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I finally got around to filling the big boy with helium.

What a surprise!

I mean, in the process I was rather astonished.

Is everything we've ever been told about these engines lies and misinformation?

I've seen in some videos that one of the reasons Stirling engines are not practical or whatever is that to get power you need a lot of very expensive Helium.

Well, this evening I filled the engine a couple times with inert welding gas, just to purge any oxygen or moisture that might be in there and also just to make sure there were no leaks.

This was a long process.

The argon gas seemed to take forever to get up to pressure. I took it up in stages to make sure I didn't put in too much, as the gas goes in extremely cold, then gradually warms up and expands. It took maybe 20 minutes to get the Argon up to 500 psi.

Well, after all that, I let out the Argon and put in a little helium a couple times and let it out to purge the argon.

Now for the Helium.

I was expecting, due to the small molecular size and how light it is, that the helium would take a really long time to pressurize.

Well, I turned the valve, barely, and the pressure shot up to about 250 psi almost immediately.

Wow. What's going on?

I let a little more out of the tank and into the engine and the pressure was already nearly 500 PSI.

I felt as though I had hardly let much helium into the engine at all.

It seemed like the helium was hardly compressible at all.

So I did some research and, among other things:

https://youtu.be/0bvfYUBotGs?si=zfxcg6yUvvk7Tc_e

The tank of helium was not expensive and It seemed like I hardly needed to use much at all to get 500 psi because of the >1 compressibility factor.

Hydrogen, apparently, is even less compressible than Helium.

I suppose though, if helium is so difficult to compress, there probably is not a lot in a tank full now that I think about it.

Anyway, I found this whole experience rather interesting.

Looking at this graph, if I'm interpreting it right:
https://chemistry.stackexchange.com/que ... er-minimum

Until you get up to some really high pressures (above 100 atmospheres or 1500 psi) from left to right; most gases get easier and easier to compress as the pressure increases

Helium and Hydrogen immediately, or always get more and more difficult to compress as the pressure increases.

What does this mean in practical terms when used in a Stirling engine ?

Well, I don't really know, but assuming it's a good thing,....

The problem with a lot of "dead air space", I believe, is compressibility of the gas or air

We want the expanding air to drive the piston, but if the air is compressible, it could be like trying to push a solid object with a rod made of foam rubber. Instead of the object moving, the foam rubber will itself compress. The foam is too "squishy".

The more "squishy" material, the longer the foam rubber push rod, the more it can compress before the object will move. Too much "squishy" air; too much "dead air space" in the engine, the less responsive it's going to be, as much of the work would go towards just compression and decompressing the working fluid rather than driving the piston.

Having experienced the difference, I couldn't find a compressibility chart that included Argon, but I imagine it must be down there with CO2, I was able to put more and more Argon into the engine and the pressure increased very gradually. Getting the Argon back out also took a very long time. As I let the argon out, it just kept coming. The pressure diminished very gradually. The engine, at 500 psi contained A LOT of Argon.

The helium on the other hand, the two times I used helium to purge the Argon, the pressure went up and back down very quickly. By comparison it took about 20 minutes to charge the engine to 500 psi with the Argon gas and another 20 minutes to let it all out again. Partly because the valves kept icing up becoming too cold to handle, but it just took a long time

When it came to the Helium, I had the engine purged and then charged to 500 psi almost before I knew it It took maybe a minute if that

I had planned on first testing the engine at lower pressures, but wasn't really sure if that was a good idea. The best information I could find said ""about 500 psi".

The pressure went to 500 psi so fast, 500 psi it is, for now anyway.

Apparently that amounts to about 12 cubic feet of unpressurized Helium (at 1 atm) which is less than one of these:


So,...

The engine apparently might need to be recharged once every 30 years ?

Somehow, I don't think the cost of an incidental expense for a birthday party puts this out of reach as prohibitively expensive for most people.

About the same as, or maybe less than an oil change in a car engine.

This is maybe not official, as I'm not entirely sure what's going on, but some very basic preliminary kind of tests with the helium filled 3 Kw Stirling are not particularly encouraging.

I've got a passive cooling system for now, which presents additional issues (convective circulation of coolant can only work (at all) with the engine in a vertical orientation, due to the placement of the inlet and outlet pipes. Also, the connections are UNDER the engine, as it is currently mounted, meaning without active forceful water circulation, there is air trapped in there.

I did manage to tip the engine upright though, onto a short section of metal stove pipe and put a 1000 wat hot plate under it. Having it upright like that got most of the air out, I think. So I could then tip it back up and hit it with the salamander kerosene heater.

Surprisingly, while charged with helium, the engine showed, what seemed to be LESS willingness to TRY to run. Trying to get it started (or just hitting it) with a rubber mallet, as I had done many times before, unpressurized, the output, as indicated by my meter, while pressurized, seemed to be LESS that before. No amount of heat applied from my two limited heat sources improved this situation.

My Thoughts & Comments:

Pressurization with helium Might (?) increase POTENTIAL output at very high temperatures, but makes startup more difficult (?)

Stirling engines ACTUALLY operate on a refrigeration/heat pump type cycle which depends heavily on HEAT OF COMPRESSION and expansion cooling, which, actually, requires a working fluid that is MORE rather than less elastic or. compressible.

The 3Kw engine, unpressurized, or only very moderately pressurized with ordinary atmospheric air, seemed almost ready and anxious to run, (recall, the engine started to make a rather loud humming sound while filling with compressed air from my shop compressor to a out 50 psi) or at least showed encouraging electrical output with just a tap from the rubber mallet.

Helium filled, wacked HARD with the mallet, I'm only seeing at best, about 1 or 2 volt, more often, fractional voltage.

After a long warm up (with the 1000 watt hot plate) , I discovered that the thermocouples on the engine were compatible with my multimeter, so I plugged them in one at a time as the meter could only accommodate one thermocouple

The thermocouples all seemed to work and provide good readings.

The highest temperature reading though only reached a little UNDER 200°C after like 30 minutes in direct contact with the hot plate.

This is, unfortunately still far below the typical startup/operating temperature for these big engines.

Keeping the hot plate going only seemed to be gradually heating up the metal housing and cooling water but not increasing the engines hot end temperature much.

The literature/published articles on the engine relate that for 3Kw electrical output the engine also "rejects" 7Kw thermal to the cooling water, which normally requires a circulating pump and a radiator with a fan.

Ah,.... So,... Normal operating conditions would then require 10,000 Watts of heat input.

My 1,000 Watt hot plate doesn't hold a candle to the designed solar heat input from a 15 foot parabolic mirror focused to a point about 4" in diameter

The 140,000 BTU kerosene heater seemed to do no better. Actually, not as well, probably because comparatively little heat can be transfered from the blast of mostly fast moving HOT AIR from the salamander, compared with the sustained direct contact with the red hot heating element of the hot plate

Honestly, I'm seeing better, more encouraging output from the little toy model engine used as a phone charger, which is actually putting out up to 9 volts at this point. I did recently see a 2%.increase in the % of charge with the 12 volt USB charger connected up

Maybe something like this DIY electric arc furnace could supply a sufficiently concentrated source of high heat to start the engine.


https://youtu.be/VTzKIs19eZE?si=KGENDeRfa-Shsf-g


Those light weight refractory bricks look like potential high temperature displacer material as well. There seems to be a number of different kinds of refractory bricks available though. Not all are so light weight.

I think I'm going to go with, or at least keep an open mind about, and test experimentally, as an alternative hypothesis, the idea that Stirling engines need a working fluid with higher compressibility.

True, helium is a more "perfect gas" in that it behaves more in accordance with the Ideal gas "Law", but it could very well be argued IMO that it is Non-ideal gas behavior that makes the operation of a Stirling engine possible.

Old, conventional theory is essentially that the working fluid needs to be heated and then cooled, externally to produce expansion and contraction, which amounts to using the gas to TRANSPORT heat from the hot to the cold side very rapidly. To take the heat in, then deposit it to the "sink". So, in such a case, light, rapid moving, highly conductive gases are desirable, theoretically, because the heat needs to be TRANSPORTED rather than "CONSUMED". The working fluid is merely a heat carrier, not a "FUEL"

If heat is ENERGY, however. ENERGY cannot be arbitrarily multiplied. That is, the energy input must equal the energy output, so if heat/energy is put in and that same energy is taken out as WORK, then the same ENERGY cannot also be transported through to the "sink', that is a violation of an ACTUAL, well established, fundamental LAW of Science.Conservation of Energy.

So, if the goal is not to TRANSPORT so much as Transform the heat, then is it necessary or important at all that the working fluid be highly mobile and highly conductive?

Energy quickly TRANSPORTED through to the sink is energy lost or "wasted" it cannot also be used for power output.

To TRANSFORM the heat/energy, it needs to NOT be conducted THROUGH, but rather, it needs to be HELD BACK while being transformed to something other than heat. As heat engines ACTUALLY operate, the sequence of energy transformation is HEAT > EXPANSION > WORK

The gas must be heated so that it expands and AS IT EXPANDS it must be made to do work. The transformation is then complete and there is literally no "heat"/energy remaining on the energy balance sheet that needs to be "rejected" to any sink.

So, what may be important for a working fluid then, may not so much be conductivity but elasticity. The capacity to be compressed and expanded. The gas needs to expand forcefully as heat is absorbed and then be easily compressed after the transformation of energy to work output.

So do we really want a gas like helium that is about as easy to compress as a ROCK?

I wouldn't think so.

Maybe what we want is a gas with a big dip on the compressibility chart.


And at what target operating temperature, as compressibility varies with pressure and temperature.

IMO it is not the role of the working fluid to simply ESCORT heat from source to "sink". It needs to be able to expand and it needs to be compressible.

Of course, this would need to be demonstrated and proven to work in practice by experiment.

Helium or Hydrogen, however, have always been considered the go-to working fluid.

Puzzling to me, however, if memory serves me, is the fact that if Stirling engines are similar in principle of operation to heat pumps or refrigerators, Why are the most common refrigerants not high on the heat conductivity tables?

I need to double check that, but I do seem to recall puzzling over that "fact" while looking over such gas property tables. Many refrigerants are NOT great conductors of heat, but don't quote me on that.

Well, you need to be careful in reading some of these tables as for refrigerants, they may be listed under normal pressurized operating conditions, which could be 600 psi or more.

Some figures from engineering toolbox in W/(mk)


Hydrogen. 0.168
Helium. 0.142
Air. 0.026
Ammonia. 0.025
Argon. 0.016
Propane. 0.015
R-134a. 0.014
R-12. 0.007


For comparison some insulating materials


Silica aerogel. 0.02
Cotton Wool insulation 0.029
Fiberglass 0.04
Perlite. 0.031


Arguably, one of the best, most popular and effective refrigerants ever developed is about as heat conductive as the vacuum. That is, not heat conductive at all.

While on the subject of compressibility, this is kind of interesting:


Looks like Ammonia (NH3) goes right off the chart.

C2H4 seems way down there, (very compressible) as well, and is a cryogenic range refrigerant.

The ammonia engine in particular, has a rather colorful history:

https://hughjyeman.wordpress.com/2016/0 ... -the-news/


Coincidence?

The principle objection to Mr Gangee's "zero motor" put forward by the various scientific commentators was:

It is true that a liquid in expanding into a vapor develops power; but it requires just as much power as is developed to reconvert it into liquid again.

If the expansion is a consequence of heat input, however, and heat is energy, and the expansion results in work output, the energy equivalent of the heat input on the expansion stroke, than this objection could not possibly be valid as it constitutes in itself, a clear violation of conservation of energy.

Just as a steam ejector converts heat to motion (velocity) so does a heat engine, convert heat into motion. Not only the motion of the expanding gas itself, but also of the entire apparatus and whatever it might be driving, such as a steamship.

The recompression or re-liquefaction could not possibly require "just as much power", since, at least SOME of the power developed by expansion is converted into WORK output

Now I have another theory about this engine:

https://youtu.be/u-YfPEFBh70?si=5M9mowBaXgvTmAEh


I found it interesting that in this, as well as another video, he states that the engine would not run when the cooling water was too cold, but liked to run when the cooling water was just about boiling, or just barely under boiling.

What does that mean in terms of compressibility?

Well, the Ammonia goes off the compressibility graph, I assume, because it undergoes phase change at the point where it bottoms out and the line ends.

Most atmospheric air, such as used in the engine in the above video, contains some percentage of water vapor. Having steam rising up around the engine likely increases that percentage.

When water changes phase from liquid to gas it expands about 800 times. (And of course, likewise contracts with condensation)

With the cold side of the engine held near the boiling/condensation point of water, you have, I can only assume, a situation similar to the ammonia graph. Any water vapor inside the engine will be near phase change, poised between a liquid and gas/vapor state and therefore have maximum compressibility for that particular substance (H2O water/water vapor)

Having the cold side near boiling/condensation would allow rapid phase transition back and forth. Much colder and this balance would be upset. The water vapor would have a tendency to condense and remain liquid rather than easily transitioning back to vapor and expanding.

The implication, or theory being then, when the working fluid temperature and pressure puts it near to its phase change temperature and pressure it will maximize the compressibility of the working fluid and therefore take the most advantage of the working fluids internal forces of molecular attraction and repulsion with the least ∆T.

That is, when a substance is near its phase change temperature and pressure, a slight decrease in temperature tips the scale towards condensation/contraction and a slight increase in temperature tips the scale towards evaporation/expansion.

Too much distance from this balance between attraction and repulsion and the engine has difficulty running and is less efficient.

So, under such ideal circumstances (the working fluid poised between the liquid and gas phase) Changes in temperature, in particular a lowering of temperature due to work output, results in contraction of the working fluid without the necessity for external cooling, or not as much external cooling, and therefore results in the greatest possible efficiency and power, or at least, greater efficiency and power than when the working fluid is far from its phase change temperature and pressure.

This would make Helium perhaps the worst possible choice for a working fluid, except for engines operating at the deepest cryogenic temperatures, or for actual Stirling Cryocoolers.

In actual fact, many of the high tech over engineered NASA type Stirling engine manufacturers have had little success in marketing power producing engines, but have, instead specialized in Cryocoolers.

To get Helium near phase change requires very very high pressure and cryogenic range temperatures. Not at all practical for deriving power from my wood stove or kerosene heater or any high heat source for that matter... Too far from phase change/high compressibility.

Of course, this is an off the top of the head theory that should be taken with a grain of salt, but, if true, what working fluid (gas) would be appropriate or more suitable for use at high temperatures,, that has a phase change temperature and pressure nearer to the actual operating temperatures and pressure?

That might be worth some investigation/experiment, I think.

Is it then, in compressing and expanding a gas in a hot air engine that we are approaching and then receding from the liquid/vapor "line of equilibrium" for the air, or whatever gas/vapor mixture is being used as a working fluid?

Certainly, it is true that compression would be easier as we approach that "line of equilibrium" where the gas molecules attractive forces are strong and the gas is about to condense due to a temperature reduction from energy expenditure due to work output even without a strong external force of compression applied.

Compression of a gas is easy near the line of equilibrium. It is condensing, the molecules are drawing together anyway.

Then with the application of heat from an external source, we move the working fluid away from that line of equilibrium again and the gas molecules repulsive forces and/or kinetic energy dominates and the gas expands.


https://youtu.be/WDgFCI_BYmo?si=nec7YjKF7Sxusk2D


If all this is true, then this might provide us with a means of designing an engine with a working fluid optimized for a particular target "operating temperature".

Certainly this sort of working fluid optimization is fairly well in hand in the field of refrigeration, air conditioning and heat pumps. No heat pump or air conditioning system would be designed without consideration being given to this area of science. The refrigerant is optimized according to the expected operating conditions. Ambient air temperature, climate, heat source temperature, etc. such systems, in fact actually straddle the phase change "line of equilibrium", condensing the working fluid into a liquid on one side of the system and expanding the working fluid to a gas on the other side of the system

Is it at all surprising or outlandish then, to suggest that a Stirling engine which driven by a motor BECOMES or actually IS, or was all along a HEAT PUMP should also take advantage of the same principles that allow it to operate or function as such?

So, with growing skepticism, I continue to look at additional data/evidence. Evidence of what exactly I'm not entirely sure, but we have this curious video.


https://youtu.be/6Zswz43Se8E?si=tcJQNp7q0Qbf3lUO


Initially it appears the helium filled engine runs very poorly, and there is a cut away.

It is difficult to read, due to the font, but some text appears on the screen:

The helium engine takes longer to get up to speed because the piston was removed, The pressure needs time to equalize.

Quite obviously it is true that the helium engine took longer to get up to speed. I'm less convinced as to the reason.

Speculating for a moment, I would tend to suspect that a helium filled engine would run poorly due to the low "compressibility" of helium.

Helium is also notoriously difficult to contain. By the time (after an indeterminate cut away) the helium engine began to speed up, how much of the helium escaped? How much helium was displaced by ordinary air?

Why might this be important or significant?

Well, maybe it isn't significant at all, but, I tend to puzzle over puzzling observations and come up with wild theories that could explain the observations rather than just dismissing the results of an experiment offhand

The results, in this case, at least initially, seemed contrary to theory, or contrary to expectations

The helium engine should have run better or faster right away. The results are a bit inconclusive and/or questionable IMO.

So, applying the logic that SOME helium very likely escaped from the engine and as it escaped it likely was displaced by ordinary air, we might assume for arguments sake that what ran better was a gas MIXTURE that included some percentage of helium and some percentage of air.

The evidence is that pure, or a high percentage of helium could cause the engine to run poorly.

There are, though, some unknown variables. For example, the helium engine was removed from the heating surface. How much did it cool down? How long did it take to warm back up, in spite of the "regulated 10°C heat differential"

Regardless, there is a fairly well known, but IMO rather strange and perplexing phenomenon refered to as "partial pressure". It has an interesting application in refrigeration, in particular ammonia absorption refrigeration.

Coincidentally, an ammonia absorption refrigerator uses two very dissimilar gases, as far as it relates to the topic of this thread (compressibility), ammonia and hydrogen.

Now, as previously shown, Hydrogen has unusual properties similar to helium. Both hydrogen and helium are not very "compressible".

Compressibility seems to be closely related related to "partial pressure".

Gases that are difficult to compress, like hydrogen and helium have low compressibility, and in a gas mixture, high "partial pressure".

That is, in a mixture that includes hydrogen or helium, the hydrogen or helium, mixed with something like ammonia, will account for nearly ALL the pressure on the container walls.

In a mixture of hydrogen and ammonia in an absorption refrigerator, for example, the hydrogen will be under high pressure, but the ammonia will be in a veritable vacuum, or under very low pressure, though the two are infact commingled within the refrigeration system on the evaporator side.

The hydrogen molecules are so small, and yet repel each other so strongly that there is a great deal of space between the hydrogen molecules, so as the ammonia gas escapes from the refrigerators expansion orifice into the hydrogen, it is as-if the ammonia were passing from high pressure into a vacuum, and so the ammonia expands and cools, effecting refrigeration.

So,... Further speculating, it seems conceivable to me that there could be a similar situation in a Stirling engine partly filled with helium and partly filled with other gases, the helium, like hydrogen, having a high "partial pressure" due to its lack of "compressibility" and the other gases (ordinary air) being much more compressible so having a low "partial pressure".

Throwing out a suggestion, perhaps such a gas mixture could increase efficiency in a Stirling engine in much the same way as putting some bricks in a toilet tank increases the "efficiency" of a toilet, in terms of water consumption.

In other words, the helium by itself is like an engine full of bricks, pretty useless.

But mixed in with ordinary air, the helium reduces some of the "dead air space". In effect, increasing the compression ratio of the air by reducing the volume. The helium molecules take up space displacing the air in the same way that bricks in a toilet tank take up space and displace water.

The bricks, of course, are altogether useless for flushing toilets.

Likewise helium may be rather useless for running Stirling engines.

But, in either case, by taking up space, some economy is realized. The engine and/or toilet, operate more efficiently, if for somewhat slightly different but at the same time similar reasons.


I can't say this is an easy or straightforward subject. We are off in the realm of NON-ideal gas properties and behavior. Partial pressure for "ideal" gases may appear simple but with real gases like the actual air we breath, things are a bit more complicated.

I did find one video that includes hydrogen and illustrates how in a mixture, a small fraction of a gas like hydrogen can contribute disproportionately to the "partial pressure".


https://youtu.be/ch61milcTUU?si=vqZyqHlqgI6Pmev1