Model LTD max power effort

[VincentG]

Thats impressive from just a displacer mod. Builders of SE's need to start realizing that metal is not the right material for anything but the heat sink(s) and maybe the power piston. Everything else should be built with low thermal conductivity in mind. The high temp material from your HTD mod seems like it would work well in that engine.

[Tom Booth]

Thats impressive from just a displacer mod. Builders of SE's need to start realizing that metal is not the right material for anything but the heat sink(s) and maybe the power piston. Everything else should be built with low thermal conductivity in mind. ....

I tend to think the power piston might be especially critical, since it is the focal point for energy transformation. Every joule of energy the piston absorbs/conducts is a joule of energy left unconverted. At least that makes logical sense to me, though I've done only one semi-unintentional "experiment" to test that, the unintended "results" seemed significant.

Which puts using graphite pistons into question, as graphite is about as heat conductive as aluminum.

Graphite: 168. (W/(mk))
Aluminum about 150 - 240 (depending on the alloy)

The heat conductivity of cast iron and steel, especially stainless steel is much better (around 50 W/(mk) or less

Still, glass and ceramics are mostly under 10

Epoxy is 0.04 which is why I tried making an epoxy piston to replace a graphite piston. That was the time when, running on ice, the engine kept "sticking" to the ice. Apparently, that is, the ice the engine was running on kept re-freezing, though the cup of ice had already started melting on the surface before I started the engine running.

The reason I think that could be significant, it seems to suggest that a highly heat conductive (graphite) piston could be a path through which unwanted heat could enter into the working fluid on the cold side of the engine due to conductivity of the piston.

What I know, I think, for sure, is running the same engine on ice many times before but with the graphite piston that came with the engine, the ice under the engine never re-froze the way it did with the epoxy (2 part epoxy JBWeld) piston.

Maybe that was in part due to the epoxy piston being tight and causing some drag covered with grinding paste (more "work" output) but maybe that is not all.

168 conductivity for graphite vs. 0.04 for epoxy at a critical energy transition point seems like a likely differentiating variable for ice melting vs. Re-freezing.

I really wish I had more free time to do follow-up experiments but unfortunately I haven't. Anyway, getting banned from all the physics forums for reporting those results isn't a tremendous incentive.

Personally, when I went to check on the engine and tried to pick it up and it wouldn't move, I was flabbergasted. Like WTF????

Then it happened again and again as the engine kept running.

Needless to say as far as heat conductivity: "maybe the power piston" is a potentially important question to get answered.

If the piston absorbs heat, the molecules of the piston itself just "vibrate" more, so it gets hot and swells and causes friction. It the piston does not absorb the "heat" then the piston can only move in response to the "pressure" when a gas molecule collides with it.

Anyway, I've rambled on long enough on that subject.

You've probably already seen this old video:

https://youtu.be/2b2dIR8Eql8?si=_90-xE63_V9sXrJU

I just took another cup of ice from the freezer to show what I started with. By the time I got things set up, the ice was already wet on the surface. Normally it would keep melting with the engine running on top, but this time, with the epoxy piston the ice kept re-freezing right to the bottom of the engine. The third time I managed to record it, (that video) but then It refroze again about five or ten minutes after ending the video, after the engine had been running about 45 minutes.

[VincentG]

True Tom, I guess we just have to prove that more work can be done using a piston with low conductivity. Even a metal piston with a ceramic coating should be a substantial improvement.

Stirling engines have the advantages and disadvantages of only needing 1/4 the rpm of a 4 stoke ice. It's a good thing to give the air more time to heat and cool when we want, but the trade off is more time to heat and cool where we don't want it, like the power piston and the displacer/chamber itself.

[Tom Booth]

Very low heat conductivity of the piston seems to be a common denominator.

Your type engine is the same as the type used in this video, which, running on ice, also appears to "stick" to the bottom of the engine.

https://youtu.be/L6Jmdve1JK8?si=9Nl35PQHjnIJDf3-

If a Stirling engines is actually a heat pump" pulling heat from the cold side, maybe using a non-heat-conducting power piston/cylinder helps to make that apparent.

Both glass and epoxy have similar very low heat conductivity. 1.0 W/(m K) for glass.

I was not able to get my SunnyTech to run on ice.
Have you tried running your engine with just ice? Just curious now.

Perhaps it needs the full contact of a larger block of ice on the bottom, like in the video. A bowl of ice cubes only provides a few contact points.

I'm tending towards zeroing in on the low heat conductivity of the power piston and cylinder at this point, as a causative factor as far as the question; how in the world could a heat engine re-freeze the block of ice it's running on?

Not the topic of this thread exactly, but if this apparent cooling effect is REAL it could, theoretically have some influence on maximizing power output.

[VincentG]

I successfully ran it on ice after the foil was added to the displacer but before the long stroke mod. I'll try it on ice now but I doubt it will run on that low delta now. I'll be ordering another one to keep stock for test comparison so I'll try ice again.

While I do think its possible for these things to pump heat away, your engine refreezing ice may have been more along the lines of Ralphie licking the pole in A Christmas Story. My ice comes out at 5 to 10F, so thats alot of room to refreeze a melted surface with insulation on it.

That said, the cooling ice I add to the top does seem to last longer after the long stroke and dwell mod. I'll do back to back tests with the stock engine when I can.

[VincentG]

Tom this is right up your alley. Here's a video of this engine running with no flywheel to demonstrate the lack of compression work needed at operating temperature. The only reason it has any air spring effect at first is due to the advanced timing of the displacer at very low starting rpm. If timing were retarded, it would surely start up much easier.

https://photos.app.goo.gl/kaQ97EW8kSPoSLWP9

[Tom Booth]

Tom this is right up your alley. Here's a video of this engine running with no flywheel to demonstrate the lack of compression work needed at operating temperature. The only reason it has any air spring effect at first is due to the advanced timing of the displacer at very low starting rpm. If timing were retarded, it would surely start up much easier.

https://photos.app.goo.gl/kaQ97EW8kSPoSLWP9

By "air spring effect" do you mean the high pressure when turning it over?

I thought that was due to your high compression ratio modification.

Does it not do the same without heat and cold applied?

[VincentG]

Yes the air spring effect is from higher compression. Point is that at operating temperature, there is no more air spring effect.

The only reason it seems like there is still an air spring effect in the last video is from the advanced timing of the displacer causing "ignition" before TDC or BDC.

Once running at speed, no forced work means no flywheel needed.

[Tom Booth]

Yes the air spring effect is from higher compression. Point is that at operating temperature, there is no more air spring effect.

The only reason it seems like there is still an air spring effect in the last video is from the advanced timing of the displacer causing "ignition" before TDC or BDC.

Once running at speed, no forced work means no flywheel needed.

So,...

To what do you attribute the going away of the "air spring effect"?

The timing, at least in terms of degrees of advance doesn't change does it?

I can see it is difficult to turn over due to the high compression. Actually, it looks like you are lifting the power piston, so it's more like high expansion ratio at that point.

Before the engine starts, it looks like you lift the piston, which would "expand" the gas, and it snaps back down once the crank passes BDC (the extremity of expansion).

I've seen it go both ways, or either way though, depending on the volume of air, or how much air has leaked out.

Generally at "operating temperature" you should be able to either give the piston a quick pull out OR a quick push in. Or rather let it snap back one way or the other.

My "pet theory" of course, is that the push or pull or "snapping back" one way or the other has to be fast or quick enough to be adiabatic.

To me, when you say: "Once running at speed, no forced work means no flywheel needed"" the "at speed" is fast enough to have adiabatic expansion and contraction.

With a quick expansion you then have adiabatic cooling to help pull the piston back and adiabatic compression (velocity) to get a bigger bang at "ignition".

In other words, I suspect, once running, the ice cube would be redundant as you have adiabatic expansion/cooling.

It might be necessary though, to replace the ice with insulation to avoid ambient heat eventually warming up the cold plate, though with the high compression/expansion you might just get away with no ice or insulation, once the engine is up and running, but insulation would probably help keep the cold side cool.

I know that probably sounds insane. Or maybe you agree?

Anyway, you've answered a long standing question. For a long time I've wondered if an LTD could possibly run without a flywheel. It seemed doubtful, but now we know. Thanks!

An actual experiment is always worth more than any amount of speculation and debate. It's great to have you on board. Your very adept with the rapid prototyping and willing to do "out of the box" unconventional experiments. You get "the most valuable player award". in my book anyway.

[Tom Booth]

In other words, when the compression is fast enough you get the diesel or fire piston effect, as well as the adiabatic cooling.

https://youtu.be/-kWF1go2J-Y?si=bVd0thbHNOVcMA0Y

[VincentG]

This will be a bit hard to describe so here goes...

What you see while I first try to start the engine is just an illusion of the air spring effect. There are no adiabatics at play, instead the explanation is as follows:

There is enough volume ratio and temperature difference that when the engine is allowed to rest steady state for even just half a second, gas either leaks into or out of the system. This greatly influences the balance of the cycle.

This is compounded by the timing advance of the displacer. Remember, in real operation the gas takes some time to heat and cool, so the displacer leads the power piston by 15 degrees or so.

Once the engine is sustaining itself, a balance is struck with the internal gas mass and the timing of the displacer becomes more ideal. At that point the air spring effect is gone. The only time the internal pressure reaches 1atm is at full cold TDC, and full hot BDC. Otherwise internal pressure is above 1atm on the hot stroke, and below 1atm on the cold stroke.

In fact, based on what I'm suggesting here viewtopic.php?f=1&t=5572 , unless the power piston is large enough to reach the "zero point", the internal pressure never reaches 1atm at full cold TDC or full hot BDC. This is because the expansion and contraction range of the gas volume is greater than the power piston volume.

*Note when I refer to TDC, it means the power piston is visually down as the LTD sits. I think I remember some confusion on this from another post one time.

And Tom, this engine certainly needs active cooling. Without cooling the cold plate becomes hotter and hotter until power is reduced greatly. I do believe that with limited heat input, and therefore an adiabatic heat stroke(as I think you are going for), cooling may not be needed. But in this case we are going for maximum power, so I think it's a bit different. And thank you for the kind words, it helps to stay motivated. I will add that your speculations have given me alot to think about. In fact, there is a video I found that I'll post in another thread that helps to explain your thoughts on the free return stroke. There seems to be that missing piece to the puzzle that you talk about and the physics forum didn't want to accept.

[Tom Booth]

This will be a bit hard to describe so here goes...

What you see while I first try to start the engine is just an illusion of the air spring effect. There are no adiabatics at play, instead the explanation is as follows:

There is enough volume ratio and temperature difference that when the engine is allowed to rest steady state for even just half a second, gas either leaks into or out of the system. This greatly influences the balance of the cycle.

(...)

I'm going to stop you there, as I don't think your explanation is supported by what is seen in the video.

In the video, after the first second, what came prior to that moment I don't know, but the piston ends up DOWN, that is, at TDC or full compression.

You turn the flywheel by hand which raises the piston, mechanically expanding the gas.

If there was any significant "leakage", instead of the gas expanding and the pressure falling, air would simply leak into the chamber and the piston would stay wherever you left it.

You "let go" at BDC, full expansion

If air had leaked in the piston would not spring back to TDC from BDC when you let go, it would just stay at BDC. Certainly if air had leaked in it would not spring all the way back to TDC but the air that had leaked in would prevent that and the engine would never start.

A Stirling engine running at all depends heavily on the absence of any significant air leaks. What is "significant"?

In the video you turn, then hold the crank at BDC with the gas mechanically expanded. If air leaked in so quickly as you say: "even just half a second", why would the piston snap back to TDC at all? How could it run? Why doesn't it stop half way down or 3/4 or 1/4 or whatever, depending on how much air "leaked" in?

In my limited experience with that type of engine those long glass power pistons in glass cylinders seal remarkably well.

[Tom Booth]

Watching the video several more times, I see that the camera angle tends to partly obscure what is actually going on.

The piston stops at a slight angle before TDC.

You actually push it on through the high compression at TDC, then the crankshaft very quickly spins all the way back around to just before TDC again.

This is repeated several times.

It seems very obvious to me you push the piston through the "hump" so to speak, of high pressure, then it suddenly "springs" loose and all the way around back to the begining of the "hump" where it hits high pressure again.

This sudden rotation is assisted by the heat input, and no doubt the heat "rejection" due to the displacer movement.

I'm still not sure what you intend to demonstrate by this exactly

[Tom Booth]

Tom this is right up your alley. Here's a video of this engine running with no flywheel to demonstrate the lack of compression work needed at operating temperature. The only reason it has any air spring effect at first is due to the advanced timing of the displacer at very low starting rpm. If timing were retarded, it would surely start up much easier.

https://photos.app.goo.gl/kaQ97EW8kSPoSLWP9

Going back and re-reading, you said: " to demonstrate the lack of compression work needed at operating temperature.

I think I more or less sort of agree.

The ENGINE at least, is not doing the compression work, especially via a flywheel that isn't there.

Is it atmospheric pressure then? Or "contraction" of the gas, as Hall would say?

The gas volume reduces. That much, I think is obvious. How and why gets a bit more complicated and involves multiple factors and influences.

Some might argue that your rather large 3D printed counterbalance acts as a flywheel.

[VincentG]

I don't think I can explain without you feeling the engine through various points in the cycle. When the engine was sitting there with the hot plate covered and the piston just before TDC, the pressure equalizes towards ambient. If I stop the engine with the cold plate covered and the piston at BDC, the reverse will happen.

As I have said, this engine is not at the zero point, so(theoretically) if you stop it after its been running for a while at the moment the piston is at TDC and the hot plate is covered(assuming 0 degrees advance), the internal pressure is still below ambient.

I can easily demonstrate with the engine up to temp that if I allow the internal pressure to equalize(using the test port) at full volume and full cold, there is much more compression work, and vice versa. However, the engine quickly sorts itself out through the leaks inherent to the design, like the piston, displacer rod and o-rings. If the engine was 100% absolutely air tight, it would be a nightmare to reach the balance point of internal gas.