The heat isn't a fluid, but the air which is doing the work via pressure is. If the heat were doing work directly, the Stirling engine would be like a solar panel using infra-red light instead of visible light. Since it's using a fluid as a working medium, the thermodynamic rules about fluids and heat transfer apply, thus, you need a sink.Tom Booth wrote: ↑Fri Jun 12, 2020 6:41 pmThat certainly does represent the conventional wisdom.Sockmonkey wrote: ↑Fri Jun 12, 2020 12:13 pm You get work out of heat by giving it somewhere to go and making it do something for you in order to get there. You can compare it to a water wheel in that way. It's usually easiest to make a hot spot and use the environment as the cool place the heat tries to flow through. Lower pressure also counts as "colder" in that the heat can now occupy more volume.
Tesla, though, was not exactly conventional. He was certainly familiar with what everybody knew about heat engines, the second law of thermodynamics and all that, but he had a different idea about how heat engines work, and to my knowledge, his ideas have never been adequately put to the test. As for Tesla, he wrote: "a misfortune befell me in the burning of my laboratory, which crippled my labors", but I think he left behind enough information to move things forward today, if his theory was correct.
More than anything, I would say; heat in a heat engine is like the kinetic energy of billiard balls on a pool table:
Translational_motion.gif
The idea is not so much giving these hot molecules somewhere to go but rather giving them something that they can hit and move.
If a ball strikes a solid immovable object it bounces off retaining its kinetic energy. But if it hits something that can move, the energy is transferred to the other object. In the case of a heat engine, the moveable object is the piston. The "heat" or kinetic energy, transferred to the piston, does not need anywhere else to go.
Tesla wrote in that regard: "But let us reflect a moment. Heat, though following certain general laws of mechanics, like a fluid, is not such; it is energy which may be converted into other forms of energy ... If the process of heat-transformation (in a heat engine) were absolutely perfect, no heat at all would arrive at the low level (or heat sink), since all of it would be converted into other forms of energy."
Think of the piston being hit by little billiard balls on both sides. Heat the gas on one side and the kinetic energy of the air molecules increases, knocking the piston out, but as each molecule hits it loses its "internal energy" and gets "cold", stops moving, but other fast moving "hot" molecules continue to hit the piston, also loosing energy, until there is equalization of energy on both sides, but the piston, having momentum continues moving, and more molecules continue to strike it and loose energy, causing the gas in the engine to become even colder, again creating an imbalance, which allows molecules outside the engine to knock the piston back inward. It seems, ALL of the heat added to the gas in the engine has been lost, and then some! The heat does not flow through the engine, rather, now more heat must be added to repeat the cycle.
That was Tesla's basic argument, that heat is not REALLY a fluid, therefore it does not REALLY need a "sink" to flow into.
There does need to be an initial temperature difference though. Why? because there must be some way of adding heat in order to create an imbalance to start the piston moving, but once moving, the heat does not need to flow through the engine into the sink, so a Stirling engine running on ice does not add heat to the ice. Rather heat is added to the engine to push the piston as previously described. The heat is converted into the kinetic motion of the piston, which might be further transformed into electricity, if the engine is driving an electric generator. The heat has been transformed into a different form of energy, now more heat can be added to continue the cycle, but no heat has been transferred to the ice!
Your description is certainly familiar to me, and is the generally recognized and accepted way that Stirling engines are supposed to work. But I think that maybe Tesla was right, and everyone else, for the past 200 years has been wrong.
Tesla's "Ambient Heat Engine" Experiment
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Sockmonkey
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Re: Tesla's "Ambient Heat Engine" Experiment
Re: Tesla's "Ambient Heat Engine" Experiment
Sockmonkey wrote: ↑Sat Jun 13, 2020 9:34 pm The heat isn't a fluid, but the air which is doing the work via pressure is. If the heat were doing work directly, the Stirling engine would be like a solar panel using infra-red light instead of visible light. Since it's using a fluid as a working medium, the thermodynamic rules about fluids and heat transfer apply, thus, you need a sink.
The mechanical equivalent of heat (heat and motion or work, are interchangeable) and the conservation of energy, indicate, when heat goes into a heat engine and that engine has produced motion (work), the heat goes out in the form of the external work performed and/or motion produced.
If the heat is transformed into mechanical motion to drive a dynamo to light lights etc. The energy represented by that light, which was originally heat, before it's transformation into light, cannot also travel through the engine to a heat sink.
Any concept that heat must of necessity flow through a heat engine to a sink, like water through a turbine is entirely wrong IMO.
Re: Tesla's "Ambient Heat Engine" Experiment
Georges Claude for example, in 1902, working on processes for liquefying gasses in the air, first took advantage of the known fact that a compressed gas, when released (expanded, or allowed to expand) drops in temperature, by some 20 degrees F.
The cold gas could then be used to pre-cool additional gas in a cyclic manner and in this way the temperature of the gas could be gradually reduced until it became cold enough to liquefy.
Claude discovered however, that if, instead of just allowing the compressed gas to escape, he used the compressed gas to drive a piston engine that powered a load (the engine was doing some work) the gas would undergo an additional sudden drop in temperature of more than 200 degrees F in one fell swoop!
When an expanding gas does work driving a piston, there is a sudden extreme drop in temperature.
This Georges Claude process greatly simplified the liquefaction process, and is still the process used today, though instead of a reciprocating engine, today a turbine is more commonly used but the principle is the same.
When an expanding gas does work in an engine it uses energy (internal heat or kinetic energy) which causes an immediate and quite sudden drop in temperature.
I believe that in reality, a Stirling engine's principle of operation is quite similar to the Claude expansion engine.
As air, or any gas expands in an engine driving a piston and doing work, heat is converted into mechanical motion. The random kinetic energy of air molecules is marshaled to drive the engine and so the energy contained in the air or gas used to drive the engine (energy in the form of heat) is transfered or transformed into the mechanical kinetic motion or "work" performed by the engine.
The whole analogy that heat goes right through a heat engine and out the other side to the heat sink and in the process turns or drives the engine just like water turning a water wheel is entirely false and misleading.
That analogy originated at a time when it was thought that heat really was some kind of invisible fluid that could pass right through solid objects. This analogy has been perpetuated for 200 years, but is no more true than saying pregnancy is caused by swallowing watermelon seeds.
The cold gas could then be used to pre-cool additional gas in a cyclic manner and in this way the temperature of the gas could be gradually reduced until it became cold enough to liquefy.
Claude discovered however, that if, instead of just allowing the compressed gas to escape, he used the compressed gas to drive a piston engine that powered a load (the engine was doing some work) the gas would undergo an additional sudden drop in temperature of more than 200 degrees F in one fell swoop!
When an expanding gas does work driving a piston, there is a sudden extreme drop in temperature.
This Georges Claude process greatly simplified the liquefaction process, and is still the process used today, though instead of a reciprocating engine, today a turbine is more commonly used but the principle is the same.
When an expanding gas does work in an engine it uses energy (internal heat or kinetic energy) which causes an immediate and quite sudden drop in temperature.
I believe that in reality, a Stirling engine's principle of operation is quite similar to the Claude expansion engine.
As air, or any gas expands in an engine driving a piston and doing work, heat is converted into mechanical motion. The random kinetic energy of air molecules is marshaled to drive the engine and so the energy contained in the air or gas used to drive the engine (energy in the form of heat) is transfered or transformed into the mechanical kinetic motion or "work" performed by the engine.
The whole analogy that heat goes right through a heat engine and out the other side to the heat sink and in the process turns or drives the engine just like water turning a water wheel is entirely false and misleading.
That analogy originated at a time when it was thought that heat really was some kind of invisible fluid that could pass right through solid objects. This analogy has been perpetuated for 200 years, but is no more true than saying pregnancy is caused by swallowing watermelon seeds.
Re: Tesla's "Ambient Heat Engine" Experiment
I've located a paper on expansion engines used for cooling, in this case to liquefy helium.
I think it should also be kept in mind while looking at this that helium liquefies at a temperature of 4.2 Kelvin that is 452 degrees below zero Fahrenheit!
A Stirling engine running normally on regular air would certainly never approach quite such a severe degree of cold, yet, in principle, one gas has the same basic properties as another as far as becoming cold when performing work in an engine.
The paper (a PDF) can be found here under the link "Cryogenics Primer":
https://operations.fnal.gov/rookie_books/rbooks.html
Or directly:
https://operations.fnal.gov/rookie_book ... Primer.pdf
The Stirling cycle is different, but in many ways the same.
At A, instead of the piston being driven by the introduction of compressed helium gas, in a Stirling the piston is driven at this stage by the introduction of heat which expands the air already contained in the cylinder, but I would argue that the effect is the same. A gas expands and drives the piston in a cylinder imparting momentum to the piston. As a result of this expansion the gas cools.
I would say the rest of the cycle is similarly the same. The piston, having picked up momentum from the initial expansion of the gas in the cylinder continues it's motion, expanding the gas past it's natural limit causing it to drop in temperature even further.
Of course, in a Stirling, there is no intake valve and the purpose of a Stirling is not to collect liquefied gas, so there is no outlet valve. But as far as the critical element of an expanding gas doing work to drive a piston in an engine which results in the conversion of heat into work, which results in a refrigerating effect which cools the gas which causes the gas to contract, from what I've seen, this is the best explanation I can find as far as answering the question: "how can a Stirling engine operate without a flywheel?"
I think that with that realization, if true, it provides some insight into how the Stirling cycle engine could be improved.
I think we want a quick initial heating and expansion that imparts maximum momentum to the piston which expands the gas in the cylinder and creates the cooling effect that keeps the engine running most efficiently and results in the greatest possible conversion of heat into work.
BTW, the paper is from Fermi National Accelerator Laboratory U.S. Department of Energy website.
https://operations.fnal.gov/
I think it should also be kept in mind while looking at this that helium liquefies at a temperature of 4.2 Kelvin that is 452 degrees below zero Fahrenheit!
A Stirling engine running normally on regular air would certainly never approach quite such a severe degree of cold, yet, in principle, one gas has the same basic properties as another as far as becoming cold when performing work in an engine.
The paper (a PDF) can be found here under the link "Cryogenics Primer":
https://operations.fnal.gov/rookie_books/rbooks.html
Or directly:
https://operations.fnal.gov/rookie_book ... Primer.pdf
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At A, instead of the piston being driven by the introduction of compressed helium gas, in a Stirling the piston is driven at this stage by the introduction of heat which expands the air already contained in the cylinder, but I would argue that the effect is the same. A gas expands and drives the piston in a cylinder imparting momentum to the piston. As a result of this expansion the gas cools.
I would say the rest of the cycle is similarly the same. The piston, having picked up momentum from the initial expansion of the gas in the cylinder continues it's motion, expanding the gas past it's natural limit causing it to drop in temperature even further.
Of course, in a Stirling, there is no intake valve and the purpose of a Stirling is not to collect liquefied gas, so there is no outlet valve. But as far as the critical element of an expanding gas doing work to drive a piston in an engine which results in the conversion of heat into work, which results in a refrigerating effect which cools the gas which causes the gas to contract, from what I've seen, this is the best explanation I can find as far as answering the question: "how can a Stirling engine operate without a flywheel?"
I think that with that realization, if true, it provides some insight into how the Stirling cycle engine could be improved.
I think we want a quick initial heating and expansion that imparts maximum momentum to the piston which expands the gas in the cylinder and creates the cooling effect that keeps the engine running most efficiently and results in the greatest possible conversion of heat into work.
BTW, the paper is from Fermi National Accelerator Laboratory U.S. Department of Energy website.
https://operations.fnal.gov/
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Sockmonkey
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Re: Tesla's "Ambient Heat Engine" Experiment
Technically true, but in practice you need the heat flow out in order to contract the working fluid so it can be heated again and expand for the next cycle.Tom Booth wrote: ↑Mon Jun 15, 2020 11:17 am The mechanical equivalent of heat (heat and motion or work, are interchangeable) and the conservation of energy, indicate, when heat goes into a heat engine and that engine has produced motion (work), the heat goes out in the form of the external work performed and/or motion produced.
If the heat is transformed into mechanical motion to drive a dynamo to light lights etc. The energy represented by that light, which was originally heat, before it's transformation into light, cannot also travel through the engine to a heat sink.
Any concept that heat must of necessity flow through a heat engine to a sink, like water through a turbine is entirely wrong IMO.
Re: Tesla's "Ambient Heat Engine" Experiment
What is technically true, and what is true by experimental observation should not contradict each other.
If heat in an expanding gas is kinetic energy that is transfered to the piston and effectively taken out of the system to power an external load, then having made such a transfer of energy, the gas could contract on its own. The piston would return without any help.
If heat is a fluid, then the engine would not actually be powered by the expanding gas. The heat would be drawn through the engine by some as yet unknown force.
Water does not power a turbine. Gravity does. Gravity pulls the water down. A turbine merely intercepts the flow. The water itself does not gain or loose any internal energy in the process and so passes through the turbine unchanged.
An expanding gas, on the other hand, by kinetic theory, does loose internal energy when expanding and doing work, and so it will contract, even to the point of condensing down to a liquid. As shown in the example of the above expansion engine described in the Fermilab paper on cryo-coolers used in their particle accelerators.
When air condenses from a gas at ambient temperature to a liquid form it contracts 800 times. But there is a gradual contraction of the gas all the way down.
In my personal observations. The piston in a laminar flow Stirling returns on its own even without a flywheel and with no help from external cooling. Cooling by conduction to a heat sink is slow. The piston in a Stirling running without a flywheel however is instantaneous.
Anyway, it looks like my experiment will be delayed. Because the little engines were sent from overseas, with border closings, it may take a month or more longer than usual for them to arrive, so I've just been informed by the tracking service.
If heat in an expanding gas is kinetic energy that is transfered to the piston and effectively taken out of the system to power an external load, then having made such a transfer of energy, the gas could contract on its own. The piston would return without any help.
If heat is a fluid, then the engine would not actually be powered by the expanding gas. The heat would be drawn through the engine by some as yet unknown force.
Water does not power a turbine. Gravity does. Gravity pulls the water down. A turbine merely intercepts the flow. The water itself does not gain or loose any internal energy in the process and so passes through the turbine unchanged.
An expanding gas, on the other hand, by kinetic theory, does loose internal energy when expanding and doing work, and so it will contract, even to the point of condensing down to a liquid. As shown in the example of the above expansion engine described in the Fermilab paper on cryo-coolers used in their particle accelerators.
When air condenses from a gas at ambient temperature to a liquid form it contracts 800 times. But there is a gradual contraction of the gas all the way down.
In my personal observations. The piston in a laminar flow Stirling returns on its own even without a flywheel and with no help from external cooling. Cooling by conduction to a heat sink is slow. The piston in a Stirling running without a flywheel however is instantaneous.
Anyway, it looks like my experiment will be delayed. Because the little engines were sent from overseas, with border closings, it may take a month or more longer than usual for them to arrive, so I've just been informed by the tracking service.
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Re: Tesla's "Ambient Heat Engine" Experiment
The video linked to the description for this engine demonstrates that it is capable of running on ice. It is stated that a 20 degree temperature difference is required, so, should run on ice where the ambient temperature is 52 F or warmer.
In theory, if Tesla was correct, and heat is converted to mechanical "work" or motion, rather that flowing through a heat engine, then two identical engines on top of two identical blocks of ice, at the same room temperature, one engine running and one idle. The system with the running engine should melt the ice more slowly, or not at all.
Early on someone predicted that the ice would melt more quickly under the running engine, which makes intuitive sense, if the engine is actively churning up warm air and transferring heat to the ice.
I should really have three engines. Two idle, one with the displacer up and one down, as it is a kind of insulator and could make a difference.
Also, there should be two running engines. One with an external load and another freewheeling. In theory, the engine with an external load should transfer less heat to the ice, as more heat is being converted.
But, several experiments can be arranged at different times using just the two engines. I might send for some additional engines anyway.
For an external load, I think just having some kind of weighted brake dragging on the flywheel and generating heat from friction should do.
https://youtu.be/2aNg87SaHeQ
This similar but opposite experiment (using a heat source rather than ice) seems to have produced results contrary to intuition, the intention was to see how much the Stirling "would actually accelerate the cooling process".
But, infact this did not happen.
https://concord.org/blog/an-infrared-in ... ng-engine/
In theory, if Tesla was correct, and heat is converted to mechanical "work" or motion, rather that flowing through a heat engine, then two identical engines on top of two identical blocks of ice, at the same room temperature, one engine running and one idle. The system with the running engine should melt the ice more slowly, or not at all.
Early on someone predicted that the ice would melt more quickly under the running engine, which makes intuitive sense, if the engine is actively churning up warm air and transferring heat to the ice.
I should really have three engines. Two idle, one with the displacer up and one down, as it is a kind of insulator and could make a difference.
Also, there should be two running engines. One with an external load and another freewheeling. In theory, the engine with an external load should transfer less heat to the ice, as more heat is being converted.
But, several experiments can be arranged at different times using just the two engines. I might send for some additional engines anyway.
For an external load, I think just having some kind of weighted brake dragging on the flywheel and generating heat from friction should do.
https://youtu.be/2aNg87SaHeQ
This similar but opposite experiment (using a heat source rather than ice) seems to have produced results contrary to intuition, the intention was to see how much the Stirling "would actually accelerate the cooling process".
But, infact this did not happen.
https://concord.org/blog/an-infrared-in ... ng-engine/
Re: Tesla's "Ambient Heat Engine" Experiment
More accurately, I mean to say, outside atmospheric pressure would push the piston back, without the necessity for transferring heat to any sink.If heat in an expanding gas is kinetic energy that is transfered to the piston and effectively taken out of the system to power an external load, then having made such a transfer of energy, the gas could contract on its own. The piston would return without any help.
Re: Tesla's "Ambient Heat Engine" Experiment
Another reason I decided to try these particular engines for some experimenting is that, as a kit, it is, or should be, easy to take apart, repeatedly if necessary, and modify as needed in various ways.
For example, another test I'd like to make is to see the difference between identical engines, one running with and one without regenerators, like in this experiment:
http://www.solarheatengines.com/2008/06 ... generator/
The regenerator, in theory, intercepts and absorbs potential excess heat, retaining the heat within the engine to be re-released for power conversion during subsequent cycles rather than dumping it to a sink.
These engines look as though the displacers could be easily modified so as to incorporate some regenerator material. Similar to these:
I'm not drawing any conclusions before doing some tests, but, I think I've seen enough evidence that Tesla may have been right, to at least do some experiments to try and find out one way or the other.
I do think that THEORY, does influence design, so that the Stirling engine may, in some ways be hampered due to what may be false assumptions regarding heat flow and utilization.
For example, another test I'd like to make is to see the difference between identical engines, one running with and one without regenerators, like in this experiment:
http://www.solarheatengines.com/2008/06 ... generator/
The regenerator, in theory, intercepts and absorbs potential excess heat, retaining the heat within the engine to be re-released for power conversion during subsequent cycles rather than dumping it to a sink.
These engines look as though the displacers could be easily modified so as to incorporate some regenerator material. Similar to these:
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I do think that THEORY, does influence design, so that the Stirling engine may, in some ways be hampered due to what may be false assumptions regarding heat flow and utilization.
Re: Tesla's "Ambient Heat Engine" Experiment
As per my above remark:
Here is a more or less typical laminar flow Stirling. Modified, as shown in this video. It not only runs without a flywheel but also is able to power a linear generator, producing a few volts.
https://youtu.be/J9ILlx3XPZ4
Observe that after being lit, the piston very gradually inches outward, then the footage is cut, presumably giving the engine time to warm up.
This slow warm up and extremely slow initial piston movement, indicates to me just how slow heat is being conducted through this glass test tube engine.
Given such a slow rate of heat conduction, I find it difficult to believe that the piston reverses course against inertia, momentum, and the counter electromagnetic force of the linear generator from mere cooling by conduction to the heat sink alone, or even at all for that matter. The speed of the engine is much to rapid for me to imagine all that gradual heat build up is being absorbed by the heat sink so instantaneously.
.In my personal observations. The piston in a laminar flow Stirling returns on its own even without a flywheel and with no help from external cooling. Cooling by conduction to a heat sink is slow. The piston in a Stirling running without a flywheel however is instantaneous
Here is a more or less typical laminar flow Stirling. Modified, as shown in this video. It not only runs without a flywheel but also is able to power a linear generator, producing a few volts.
https://youtu.be/J9ILlx3XPZ4
Observe that after being lit, the piston very gradually inches outward, then the footage is cut, presumably giving the engine time to warm up.
This slow warm up and extremely slow initial piston movement, indicates to me just how slow heat is being conducted through this glass test tube engine.
Given such a slow rate of heat conduction, I find it difficult to believe that the piston reverses course against inertia, momentum, and the counter electromagnetic force of the linear generator from mere cooling by conduction to the heat sink alone, or even at all for that matter. The speed of the engine is much to rapid for me to imagine all that gradual heat build up is being absorbed by the heat sink so instantaneously.
Re: Tesla's "Ambient Heat Engine" Experiment
Also, another observation I've made many times with this type of Stirling is that, as in the above video, at high RPM the entire engine begins to move towards the flame.
Why?
This indicates to me that the piston is moving inward during the cooling/contraction phase, more forcefully than it moves outward during the heating phase.
How can this be explained other than that the gas inside the engine is getting very cold very suddenly and that therefore the piston is being driven inwards with a great deal of force, so much so that the excess force on the inward stroke is jerking the weight of the entire engine in that direction.
Heat being slowly conducted away by the sink does not adequately explain this whole engine movement IMO.
Why?
This indicates to me that the piston is moving inward during the cooling/contraction phase, more forcefully than it moves outward during the heating phase.
How can this be explained other than that the gas inside the engine is getting very cold very suddenly and that therefore the piston is being driven inwards with a great deal of force, so much so that the excess force on the inward stroke is jerking the weight of the entire engine in that direction.
Heat being slowly conducted away by the sink does not adequately explain this whole engine movement IMO.
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Sockmonkey
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Re: Tesla's "Ambient Heat Engine" Experiment
Quite so, and ideally the stroke would be made long enough to do that for max efficiency. But that's going to limit how powerful it can be. If it's pressurized for faster heat transfer and a more powerful stroke, there's going to be leftover heat/pressure which will fight the upstroke so you would need to radiate that excess heat, or have a flywheel/spring/bounce chamber in order for the stroke not to be impractically long.Tom Booth wrote: ↑Fri Jun 19, 2020 1:19 pmMore accurately, I mean to say, outside atmospheric pressure would push the piston back, without the necessity for transferring heat to any sink.If heat in an expanding gas is kinetic energy that is transfered to the piston and effectively taken out of the system to power an external load, then having made such a transfer of energy, the gas could contract on its own. The piston would return without any help.
That could also mean that the piston pushes away so fast during the first part of the down stroke that it's shoving the engine forward. Think of the recoil of a gun. Cylinder = barrel Piston=bullet. The slowdown at the end of the down stroke is going to be more gradual, so it's not quite enough to over come the friction of the engine base resting on the table.Tom Booth wrote: ↑Fri Jun 19, 2020 8:02 pm Also, another observation I've made many times with this type of Stirling is that, as in the above video, at high RPM the entire engine begins to move towards the flame.
Why?
This indicates to me that the piston is moving inward during the cooling/contraction phase, more forcefully than it moves outward during the heating phase.
How can this be explained other than that the gas inside the engine is getting very cold very suddenly and that therefore the piston is being driven inwards with a great deal of force, so much so that the excess force on the inward stroke is jerking the weight of the entire engine in that direction.
Heat being slowly conducted away by the sink does not adequately explain this whole engine movement IMO.
The start of the down stoke has to be stronger than the up stroke in order to get the piston moving fast enough to create low enough pressure in the cylinder at the end of the down stroke for the atmospheric pressure to send it back.
Re: Tesla's "Ambient Heat Engine" Experiment
Recoil is certainly a factor, and a good explanation for the engine traveling as it does, but I'm also going by accounts like this:
https://youtu.be/DyPxNNJQo9M
About one minute in he mentions how he had to install a spring (edit: piece of a silicone tube rather, I believe a spring was used in another video to solve the same issue if I recall, but I'd have to locate it) to prevent the glass piston from smashing into the brass orifice (on the return stroke).
If the piston is driven out with such an explosive force it causes recoil of the entire engine, which jumps slightly back, I would not expect the piston bottoming out against the bottom of the cylinder to be a problem, yet, he found it necessary to prevent this.
Given a volume of air, where no air is added or subtracted throughout the cycle. Why would the piston ever bottom out, hitting the orifice, unless the contraction phase were actually stronger than the expansion?
https://youtu.be/DyPxNNJQo9M
About one minute in he mentions how he had to install a spring (edit: piece of a silicone tube rather, I believe a spring was used in another video to solve the same issue if I recall, but I'd have to locate it) to prevent the glass piston from smashing into the brass orifice (on the return stroke).
If the piston is driven out with such an explosive force it causes recoil of the entire engine, which jumps slightly back, I would not expect the piston bottoming out against the bottom of the cylinder to be a problem, yet, he found it necessary to prevent this.
Given a volume of air, where no air is added or subtracted throughout the cycle. Why would the piston ever bottom out, hitting the orifice, unless the contraction phase were actually stronger than the expansion?
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Sockmonkey
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Re: Tesla's "Ambient Heat Engine" Experiment
If the given volume of air is small enough that the "neutral" point of the piston where it naturally sits when the engine isn't running is closer to where the top of the stroke would be when connected to the flywheel than where the bottom of the stroke is, then I would expect it to hit the orifice when run disconnected. He could solve that by running the engine a little hotter I think, so the range of piston motion would be a little farther from the orifice because the air wouldn't be cooling and contracting as much. That might screw up the geometry of the engine though, so a spring or something is probably best.