Stirlingengineforum.com

VincentG wrote: ↑Sun Mar 12, 2023 4:00 pm
I have to add here that the point of this post is for home power generation with a biomass or solar fuel source. We are not concerned about absolute efficiency, just power production with a real world engine build. And big industries have proven it can be done.

When I first got into SE, biomass was all the rage. I later stumbled across some basic combustion science that eluded the difference between theoretical and real heating values. The average guy burning something is blind to the low heat value (LHV) vs the high heat value (HHV). The basic difference is that HHV is the 'chemical' value of combustion vs LHV is the chemical value minus the loss to latent heat of water formation. So, something like coal (think 100% carbon) has HHV=LHV since there's no water byproduct, but some like gasoline has LHV=.92 HHV and losses 8% to water byproduct, while hydrogen losses 18%. Yep, those guys pitching hydrogen as the fuel of tomorrow are stoned or conmen, kinda like those guys pitching ethanol in gasoline. Ethanol fuel additives probably takes more energy to produce than they supply (aka negative energy balance) but their real purpose is to drive up the price of grain, not shrink oil consumption.

Most 'dry' biomass is assumed LHV=.6 HHV but rarely seen in this simple technical reduction.

VincentG wrote: ↑Sun Mar 12, 2023 7:46 pm

but to use all this heat would require the pressure to be reduced by 2.64 as in 1.32 atm>>>.5 atm with a 1:2 adiabatic expansion. No way, Jose.

This does not seem to be taking into account that the displacer will shut the heat off for us. There is no need to use all the heat on the expansion stroke, we are just using the pressure spike to drive a piston. The temperature will drop 200 degrees at BDC(play along with me here) and return the piston under vacuum.

One of us is missing something here. The way I see it is that (ideally) the whole gas volume most be heated before the working piston volume expands. I think you're falling into the reservoir scheme trap where you imagine the working cylinder can selectively take little 'bites' out of the reservoir gas at will, use it, then return it at will.

matt brown wrote: ↑Sun Mar 12, 2023 7:28 pm
(...)

Interestingly, Tom will probably spot the way out...

Not likely.

AS far as I'm concerned, as usual, your imagined issues are jumbled up theoretical nonsense you spew off the top of your head like a fountain of BS that's not worth the brain strain it would take to try and untangle.

matt brown wrote: ↑Sun Mar 12, 2023 7:41 pm

Tom Booth wrote: ↑Sun Mar 12, 2023 5:37 pm
Compressed air STORED in a tank certainly does frost up when discharged.

Requires really dry air and really large pressure drop, both of which are rarely encountered. I was surprised that even the large compressor brands shrug off the moisture issue.

All I can do is shake my head in disbelief at your comments. It seems pretty obvious you have no first hand practical experience with compressors or much of anything else, sorry to say. I'd debate the issue further but this is not my thread to derail. I can only marvel at how nearly every word out of your mouth is so wrong, or off base.

One of us is missing something here. The way I see it is that (ideally) the whole gas volume most be heated before the working piston volume expands. I think you're falling into the reservoir scheme trap where you imagine the working cylinder can selectively take little 'bites' out of the reservoir gas at will, use it, then return it at will.

Matt, not only am I claiming this, but in practice it seems to be the biggest hurdle to overcome. The pressure in the cylinder is directly proportional to the position of the displacer. In other words, lift the displacer a little, and the piston rises a little, it will not continue to rise unless the displacer is lifted further. The same goes for lowering the displacer from its full upward position.

If I had my way, the air would continue to expand with just a small lift of the displacer. But that is simply not the case, as it seems the cold side of the cylinder takes this heat away. Perhaps Tom is right here and there should really be no cold side. Just perfect insulation, and only just the necessary amount of heat input, then let the pistons work alone cool the gas.

The more I experiment with this engine, the more I see how fast the gas actually gains and loses temperature, due to its incredibly low specific heat.

Also, the discussion of water content in the air is extremely relevant to the operation of hot air engines and does deserve its own thread to be investigated thoroughly.

VincentG wrote: ↑Mon Mar 13, 2023 7:33 am ...The pressure in the cylinder is directly proportional to the position of the displacer. In other words, lift the displacer a little, and the piston rises a little, it will not continue to rise unless the displacer is lifted further. The same goes for lowering the displacer from its full upward position.

If I had my way, the air would continue to expand with just a small lift of the displacer. But that is simply not the case, as it seems the cold side of the cylinder takes this heat away. ...

I suspect when the engine is running at some speed, expansion continues due to momentum regardless of the heat source being blocked by the displacer, but this results in cooling

As you suggested earlier:

The overextension towards BDC and TDC due to momentum is what must help change the temperature of the gas much quicker than any heat exchanger could ever hope to.

I will film a quick video tonight to demonstrate what I mean. But in short, we can eliminate the movement of the piston and focus on the ability of the displacer alone to effect pressure change in the system. What I find by doing this is that the pressure change is nearly instantaneous, at least to the degree I am able to record, and within the rpm range of the engine. And again, there seems to be no thermal lag. Just pressure change in proportion to the position of the displacer. The cold sink is removing heat as fast as the hot side can generate it.

Interestingly, I assembled the engine with the extremely thin aluminum hot plate. Using a small candle, the engine starts almost immediately after the application of heat, as I was hoping. The problem is that its so effective at delivering heat, the engine quickly heat soaks the cold plate and displacer foam. After that it has nearly no cold stroke, and must be run in open cycle to have any power. It does however run nicely on my wood stove with a more moderate temperature input, at least for longer until heat soak. I would need a much smaller candle flame to deliver a low enough btu output, which I may try to achieve.

I believe I could modify the cold plate to a similar thin aluminum design to effectively remove the heat as fast as its coming in and restore the balance in the system. But I think this is a down hill spiral, trading power for efficiency.

It seems to me, there is an inherent design flaw in the basic design of the displacer system that causes excessive heat transfer loss. As Tom so often states, the cold side maybe should not be "cold" but instead a perfect insulator, and the heat source dialed back to just the right amount to power the piston to BDC. But we are dealing with real world materials(the displacer and its cylinder) and as such they will become heat soaked I believe. The question is how to provide cooling without nixing the heat of the expanding gas.

Maybe the overextension toward BDC can provide this cooling effect for us, but it seems the system must be in perfect harmony to achieve this result. Perhaps there is a more robust solution.

VincentG wrote: ↑Mon Mar 13, 2023 11:16 am I will film a quick video tonight to demonstrate what I mean.

In slow motion would be interesting if possible.

What do you mean by "open cycle"?
", the engine quickly heat soaks the cold plate and displacer foam. After that it has nearly no cold stroke, and must be run in open cycle to have any power."

...the heat source dialed back to just the right amount to power the piston to BDC. But we are dealing with real world materials(the displacer and its cylinder) and as such they will become heat soaked I believe. The question is how to provide cooling without nixing the heat of the expanding gas.

Maybe the overextension toward BDC can provide this cooling effect for us, but it seems the system must be in perfect harmony to achieve this result. Perhaps there is a more robust solution.

I should mention, I don't, with 100% certainty know what's going on as far as the working fluid temperature in an acrylic top engine insulated with silica Aerogel.

Does the gas actually get cold every 1/2 cycle or does heat build up inside the engine and it just doesn't care?

On the Science forum some have suggested that the insulation builds up heat on both the hot and cold side which still maintains a temperature differential sufficient to keep the engine running.

Is it possible for the working fluid to get very hot on the cold side and still contract enough for atmospheric pressure to drive the piston back regardless?

Unfortunately I haven't yet done any experiments involving drilling holes and inserting temperature probes, mainly because I've been trying to find probes responsive enough to get meaningful live readings rather than just some average temperatures

As far as "perfect insulation" I also need to do that back to back experiment. (2 engines with Cold sides together).

That has been delayed because I found that most of my experimental engines are pretty warn out, and no longer have a good piston seal. The graphite pistons tend to get warn and loose compression after so many hours.

I sent for a bunch of new glass piston & cylinders but will need to rebuild the engines.

I had previously drilled the hole in the cylinder at BDC to let excess expanding air out. So when that hole is open it exhausts expanding air and intakes fresh air at BDC.

I apologize for the poor video, my camera was not focusing well. I am moving the displacer very rapidly and have placed a weight on the piston to simulate a load.

https://photos.app.goo.gl/qQyZaFMJTxoFa3Jp9

I received the other LTD engine in the mail and installed a pressure port to make a comparison. I think maybe I never had ice on the first engine when I said it would not drive the gears when it was stock. This engine has no trouble driving the gear on simmering water and ice. But that is not the full story. Three things are clear after having the engines side by side. First, it is alarming how much extra drag my cam system adds. Second, the gross power gains are SIGNIFICANT. Third, these crank driven Stirling engines are really a marvel of simplicity and a beautiful thing to watch, regardless of practicality. I feel quite happy with the results and I think when scaled up, the drag would no longer be a significant reduction of net power.

These are the tests with the pressure gauge, both engines on simmering water and with a single ice cube on top. The pressure swing of the modified engine seems nearly double. Interesting to note but not filmed here, at full unloaded speed the stock engine shows 0 gauge movement. Its has such low friction it coasts along with effectively no net power output.

Here is the stock engine.
https://photos.app.goo.gl/2Fvg6kRerBbFF9Vx9

Here is the engine with the cam driven displacer. Note: this engine has ran much stronger and is quite finicky to adjust just right.
https://photos.app.goo.gl/K96FpDia8WMVbe7W8

VincentG wrote: ↑Mon Mar 13, 2023 5:26 pm I apologize for the poor video, my camera was not focusing well. I am moving the displacer very rapidly and have placed a weight on the piston to simulate a load.

https://photos.app.goo.gl/qQyZaFMJTxoFa3Jp9

The video seems clear enough. As far as "I am moving the displacer very rapidly", relatively speaking, I would say that is rather slow in comparison with a running engine.

What I find especially interesting is how the weighted down piston appears to bounce or oscillate up and down at the end of the stroke, especially after, or at the end of the compression/contraction.

It looks very much like a buoyant object dropped into water.

In poking around for a video of some buoyant object being dropped into water I came across this which, if not particularly applicable is at least interesting.

https://youtu.be/bodsuTucSxQ

This is interesting, but rather than more weight increasing "displacement", in a confined container of air the pressure increases with weight.

https://youtu.be/K4nFoU2Wa_k

Needless to say, your heavily weighted piston is much heavier than any normal small graphite or other LTD piston. The buoyancy-like effect is quite interesting.

Yea I did notice that. I just wrote it off to an air spring effect. But you have to wonder what influence it has on the engine's operation.

VincentG wrote: ↑Tue Mar 14, 2023 9:12 am Yea I did notice that. I just wrote it off to an air spring effect. But you have to wonder what influence it has on the engine's operation.

The weight of the power piston or diaphragm can have a huge influence on free piston type engines.

There are several videos on YouTube of people stacking nuts or magnets on the piston to get the weight just right to match or change the frequency of oscillation at which the engine will run.

Here is an extreme example

https://youtu.be/N20oUFN-mXo

That demonstration of the free piston shows exactly how picky these engines are to be in the sweet spot. Now imagine the difficulty in containing that movement with a crankshaft.

In light of my most recent observations, I am temporarily abandoning further modification to the LTD as it exists in its basic form. I believe there is no amount of trickery to mask the fact that the displacer consumes mechanical power and acts as a heat sink of its own. Also, it seems you can only band aid the main issue as I see it. That is, as long as the hot and cold side are linked together by air, there is a significant loss of heat energy that should be used to do work on the piston.

Work will begin on a design that:
-Uses the piston itself as a displacer
-Uses the piston itself for timing control
-Separates the hot and cold side through insulation
-Isolates the hot and cold gas until mixing is necessary
-Combines elements of Otto and Stirling cycles

If this tangent leads nowhere, I'll return to the LTD for further refinements.