Stirlingengineforum.com

Bumpkin's comment on this thread seems to be on point:


viewtopic.php?f=1&t=2700

That's an average. It could be up or down a kPa or two, depending on the weather.

Yes, but Altitude is what matters, Temp, humidity... matters if you want to go down to the nitty-gritty.
Atmospheric pressure varies widely on Earth
At low altitudes above sea level, the pressure decreases by about 1.2 kPa for every 100 metres. (328 ft)

To go down 1 or 2 kPa (mean 101325, lowest on chart is ~100980 )
100325 83 m (274 ft)
99325 167 m (550 ft)

up one or two kPa (below mean sea-level)
102325 -82 m (-272 ft)
103325 -165 m (-541 ft)
106500 kPa the lowest place on Earth at 430 metres (1,410 ft) below sea level, at the Dead Sea

Yes it would be nice to know the barometric pressure, wherever these readings were taken

There may also be some (slight) pressure variations, depending on where, or at what point in the interior of the engine pressure readings were taken.

For example, if the displacer is moving up, pressure would be a little higher ABOVE the displacer, lower below. Reverse that if the displacer is moving down, pressure under the displacer might be slightly higher.

Anyway, assuming a clear day (high pressure system) near sea level, I think it is in the ballpark, aside from possible instrument calibration variations.

With such minute, fractional pressure variations, without more testing, it remains questionable/theoretical.

In this diagram I posted a long time ago in this forum, the numbered illustrations were meant to depict important or key moments in a typical LTD engine cycle. So, in other words, the divisions between the images around the cycle are not symmetrical or evenly divided.


So, to try to make things more understandable, I took some time and plotted the numbers corresponding to the images, along with some additional explanatory notations onto the PV readings diagram.


5 may extend down further towards 1, the "constant volume" cross over, where internal pressure drops below buffer/atmosphere with a corresponding drop in temperature.


It might be interesting to see where isotherms line up exactly as well, though in general, temperature increases in a top-right direction and decreases to the bottom left.

The blue a-b arrow can be ignored, I'm just robbing an image from here: https://physics.stackexchange.com/quest ... pe-at-that and that was there.

From the above, it appears there is a very sharp drop in temperature from 5 to 1

Well, naturally, that is when the hot plate is covered and the cold plate is exposed, but I wonder, is that all?

Air has elastic properties. So,..

At the end of the power stroke, the working gas is, I think, expanded, mechanically, to one degree or another.

That is, the momentum of the piston "stretches" out the working fluid like a rubber band.

On the return stroke, the working fluid is able to "relax", and like a rubber band that has been stretched, it cools. Or does it?


https://youtu.be/lfmrvxB154w


Is that possible?

When a rubber band is stretched out and held in that position, then allowed to relax, this has a refrigerating effect. It gets cold.

Does the stretching and relaxing of a rubber band parallel the expanding and contraction of the gas in a Stirling engine?

Why does the rubber band get cold and where does the heat go?

The molecules of the rubber band absorb heat as it relaxes. So, does the gas in the engine also absorb heat when allowed to contract after having been mechanically expanded?

I tend to think that because the engine is moving at a relatively high RPM, generally, the "compression" is mostly adiabatic (without heat exchange with the surounding) so, this is a potential alternative explaination for the rather precipitous drop in temperature between 5 and 1.

In other words, there is a sudden sharp drop in temperature, but not necessarily due to heat "rejection" to the sink, but rather like the rubber band, due to the elastic properties of the working fluid itself.

I think this is also interesting:


The PV tracing with and without a load. (Screenshot from this video:

https://youtu.be/dvomod6SsA0


Adding a load would appear to extend and increase the cooling (5-1 point of highest volume to lowest pressure)

Does the orange line correspond to atmospheric pressure?

It is hard to know what is actually happening with temperature.

Is the temperature of the working fluid different above or below the displacer? At least on average, I would think so. So, probably at least two different temperature readings upper and lower would be needed to give a somewhat accurate picture. As in this graph


But, judging as best I can, it looks to me like between load and no load, the period of heat input (up and to the right) stays about the same, but the period for cooling (down and to the left) is considerably extended.

Could it be that with a load the engine is running slower and can absorb more heat and cold.
They don't state the rpm, do they?
Yes very interesting.

skypupbob wrote: ↑Wed Mar 09, 2022 9:26 am Could it be that with a load the engine is running slower and can absorb more heat and cold.
They don't state the rpm, do they?
Yes very interesting.

Certainly, and quite obviously, just watching the video, there is a big change in RPM when the load is removed.

I think though, the temperature drops, and continues dropping during "compression"..

The volume is decreasing, so logically, the temperature should be rising. At least it is not "isothermal" which would follow the isotherm. The temperature is falling like a stone, creating a vacuum that is drawing the piston in. Even at a slower speed the RPM is too fast to explain such rapid heat loss by conduction into the "sink", especially where the alleged "sink" is made of acrylic and insulated to prevent heat loss.

So my theory is that with a load, more of the heat is converted to work. (And back into heat at the external break applied to the flywheel due to friction).

I need to do more experiments, but having a load on my engine seemed to cause a cooling effect, when running on ice, that caused the ice to keep re-freezing.

Logically, IMO, if pressure drops below ambient (the "sink") the temperature would also drop below ambient temperature. And isn't that what this chart shows?

The curved blue line at the bottom is the temperature of the working fluid the straight blue line is the temperature of the "sink". For a good portion of the cycle the internal temperature is below the "sink" temperature.

So,...

If the engine is running on ice, the engine is, internally, at times, colder than the ice it is running on.


https://youtu.be/2b2dIR8Eql8


About 15 minutes after the end of this video, continuing to run with a load (a thick grinding paste on the piston/cylinder) the engine was again "stuck". Frozen to the ice.

With the engine running very slowly, giving plenty of time for heat to be "rejected" to the ice, the ice should be melting more rapidly, not re-freezing

A little more meaningful stirling cycle diagram at it starts form Atmospheric pressure. I have no idea of accuracy or any other info about it but its close to what i was expecting it to look like in relation to Barometric pressure. A small overshoot makes sense.
source: https://www.stirlingengine.com/wp-conte ... m-real.png
Pressure Volume Diagram of a Real Stirling Engine

airpower wrote: ↑Thu Mar 10, 2022 12:56 am A little more meaningful stirling cycle diagram at it starts form Atmospheric pressure. I have no idea of accuracy or any other info about it but its close to what i was expecting it to look like in relation to Barometric pressure. A small overshoot makes sense.
source: https://www.stirlingengine.com/wp-conte ... m-real.png
Pressure Volume Diagram of a Real Stirling Engine

As far as I can discover, that appears to be a "model" (simulation using COMSOL software : https://www.comsol.com/comsol-multiphys ... gJ7zvD_BwE
some kind of theoretical Stirling heat pump cycle.

https://www.comsol.com/blogs/how-can-i- ... heat-pump/


The mostly negative pressure would seem to suggest some such thing as well.

IMO not a real or typical Stirling cycle at all, though I see it also appears on the American Stirling site, presented as such, they credit "console" for the image, which led me to the above links.

^^^

The mostly negative pressure

atmosphere is ~ 850 Pa below
The overshoot, negative presure is ~ 55 Pa
Technicaly there is no such thing as negative pressure, atmosphere is 1, everything else below, cant go below 0, only below 1
I like to view pressure as percent 1 = 100%

In a typical Stirling engine there is nothing there to generate negative pressure. Cooling fins can not in a hundredmillion years get to negative pressure.
The only other way to go way below atmospere is with a very poor designed / build engine and the flywheel takes it down.

By "negative pressure" I mean below the atmospheric pressure we all walk around in. What, in practical terms, would be considered a partial vacuum, where atmospheric pressure would exert a force to push a piston inward.

Normal atmospheric pressure is something like 100000 Pascals, right?

I'm not familiar with the COMSOL software, but I'm assuming p, or Pa is supposed to represent pascals.

I would guess 0 zero on that scale represents the equivalent of normal atmospheric pressure, but who knows?

At any rate a Stirling HEAT PUMP is entirely driven by an outside power source. At least that is what is described in the COMSOL article associated with that chart.

It looks like that HEAT PUMP is operating entirely at some kind of partial vacuum, which maybe makes sense for a Stirling heat pump/Cryocooler of some sort

At any rate it is apparently just a computer simulation.

I disagree. On my solar engine, which has a diaphragm power piston, when the displacer moves the air to the cold side you can see a strong negative pressure pulling the diaphragm in. If I stop it with my hand I can feel the negative pressure for a few seconds till the vacuum equalizes.
The engine seems to have equal pressure and vacuum on corresponding power strokes.
Nothing scientific, just my observations.

airpower wrote: ↑Fri Mar 11, 2022 5:22 am ...

In a typical Stirling engine there is nothing there to generate negative pressure. Cooling fins can not in a hundredmillion years get to negative pressure.
The only other way to go way below atmospere is with a very poor designed / build engine and the flywheel takes it down.

I can, I think, agree that there is no such thing as LITERAL "negative pressure", (below a total vacuum).

However, as skypupbob relates, cooling the air can result in below atmosphere pressure or a partial vacuum.

Another means of cooling, other than movement of the gas to the cold "sink" is conversion of heat to work output.

I would not consider an engine that results in an internal pressure below atmosphere to be "very poorly designed", quite the opposite, it results in atmospheric pressure doing roughly 1/2 the work to not only complete the cycle, but also contributing to power output.

The "negative pressure" also produces a cooling effect that maximizes the ∆T if not actually increasing it.

The more adiabatic expansion (with conversion of heat into work) results in internal cooling and pressure drop, below atmosphere/buffer, the more efficient the engine.

Tom Booth wrote: ↑Fri Mar 11, 2022 8:34 am By "negative pressure" I mean below the atmospheric pressure we all walk around in. What, in practical terms, would be considered a partial vacuum, where atmospheric pressure would exert a force to push a piston inward.

Normal atmospheric pressure is something like 100000 Pascals, right?


I would guess 0 zero on that scale represents the equivalent of normal atmospheric pressure, but who knows?

Yes 0 on that scale represents normal atmospheric pressure, what ever that was at the moment.
Atmospheric pressure various with location and weather changes so mean sea-level pressure of 101325 Pa at 15°C (59°F) is used as base.

Atmospheric pressure is 1, you can not go below 0 (negative) only a fraction of 1. Even when combining all available energy in the world no chance of going below 0. Of the shelf vacuum pump can go to 0.000049346164 atm, 5 Pa,

Most people underestimate the weight of Air, of how fast nature will correct and will try to go back to normal pressure.
The failure is at a pressure of about 21.2 inch of Mercury (inHg) or 71589 Pa, 0.71 bar, 10 psi, 287 inH2O, 0.70653 standard atmosphere (atm)
https://youtu.be/Zz95_VvTxZM

From my personal experience last summer. The weight of the whole thing is 42.6 n, 4.34 kg, 9.57 lb and lift is somewhere between 1cm and ½ in.
Nature will correct and fast
(Right click for "loop", That little vacuum pump could reach a pressure of about 55995 Pa if used as such)

99% of Stirling engines have no active cooling, without it, it is impossible to go below atmospheric pressure. There simple is nothing there to do so.
The only thing what can happen is the flywheel is working against nature and taking it down, aka inefficient.
The flame rising the pressure, the passive cooling fins bring it back to near atmosphere and somewhere in between the re-generator catches some of the heat to give back on the way back.

Below atmosphere is not negative, it is lower (less) pressure.