I'm still just learning about stirling engines, and i'm curious as to the need for the compression cycle.
As I understand it, the compression cycle takes power that you have previously generated (at < 100% efficiency) and uses it to compress/expand air (at < 100% efficiency) which either heats it while it's being cooled, or cools it while it's being heated.
It seems like this extra compression/expansion is more a side-effect of the flywheel design than an integral part of the engine.
I would picture the ideal operation being that the compression/expansion would push or pull the piston, and then some of that extra energy would move both pistons simultaneously (ie unloaded.. no pressure) to move the air between chambers... in the case of a beta stirling, it would move the displacer.
Do i have it wrong? why the compression cycles?
-tmk
Compression cycle.. worthwhile?
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alpha stirling
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Under ideal conditions there is little actual pressure rise inside a stirling engine under the compression stroke. The compression stroke happens when the piston is moving inwards, but in the meanwhile the air temperature in the engine is decreased (by most of the air being on the cold side). This actually decreases the power needed for the compression, but doesn't eliminate it totally (nothing is perfect in our world).
The compression is actually beneficial to powerfull stirlings: When the air is compressed at the end of the compression stroke, it fits in smaller space. When the compressed air meets the hot heat exhanger, the expansion will have much more kick to be turned into work by the piston. Furthermore, If you pressurise your engine (ie. under normal conditions the working fluid has...say 10 bars overpressure, the power output of the engine will be much higher compared to a non-pressurised version.
So the compression is one important part of the stirling cycle, and by eliminating it you'll eliminate your engine's chances of running...
The compression is actually beneficial to powerfull stirlings: When the air is compressed at the end of the compression stroke, it fits in smaller space. When the compressed air meets the hot heat exhanger, the expansion will have much more kick to be turned into work by the piston. Furthermore, If you pressurise your engine (ie. under normal conditions the working fluid has...say 10 bars overpressure, the power output of the engine will be much higher compared to a non-pressurised version.
So the compression is one important part of the stirling cycle, and by eliminating it you'll eliminate your engine's chances of running...
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alpha stirling
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- Joined: Sun Apr 29, 2007 9:24 am
... what if
What if the timing was changed slightly, and we could move the displacer quickly at set points in the cycle, rather than a fixed offset from the power piston..alpha stirling wrote:The compression stroke happens when the piston is moving inwards, but in the meanwhile the air temperature in the engine is decreased (by most of the air being on the cold side).
Eg when the power-piston is at/near its highest point, we move the displacer to push the air to the cold side. At this point the air will contract, and *pull* the power piston down, gaining energy instead of spending it compressing the gas.
I realize that the compression/expansion does 'give back' later in the cycle, but i think some energy is lost in the process.
-tmk
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alpha stirling
- Posts: 35
- Joined: Sun Apr 29, 2007 9:24 am
This (I quess) Have been tried by several groups. I quess one japanese group actually got a working engine. I have no information about the performance...
I also have lost the document where I had all of the equations to describe the stirling cycle in mathematical ways. Would have been possible to test this idea in theory... maybe.
I quess this question will stay unanswered until someone builds a prototype engine with measurable properties...
I also have lost the document where I had all of the equations to describe the stirling cycle in mathematical ways. Would have been possible to test this idea in theory... maybe.
I quess this question will stay unanswered until someone builds a prototype engine with measurable properties...
Correct me if I'm wrong but I think a Stirling that is running has almost no compression since it is basically a one stroke engine cycle. Hot pushing out and cold drawing back in. However, a cold, not running Stirling will have compression when it's spun by hand due to it being a sealed system. You will actualy have a vacuun and compression force at this time unless you engine leaks. If you remove your power piston connecting rod and heat your displacer you will see the power piston move up and down (remember "free" piston Stirling's?) when you spin the flywheel by hand. This is actually a good test if your having trouble getting your Stirling to run for the first time. So my point is, you should have compression with a cold engine and if you don't, it may not run at all when heated.
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alpha stirling
- Posts: 35
- Joined: Sun Apr 29, 2007 9:24 am
The terms "Compression" and "compression" are amazingly similar. In the internal combustion engine compression really means compressing air and fuel mixture into high pressure to allow it to burn efficiently. In the stirling engine, however, compression occuer when the working fluid is cooled, making it actually contract. The compression here doens't mean the rise of pressure unlike in the internal combustion engines. On the contrary, in stirling engine compression means that the working fluid's VOLUME decreases, pulling the power piston inwards...
ahh.. i must have misunderstood
I was under the impression that the '90degrees out of phase' orientation of the power piston vs the displacer actually was meant to cause some 'real' compression, for example take this animation:
http://www.grc.nasa.gov/WWW/tmsb/stirli ... nim_6.html
The displacer is shown to be stationary while the air is on the hot side, but at the same time, the power piston is moving downwards which (in a well sealed engine) should cause a vacuum and a cooling effect, which the hot side is supposed to counteract by adding heat and causing the gas to expand, equalizing the pressure.
Same deal with the cold cycle, only in reverse (power piston compresses = heats/dissapated by cold)
I saw one article that said the ideal stirling should be isothermic, ie the working gas temperature never changes. This would certainly require compression/expansion.
Cheers,
-tmk
http://www.grc.nasa.gov/WWW/tmsb/stirli ... nim_6.html
The displacer is shown to be stationary while the air is on the hot side, but at the same time, the power piston is moving downwards which (in a well sealed engine) should cause a vacuum and a cooling effect, which the hot side is supposed to counteract by adding heat and causing the gas to expand, equalizing the pressure.
Same deal with the cold cycle, only in reverse (power piston compresses = heats/dissapated by cold)
I saw one article that said the ideal stirling should be isothermic, ie the working gas temperature never changes. This would certainly require compression/expansion.
Cheers,
-tmk
Re: Compression cycle.. worthwhile?
A very old but still ongoing debate.
One thing not addressed in the above exchange is the "vacuum" produced to initiate the return "power stroke" that is cooling by the displacer shifting the working fluid to the cold side causing the gas to "contract", being assisted or augmented by the very conversion of heat into work which is the function of a heat engine in the first place.
I wonder why this elimination of heat through "conversion" is so often left out of the discussion as if an insignificant or even non-existent, unrecognized factor.
Since the invention of the Claude cycle expansion engine it has been recognized by those familiar with the process, that cooling or the removal of heat for gas liquefaction purposes is MUCH more effective when adiabatic expansion of a gas is accompanied by actual external shaft work.
That is, the expanding gas does not just undergo "free expansion" i.e. Joule-Thomson cooling, but simultaneously drives a load (is made to do additional work as it expands).
This kind of compression followed by expansion cooling, with work output is how cryogenic temperatures are reached to liquify the most difficult to liquify gases.
"Ah..., that doesn't apply to Stirling engines", I've been told again and again.
If not, why not. An expanding gas doing work is an expanding gas doing work. How is an "expansion engine" different from a Stirling engine or vice versa.
Both have a piston and cylinder driven by a previously compressed gas expanding and doing work.
Granted, the compression stage in a gas liquefaction engine involves very high pressure. The more the gas is compressed the more "work" can be extracted during subsequent expansion.
The goal of a Stirling engine is not cryogenic cold to liquify the working fluid but the work output, in principle however, the same process, just not so extreme and with a different goal in mind.
The shifting of the gas to the cold side by the displacer is I think, almost completely ineffectual in comparison to cooling via CONVERSION of heat into work output.
One thing not addressed in the above exchange is the "vacuum" produced to initiate the return "power stroke" that is cooling by the displacer shifting the working fluid to the cold side causing the gas to "contract", being assisted or augmented by the very conversion of heat into work which is the function of a heat engine in the first place.
I wonder why this elimination of heat through "conversion" is so often left out of the discussion as if an insignificant or even non-existent, unrecognized factor.
Since the invention of the Claude cycle expansion engine it has been recognized by those familiar with the process, that cooling or the removal of heat for gas liquefaction purposes is MUCH more effective when adiabatic expansion of a gas is accompanied by actual external shaft work.
That is, the expanding gas does not just undergo "free expansion" i.e. Joule-Thomson cooling, but simultaneously drives a load (is made to do additional work as it expands).
This kind of compression followed by expansion cooling, with work output is how cryogenic temperatures are reached to liquify the most difficult to liquify gases.
"Ah..., that doesn't apply to Stirling engines", I've been told again and again.
If not, why not. An expanding gas doing work is an expanding gas doing work. How is an "expansion engine" different from a Stirling engine or vice versa.
Both have a piston and cylinder driven by a previously compressed gas expanding and doing work.
Granted, the compression stage in a gas liquefaction engine involves very high pressure. The more the gas is compressed the more "work" can be extracted during subsequent expansion.
The goal of a Stirling engine is not cryogenic cold to liquify the working fluid but the work output, in principle however, the same process, just not so extreme and with a different goal in mind.
The shifting of the gas to the cold side by the displacer is I think, almost completely ineffectual in comparison to cooling via CONVERSION of heat into work output.
Re: Compression cycle.. worthwhile?
An interesting video on adiabatic expansion-cooling.
https://youtu.be/4GL2jj6XJGk
Note however, these demonstrations do not involve expanding the gas inside an engine cylinder to drive a piston to do shaft work, which is much more effective.
The only "work" involved in these demonstrations in the video is the relatively minor work of pushing air out of the way as the gas expands. Doing real work in an engine cylinder to drive the engine and an attached load, such as a generator, takes much more internal energy out of the gas resulting in much colder temperatures, potentially to near absolute zero for the liquefaction of helium.
https://youtu.be/4GL2jj6XJGk
Note however, these demonstrations do not involve expanding the gas inside an engine cylinder to drive a piston to do shaft work, which is much more effective.
The only "work" involved in these demonstrations in the video is the relatively minor work of pushing air out of the way as the gas expands. Doing real work in an engine cylinder to drive the engine and an attached load, such as a generator, takes much more internal energy out of the gas resulting in much colder temperatures, potentially to near absolute zero for the liquefaction of helium.
Re: Compression cycle.. worthwhile?
The gas liquefaction process is pretty simple.
1) Cool and compress the gas.
2) Expand the gas while making it do work.
Well for a power producing engine we really don't want to throw the heat/energy away to just make cold to liquify the gas, we want to get the work out of the gas as it expands.
If only there were a way to cool the gas without throwing away the heat.
That would make the gas easy to compress while it is cold.
But if we just move the heat to a 'sink" to cool it, the energy is lost.
What if we could somehow cool the gas to make it easier to compress, but somehow at the same time save the heat so we can put it back to expand it more rapidly and get work out of that heat as the gas expands driving a piston.
Then the expansion and work output would help cool it right back down so it can be easily compressed again.
But how could we temporarily take heat out of the gas, store it somewhere and then put it back in again?
Hmmm... I wonder.
1) Cool and compress the gas.
2) Expand the gas while making it do work.
Well for a power producing engine we really don't want to throw the heat/energy away to just make cold to liquify the gas, we want to get the work out of the gas as it expands.
If only there were a way to cool the gas without throwing away the heat.
That would make the gas easy to compress while it is cold.
But if we just move the heat to a 'sink" to cool it, the energy is lost.
What if we could somehow cool the gas to make it easier to compress, but somehow at the same time save the heat so we can put it back to expand it more rapidly and get work out of that heat as the gas expands driving a piston.
Then the expansion and work output would help cool it right back down so it can be easily compressed again.
But how could we temporarily take heat out of the gas, store it somewhere and then put it back in again?
Hmmm... I wonder.
Re: Compression cycle.. worthwhile?
The energy represented on a PV diagram depends on path, and the direction traveled on the path. The initial difference between a Stirling Engine and a Stirling cooler is the direction taken on the path. Typically clockwise for an engine representing energy-out equal to the area. And for a cooler, counter clockwise, with energy-in equal to the area.Tom Booth wrote:"Ah..., that doesn't apply to Stirling engines", I've been told again and again.
If not, why not.
The heats travel in opposite directions.
.
- Screenshot_20211119-102348.jpg (48.32 KiB) Viewed 23 times
I could not find a stiling cooler real indicator diagram so will try to guess at how it differs from above ideal and real engine diagrams. The real engine diagram is completely inside the ideal. I believe the cooler will be outside the real diagram. I don't know if it will be completely outside the ideal, or if it will cut the corners.
The larger area of the cooler is the extra energy that would be needed, thermodynamically, to power the engine cooler combination. The temperature gaps between the ideal and reals are needed for heat transfer. Volume top, and bottom, dead center will be approximately the same volume for all three.
The other possibility is the the cooler would be inside the real engine diagram. That would make the cooler ineffective at providing a cold hole. It wouldn't work as desired.
Re: Compression cycle.. worthwhile?
An admirable pile of guesswork. It doesn't answer the question though.Fool wrote: ↑Wed May 08, 2024 8:30 amThe energy represented on a PV diagram depends on path, and the direction traveled on the path. The initial difference between a Stirling Engine and a Stirling cooler is the direction taken on the path. Typically clockwise for an engine representing energy-out equal to the area. And for a cooler, counter clockwise, with energy-in equal to the area.Tom Booth wrote:"Ah..., that doesn't apply to Stirling engines", I've been told again and again.
If not, why not.
The heats travel in opposite directions.
.
Screenshot_20211119-102348.jpg.
I could not find a stiling cooler real indicator diagram so will try to guess at how it differs from above ideal and real engine diagrams. The real engine diagram is completely inside the ideal. I believe the cooler will be outside the real diagram. I don't know if it will be completely outside the ideal, or if it will cut the corners.
The larger area of the cooler is the extra energy that would be needed, thermodynamically, to power the engine cooler combination. The temperature gaps between the ideal and reals are needed for heat transfer. Volume top, and bottom, dead center will be approximately the same volume for all three.
The other possibility is the the cooler would be inside the real engine diagram. That would make the cooler ineffective at providing a cold hole. It wouldn't work as desired.