Stirlingengineforum.com

Your video does raise some significant questions Tom. My engine with screws and metal top plate will quickly heat soak even on a rapidly cooling cup of coffee. We waste energy on ICE motors to run water pumps and fans, so even if the expansive cooling phase is a slight "waste" of energy, the cooling effects may be substantial enough to be worth it. My modified engine moves much more heat into the system that stock. Clearly more than the working displacement can handle, so that why I have the need for more cooling.

The question becomes open cycle vs closed cycle. Don't read this Matt but I have my hopes on a closed cycle or this "hybrid cycle I seem to have stumbled on.

VincentG wrote: ↑Wed Mar 08, 2023 10:50 am ... My modified engine moves much more heat into the system that stock. Clearly more than the working displacement can handle, so that why I have the need for more cooling. ...

Not to be critical, but this seems like just throwing away the gains made for a return to mediocrity.

But my ideas are a hard sell. As someone on the science/physics forum told me recently, before I was banned again:

And don't try the Galileo Gambit here. I've told you before, several times, you are not going to overturn 150 years of engineering experience and thermodynamic theory with some badly done Mickey Mouse experiments in your garage.

But surely, I would think, having gone to all the trouble of putting more energy in, the best way to handle it is not to just take it away again without having been utilized.

To my mind, to be consistent with conservation of energy, that energy input has to go one way or the other. Either figure out a way to utilize it for power output or throw it away to the sink, but it can't possibly do both.

On the other hand.

Is it possible cooling in some way taps into molecular attraction forces?

Cooling water vapor for example, results in condensation to a liquid, a reduction in volume of some 800X.

I agree Tom, this is just for now. I plan to use the extra heat of the system to do more work. Just have to get around to the hardest part of making a new crank shaft and crank support stand. It will have a single support in the center to allow for easy stroke adjustments and cam changes. I hope to have enough stroke in this little cylinder to start the piston into expansive cooling. I think that will unlock significant power increases. Otherwise, I have contacted stirlingengines.co.uk and they can supply me with much bigger glass cylinders and graphite pistons.

What you've done so far is quite amazing.

I keep thinking about the little cox engines we used to play with as kids on radio controlled model cars and aeroplanes, no bigger than a thimble, but still quite powerful, like their larger IC counterparts.

So I always figured if someone could get some power into a small model Stirling engine it could be scalled up, in principle.

Personally I don't see a whole lot of difference between IC and external combustion engines. One heats the air from outside, the other from inside, but both rely on heating the air in a cylinder.

I've just ordered the exact same model for side by side testing, maybe I can have them lift a weight with a pulley and string. I want to be sure I'm not getting high on my own supply here.
I couldn't get the stock engine to turn the gear after a few attempts but it could have been a fluke. Either way I should test again to be sure.

I have though the same thing about ICE just being a hot air engine as well. Part of me thinks the ECE can have even higher power density due to 4 times as many power stokes per cycle AND there is no more issues with pre-ignition or knock. We can work with a compression ratio only limited by cooling and mechanical components.

VincentG wrote: ↑Wed Mar 08, 2023 5:57 pm... Part of me thinks the ECE can have even higher power density due to 4 times as many power stokes per cycle AND there is no more issues with pre-ignition or knock. We can work with a compression ratio only limited by cooling and mechanical components.

For some reason, your modifications got me thinking about the possibility of a double acting power piston on a Stirling Hot air engine. It seemed to me that being able to decouple the displacer from the crank, instead actuated by a cam, might make that possible one way or another.

I've read many times that it is possible to get 100% efficiency from the expansion of air pushing a piston, and that it is returning the piston to start over which introduces loses,.

With a double acting piston you basically have two expansion strokes, twice the power, I think, at least without the loses involved in just bringing the piston back to the starting position after each power stroke.

What I had in mind is the displacer lifting twice, once to drive the piston out and then again to drive it back with some kind of ports or slider valves, but haven't given the actual mechanics all that much thought, but something along the lines of a double acting steam engine, I suppose, except there might not be any need for separate intake and exhaust ports, if, while the gas is expanding on one side it is contracting on the other, due to expansion cooling, as your port system seems to create.

Maybe something to think about anyway.

There's one amazing site which not only provides simulated animation of Stirling engines but also calculator for engine specs:
Here's the engine calculator for gamma type:

http://jordaan.info/greengasoline/SGAMMA.HTM

You can see an engine graph in the top and if you hit the green "start" button in the block below, the animation will start.
Scroll down and you will see a list of text areas which you can input your engine's setting and calculate the result.
Scroll even lower and there's calculated data.

This site is called GreenGasoline.eu and made by a gentleman from Netherlands.

http://jordaan.info/greengasoline/

You can also find calculator/simulation for other types of Stirling engines on this site.
I hope this will help you in you project.

--------------------------------------------
I do want to build my own Stirling engine for home use(200~500 Watt is good enough).
The resources like biomass and sunlight is highly available around my home but I just don't have a proper device to turn them into power.
However, I don't have the skill for wielding and machining and commercial FDM 3D printer is my limit

There's one amazing site which not only provides simulated animation of Stirling engines but also calculator for engine specs:
Here's the engine calculator for gamma type:

Thank you I may play around with this but all the numbers make my head spin. It would be interesting to make real world adjustments to get the engine running best and then plug those numbers back into the calculator.

I have assembled the engine for testing with the new setup. I have to address some air sealing issues with the top plate on the 3d printed housing, but the engine ran much worse. I ended up finding that the heat is being introduced so much faster the cam timing had to be retarded 10 degrees or so. Doing this made it run better but still not well. Further testing seems to suggest I need, as Tom stated, reduce the "on" time to just a blip. The heat is being introduced so much more effectively that its overwhelming the small system.

When I independently move the displacer and piston by hand, the hot stroke now has a very noticeable "punch" on the working piston. No longer does it feel like just a steady "push". This failure to run better was disappointing at first but I think I need to take a step back and start from the beginning on cam design to make use of these different expansion characteristics.

VincentG wrote: ↑Thu Mar 09, 2023 11:44 am

There's one amazing site which not only provides simulated animation of Stirling engines but also calculator for engine specs:
Here's the engine calculator for gamma type:

Thank you I may play around with this but all the numbers make my head spin. It would be interesting to make real world adjustments to get the engine running best and then plug those numbers back into the calculator.

I have assembled the engine for testing with the new setup. I have to address some air sealing issues with the top plate on the 3d printed housing, but the engine ran much worse. I ended up finding that the heat is being introduced so much faster the cam timing had to be retarded 10 degrees or so. Doing this made it run better but still not well. Further testing seems to suggest I need, as Tom stated, reduce the "on" time to just a blip. The heat is being introduced so much more effectively that its overwhelming the small system.

When I independently move the displacer and piston by hand, the hot stroke now has a very noticeable "punch" on the working piston. No longer does it feel like just a steady "push". This failure to run better was disappointing at first but I think I need to take a step back and start from the beginning on cam design to make use of these different expansion characteristics.

I would assume that as the compression/expansion characteristics more closely resembles an IC engines "explosive" ignition, the timing would also need to be adjusted accordingly. i.e. much closer to TDC. At least at low speed. (Typically about 10° BTDC for an IC engine rather than the standard 90° recommended for most Stirling engines). The magnetic type displacer Stirling engines seem to be closer to TDC as well.

This is the kind of situation where some kind of live timing adjustment mechanism would be useful.

I was just about to post this when I read your response Tom. https://www.researchgate.net/figure/Cyl ... _328074542

I believe you are correct. Above is a chart of diesel cylinder pressure v. crank angle. Most of the cycle is within 50 degrees and the combustion itself, from other sources I've seen, can be as short at 8 degrees with modern direct injection. My current cam is lifting for nearly 180 degrees. I feel like I need a team of researchers for this little engine while I'm at work all day. There are more variables than I expected. Since the diesel engine is the most efficient and powerful piston engine we have today, I think its best to start by copying its timing/ cylinder pressure curves.

This seems promising not only for matching cylinder pressure v. crank angle, but also there will be far less drag from the cam mechanism. If the rpm goes up a bit, the cam does not even have to lift the whole amount. The follower will essentially jump off the ramp and continue to rise just a bit, as high speed filming has shown me already.

There might be some advantage using "Desaxe" crank offset due to TDC dwell (tho attached animation doesn't show this well).

https://en.wikipedia.org/wiki/Desaxe

I wouldn't worry about miming diesel, it's eff is solely due to high compression ratio. You guys are on to the main Otto ECE 'trick' which is a BUNCH of small heat inputs vs typical ECE thinking of simply more continuous heat input (which quickly stalls small ECE).

Another interestingly thing about Otto vs Stirling ECE is that Otto has less heat input per cycle than comparable Stirling has during regen per cycle. So, for many decades the Stirling camp has suffered many sleepless nights scheming regen improvements while the Otto camp laughs themselves asleep. IOW any means that can provide Stirling regen should be able to provide Otto input.

matt brown wrote: ↑Thu Mar 09, 2023 10:29 pm (...)
I wouldn't worry about miming diesel, it's eff is solely due to high compression ratio. ...

Sometimes Matt, I can't help but come to the conclusion you are intentionally throwing out red herrings, Or do you not understand that high compression results in a high degree of adiabatic expansion?

Diesel engines are efficient because the fuel injection is very brief at TDC and cut off quickly. The fuel is then free to burn as the gas expands greatly, as a result the combustion is very thorough and the heat generated more fully utilized.

I think a focus on Diesel engine efficiency is right on the money.

Sorry Matt, but your statement is so completely off base and contrary to known fact regarding how a Diesel engine actually operates I can't just let it pass. As usual we are in complete disagreement.

IMO what makes any engine, generally, very efficient is a very brief explosive impact at TDC, much like a bat striking a baseball. From that point on energy is extracted from the rapid expansion, including the momentum imparted to the piston (and flywheel if present) to drive the load, until the energy of the initial impact is fully exhausted - or infact, or in a sense, beyond, into expansive cooling, so that, in the words of Andrew Hall, the return stroke "is also a power stroke" .

viewtopic.php?f=1&t=5497

The Diesel engine however, is somewhat wasteful in that it has to constantly exhaust spent fuel. A Stirling engine does not have that drawback, but understanding what makes a Diesel engine so efficient is certainly important as a starting point for further development.

BTW, an early cut-off with a long adiabatic expansion is what made the most efficient steam engines most efficient.

That is, the steam would reach high pressure in the boiler and be introduced into the cylinder BRIEFLY, at TDC then cut off. From there the high pressure steam would be able to expand driving the piston, the steam cooling down and condensing back into water vapor in the process.

I believe this is also what makes the rice engine so powerful:

viewtopic.php?f=1&t=5483

Again, what do you have to say on that topic Matt?

viewtopic.php?f=1&t=5491#p18311

I don't know if it is deliberate or you just have a blind spot, but I'm almost inclined to advise: whatever Matt Brown says, do the opposite.