final compression work need not necessarily result from stored expansion work, but rather atmospheric pressure " work" STORED temporarily in the form of acceleration/velocity of the piston.

[airpower]

Interestingly, the right hand "point" of the cycle is right at about 101.325 kPa

But the 101.325 kPa is mean sea-level and 15° (59°F), 6 ft up it is 101303 kPa
Even the lowest value (100960 kPa) on the graph is only 100 ft (30 m) above water line
Would be handy to know what the atmospheric pressure (and temp) was at the time.

[Tom Booth]

Interestingly, the right hand "point" of the cycle is right at about 101.325 kPa

But the 101.325 kPa is mean sea-level and 15° (59°F), 6 ft up it is 101303 kPa
Even the lowest value (100960 kPa) on the graph is only 100 ft (30 m) above water line
Would be handy to know what the atmospheric pressure (and temp) was at the time.

That's an average. It could be up or down a kPa or two, depending on the weather.

It would be quite interesting to correlate engine start up "operating temperature" with barometric pressure.

For now I'll assume the barometric pressure, wherever and whenever these readings were taken, lined up at the red line, just because it fits my theory at the moment.

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