Programmatic control of Stirling Engines

[MikeB]

"It is well know that it takes TIME to transfer heat by conduction"

Yes, that is one of the real-world compromises that we face - the longer we have for heat/energy transfer (in either direction) the more will transfer, but even at zero RPM the air on the inside will never reach the temperature of the flame outside. That was a large part of my point about Wind (or water) turbines - the mechanisms are different, but the compromises are very similar - the more ENERGY you extract the harder it is to extract any more, and for turbines you have a similar issue to heat engines (of any kind) - an exhaust is required even for a wind turbine.

BTW While Carnot may have been confused, 99% of the time when I talk about heat, I really mean energy; and 98% of the time when I talk about wind, I might as well be saying 'energy' too. It is far more reasonable to describe wind as a fluid, than heat, but if we think of them both as pure energy, then surely it becomes obvious why people frequently talk about 'energy flows'?

[Tom Booth]

My point in bringing up the generally slow rate at which heat travels by conduction was in relation to the need to lose heat in order to cool the air in the cylinder and displacer chamber so the air can contract, allowing the piston to return to it's starting point after being pushed out by the heated expanding air.

Let's say, just for arguments sake that it normally takes 20 seconds to cool the air down by conduction to a sink in order for the piston to return.

The piston, in a running engine, may however, actually return in 1/100th of a second. (1/2 cycle at 3,000 RPM).

Looking at that logically, I see a glaring discrepancy.

The presence of a heat sink to conduct away the added applied heat so the piston can return after the air is heated, expanded and the piston driven out, does not explain how the piston is able to return in a mere fraction of a second and without the need for a flywheel.

The gas contracts and "pulls" the piston back, or rather, allows it to be pushed back by atmospheric pressure, for all intents and purposes, INSTANTANEOUSLY

The only instantaneous process for removing heat would be by conversion to another form of energy or the transfer of kinetic energy resulting in a sudden drop in temperature.

I'm not otherwise able to explain how some Stirling engines operate quite well without a flywheel to store energy to help push the piston back against the hot expanded gas which has only lost a negligible amount of heat by conduction to a sink

Not too long ago: I'm talking, within the time I've been on this forum, the general consensus seemed to be that it would be impossible for any kind of engine to operate without a flywheel, for the reason that heat dissipation by conduction and conversion combined would never be enough to dissipate enough heat for the piston to return without help from the stored energy in the flywheel.

It would seem, from my research on the subject, that in an engine running at a high RPM, there is not enough time to conduct hardly any heat away by conduction.

My conclusion then is that, in spite of wide spread belief to the contrary, a Stirling engine running at high speed without a flywheel must be converting very near 100% of all the heat, entering into the working gas inside the engine, is being converted to mechanical motion.

That is not to say that there are no loses. Probably 99% of the heat from an alcohol burner running a typical Stirling engine is lost to the air and never gets inside the engine at all Additional heat is lost due to heat conducting through the body of the engine and never reaching the working gas inside.

But, of the fraction of heat that actually makes it through to the interior of the engine to expand the air in the cylinder, very nearly all of that heat is effectively converted to work, as there is not time for that added heat to dissipate by conduction. That is how it looks to me anyway, and so far, my experiments seem to support this.

In terms of the actual "working gas" inside the engine, almost no heat at all passes through the engine from the heat source - through that gas - to the sink.

If watched closely, while applying a propane torch to this engine, it takes quite some time for heat to conduct through and into the gas and for the gas inside the engine to expand and push out the piston. (About 20 seconds)

I sealed the end of the cylinder with the balloon to see if gas was escaping past the piston to atmosphere. It seems no gas was escaping, so that is no explanation.

I also removed the flywheel, so the flywheel cannot be bringing the piston back, nor any revolving crankshaft, as there is none.

If it takes several seconds for the applied heat to expand the gas, would it not also take a comparatively long time for that gas to cool down after the heat is removed?

And yet, the heat is not removed. The heat source is not "replaced" with a heat sink to cool the gas back down so the piston can gradually return, the same way it was slowly driven out by heat conduction into the engine.

I have to conclude some other mechanism is at work which allows the engine to operate at such a high frequency without a flywheel.

https://youtu.be/iOs3BADFeKI

[Tom Booth]

Here is another interesting phenomenon I've seen several times, at the startup of an engine.

https://youtu.be/XD4Q2bxEBYU

At times, not only does the piston return, but does so against, or in opposition to the inertia/momentum of the flywheel.

The flywheel is, say, turning clockwise, the piston driving it out due to the expanding gas, then the piston returns, dragging the flywheel with it in the reverse direction of the stored momentum in the flywheel.

How is that possible?

It would seem that, not only is the gas in the engine cooling to the point of equalization with the outside atmosphere, but the pressure is dropping below outside atmospheric pressure to the extent that the piston stops the forward revolution of the flywheel and pulls it back in the opposite direction.

Presumably, to produce this low pressure inside the engine the air in the engine must also drop in temperature.

It drops in temperature enough to contract the air, producing a vacuum, all the while, as heat is being continually applied.

In contrast, taking the Carnot theory of heat, comparing a heat engine to a water wheel;

Whatever water gets taken in by a water wheel must ALL be dumped out, there is no conversion of the water into anything else.

A water wheel does not ever ossilates back and forth, reversing direction as seen with a Stirling engine.


https://youtu.be/uiRHah9dujw


There is a steady flow.


https://youtu.be/x2qB0sR5IWA


Whatever is going on inside a Stirling engine, it is not a steady flow of heat from the heat source to the sink, like water taken down to a lower level by a mill wheel.

[MikeB]

Water is not energy, only a carrier of gravitational energy. The bottom video shows that that particular water wheel is rather inefficient since much of the water is being thrown clear.
My problem with the experiment you describe above is that heat transfer is happening in three phases, not one:
Flame > end cap
End cap outside > inside
End cap inside > air -- This stage is the only bit that really matters to the operation

Also, you appear to be measuring the time for that whole process, heating the air from cold, rather than from warm?

[Tom Booth]

Water is not energy, only a carrier of gravitational energy. The bottom video shows that that particular water wheel is rather inefficient since much of the water is being thrown clear.
My problem with the experiment you describe above is that heat transfer is happening in three phases, not one:
Flame > end cap
End cap outside > inside
End cap inside > air -- This stage is the only bit that really matters to the operation

Also, you appear to be measuring the time for that whole process, heating the air from cold, rather than from warm?

Air is almost a perfect insulator, the next best thing after a complete vacuum. Without some air circulation, (heat transfer due to cooling by convection) the transfer of heat from end cap to inside air would be perhaps the slowest, most difficult of all.

Add to that the fact that for heat transfer to take place at all between any two materials, or even within the self same material requires a temperature difference.

This heat transfer from the end cap to the interior gas is generally supposed to, or thought to take place during compression at the time when the gas is hottest due to compression, so the temperature differential at a minimum, assuming any temperature difference exists at all, or is not entirely reversed at that point (i.e the interior gas being hotter than the heat "source" at full compression).

Even Carnot, in his description of the so-called Carnot Cycle states that the gas reaches equilibrium with the heat source before being put in contact with the heat source and cools to the point of reaching equilibrium with the sink before being put into contact with the sink.

There can be no heat transfer between objects in thermal equilibrium, so what's wrong with this picture?

Screenshot_20210929-151947.jpg (410.59 KiB) Viewed 7859 times

Carnot describes heat transfer between bodies that have already reached thermal equilibrium due to adiabatic expansion and contraction, which, incidentally is impossible.

I don't actually dispute Carnot's basic description. Adiabatic compression DOES bring the gas into thermal equilibrium with the heat source as described, then adiabatic expansion DOES bring the gas into thermal equalibrium with the so-called "sink".

A "flow" of heat, between two materials in thermal equalibrium, however, is impossible, so it would seem something else is going on.

I would offer the analogy of a heavy weight dropped on a strong spring. The spring being the heat source. Energy is transfered to the heat source and back.

The oscillation, perhaps, is between two molecular states of the Gas/air.

When compressed, gas heats up. When expanded it cools

When compressed and hot, the gas has enough kinetic energy that it expands.

When expanded and cold, the attractive force between the gas molecules, as when a gas condenses into a liquid causes the gas to contract.

The gas oscillates between these two extremes.

As in the video Rüchardt oscillating ball experiment, this can take place without any heat input, or work output at all. Not overt anyway, moving the steel ball up and down involves SOME slight amount of work I suppose, and with adiabatic heating and cooling in the process, there is some passive heat transfer going on.

https://youtu.be/vT6n7VVBvqw

The only heat input required to maintain the oscillation is enough to compensate for any work output and friction loses. There is no "flow through" of heat from source to sink is how it looks to me, just a kind of "bouncing" with enough heat input to compensate for work output and any incidental loses to friction or undesirable heat conduction through the engine body.

[Tom Booth]

To be fair, a reciprocating water powered engine, rather than a rotating wheel, is possible, as with this contraption:

https://youtu.be/8W5SY651wxg

[MikeB]

That video of adiabatic bounce is quite a clever demonstration, but I have to point out:

As you note, no REAL work is done - in our nice, perfect frictionless theoretical world, the 'piston' will continue to oscillate perpetually.
But without doing any work, which is the fundamental point of any engine.
And also by isolating a single portion of the process, we are ignoring the whole point of this discussion, which is that any engine is complex, with different types of process interacting with each other, and equilibrium pretty much non-existent.

This is reflected in some of you points in the previous post, such as when you say a gas contracts when it cools, which is not entirely correct - it will only contract if/until it reaches equilibrium with atmospheric pressure (or whatever.) 99% of the time this would be nit-picking on my part, but in most Stirling engines, atmospheric pressure plays at least some part in returning the power piston.

[Tom Booth]

.... when you say a gas contracts when it cools, which is not entirely correct - it will only contract if/until it reaches equilibrium with atmospheric pressure (or whatever.) 99% of the time this would be nit-picking on my part, but in most Stirling engines, atmospheric pressure plays at least some part in returning the power piston.

Of course.

The gas "contracts" due to the imbalance of all forces involved relative to each other.

The gas that has expanded and done some "work" great or small, drops in temperature and pressure which allows, or causes the gas to then be reduced in volume, partly due to lower pressure relative to the outside atmospheric pressure, partly due to molecular attraction becoming greater than molecular repulsion at relatively low temperature.

The thing that interests me is the potential drop in temperature that may take place due to air expanding and doing work, as would ordinarily take place in an air motor.

In an air motor, the gas is first compressed into a tank and usually the heat of compression dissipates. Then when the air is expanded through an air motor to do work of some sort, the exhaust may drop in temperature drastically, as much as 200° below ambient. According to this PDF for example:

https://papers.ssrn.com/sol3/papers.cfm ... id=3760428

To quote a passage: ". It has been found that temperature at the outlet of the pneumatic motor can reach values up to −122.5 °С, which is unacceptable"

There is a problem with air motors getting so cold that they freeze up. Expanding compressed gas through an air powered motor or engine is a method used to reach cryogenic temperatures otherwise impossible by other methods of cooling for the liquefaction of various gases.

-187°F (-122°C) is certainly colder than outside atmospheric temperature. I'm not trying to say that such cold temperatures are necessarily reached inside a Stirling engine, but if such extremely cold temperatures are possible at the exhaust of an air motor under a load, fed by compressed air at ambient temperature, due to the gas simply expanding under a load and doing "work", I do wonder what kind of low temperatures might be reached in the cylinder of a Stirling engine under ordinary operation as a result of the same phenomenon that is, compression followed by expansion + work output.

And then, during the compression, the opposite takes place and the compressed gas gets hotter and the pressure rises above that of the outside ambient, much like a fire piston.

https://youtu.be/Bjy6m6MR-PQ

I don't know, but I imagine that a running engine of say 1 or 2 horsepower would be able to achieve pressures and temperatures greater than what can be reached by a hand powered fire piston, which reaches temperatures hundreds of degrees above ambient.

Logically, to me, anyway, the degree of cooling (below ambient) by adiabatic expansion + work should be roughly equal to or possibly greater than the degree of heating (above ambient) by adiabatic compression.

By human hand pressure, the fire piston is said to generate heat as high as 900°K, or over 1000° F

[Tom Booth]

Say we have an engine, simar to this:

termo.jpg (20.99 KiB) Viewed 7817 times

Apply heat until the piston moves out slowly and stops, at say, the half way point.

At this point, the heat entering the engine is the same as that leaving and the piston is at rest. There is equalibrium of pressure on either side of the piston. Let's mark the cylinder with a magic marker at the center of the piston, the half-way point.

Now rotate the flywheel so the piston travels inward. What happens?

Logically the pressure and temperature rises.

As the piston passes TDC the pressure and temperature in the piston cylinder reaches a maximum, above ambient and the piston is driven outward.

As it crosses the point marked, the half-way point, what happens?

Logically, I think, the temperature and pressure now drops past or below the point of equalibrium, the pressure in the cylinder is lower, and the temperature is lower than at the start when the heat input and output were equal. The piston continues traveling outward, however, due to its momentum, the temperature and pressure continuing to drop.

At some point the pressure differential is so great, the internal temperature and pressure so low that the motion of the piston is abruptly halted and it's motion reversed.

This is a hypothesis that would be difficult to verify, as the moment of maximum cooling of the gas only lasts a fraction of a second and is entirely internal. There would not be sufficient time to get an accurate temperature reading even with an internal probe. I don't really know how such a fleeting instant of cold could be measured, if it existed.

Likewise, as far as a rise in temperature when the piston returns.

Logically, on the return stroke the gas again gets hot once past the half way point marked on the cylinder. The outside atmospheric pressure is putting "work" into compressing the gas, which translates into heat. For an instant, the temperature of the gas in the cylinder is elevated above the temperature produced due to the heat source alone. That is my assumption anyway. It seems like a logical conclusion.

So, does the ossillation of the piston cause the hot end of the engine to become intermittently hotter and the cold end colder?

[MikeB]

To be positive for once - you don't need to measure temperature - you have a known (though not entirely fixed) volume of gas, so if you are able to measure the pressure then you can calculate the temperature from that.
Whether pressure sensors are significantly faster than temperature sensors I have no idea.

[Tom Booth]

To be positive for once - you don't need to measure temperature - you have a known (though not entirely fixed) volume of gas, so if you are able to measure the pressure then you can calculate the temperature from that.
Whether pressure sensors are significantly faster than temperature sensors I have no idea.

One way of testing the theory, I think, without actually getting an internal temperature/pressure reading of any sort would, perhaps, be to simply measure the outside temperature of the cylinder at the high compression and low compression points, the cylinder itself enclosed within insulation. Preferably a vacuum.

If the cylinder could be non-heat conducting, that would be helpful to prevent heat being conducted from the hot to the cold end via the cylinder body.

iMarkup_20211007_151003.jpg (20 KiB) Viewed 7799 times

The point on the cylinder near the blue arrow should grow progressively cooler as the engine runs while the point at the red arrow should, well, not quite sure. Probably remain stable.

When the piston is fully extended and the gas/air expanded completely, in theory, the temperature of the gas inside should be lower than the surrounding ambient temperature air outside.

That this cooling only takes place very fleetingly leaves virtually no time for this drop in temperature to migrate to the outside and with ambient heat all around, the brief instant of cold would be nearly instantly destroyed by the overwhelming heat of the ambient surroundings, as well as from heat migrating up the tube by conduction from the hot end.

But, if the cold end could be completely insulated and the cylinder made non-heat conducting, this cold point at the extremity of the cylinder exposed to cold expanded gas might become colder than the surrounding ambient.

The piston would also need to be of sufficient length so as to exclude ambient air, or some other means of excluding outside air from contacting that inside cold area of the cylinder when the piston is at the hot end.

The piston itself should be as non-heat conducting as possible as well, and friction reduced as much as possible.

With all that, even with a rapid running engine, over time, some cooling of the cylinder should eventually take place due to the cylinders nearly constant contact with the cold expanded air inside the cylinder.

That's what I imagine anyway.

I've almost exclusively been experimenting with LTD type engines rather than the thermal Lag type, but even with the LTD there is some indication that some such action is taking place, though I suspect it to be more pronounced in a thermal Lag type Stirling.

[Alphax]

Hi Tom

Just a quick thought in passing - it may or may not appeal.

Your first post describes your general interest at the moment in programming, timing, rates and frequency (of heat delivery).

You mention the possible need for sensors to synchronise displacer and piston motion.

Can I suggest that you do a bit or googling on reed switches. I build electric motors, somewhat similar to demonstration motors (when there was limited understanding of current, voltage, magnetism and electromagnetism). I particularly like large, slow rotation. Fast, powerful rotation is easy, but designing a slow rotation - especially one that can run for weeks on a small battery - is an nice problem to work on. As with Stirling engines, reducing friction is vital, and so is optimising synchronisation and timing.

I find that reed switches (triggered by tiny neodymium magnets embedded in the relevant reciprocating components) are extremely adaptable to all kinds of simple autonomous timing functions, obviating the need for programming and electrical components. Reed switches are amongst the commonest (and cheapest) electrical devices made and come in a vast array of specs, yet hardly anyone outside electronic design groups seem to be aware of their usefulness. One thing that has surprised me is just how fast their response times can be (thousands of rpm) and how useful they are in reaching system resonance in decaying and restoring electromagnetic fields within inductors.

Anyway.. have a look on Youtube at how enthusiasts use reed switches to control and time things like simple pulse motors with home made coils (inductors) as drive units.

[Tom Booth]

Hi Tom

Can I suggest ...reed switches.

Thanks for the tip

I bought some of those shake flashlights a while back, I have plans on tearing apart. I noticed a couple of the water proof type have magnetic reed switches I thought might be handy for something.

With modern internal combustion engines the ignition timing is computer controlled and variable with RPM, fuel octane, engine load etc. for improved efficiency.

About any kind of mechanical switching device I could imagine is not going to have that kind of responsiveness to varying real time conditions, sensor reading inputs.

I'm basically thinking far into the future. At this point, my electronics kit doesn't even have a servo responsive enough to be of any real use, so that project is on the back burner.

[Tom Booth]

Taking a trip down memory lane and reviewing some very old posts/threads, I came across this:

Hi Tom, ...

I read an article in which the builder had linked the actuator to a computer and was able to "throttle" the engine by changing the speed of the actuator's movement. And yes, it was self-starting....

Posted by SScandizzo (signed Stefan) back in 2006.

Unfortunately no link or detailed reference, but interesting that this idea had already been implemented and perhaps the idea seeded in my mind, further back in time than I could even recall. This likely flew right over my head at the time.

Knowing an article existed before 2006 on the subject maybe some chance of still finding it.

[Fool]

Have you thought of using a solenoid? They won't go partway but they are very fast.