Stirlingengineforum.com

In my general quest to understand what makes a Stirling engine tick, and in particular, how can it (sometimes) run, without any need for a flywheel, I spent some time watching this video.

https://youtu.be/_5fv-xFuozs

As can be seen, when heat is first applied, (around 2:00) the piston immediately moves outward to the furthest existent possible with the expansion of the heated air.

With every subsequent attempt to get the engine going, the piston returns to this position, indicating, to me, anyway, that the air in the cylinder is still taking in heat and expanding, pushing the piston outward to the farthest limit.

After a certain time, however, some change takes place, and the engine begins to rock back and forth "on its own".

At this point, the piston is not only pushed out by the expanding gas, but is also in some way pushed or drawn back in.

This "drawing in" increases more and more, the direction of the flywheel actually reversing, so that, it would seem to me, at least, that not only is the piston NOT helping to push the piston in, but this "drawing inward" takes place, even in direct opposition to the inertia of the flywheel.

At some point, the rocking back and forth builds up to a point where the engine gats over the "hump" and takes off. From there it gradually picks up speed.

So what is going on?

Why the sudden change.from only pushing the piston outward to its furthest extent to eventually pulling or drawing the piston back inward?

My tentative speculation by logical deduction at this point goes something like this.

Of course, the engine has to reach "operating temperature", but what is so special about some specific operating temperature?

As the engine heats up more and more, the air inside expands more and more, gets hotter and hotter.

Logically, the gas, having nowhere to go LEAKS OUT, around the piston.

So, as time goes by, the air in the engine gets thinner and thinner, or more and more rarefied, as it gets hotter and hotter and more and more leaks out.

At some point, the gas becomes so uniformly hot and "thin" or rarefied that the few molecules left, are moving fast, striking the piston with much force and transferring a high proportion of their kinetic energy with each cycle.

A certain degree of rarefication has been reached where the heat added each cycle is balanced by the heat converted (kinetic energy transfered to the piston/flywheel/load).

Why should this rarefication be necessary?

Because otherwise there are too many hot molecules pushing out, all crowded together, bumping into each other, but not hot or fast enough to all impact and transfer energy to the piston.

Remember that, for a transfer of energy to take place, the piston has to be able to MOVE. It is the "give" of the piston that allows it to take in kinetic energy. The walls of the cylinder do not move, do not "give" so the speedy, fast, freely moving molecules mostly just bounce off the walls of the cylinder and do not give up their kinetic energy until they happen to strike the piston, which does give or move, so a transfer or conversion of energy takes place.

The "thinner" the air, the more freely the molecules can move and the more likely nearly all of them, moving at nearly the speed of light, will have a chance to strike the piston in a given cycle, so the more thorough is the energy transfer, and therefore, the more pronounced becomes the adiabatic cooling of the gas, the conversion of heat into mechanical movement, which results in the "weakening" of the molecules, or cooling, or lack of kinetic energy, which then gives the more dense, relatively MORE energetic outside atmospheric air molecules a chance to push the piston back inward, the very few internal air molecules having become suddenly weakened from impacting the piston and giving up their energy to it.

So, my conclusion is, then, for a Stirling engine to be able to run efficiently, and especially, to be able to do so without a flywheel, the gas inside the engine has to be sufficiently rarefied so that adiabatic cooling can be most effective.

Without the freedom to move, the molecules are too crowded, too sluggish, too dense, so that they spend more time bumping into each other than they do impacting the piston.

Perhaps this is why a very rarefied gas, like helium, gives a Stirling engine such a boost, even at relatively low temperatures, because the gas is already rarefied. Helium has such a very low boiling point that even ambient heat is extremely hot to it, so helium is very light and thin at ambient temperature.

https://youtu.be/Tlwu2UUIImo

Here is a similar engine, but without a flywheel.

The sequence of events is similar.

Watch the piston closely. As soon as the tea candle gets close to the engine, the piston begins moving outward, and continues to move steadily outward in one direction as might be expected where the air inside is gradually heating and expanding.

https://youtu.be/ErlvMZI0tlA

This video includes a lengthy textual explanation of how this engine works which is quite fascinating, but IMO probably completely wrong.

An accustic device or instrument generally has open ends and a fixed length. I don't believe a "standing wave" such as is developed in a pipe organ, could in and way be sustained in an operating engine with moving parts, nor does this acoustic theory of operation explain how the air can possibly expand and then fully and completely contract, drawing the piston inward in a continuous cycle while heat is being continuously added.

It is also, IMO, inconsistent to attribute some entirely exotic mechanism of operation to one type of heat engine, which cannot possibly apply to other very similar, or nearly identical heat engines which do not have an acoustic type "stack" (or Regenerator).

This engine, for example, in a comparative test, does run faster with a regenerator, but without a regenerator, it still runs and functions normally.

It might be argued that the walls of the cylinder serve as a regenerator, which may be true, but I don't believe the walls can serve as an acoustic type "stack" for developing a "standing wave".

https://youtu.be/5OdqD0wCyaQ

I agree it looks like in the top video, that the internal pressure needs to drop to get the engine going, but I think it's because it needs to (on average throughout it's cycle) balance with the external atmospheric pressure to get "over the hump." I suspect that if the engine were encased in a higher pressure environment it would balance to that pressure and still behave the same on start-up, but have more power. Just my guess.

Bumpkin

Bumpkin wrote: ↑Tue Sep 01, 2020 9:32 am I agree it looks like in the top video, that the internal pressure needs to drop to get the engine going, but I think it's because it needs to (on average throughout it's cycle) balance with the external atmospheric pressure to get "over the hump." I suspect that if the engine were encased in a higher pressure environment it would balance to that pressure and still behave the same on start-up, but have more power. Just my guess.

Bumpkin

It is true, the advanced, high power engines used on solar dish generators, for example, are so pressurized.

I've also seen engines that seem to start up quickly, though I think in most cases they have been pre-heated.

This engine also had a comparatively large regenerator. There are many variables to consider and it is hard to draw conclusions without comparing identical engines, under identical conditions.

A simple BALANCE of forces, though, does not, I don't think, explain how a piston, traveling down a cylinder at high speed, in an engine running at a high frequency (Frequency rather than RPM, as I'm referring to no crank/flywheel) due to a heated expanding gas, almost instantaneously reverses course against it's own momentum.

If the pressure were only equal, the piston would, I imagine, just go out to the point of equalization of pressure and stop.

In my observations, I get the impression that the piston returns with as much, if not more force on the return, inward stroke, as if more heat were removed than added for any given cycle.

The slow, gradual movement outward of the piston at the very beginning when heat is added, indicates the speed at which the air is heated by conduction, from the heat source.

If the heat source were replaced with ice, the contraction or cooling would be equally slow and gradual, if the cooling were due to heat being conducted to a sink.

Clearly, IMO, there is something else involved in producing such rapid, instantaneous cooling, which allows the engine to operate at such a high frequency.

I think, also, even in a pressurized engine, the "thinning" or rarefication of the air, or gas, I'm suggesting, only applies to the heated air on the "inside" of the cylinder relative to the "outside", regardless if the "outside" is actual atmosphere or a sealed buffer chamber.

Of course the density of the heated air changes as it expends and then looses heat and contracts, but I think what it comes down to is a relatively few gas molecules doing the work, taking in heat and expanding,

The main question is, where is the heat actually going? Is it all (or most of it) being converted into work and "disappearing", or is it all (or most of it) going to the sink?

Is it really necessary for ANY of the heat to be transmitted to the sink?

There is, IMO, a vast conceptual as well as practical difference between a piston that is pushed back inward by a flywheel to "compress" a gas and a piston that is, in effect, pulled back inward due to the gas contracting "on its own" due to the energy it once contained having been transfered or converted.

The concept or theory behind how the engine operates could greatly influence the design and construction of the engine.

I think a long standing misconception in regard to how a Stirling engine actually works, could be the main reason why development has not progressed as much as it should have or could have by now.

The idea that there is some cosmic force at work in the universe that REQUIRES MOST of the heat passing THROUGH a heat engine to be WASTED to the sink, that the heat REQUIRES an outlet in the form of a cold "sink", and that it is not even worth trying to develop a more efficient heat engine because it would just be a complete waste of time, has discouraged work and intellectual involvement in the problem, discouraged funding for research and development, turned away potential investors, removed whole potential fields or research from possible grant funding by governments, discouraged inventors from dabbling in the area, and IMO has also been a source of unjustified accusations of fraud against people who might actually have been onto something. The list of historical inventors who suffered character assassination because "science" dictated that their WORKING PROTOTYPES were "impossible" and/or a "violation of the second law of thermodynamics" is quite long.

Nicola Tesla, John Gorrie, Charles E. Tripler, to name a few. IMO, honest men who had their reputations destroyed and their life work discarded because their inventions did not comport with "scientific" suppositions, elevated to universal "Law", like an airplane can never fly because it is heavier than air, and that would violate the "Law of gravity". We should content ourselves with hot air balloons.

I found John Gorrie's invention; an ice making machine, very interesting, for a few reasons. It used an "Air-cycle" refrigeration system, and the exhausted cold air used for making ice was vented to atmosphere. Also the heat of compression was largely taken away by water cooling, and yet, Gorrie suggested that his ice making machine required virtually no (or very little) external power to operate.

His ice making engine was really a kind of expansion heat engine. The heat to run the engine, in part, being supplied by the heat taken out of the water to be frozen as well as ambient heat in the air.

He also utilized the phenomenon of removing heat by having compressed air performing work in a piston expansion engine, which I think must have greatly increased the effectiveness of his machine several years before Prescott Joule would perform his experiments, demonstrating the equivalence of heat and work.

In his patent Gorrie makes note of this additional cooling effect of having the expanding air perform work, helping to drive the air compressor that supplied the compressed air in the first place, but attributes this extra cooling to the extended time required for the expanding air to work the piston, which he imagined allowed additional time for cooling, as opposed to free expansion, rather than the work performed itself.

How Gorrie's near "perpetual" ice maker worked may have been something of a mystery, even to Gorrie himself, and he also faced much opposition from an established natural ice making industry, yet his machine was demonstrated.

Apparently, all that remains of this marvelous achievement is a replica in the Gorrie museum.

Sadi wrote: ↑Sat Sep 05, 2020 4:00 am (...)
STIRLING ENGINE LAMINA - (...)

The illusion is that the test tube is completely clogged with a working piston! If someone heated a completely sealed test tube like that, it would soon explode!

(...)

THE "THERMOACUSTIC ENGINE" is just fiction! This is the same double engine from the first example, whose work I have already explained. The difference between these two examples is that this second engine uses a very massive piston and a piston rod instead of a flywheel that oscillate based on expanding and sucking air into the test tube. That the tube is not tightly closed is shown by the fact that the piston rod hangs quite well - before the engine moves.

There seems to be quite a lot of "hot air" there.

I've been trying to get the idea across, as Nicola Tesla pointed out more than a century ago, Heat engines CONVERT heat into mechanical motion, so, unless a lot of excess heat is applied, the heat does not build up. So no explosions.

The engine (running without a flywheel) is not sucking in cold air or loosing hot air past a loose piston as some kind of substitute for a flywheel. If the piston were that loose, the engine wouldn't run at all.

A sealed engine is not going to "explode" nor will the piston fly out the cylinder. Not under any normal operating conditions. There are plenty of sealed Stirling engines of all descriptions. Many use a membrane or diaphragm instead of a piston, or are entirely hermetically sealed.

I don't know where you come up with this stuff.

But, to be fair and open minded, I took one of my little Laminar flow, thermoacoustic or whatever toy engines, removed the flywheel and sealed up the end of the cylinder and connecting rod good and tight with a finger from a rubber glove, glue and rubber bands. I did a pressure test on my seal , there were no air leaks.

I pushed the piston in as far as it would go when sealing the engine to allow room for expansion.

As can hopefully be seen, (in the video) the piston did not fly out, there was no leaking hot air to inflate the "balloon" (rubber gloves finger). For better or worse, the engine did not explode.

I did not have alcohol or a candle, so I used a propane torch as a heat source, which I'm sure was excessive, yet still, no air leaks into the finger and certainly no explosions.

As a matter of fact, without a flywheel, the little "toy" ran better than I've ever been able to get it to run WITH a flywheel, and at a very much higher RPM or frequency.

It worked so well, I may.try using a neodymium magnet for a piston and wrap some coil wire around the cylinder for a linear generator.

https://youtu.be/iOs3BADFeKI

I may never build another Stirling engine with a flywheel again. It seems to be nothing more than dead weight that slows the engine down, and robs some energy.

Thank you Said, if not for your &#!$ comments, I would probably never have bothered doing such an experiment as a rebuttal.

Some description of how this "thermoacoustic" engine works is interesting.


https://youtu.be/Zq3nhhsjve4?si=RK5PKT1hkY8_sbX6


The part about "less air takes up more space" resulting in a partial vacuum sounds a lot like what I've been saying in here about "rarefaction" in a slightly different way.

In one video this guy measures the output of his engine, that looks to be around 35 or 40 watts not really sure.

I can't vouch for the quality, worth, reliability or anything of this engine, but it does appear that he is making them for sale (on back order). There are no reviews.

The engine can, he says, be placed on a wood stove or in a campfire.

There was a time I would have been overjoyed to be getting 30 watts continuous from my wood stove living off grid.

The, presumably, high frequency AC would have to be rectified for charging a battery, then run through an inverter, so after loses, there might not be much usable power left, but back in those days I was peddling a bike generator just to run a laptop a few minutes a day

Anyway, the price seems reasonable for a large, rugged, stainless steel Stirling generator that at least puts out a few watts of probably unusable high frequency AC (not sure, can't find any specs on output on the website, nevertheless)

https://longearthsociety.com/product/th ... ic-engine/

I have a feeling an engine of this general type could put out some useable power with a larger "star" type (multiple coil) linear generator and rectifier circuit

Anyway, it's nice to see someone making the effort to make at least something available. I'd buy one right now if I had the money to spend.

I was mistaken regarding the linear generator output being "unusable".

Some text on the website reads:

All spools come with 30 AWG magnet wire with a 3A bridge rectifier, unless otherwise requested

So a rectifier is already included apparently, though that wasn't specifically mentioned on the "what's included" page. I found the above on a separate page for ordering just the coil, which can be purchased separately.

I had commented on one of his YouTube videos and asked if he was on this forum. He said he was not on any. I'd like to hear him weigh in on this.

I would guess that for any particular engine, there is a sweet spot between the surface area of the heating end, the heat input, and the frequency, where power would peak substantially. That frequency where the lag of thermal transfer by the heat source matches perfectly with the compression ratio and the subsequent adiabatic cooling/expansion and the weight of the piston, etc., etc.

Add to that this apparent monothermal effect and who knows what's going on in there.

Hi all, I was redirected here from reference traffic to my website. I am the creator of the Stirling engine in this thread.

AMA!

Hi all, I was redirected here from reference traffic to my website. I am the creator of the Stirling engine in this thread.

AMA!

Welcome Les, glad to have you here. This is Tom's thread, but I'd like to hear your take on overall theory and operation of a Stirling cycle in as much detail as you'd like to get into.

Sure! I will only talk about my specific engine, since that is what I have the most experience with.

Thermoacoustic Effect: In my vertically oriented design, when the base is heated, air ascends through an internal stack. Adjacent to the upper portion of this stack, there's an ice sleeve, facilitating rapid cooling. This pronounced temperature gradient likely induces a thermoacoustic effect, reminiscent of the phenomena observed in a Rijke tube [Swift, G. W. (1995). Thermoacoustic engines. The Journal of the Acoustical Society of America].

Electromagnetic Induction: Resulting sound waves prompt a high-temperature silicone diaphragm with an affixed magnet to oscillate. This magnet's motion within a coil generates electricity, adhering to Faraday's law of electromagnetic induction [Griffiths, D. J. (2013). Introduction to electrodynamics. Pearson Education].

Pressure Regulation with Duckbill Valve: A distinct feature in my design is a pipe with a low-PSI-rated duckbill valve. When heated, expanding air is vented, potentially stabilizing the internal pressure. Observations suggest a small pocket of air remains, undergoing consistent heating and cooling.

Diaphragm Dynamics: During heating, this air pocket expands, leading the diaphragm to ascend. In the cooling phase, the air contracts, causing a downward diaphragm movement. The frequency of this oscillation is approximately 2000 cycles per minute. I believe this feature lends additional power to the engine, since both up and down could be considered a power stroke. Without this, it would be observed at the diaphragm only moving up with the pressure or sound wave.

Cool Down Phase: Post-operation, the engine exhibits a pronounced vacuum effect, drawing the diaphragm inward, supporting the theory of a confined air pocket.

Also adding this video to show the thermoacoustic effect without a diaphragm. (Watch your volume)

https://youtube.com/shorts/vCZ-Nm5N9aU? ... pbEkkPxypI

Once the engine running, do you attribute any extra heating or cooling effect to the expansion and compression of the gas? In other words, temperature changes not due to direct heat transfer with the heat sinks.