As a result of a recent video I posted on YouTube, I received some comments to the effect that the engine depicted in the video was NOT a Stirling cycle engine because it does not have a regenerator or "economizer".
For reasons not entirely known to me, the comments, along with my responses disappeared.
What do you think? Is this a "Stirling engine" or a "Hot Air" engine only?
https://youtu.be/R_QB5amihko
"Stirling engine" or "Hot Air engine"?
Re: "Stirling engine" or "Hot Air engine"?
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Why do you say that. Other than the orientation and size of the engine, It appears to be virtually identical to Robert Stirling's original model:
https://youtu.be/FZVOnw0nlWc
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Re: "Stirling engine" or "Hot Air engine"?
This page, http://hotairengines.org/closed-cycle-e ... rling-1816 which reproduces portions of Robert Stirling' patent, much of it in his own words and diagrams, depicts the regenerator in various ways, some as a simple "passage":
Further, Stirling's original patent drawing or engraving depict the regenerator or "economiser" as "the annular space between the displacer and the cylinder."
The page also relates: "The economiser (not shown in the engraving) is in the annular space between the displacer and the cylinder, and probably consisted of thin wire wound around the plunger."
My recollection, in reading the description in Stirlings actual patent was that the passageway between the cylinder wall and displacer served as the regenerator and that the passage COULD also contain some wire or other material to increase its effectiveness.
I do not know that Stirlings original model in the museum did or did not contain any wire.
I have no strong opinion either way, but overall, it seems to me, that taken all together with Stirlings own statements and drawings, an engine of the type in the video with a long metal tube containing a long metal internal displacer with at fairly close tolerance or narrow passageway between them, through which the air can pass back and forth, picking up and loosing heat, does meet the description of a Stirling engine complete with regenerator.
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Further, Stirling's original patent drawing or engraving depict the regenerator or "economiser" as "the annular space between the displacer and the cylinder."
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The page also relates: "The economiser (not shown in the engraving) is in the annular space between the displacer and the cylinder, and probably consisted of thin wire wound around the plunger."
My recollection, in reading the description in Stirlings actual patent was that the passageway between the cylinder wall and displacer served as the regenerator and that the passage COULD also contain some wire or other material to increase its effectiveness.
I do not know that Stirlings original model in the museum did or did not contain any wire.
I have no strong opinion either way, but overall, it seems to me, that taken all together with Stirlings own statements and drawings, an engine of the type in the video with a long metal tube containing a long metal internal displacer with at fairly close tolerance or narrow passageway between them, through which the air can pass back and forth, picking up and loosing heat, does meet the description of a Stirling engine complete with regenerator.
In the construction of the passage in both Modification of my contrivance for heating and cooling liquids and other bodies I observe the following rules:
When the passage are made of metal or any other substance that conducts or transmits heat easily I make the metal or other substance as thin as possible to prevent the heat from being transmitted in this manner from the hot to the cold extremity of the passage.
Liquids and airs being very imperfect conductors of heat I make the passages very narrow (at least in one direction) in proportion to their length, for the purpose of heating and cooling more completely the liquids or airs that pass through them. A transverse section of the passages is given at A, B and C fig. 3.
When the passage cannot be made sufficiently narrow I make their sides jagged or rough by bodies projecting form them as represented at fig 4 or I adopt any similar method for promoting the internal motions of the fluids and the ready communication of heat to them or to the passage.
When the width of the passage cannot be sufficiently diminished increase its length in order to attain the same end. The form and construction of the tubes, passages and plates in both the modifications of my general contrivance or arrangement may be varied according to circumstances; but the benefit to be derived from this contrivance arises from the fluids and other bodies to be heated and those to be cooled being made to move in opposite directions and it is for the invention or improvement of this arrangement that I have applied for and obtained His Majesty’s Letter Patent.
Re: "Stirling engine" or "Hot Air engine"?
On the other hand, the engine does violate a couple of Robert Stirling's specifications.
The internal displacer of this engine is not a thin walled hollow tube or can of any kind that would prevent heat from being readily conducted from the hot to the cold end, but rather a SOLID slug of aluminum.
Stirling further specified that his regenerator passages or tubes should be surrounded by masonry or other insulating material so as to prevent needless loss of heat.
Understanding such considerations and the reasoning behind them is important in engine design and optimization, which is why I thought this conversation was important enough to preserve.
It also appears, or could be argued that Stirlings own model violates the standards for a "real" engine, as his long metal regenerator/displacer tube is not insulated with brick and mortar or other insulation. Though, in reality, air is one of, if not the best insulator.
The internal displacer of this engine is not a thin walled hollow tube or can of any kind that would prevent heat from being readily conducted from the hot to the cold end, but rather a SOLID slug of aluminum.
Stirling further specified that his regenerator passages or tubes should be surrounded by masonry or other insulating material so as to prevent needless loss of heat.
Understanding such considerations and the reasoning behind them is important in engine design and optimization, which is why I thought this conversation was important enough to preserve.
It also appears, or could be argued that Stirlings own model violates the standards for a "real" engine, as his long metal regenerator/displacer tube is not insulated with brick and mortar or other insulation. Though, in reality, air is one of, if not the best insulator.
The waste or escape of heat from the passage is prevented by their being surrounded with charcoal powder, wood, bricks or any substance that does not easily permit heat to pass through it.
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Alphax
Re: "Stirling engine" or "Hot Air engine"?
Tom, I think that your model (I happen to have the same one) can be fairly described as either a Stirling engine or as a hot air engine - whichever you prefer yourself is fine. Either description is OK and neither description is wrong, as far as I can understand.
If you look up all the definitions for "Stirling engine" you will find lots of conflicting definitions. You similarly get conflicting definitions for "Hot air engines". Both are often described as "External Combustion Engines" (ECE) in order to distinguish them from Internal Combustion Engines (ICE) which really doesn't make a lot of sense when you consider that combustion is far from the only external heat source used to power Stirlings (NASA plan to use nuclear heat sources for Stirling engines used to generate electrical power for space missions). Not to mention solar heat sources or geothermal heat sources for terrestrial uses.
Changing the working fluid from air to, say, helium means that perhaps it shouldn't be called a Hot Air Engine, even though that is what you could have called it when it used air!
It seems to be a feature of these engines that they have been made to work in so many different configurations that definitions (other than ones based on thermodynamics) are unlikely to mean a lot. I happen to think that variation and difficulty of agreeing which definition a particular Stirling engine fits into is one of their more defining characteristics! You can take off the regenerator, get rid of the displacer and displacer cylinder, use a different gas, heat the end cap any way you like, throw the crankshaft away and not have a flywheel at all and some designs still work and still meet the thermodynamic requirements of a Stirling engine. As if that was not enough, it is possible to also do away with the last remaining piston and have a diaphragm instead which reduces the entire engine down to one cylinder with a single moving part.
You do not have such versatility or variation in physical form with Internal Combustion Engines - they generally need more moving parts and won't take kindly to being run on anything other than combustible fuels and oxygen.
If you look up all the definitions for "Stirling engine" you will find lots of conflicting definitions. You similarly get conflicting definitions for "Hot air engines". Both are often described as "External Combustion Engines" (ECE) in order to distinguish them from Internal Combustion Engines (ICE) which really doesn't make a lot of sense when you consider that combustion is far from the only external heat source used to power Stirlings (NASA plan to use nuclear heat sources for Stirling engines used to generate electrical power for space missions). Not to mention solar heat sources or geothermal heat sources for terrestrial uses.
Changing the working fluid from air to, say, helium means that perhaps it shouldn't be called a Hot Air Engine, even though that is what you could have called it when it used air!
It seems to be a feature of these engines that they have been made to work in so many different configurations that definitions (other than ones based on thermodynamics) are unlikely to mean a lot. I happen to think that variation and difficulty of agreeing which definition a particular Stirling engine fits into is one of their more defining characteristics! You can take off the regenerator, get rid of the displacer and displacer cylinder, use a different gas, heat the end cap any way you like, throw the crankshaft away and not have a flywheel at all and some designs still work and still meet the thermodynamic requirements of a Stirling engine. As if that was not enough, it is possible to also do away with the last remaining piston and have a diaphragm instead which reduces the entire engine down to one cylinder with a single moving part.
You do not have such versatility or variation in physical form with Internal Combustion Engines - they generally need more moving parts and won't take kindly to being run on anything other than combustible fuels and oxygen.
Re: "Stirling engine" or "Hot Air engine"?
And then, there is this sort of thing:
https://youtu.be/Nxu6g_9JsVk
"Thermoacoustic"?
I guess. Not even a piston or diaphragm. Just pulsating air.
The turbine doesn't seem to actually be a part of the engine, not physically connected anyway, though I suppose could be.
https://youtu.be/Nxu6g_9JsVk
"Thermoacoustic"?
I guess. Not even a piston or diaphragm. Just pulsating air.
The turbine doesn't seem to actually be a part of the engine, not physically connected anyway, though I suppose could be.
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Alphax
Re: "Stirling engine" or "Hot Air engine"?
Hi Tom
Yes... isn't that amazing! Not even a piston - no moving parts at all. I think it could be called a Thermoacoustic "engine". It seems to pulse enough power to spin an unconnected rotor, so probably is an "engine", or at least half an engine! It is a modern version of the Rijke tube.
Yes... isn't that amazing! Not even a piston - no moving parts at all. I think it could be called a Thermoacoustic "engine". It seems to pulse enough power to spin an unconnected rotor, so probably is an "engine", or at least half an engine! It is a modern version of the Rijke tube.