I don't want to be dismissive. I think you have a point. I still, though find this concept rather intriguing. Partly due to it being based on the old Atkinson engine. Part because it seems to be a proven method for rather dramatically increasing efficiency. Some of the hybrid electric vehicles using the Atkinson engine approach 80 mpg:
More recently as a stand alone Atkinson:
https://newatlas.com/toyota-atkinson-en ... ncy/31615/
This video (below) and some other videos have me thinking that some kind of port or other pressure relief mechanism might be possible in an external combustion type engine.
Apparently a variation? of the Atkinson cycle (I don't really know anything about it, but this valve aspect is not always mentioned) is to have the intake valve stay open part way through the compression cycle which effectively shortens the compression stroke making the expansion stroke relatively longer.
Anyway I like this guy's attitude
https://youtu.be/gE7B_I00bDY?si=sSpcEVkv2250G1Uj
Aligning heat "vectors"
Re: Aligning heat "vectors"
The monotherm involves ordinary heat of compression and adiabatic expansion cooling. Quite "ordinary" ideal gas behavior.
Joule Thomson cooling is something different involving throttling through a valve or orifice, a slight departure from "ideal".
Comparatively expansion-work, that is, gas expanding and doing actual "shaft work" driving a piston with an external load results in a much greater temperature drop than either Joule Thomson or adiabatic expansion alone.
Generally speaking, thermodynamics doesn't seem to even have terminology to describe the actual conversion of heat into work that that involves.
Re: Aligning heat "vectors"
This is again from Wiki, though maybe not a direct comparison, I think it can be extended to explain these effects.The adiabatic (no heat exchanged) expansion of a gas may be carried out in a number of ways. The change in temperature experienced by the gas during expansion depends not only on the initial and final pressure, but also on the manner in which the expansion is carried out.
If the expansion process is reversible, meaning that the gas is in thermodynamic equilibrium at all times, it is called an isentropic expansion. In this scenario, the gas does positive work during the expansion, and its temperature decreases.
In a free expansion, on the other hand, the gas does no work and absorbs no heat, so the internal energy is conserved. Expanded in this manner, the temperature of an ideal gas would remain constant, but the temperature of a real gas decreases, except at very high temperature.[10]
The method of expansion discussed in this article, in which a gas or liquid at pressure P1 flows into a region of lower pressure P2 without significant change in kinetic energy, is called the Joule–Thomson expansion. The expansion is inherently irreversible. During this expansion, enthalpy remains unchanged (see proof below). Unlike a free expansion, work is done, causing a change in internal energy. Whether the internal energy increases or decreases is determined by whether work is done on or by the fluid; that is determined by the initial and final states of the expansion and the properties of the fluid.
The temperature change produced during a Joule–Thomson expansion is quantified by the Joule–Thomson coefficient. This coefficient may be either positive (corresponding to cooling) or negative (heating)
The Monotherm concept is to me classified under this description. There is no "work" produced from its cycle, as would be classically described.
So we are aiming to capture the sum of this temperature delta(be it heat or cooling) produced by constant volume adiabatic heat addition(or subtraction) of ideal gasses , in addition to the extra temperature gain(or loss) from the behavior of real gasses during this constant volume adiabatic process.
Re: Aligning heat "vectors"
That Wiki passage covers several different topics:
"The adiabatic (no heat exchanged) expansion of a gas may be carried out in a number of ways."
The last part: "The method of expansion discussed in this article..." is meant to differentiate Joule Thomson expansion from the other types of expansion.
So when you say: "This is again from Wiki, though maybe not a direct comparison, I think it can be extended to explain these effects." It is not really clear what you might be referring to in that passage that "can be extended" as it summarizes several different topics.
I am also not sure what you mean by "...constant volume adiabatic heat addition(or subtraction)".
This appears to be self-contradictory
When you say: "So we are aiming to capture the sum of this temperature delta(be it heat or cooling) ..." are you referring to the topic heading:
"Aligning heat 'vectors'"?
I think I would have to say no. The monothermal thing and Joule Thomson are really unrelated topics that basically have little or no bearing on the heating and cooling effects in a Stirling engine.
"The adiabatic (no heat exchanged) expansion of a gas may be carried out in a number of ways."
The last part: "The method of expansion discussed in this article..." is meant to differentiate Joule Thomson expansion from the other types of expansion.
So when you say: "This is again from Wiki, though maybe not a direct comparison, I think it can be extended to explain these effects." It is not really clear what you might be referring to in that passage that "can be extended" as it summarizes several different topics.
I am also not sure what you mean by "...constant volume adiabatic heat addition(or subtraction)".
This appears to be self-contradictory
When you say: "So we are aiming to capture the sum of this temperature delta(be it heat or cooling) ..." are you referring to the topic heading:
"Aligning heat 'vectors'"?
I think I would have to say no. The monothermal thing and Joule Thomson are really unrelated topics that basically have little or no bearing on the heating and cooling effects in a Stirling engine.
Re: Aligning heat "vectors"
To be fair, I'm probably responsible for the confusion in referring to the monothermal contraption as utilizing "adiabatic" expansion/cooling.
The monothermal thing, or anything I suppose, involving a regenerator and/or displacer can be confusing as there are different "volumes" that are really connected into one volume.
I often see reference to "expansion space" and "contraction space" or "hot volume" and "cold volume", displacer volume vs. Power piston volume etc. That are actually just parts of one total volume.
The monothermal thing is particularly confusing in that respect as within a single volume you have heat addition in one section causing expansion which simultaneously compresses the gas in another section. Then cooling that causes contraction of the gas in one section but "adiabatic expansion" in another section.
If you cool one end of a cylinder, the gas in the other end will expand as the gas in the cold end contracts.
As in actuality, heat IS being added or removed from the TOTAL volume, calling anything "adiabatic" in that scenario seems contradictory.
However the expansion in one section would be "adiabatic" as far as that semi-isolated section is concerned.
If for example I have two springs attached end to end between fixed points, if one spring contracts, the other spring will be forced to expand. Taken together however, the two springs could be viewed as one long continuous spring.
Generally, with such difficult concepts, it's probably best to avoid generalizations.
I said Joule Thomson has no bearing on Stirling engine operation generally, but it actually might in a "thermal-acoustic" type engine with the gas passing back and forth through a narrow orifice.
Tossing all these different things together into one big bowl of soup doesn't really get us anywhere.
The monothermal thing, or anything I suppose, involving a regenerator and/or displacer can be confusing as there are different "volumes" that are really connected into one volume.
I often see reference to "expansion space" and "contraction space" or "hot volume" and "cold volume", displacer volume vs. Power piston volume etc. That are actually just parts of one total volume.
The monothermal thing is particularly confusing in that respect as within a single volume you have heat addition in one section causing expansion which simultaneously compresses the gas in another section. Then cooling that causes contraction of the gas in one section but "adiabatic expansion" in another section.
If you cool one end of a cylinder, the gas in the other end will expand as the gas in the cold end contracts.
As in actuality, heat IS being added or removed from the TOTAL volume, calling anything "adiabatic" in that scenario seems contradictory.
However the expansion in one section would be "adiabatic" as far as that semi-isolated section is concerned.
If for example I have two springs attached end to end between fixed points, if one spring contracts, the other spring will be forced to expand. Taken together however, the two springs could be viewed as one long continuous spring.
Generally, with such difficult concepts, it's probably best to avoid generalizations.
I said Joule Thomson has no bearing on Stirling engine operation generally, but it actually might in a "thermal-acoustic" type engine with the gas passing back and forth through a narrow orifice.
Tossing all these different things together into one big bowl of soup doesn't really get us anywhere.
Re: Aligning heat "vectors"
There is another Atkinson engine that has opposed cylinders called a differential Atkinson engine.
First, I found this video that does a good job of explaining the difference between the original Atkinson linkage that resulted in an extended expansion stroke and the, I think, new method of simply delaying the closing of the intake valve to effectively make the compression stroke shorter.
https://youtu.be/ucGRC7apI28?si=MrmNAB9USXIXLvu1
The Atkinson differential engine has two opposed pistons, though only one actually acts as a power piston, the other, I believe, acts as a kind of follower that varies the cylinder length.
The second opposed piston, you could think of as the cylinder head. But the "cylinder head" moves in such a way as to make the cylinder longer during expansion and effectively shorter during compression, though in reality, the two pistons work together to accomplish this.
The Atkinson is, however, a four stroke internal combustion engine with intake and exhaust as well as expansion and compression, so half of its functions would not be applicable to an internal combustion adaptation.
An Alpha Stirling, though, is, in a sense, an Opposed piston engine, just not opposed at 180° but at a 90° and not in the same cylinder but connected through the regenerator.
There were/are some opposed piston hot air engines, the Essex for example, though really opposed piston and displacer I guess.
Anyway, I have an idea that maybe this variable length cylinder, opposed piston, extended expansion Atkinson concept could be applied to a more or less Alpha or some such external combustion Stirling type engine
I would think that if there are at least three ways Atkinson managed to accomplish this with his internal combustion engines, there should be one or more ways to accomplish an extended expansion stroke in an external combustion hot air engine as well.
Why do this?
Well for one thing, I've always thought that the Stirling engines was a potential candidate for powering hybrid electric cars. The Atkinson does this very well the high efficiency, low power steady running engine and electric motor making a good combination.
Stirling engines already have very good efficiency, at least in theory. Could the efficiency be improved further with this extended expansion concept? I don't see any reason why not.
I'm not all that interested in high power, high torque applications. I would like something that could just provide battery charging to an off grid power supply. Something that would run steady and efficiently to keep the batteries topped off.
This, according to the title is a two stroke Atkinson Diesel engine, it apparently runs quite cool as he can put his hand on the exhaust muffler without any problem and comments on how cool the engine runs. Testimony to the high efficiency of this engine I think.
https://youtu.be/1xjz1wwaNeI?si=jm7NRCizh3xPIGzT
Here are a few Atkinson differential engine videos explaining the mechanism.
https://youtu.be/fgcOYEpKrMY?si=s-PISRSCkQ8_KCZn
Not sure what the issue might have been with this engine actually running, mentioned at the end of the video, but some good detail of the piston motion.
https://youtu.be/Z4JfejLWcso?si=-RbJxHcSGLSTq8Tu
During expansion the power piston moves quickly and independently to the right, but during compression the "follower" piston catches up and both pistons move together to the left during compression.
If instead of a spark plug, the left end of the cylinder was heated, like an Alpha, the pistons would together move the air over to the hot "ignition" zone, then the power piston would expand independently to the right while the left "displacer" piston moves part way to the right to cover the hot cylinder walls, ending heat input in preparation for "contraction".
It almost appears that this linkage could actually work almost without modification, adapted for external combustion.
First, I found this video that does a good job of explaining the difference between the original Atkinson linkage that resulted in an extended expansion stroke and the, I think, new method of simply delaying the closing of the intake valve to effectively make the compression stroke shorter.
https://youtu.be/ucGRC7apI28?si=MrmNAB9USXIXLvu1
The Atkinson differential engine has two opposed pistons, though only one actually acts as a power piston, the other, I believe, acts as a kind of follower that varies the cylinder length.
The second opposed piston, you could think of as the cylinder head. But the "cylinder head" moves in such a way as to make the cylinder longer during expansion and effectively shorter during compression, though in reality, the two pistons work together to accomplish this.
The Atkinson is, however, a four stroke internal combustion engine with intake and exhaust as well as expansion and compression, so half of its functions would not be applicable to an internal combustion adaptation.
An Alpha Stirling, though, is, in a sense, an Opposed piston engine, just not opposed at 180° but at a 90° and not in the same cylinder but connected through the regenerator.
There were/are some opposed piston hot air engines, the Essex for example, though really opposed piston and displacer I guess.
Anyway, I have an idea that maybe this variable length cylinder, opposed piston, extended expansion Atkinson concept could be applied to a more or less Alpha or some such external combustion Stirling type engine
I would think that if there are at least three ways Atkinson managed to accomplish this with his internal combustion engines, there should be one or more ways to accomplish an extended expansion stroke in an external combustion hot air engine as well.
Why do this?
Well for one thing, I've always thought that the Stirling engines was a potential candidate for powering hybrid electric cars. The Atkinson does this very well the high efficiency, low power steady running engine and electric motor making a good combination.
Stirling engines already have very good efficiency, at least in theory. Could the efficiency be improved further with this extended expansion concept? I don't see any reason why not.
I'm not all that interested in high power, high torque applications. I would like something that could just provide battery charging to an off grid power supply. Something that would run steady and efficiently to keep the batteries topped off.
This, according to the title is a two stroke Atkinson Diesel engine, it apparently runs quite cool as he can put his hand on the exhaust muffler without any problem and comments on how cool the engine runs. Testimony to the high efficiency of this engine I think.
https://youtu.be/1xjz1wwaNeI?si=jm7NRCizh3xPIGzT
Here are a few Atkinson differential engine videos explaining the mechanism.
https://youtu.be/fgcOYEpKrMY?si=s-PISRSCkQ8_KCZn
Not sure what the issue might have been with this engine actually running, mentioned at the end of the video, but some good detail of the piston motion.
https://youtu.be/Z4JfejLWcso?si=-RbJxHcSGLSTq8Tu
During expansion the power piston moves quickly and independently to the right, but during compression the "follower" piston catches up and both pistons move together to the left during compression.
If instead of a spark plug, the left end of the cylinder was heated, like an Alpha, the pistons would together move the air over to the hot "ignition" zone, then the power piston would expand independently to the right while the left "displacer" piston moves part way to the right to cover the hot cylinder walls, ending heat input in preparation for "contraction".
It almost appears that this linkage could actually work almost without modification, adapted for external combustion.
Re: Aligning heat "vectors"
An interesting passage from the Wikipedia article reads:
I suppose in that case, where the working fluid "contracts" or is pushed in by atmospheric pressure, the effective compression phase would not actually begin until the pressure equals atmospheric.
In this PV graph, that does not happen until the piston has returned about 4/5 ths of the way back to TDC !
Assuming a standard atmospheric pressure of 101.325 kPa
Interesting in that at least some Stirling engines apparently expand to a pressure BELOW atmospheric pressure.The goal of the modern Atkinson cycle is to make the pressure in the combustion chamber at the end of the power stroke equal to atmospheric pressure. When this occurs, all available energy has been obtained from the combustion process. For any given portion of air, the greater expansion ratio converts more energy from heat to useful mechanical energy—meaning the engine is more efficient.
I suppose in that case, where the working fluid "contracts" or is pushed in by atmospheric pressure, the effective compression phase would not actually begin until the pressure equals atmospheric.
In this PV graph, that does not happen until the piston has returned about 4/5 ths of the way back to TDC !
- Resize_20230522_110819_9316.jpg (272.58 KiB) Viewed 8993 times
Assuming a standard atmospheric pressure of 101.325 kPa
Re: Aligning heat "vectors"
Comparing the Alpha Stirling with the Atkinson differential engine,, in this position (turning.clockwise):
The hot and cold pistons are moving together in the same way as the Atkinson which theoretically could have the same. "differential" effect of reducing compression. A corresponding lengthening of expansion is also evident later in the cycle.
The problem that arises with the Alpha is that there is no concise ignition. Rather, as soon as; what Atkinson calls in his patent, the "pump piston", or what I might term the hot "displacer" piston or "heat valve" starts to move, it also begins to expose the working fluid to tbe hot cylinder walls. This, likely, I would imagine, expands the working fluid prematurely nullifying the potential "differential" or compression easing effect.
This might be at least partially remedied by leaving an unheated collar for the hot piston to move through before heating begins, keeping the heated zone further out towards the end of the hot cylinder so heat input is delayed..
As already suggested, a dwell mechanism would delay heat input, but lacks the differential effect.
I have an idea, which is really a complete departure from the Alpha.
A kind of single cylinder opposed piston Ringbom.
As the power piston executes the compression stroke it approaches the opposed "displacer" piston. As pressure increases the "displacer" piston moves away, easing up compression having an Atkinson engine-like "differential" effect. The power piston and displacer piston then move together into the hot zone relatively briefly. Once the displacer runs out of room, the hot zone is fully exposed, final compression completes and the power piston begins expansion.
As expansion proceeds, pressure reduces and the displacer piston can return to discontinue heat input as expansion continues.
I did find an example of a single cylinder Ringbom engine demonstrated by Blade Attila:
https://youtu.be/MIS11qIoM8I?si=1Q57n-0jGAM5g28D
While not opposed piston exactly, I think this at least demonstrates that a single cylinder Ringbom is possible.
I could not find any examples of an actual opposed piston Ringbom. I'm not sure why, it seems like a no brainier.
The only thing I can think of is that such an arrangement has been avoided as it leaves no room for a cold side, which has formerly been considered an essential feature.
- Resize_20230929_094023_3058.jpg (76.94 KiB) Viewed 8972 times
The problem that arises with the Alpha is that there is no concise ignition. Rather, as soon as; what Atkinson calls in his patent, the "pump piston", or what I might term the hot "displacer" piston or "heat valve" starts to move, it also begins to expose the working fluid to tbe hot cylinder walls. This, likely, I would imagine, expands the working fluid prematurely nullifying the potential "differential" or compression easing effect.
This might be at least partially remedied by leaving an unheated collar for the hot piston to move through before heating begins, keeping the heated zone further out towards the end of the hot cylinder so heat input is delayed..
As already suggested, a dwell mechanism would delay heat input, but lacks the differential effect.
I have an idea, which is really a complete departure from the Alpha.
A kind of single cylinder opposed piston Ringbom.
As the power piston executes the compression stroke it approaches the opposed "displacer" piston. As pressure increases the "displacer" piston moves away, easing up compression having an Atkinson engine-like "differential" effect. The power piston and displacer piston then move together into the hot zone relatively briefly. Once the displacer runs out of room, the hot zone is fully exposed, final compression completes and the power piston begins expansion.
As expansion proceeds, pressure reduces and the displacer piston can return to discontinue heat input as expansion continues.
I did find an example of a single cylinder Ringbom engine demonstrated by Blade Attila:
https://youtu.be/MIS11qIoM8I?si=1Q57n-0jGAM5g28D
While not opposed piston exactly, I think this at least demonstrates that a single cylinder Ringbom is possible.
I could not find any examples of an actual opposed piston Ringbom. I'm not sure why, it seems like a no brainier.
The only thing I can think of is that such an arrangement has been avoided as it leaves no room for a cold side, which has formerly been considered an essential feature.
Re: Aligning heat "vectors"
Something that should be noted about Blade Attila's single cylinder Ringbom is that the Ringbom mechanism is entirely internal, effectively then, the Ringbom displacer acts as an internal "air spring" without being in any way associated with external atmospheric pressure.
This is very similar to what I was trying to achieve with my portable generator conversion using a displacer with an air spring:
viewtopic.php?f=1&t=5535#p19410
This may perhaps all be achieved more easily by simply turning the displacer around and putting the air spring behind the displacer (relative to the piston) in an Atkinson opposed piston differential engine type arrangement.
This is very similar to what I was trying to achieve with my portable generator conversion using a displacer with an air spring:
viewtopic.php?f=1&t=5535#p19410
This may perhaps all be achieved more easily by simply turning the displacer around and putting the air spring behind the displacer (relative to the piston) in an Atkinson opposed piston differential engine type arrangement.
Re: Aligning heat "vectors"
Combining a few ideas:
Something like this maybe:
(I had put the heat tubes in a location I had not intended but didn't bother redrawing)
The illustration is intended to show the engine at or near full compression or TDC.
The forked thing with the spring attached is the Ringbom-ish displacer/opposed piston/Atkinson differential compression easing mechanism/air spring.
The other darker forked thing closer to the power piston is stationary, mostly just to take up "dead air space" but also might add some additional surface area for heat input. The face adjacent to the power cylinder should be non-heat conducting or of some material with heat insulating properties. The inner tines could hold heat. The horizontal lines are holes for air flow between the power cylinder and the heating chamber (area between and within all the forked tines.
The hot space is closed, the tines meshed together so that the hot space, essentially does not exist until the power piston compresses the working fluid sufficiently to actuate the "air spring" moving the forked displacer thing back
Now that I look at this, it is quite similar to the "hot potato" engine proposed years ago:
viewtopic.php?f=1&t=5535#p19410
The newly designed "potato" however has less internal "dead air space" until opened (actuated) by increased pressure when the piston reaches TDC. Also the "air spring" air is isolated from the working fluid in the power cylinder.
Something like this maybe:
(I had put the heat tubes in a location I had not intended but didn't bother redrawing)
- Resize_20230929_120924_4460.jpg (99.58 KiB) Viewed 8960 times
The forked thing with the spring attached is the Ringbom-ish displacer/opposed piston/Atkinson differential compression easing mechanism/air spring.
The other darker forked thing closer to the power piston is stationary, mostly just to take up "dead air space" but also might add some additional surface area for heat input. The face adjacent to the power cylinder should be non-heat conducting or of some material with heat insulating properties. The inner tines could hold heat. The horizontal lines are holes for air flow between the power cylinder and the heating chamber (area between and within all the forked tines.
The hot space is closed, the tines meshed together so that the hot space, essentially does not exist until the power piston compresses the working fluid sufficiently to actuate the "air spring" moving the forked displacer thing back
Now that I look at this, it is quite similar to the "hot potato" engine proposed years ago:
viewtopic.php?f=1&t=5535#p19410
- hot_potato_engine.gif (72.02 KiB) Viewed 8960 times
Re: Aligning heat "vectors"
One more point about this phase in an Alpha:
Regarding the concept of "aligning the heat vectors"
The goal would be to have the maximum heat input and the maximum heat of compression coincide at TDC. With sinusoidal motion, the closest approach to that goal is to start adding heat 90° early so that heat input can peak around TDC. This however, results in some trade off. The cold piston is compressing but the hot piston is already adding heat resulting in expansion.
Theoretically, if the hot piston could "dwell" a bit longer, not adding heat until closer to TDC and then expose the heat to the working fluid much more quickly the engine would work against itself less..
If the animation is obsererved carefully, a similar situation occurs just after BDC. The "heat valve" stays open too long. Then the cold piston is already starting compression/contraction but the hot piston is still working on shutting down the heat supply. Again, with sinusoidal motion that is about the best that can be achieved. It would be much better if the heat input could have been completely shut off by the time the cold power piston crossed BDC at the latest!
In fact, the hot piston is already adding heat AGAIN before the cold power piston can complete the compression stroke! The hot piston barely even takes a break.
Needless to say, the situation is the same for all Stirling engines with sinusoidal motion and a 90° phase angle between the power piston and the displacer/heat valve.
Regarding the concept of "aligning the heat vectors"
- Resize_20230929_204056_6751.jpg (116.74 KiB) Viewed 8950 times
The goal would be to have the maximum heat input and the maximum heat of compression coincide at TDC. With sinusoidal motion, the closest approach to that goal is to start adding heat 90° early so that heat input can peak around TDC. This however, results in some trade off. The cold piston is compressing but the hot piston is already adding heat resulting in expansion.
Theoretically, if the hot piston could "dwell" a bit longer, not adding heat until closer to TDC and then expose the heat to the working fluid much more quickly the engine would work against itself less..
If the animation is obsererved carefully, a similar situation occurs just after BDC. The "heat valve" stays open too long. Then the cold piston is already starting compression/contraction but the hot piston is still working on shutting down the heat supply. Again, with sinusoidal motion that is about the best that can be achieved. It would be much better if the heat input could have been completely shut off by the time the cold power piston crossed BDC at the latest!
- ezgif.com-gif-maker (1).gif (289.92 KiB) Viewed 8950 times
Needless to say, the situation is the same for all Stirling engines with sinusoidal motion and a 90° phase angle between the power piston and the displacer/heat valve.