Stirling Engine Thermodynamics

[Tom Booth]

VincentG your drawing is not showing up for me, but I think I understand you.

[VincentG]

Maybe this will work?

[Tom Booth]

Maybe this will work?

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

Not so far, but yikes, what a mess google makes of things.

Rather than remote linking I would just upload the image (as an attachment using the "full editor & preview") to the forum, it is generally more reliable.

Then use the "place inline" button, will insert the image wherever the curser is in the text edit box.

It may be necessary to reduce the image size though.

[Tom Booth]

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.

I think your plan is pretty sound, based on your description, to increase the surface area for better heat exchange, and also increasing the idle time or dwell.

There is, or was at one time, on the earliest, original LTD type engines by Ivo Kolin and James Senft, a particular kind of linkage to create the kind of momentary "rest" you are aiming to achieve.

This was referred to as a "lost motion mechanism", an example of which can be seen on this engine:

https://youtu.be/9Ckd7N3MxUU

The rod closest to the camera has two stops or bumpers so the motion of the displacer is discontinuous.

There was another variation on Senft's P-19 Ultra LTD. Just an elongated loop in the displacers connecting rod.

2021-11-22 10-48-34.jpg (121.87 KiB) Viewed 9187 times

[Tom Booth]

The only thing I might add, based on my own research is that in actuality, in a Stirling engine, the heat going into the engine is converted into mechanical work output. So adding ribs or groves to the cold side for additional surface area may be redundant. That is, trying to remove excess heat that is no longer there. So, if that can be accepted, you might want to focus attention on the hot side, getting more heat in, as the heat "rejection" on the cold side may be pretty much nihil.

[Tom Booth]

Another means of achieving this "rest" period is with a magnetic lift on the piston for lifting the displacer, such as in these type engines:

https://youtu.be/lx1tet8aHJU

The displacer just sits on the bottom until the piston gets close enough for the magnet on the end of the piston to lift the displacer.

Note also, the bottom of the engine is metal for rapid heat input. The rest of the engine however is acrylic, which is actually a heat insulator.

I called this an "impossible" heat engine because according to the "accepted laws of physics", the second law of thermodynamics, in particular, and the "Carnot efficiency limit" a heat engine is not supposed to be able to complete a cycle without a great deal of heat "rejection" on the cold side.

To illustrate that the heat "rejection" on the cold side is apparently unnecessary, I've done many experiments, covering the cold side with additional insulation.

In this case, the same acrylic engine with magnetic lift is provided a continuous supply of heat on the metal heat input side, but is covered with a Silica Aerogel blanket on the upper cold side on top of the acrylic, not to help "reject" heat on the cold side, but to retain it.

https://youtu.be/l2XcnN6QdfA

Contrary to so-called "established physics" this has no detrimental effect on the engine performance.

[VincentG]

Thank you I did not see that option.

displacer theory.jpg (205.18 KiB) Viewed 9178 times

Tom, as I've said, what I believe is happening, is that the engine is poorly timed and is robbing its own power to cool the cold side. By doing so it is losing out on most if not all of the second (atmospheric driven) power stroke. You could be correct however, and then the cold side should be constructed of the best insulator available.

I think a good way to test this is to add some preheat to the top plate of two engines over hot water. Then start one engine and see if it cools the top plate faster than the engine at rest. It's pretty unfortunate that those in the science community would just ban you without looking for answers first.

[Tom Booth]

Some would consider that you still have a regenerator or heat gradient in the space between the displacer and chamber walls. If you don't mind my adding some color to your whiteboard.

Resize_20230225_121711_1561.jpg (121.78 KiB) Viewed 9168 times

Actually, that is how Robert Stirlings original,minimal regenerator, or economizer concept was portrayed in the 1816 patent:

Resize_20230225_122748_8420.jpg (22.58 KiB) Viewed 9168 times

The hot liquid, gas, or body to be cooled is by any means made to enter the passage at A and to pass along to its other extremity B.
In its progress it gives out its heat to the sides of the passage or to any bodies contained in it and issues at B at nearly the original temperature of the passage.

In this manner the extremity at A and a considerable portion of the passage is heated to nearly the temperature of the hot liquid while the extremity B still retains its original temperature nearly.

http://hotairengines.org/closed-cycle-e ... rling-1816

[Tom Booth]

Tom, as I've said, what I believe is happening, is that the engine is poorly timed and is robbing its own power to cool the cold side. By doing so it is losing out on most if not all of the second (atmospheric driven) power stroke.

Without such cooling, there would not be any atmospheric driven power stroke. You would have to remove the heat to a cold sink and then force the piston back with the stored energy in a flywheel.

Be that as it may, it should not be possible to violate a veritable Law of the universe, by simply moving the connecting rod on the engine a few millimeters further out on the flywheel.

You could be correct however, and then the cold side should be constructed of the best insulator available.

I think a good way to test this is to add some preheat to the top plate of two engines over hot water. Then start one engine and see if it cools the top plate faster than the engine at rest

That's a possibility.

It's pretty unfortunate that those in the science community would just ban you without looking for answers first.

I think the concept of heat being converted into mechanical motion, and leaving behind cold runs so contrary to every day human experience it is difficult to even contemplate. As I said at the very beginning of this thread, 12, or however many years ago that was, I had a hard time wrapping my heat around it. But that is what I was reading in these old physics books.

Practically speaking, it certainly makes a difference in how I would design a heat, or hot air engine, if there is actually no law of the universe dictating that 85% of the heat must absolutely be thrown off for the engine to run at all.

[matt brown]

. 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?

I'm waiting for your eureka moment when you realize that ALL closed cycle compression cycles require some type of heat sink !!! Gadz, even a crude Cayley type (double-diameter) requires a heat sink, tho typically results in (or nearly so) Wpos=Wneg, thus Wnet=0.

If you preaching no heat sink closed cycle, let's see some type of PV plot for such...

[Tom Booth]

(...)

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. ...

I think that is basically correct (highlighted in bold). Of course, IMO I don't believe this is due to "poor" displacer timing. It's just what a Stirling engine does under normal conditions. It's normal mode of operation.

I think it should also be kept in mind that the greater the temperature difference, the more potential energy becomes available to the engine.

In my experiments, simply putting insulation over the cold side of the engine improves performance. The RPM increases. Power output and torque increases.

I don't believe that this is due to retaining heat, it is due to preventing ambient heat from destroying the slight cooling the engine is able to muster. Protect the cold side of the engine from the surrounding ambient heat and it can gradually grow colder and colder increasing the temperature difference which makes more of the heat available for conversion to power output.

Putting insulation in the cold side has an effect similar to putting ice on the cold side, but rather than increasing the ∆T by an external application of cold, the ∆T is increased due to the engines own internal cooling, partly due to a heat pump-like action, but also due, primarily, to the conversion of heat into work. The heat "disappears" and re-emerges as the mechanical motion of the engine and the powering of a load.

Anyway, again, it should not be possible for an engine to violate a Law of the universe, simply due to a bad timing adjustment, if that were the case.

[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?

I'm waiting for your eureka moment when you realize that ALL closed cycle compression cycles require some type of heat sink !!! Gadz, even a crude Cayley type (double-diameter) requires a heat sink, tho typically results in (or nearly so) Wpos=Wneg, thus Wnet=0.

If you preaching no heat sink closed cycle, let's see some type of PV plot for such...

So you have appointed yourself as my personal debunker.

Anyway, I'm sure you've seen a real time PV diagram of a Stirling engine cycle before. Here's one example:

Resize_20221108_045355_5291.jpg (113.18 KiB) Viewed 9152 times

The actual readings tell the story. The horizontal line at 0 is atmospheric pressure There is a sharp drop in pressure around BDC (full expansion, piston all the way out, to the extreme right of the graph) and a presumed corresponding sharp drop in temperature. Nearly the entire return strike of the piston takes place with the working fluid below atmospheric pressure.

You look at this and imagine that isothermal expansion and compression are responsible due to heat intake and heat rejection, but that makes absolutely no sense in a real engine operating at any speed whatsoever.

Obviously the rapid, near instantaneous cooling and pressure drop are the result of rapid adiabatic expansion, not any slow heat conduction out of the engine to a heat sink. That much heat does not conduct anywhere in the fraction of a second it takes for the engine to complete a cycle, particularly when the internal temperature of the engine is below the external ambient temperature. Heat does not flow uphill from the cold expanded gas inside the engine to the warmer ambient air outside the engine

[VincentG]

Some would consider that you still have a regenerator or heat gradient in the space between the displacer and chamber walls. If you don't mind my adding some color to your whiteboard.

Of course not. I agree, that area can be viewed as a regenerator. Though the lack of relative surface area against the insulating displacer center and the cylinder(hopefully of low thermal conductivity and capacity itself) is meant to minimize regenerative effects. Also of great importance, I think, is to eliminate as much air space as possible when the displacer is at its resting position, thus reducing dead space.

but rather than increasing the ∆T by an external application of cold, the ∆T is increased due to the engines own internal cooling, partly due to a heat pump-like action, but also due, primarily, to the conversion of heat into work. The heat "disappears" and re-emerges as the mechanical motion of the engine and the powering of a load.

I am not at all discounting that the air temp is reduced as work is performed. Just that the further reduction in temp caused by volumetric expansion is purely consequential to the operation and should be negated by the same energy used for compression. Furthermore, I argue that energy used both for compression and expansion of the working gas is wasted. And that the swept volume of the working cylinder should be kept low compared to the volume of the displacer chamber.

[matt brown]

No Tom, you got me wrong. What I mean is for you to simply draw ANY closed cycle compression cycle on a piece of paper without cooling somewhere...

(1) isobaric compression requires cooling
(2) isothermal compression requires cooling

(3) adiabatic compression has no cooling but will require cooling (process) somewhere in cycle, otherwise it's a gas spring

I can doodle up lots of cycles with adiabatic compression (3, 4, 5 legged, whatever) but all these cycles will require a cooling process to return to start state. What I see you to be suggesting is something like a 3 leg cycle with adiabatic compression & expansion which meet at the 'tail' (aka BDC) on a PV plot and heat is supply at the 'head'. To avoid gas spring (and have any measure of Wpos>Wneg) expansion would have to exceed compression, somehow. How any heat would enter this cycle is what I'm wondering, even when allowing for 'irregular' volumetrics due to phasing. My point is that PV plots never lie in theory, but realities are always mere approximations. So again, show me some closed cycle compression cycle PV plot 'theory' with no cooling...

All early steam & HAE were open cycle. Watt's condenser closed steam, but HAE like Cayley, Stirling, Ericsson, etc continued with open cycle into obscurity. After Otto proved the advantage of compression cycles, open ICE & closed ECE became the favored formats. ICE have evolved a long way since Otto due to compression process, but ECE have remained bogged down by their closed cycle. Even Dean Kamen hasn't solved the ECE riddle...

[Tom Booth]

... I am not at all discounting that the air temp is reduced as work is performed. Just that the further reduction in temp caused by volumetric expansion is purely consequential to the operation and should be negated by the same energy used for compression. Furthermore, I argue that energy used both for compression and expansion of the working gas is wasted. And that the swept volume of the working cylinder should be kept low compared to the volume of the displacer chamber.

Perhaps you are right. There are some subtleties in the operation of the engine that probably can't be known for sure without a lot of trial and error, experimentation.

I do believe, however, or I have come to, or been led to the understanding, through information such as in this video:

https://youtu.be/PMKPZuCj9a0

that as long as the piston is moving outward and the gas expanding, as far as the gas or working fluid is concerned, the piston is a "moving target" so that when a gas molecule collides with the piston, moving away from it, the gas looses energy to the piston, does "work" on the piston and cools down as a consequence. The greater the cooling, the more "free" so to speak, power can be returned by atmospheric pressure. Atmosphere then does work on the gas, increasing the temperature, adding to the heat that would result from simple compression (or reducing the volume, putting the gas into a smaller space) alone. As a result of doing work during expansion/cooling, the greater the cooling, the greater the payback in work accomplished by atmospheric pressure, adding energy to the system. So for that reason, I don't think it is a simple matter of positive work out during expansion and negative work during compression When the gas is compressed by atmospheric pressure pushing the piston inward, that is not taking energy out of the system, it is adding energy to the system.

Having said all that, I have seen evidence that indicates a very short stroke on the power piston does have its advantages. In "NASA type" free piston engines the piston tends to "vibrate" or oscillate at a very high frequency. Take this engine, which has a pretty decent power output for what it is.

https://youtu.be/QcppEhp2RfA

What I also find interesting about this engine is how well it seems to keep itself cool, regardless of how long it is left running, and without a water jacket.