Stirling Engine Thermodynamics

[Tom Booth]

...Less heat input will not result in more work.

I'm not suggesting less heat input at the hot end, just less heat input (from the atmosphere) at the cold end, where heat is not wanted.

Regardless of the potential torque output, actual or theoretical, it seems to me that ANY work done, no matter how small or seemingly insignificant would have to result in cooler temperatures at the unheated end of the displacer where heat is not wanted resulting in greater efficiency and possibly a gradual increase in torque as the temperature differential increases.

In other words, by making a model (or any other) Stirling Engine do some work rather than just freewheeling, more heat would be converted into work and the temperature differential would increase proportionately to the amount of work being done.

It seems very difficult to find information on this subject and it is a rather obscure point, mostly discussed in relation to liquefying gases but I've found several references I can post here:

Hello Tom,

I will try to test this. I'm working on making an electric generator with the flywheel and I will load the electric circuit. So more load should result in a colder end. I will keep you updated when I have done the tests.

OK, Thanks, that should be interesting.

[Tom Booth]

That was many years ago, but presumably, physics has not changed in that time.

Delving into this again, I did just now find a PDF on the subject. Unfortunately the formatting of the PDF makes the latter portion of the brief article impossible to copy/paste easily but I can reproduce the first portion as the one brief itself is very small but the PDF is a compilation of many. (56.5 MB)

https://ntrs.nasa.gov/api/citations/201 ... 028124.pdf

NASA Tech Briefs, Spring/Summer 1982

Overheat Prevention in Solar-Powered Stirling Engines

Power output would be regulated
according to solar-energy input.

NASA's Jet Propulsion Laboratory, Pasadena, California

A proposed controller for a solar powered Stirling engine would prevent the engine from burning up when the energy added by the Sun exceeds that withdrawn by the load.

The controller is suitable for point-focus solar engines, in which reducing the input energy by shuttering the Sunlight or bypassing part of the primary working-
fluid flow would reduce the engine efficiency. In a solar-powered Stirling engine (see figure), concentrated Sunlight evaporates a primary working fluid (liquid sodium). The sodium vapor heats a secondary working fluid (helium) in the cylinder-head heat exchanger. Expanding in the cylinder, the helium drives a piston.

The proposed controller maintains constant cylinder-head temperature regardless of variations in the Sun's intensity; it ensures that all the energy directed to the receiver is used. An electronic circuit senses the head temperature and signals the engine electrical control unit to increase or when the solar energy decreases decrease the engine power.

There is a diagram of the proposed controller:

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I previously referenced a patent or patent application for a different type of controller that mentioned the same problem (overheating when the load drops).

I assume this is a real phenomenon for NASA to bother writing up papers about it, patents for controllers to address the problem being applied for, but certainly some current, real world testing of the response in a small model would also be interesting.

[Tom Booth]

The previous link provided earlier in the tread to a patent application addressing the same issue is broken so here is an updated link

https://patents.google.com/patent/WO2006002545A1/en

... a Stirling engine has no control that corresponds to a throttle whereby the energy supplied to the engine corresponds to the load. When powering a Stirling engine with solar energy from a mirror array, the solar energy supplied to the engine is substantially constant, and so the load must be maintained at a sufficient level to use all the solar energy supplied by the mirror array. If the load drops, the engine very quickly overheats and is damaged. Similar overheating and damage can occur with other solar receptors as well.

To provide a level of control, the flat mirror segments on the mirror array can be mounted such that they can be moved by an actuator. Controllers activate the actuators and pivot the mirrors to produce the focused cone-shaped solar beam. The amount of energy received by the receptor can thus be varied. Thus when overheating is detected the mirror segments are moved out of focus to reduce the amount of energy received, such as when the load on a Stirling engine drops.

A patent application and now a proposal in a NASA brief are perhaps not conclusive, however these references demonstrate that there was at least a high level knowledge of this phenomenon i.e. heat input can be siphoned off by the electrical load on a Stirling generator. Or cooling/temperature reduction of an overheating Stirling engine can be accomplished through heat input conversion to electrical power output.

The statements in these two paper do not seem ambiguous or open to interpretation. The implication seems quite clear. A refrigerating effect can be achieved through conversion of heat to "work" or power output.

In a very real way, the electrical load on the engine or the power output from the engine serves as a "heat sink".

[Tom Booth]

There has been some discussion by Matt Brown where he uses the word "load" in a different way.

The diagram from the NASA paper makes clear that the "load" in question is the electrical output of the alternator/generator being driven by the Stirling engine, not anything to do with the internal regenerator

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[KristofB1982]

Tom thanks for this info. So my load will be milli amps on the Ltd engine. I know how I can heat up te cold Side plate. I Will try to measure the temperature with a contact probe. I hope we can see Some things happen.

[Tom Booth]

Tom thanks for this info. So my load will be milli amps on the Ltd engine. I know how I can heat up te cold Side plate. I Will try to measure the temperature with a contact probe. I hope we can see Some things happen.

OK, should be interesting.

[Tom Booth]

After running this "bullet" engine "free piston" for a while,


https://youtu.be/PEhvHg-Gruk


or rather trying to.

I found that it tended to do one or the other of two things.

Either the outward throw gradually increased until the piston popped right out the cylinder, or the inward throw gradually increased until the piston was smashing into the center metal orifice.

It seemed that the only way to get it to run relatively stable was to drape a strip of wet cloth over the central "heat sink" - cooling fins, minimal as they are (basically just groves in the engine body).

The "stability" achieved by this however was not so much a stable operation as a kind of secondary oscillation.

The outward throw would gradually increase to a certain point, the piston moving further and further outward while simultaneously "running" rapidly as usual, then there would be a reversal and the piston would gradually move further and further inward, all the while, of course, running with a rapid normal oscillation.

After moving inward, it would move back outward etc.

This secondary oscillation was observed by "Mower of Doom" with his free piston thermoacoustic engine as well, but he attributed it to a different cause (explanation at about 1:30 into the video, a little past 1/2)


https://youtu.be/cAyw_dOioMU


If you watch this segment carefully, you may observe what I'm talking about, the piston throw moving alternately inward and back outward repeatedly with the "pulsation" of the LED's

My engine however, doing the same thing, this secondary oscillation or "pulsation" was not attached to any linear generator. Note was it revolving.

I would attribute this pulsation to the purely thermal effects. This appears to be having a direct effect on power output. The brightness of the LED's correlates directly with the in and out movement of the piston.

I haven't made a close enough observation as yet to say what that correlation might be.

Is there more power output (LED's brighter) on the inward or outward oscillation? Or when the piston is running between the two extremes?

All I can say at this time is that I think this secondary oscillation deserves additional attention and study in that it seems to have a direct influence on efficiency and power output, as well as the general stability and smooth operation of the engine.

[Tom Booth]

With further observation of the Mower of Doom video, it seems obvious the LED's are brightest when the piston throw moves furthest outward.

However, this is synchronized also with the piston rotation, so I don't think it can be rulled out that the apparent increase in power output might be due to the magnet running off center within the coil as it rotated or perhaps the magnet is better aligned with the coil as it moves out.

I would speculate that perhaps the greater power output when the LED's are brightest directly causes a cooling effect on the internal working fluid. This cooling causes the gas to contract and the piston moves back inward.

In other words, the electric load on the engine has roughly the same effect as my wet cloth, serving as a "sink".

[Tom Booth]

Put another way;

Let's suppose there is an ideal position where the linear generator outputs the most power.

The gas,on average, heats up and expands gradually pushing the piston further and further outward as it reciprocates.

Once the piston and magnet reach the "sweet spot" for maximum power output, lighting the LED's brightly, the additional power output draws on the "internal "energy of the working fluid in addition to the energy input from the heat source, which at that point does not keep up with the energy output. As a result the working fluid, on average, grows slightly cooler, contracting, drawing the piston back inward, gradually, as it reciprocates, pulling the magnet back from the sweet spot, again reducing power, so the oscillation continues to repeat. At that point, the reduction of power output tips the balance.back towards excess heat input and the piston moves out again.

This could, in theory, be regulated (stabilized) by increasing the heat input when the load (power output) increases.

With my wet cloth, the partial stabilization produced by the additional sinking of the heat goes away as the cloth heats up and the engine stops running. It is also necessary to increase the heat input to compensate.

I might mention that I've also witnessed the same kind of secondary oscillation (or "pulsation") in IC engines from time to time but never thought much about it in these terms as a general tune up would resolve the problem.

In this video the "surging" is attributed to insufficient fuel, which I think correlates with insufficient heat.


https://youtu.be/gV6IOkevloM


BTW I do not recommend following the directions further on in the video. Especially drilling out a jet AFTER the carb has already been reassembled, putting metal fragments into the carb that could clog it back up and/or eventually get into the engine cylinder.

[matt brown]

Hey, Tom, imagine what the grid operators go thru to maintain load balance, then imagine how a rolling blackout is possible without crashing the grid.

[Tom Booth]

Hey, Tom, imagine what the grid operators go thru to maintain load balance, then imagine how a rolling blackout is possible without crashing the grid.

I thought a "rolling blackout" was a controlled shutdown to prevent the grid from crashing.

Anyway, reviewing this old video I noticed something which was unexpected at the time. I remember being puzzled about it


https://youtu.be/l2XcnN6QdfA


There is a steam generator putting steam up into a radiator hose. The Stirling engine on top.

There was obviously condensation on the bottom of the engine.

I imagined that it would be much hotter at the top of the hose, given the steam is rising. But the further up the hose I pointed the thermometer, it got up to about 180°F but then the temperature dropped off close to the bottom of the engine.

Later on, measuring the temperature of the bottom of the engine it was quite a bit cooler than the hose. I had expected it must be much hotter inside the hose than outside. I was expecting the bottom of the engine to be closer to boiling, hotter rather than cooler.

So, how to explain this?

It might be worthwhile to make a comparison, a running vs non-running engine on identical steam setups.

Is the engine keeping the hose cooler? Is the bottom of the engine cooler than the steam hitting it? Is this due to the engine running, converting the heat to "work" producing a cooling effect, or is there some other explanation.

Maybe the rubber radiator hose is more conducting heat from the steam generator heating base directly rather than being heated from the inside by the steam.

However, if that were the case, I would think the hose would be hottest at the bottom, but it was hotter nearer to the top, but dropped off, getting cooler again near the bottom of the engine.

Unfortunately I managed to destroy my magnetic engine trying to run it on the ceramic heat lamp, and also burned out the facial sauna steam generator.

These experiments can get rather costly at times, as well as time consuming, but I have an insatiable curiosity and don't like guesswork.

Well, I guess the rubber hose is conducting heat, or I guess the engine is having a refrigerating effect.

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It looks like about 30° cooler right near the base of the engine.

Another possibility is inaccuracy of the thermal camera I suppose.

[Tom Booth]

Note the + is the target reading of 142°

The red square below that indicates a thermal hot spot, the hottest point in the field of view.

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