More ran-dumb thoughts: Yes reciprocating stuff is crude, but reciprocating the working gas is the only way regeneration works as far as I know. Stirlings basically got put out of business by engines that don’t use it though, so…
I think the vane pump could make an engine on it’s own without any other complication by heating one side and cooling the other, and moving the axis to where the working volume and timing was right. But — it’s still a matter of getting enough power to overcome friction.
Even your mentioned 4/1 displacer swept volume to power piston swept volume seems low to me for biomass fuel without fancy gasification temperatures, but then I think in terms of cogeneration where you can set a slow burn and just walk away. I don’t know how much if any, heat other than cooking and hot water you would want; but if you were always right there cooking anyway you could easily tend a shorter hotter fire and 4/1 should be fine.
Back to rotary displacers; I once had a notion of double acting rotary displacers on one shaft with a bi-directional turbine between them. The turbine would simply be a flat fan with light flexible blades so it would turn the same way whichever way the air passed through.
Anyway, like everybody, I have so many unused ideas I trip over them every time I turn around. I hope you put some of yours to good use and share.
Bumpkin
I'm planning to build a Stirling generator to work off heat from our kitchen stove.
Re: I'm planning to build a Stirling generator to work off heat from our kitchen stove.
I've ordered the first parts to try and make a prototype rotary vane stirling engine. It's giong to be small and somewhat limited in provisions for a higher delta T. But I'm hoping to get a feeling for the viability of it.
I've got some ideas on how to work sort of a regenerator into that.
The only worry I still have is the bending of a cylinder when one side is hot and the other cold. Should be better with shorter cylinders, but ideally I'd like it to be around 30cm long to fit nicely into our stove.
Let's see.
I've got some ideas on how to work sort of a regenerator into that.
The only worry I still have is the bending of a cylinder when one side is hot and the other cold. Should be better with shorter cylinders, but ideally I'd like it to be around 30cm long to fit nicely into our stove.
Let's see.
Re: I'm planning to build a Stirling generator to work off heat from our kitchen stove.
The fire is usually only on for about an hour per time. The embers will last a bit longer after that. That's all I've got to use.Bumpkin wrote: ↑Tue Nov 14, 2023 9:41 pm Even your mentioned 4/1 displacer swept volume to power piston swept volume seems low to me for biomass fuel without fancy gasification temperatures, but then I think in terms of cogeneration where you can set a slow burn and just walk away. I don’t know how much if any, heat other than cooking and hot water you would want; but if you were always right there cooking anyway you could easily tend a shorter hotter fire and 4/1 should be fine.
I'll give this ratio a try in my prototype.
Re: I'm planning to build a Stirling generator to work off heat from our kitchen stove.
I'm connecting two things about regeneration. Please tell me if I'm missing something.
Regeneration is basically pre-heating and pre-cooling of the working fluid depending on which part of the cycle you're in.
Isn't that the same or similar to changing the timing?
I've read some research where they end up with an 130 degree angle as the most effective in stead of the normal 90 degree. A regenerator would basically change this 90 degree more towards the 130 degree timing.
I see timing as the placement of the piston at a certain part of the heat and cool cycle of the displacer. So if the air comming into the displacer cylinder pre-heated, that's basically the same as moving the displacer sooner compared to the piston.
Re: I'm planning to build a Stirling generator to work off heat from our kitchen stove.
Ideally the displacer should change positions at top and bottom dead center of the power piston position. That way the gas reaches full hot temperature at the beginning of the expansion stroke. The opposite for the cold compression stroke. Lost motion links, dwell, have been used to improve that over the more common harmonic motion, sinusoidal crank and slider motion.
Going to lost link, basically a Bang-Bang motion, is a compromise on beneficial vs detrimental effects. The energy of moving the displacer is lost, not saved by the crank-flywheel speed, and there will be more, noise and pounding. Meaning it will only work for small low mass engines and displacers. Great for proving a small toy model engine will run at extremely low temperature differentials and extremely slow speeds, rpms. However, it will provide very very low power, I.E., microwatts, with an incessant clicking.
Having harmonic motion for the crankshaft and piston means that timing is a compromise. At some advanced adjustment point, the heat will be added before top dead center enticing the engine to move backward. In the extreme, it may run excellent but be very difficult to start. Momentum of the crankshaft will have to push it through exactly the same as for an ICE. So timing other than 90° may very well be beneficial. Variable timing may also be an option. How to do that???
The regenerator is a heat suppler and saver, as you've said, preheater. A bit more complicated, okay for now. It's timing will be slaved to the displacer motions. It is plain and simply an efficiency enhancer, a very very important one.
Matt Brown pointed out that the regenerator saves multiple times the heat entering the engine per cycle. Imagine, at extreme, an engine getting 25% efficiency with regeneration, getting 5% without. Or 5% with and 1% without. With "free heat", efficiency doesn't seem important, but imagine getting five times more electricity from the same engine? 20 Watts without vs 100 Watts with. Build one with and without to see which has more kick.
The lack of regeneration is probably the main reason rotary displacers have not replaced their sliding counterparts.
Rotary expanders, and compressors too, suffer from an inability of either being heated or cooled, and worse if both are done. The rotor will heat on one side and cool on the other making heat travel straight through to the cold side. Please keep trying to solve those problems.
It might help to think like a heat particle. You go into an engine big and hot. Make the gas big and hot. Be expanded, cooled and rejected smaller. The bigger you go in, plus, the smaller you come out, the more the heat energy goes into work. If you short circuit the gas, no pressure, no work. If you can be saved from going out and put back in before more comes in, great.
Bigger hotter higher pressure in, more energy entering.
Smaller cooler lower pressure out, less energy discarded.
The more heat saved and reused the less needs to come in and go out. Savings on both sides.
Heat goes, in during expansion, out during compression. Work comes out.
Opposite for a refrigerator. Work goes in.
I know everyone here knows that. Sorry. I get off onto a tangent. I find this fascinating and helpful. Thanks.
Going to lost link, basically a Bang-Bang motion, is a compromise on beneficial vs detrimental effects. The energy of moving the displacer is lost, not saved by the crank-flywheel speed, and there will be more, noise and pounding. Meaning it will only work for small low mass engines and displacers. Great for proving a small toy model engine will run at extremely low temperature differentials and extremely slow speeds, rpms. However, it will provide very very low power, I.E., microwatts, with an incessant clicking.
Having harmonic motion for the crankshaft and piston means that timing is a compromise. At some advanced adjustment point, the heat will be added before top dead center enticing the engine to move backward. In the extreme, it may run excellent but be very difficult to start. Momentum of the crankshaft will have to push it through exactly the same as for an ICE. So timing other than 90° may very well be beneficial. Variable timing may also be an option. How to do that???
The regenerator is a heat suppler and saver, as you've said, preheater. A bit more complicated, okay for now. It's timing will be slaved to the displacer motions. It is plain and simply an efficiency enhancer, a very very important one.
Matt Brown pointed out that the regenerator saves multiple times the heat entering the engine per cycle. Imagine, at extreme, an engine getting 25% efficiency with regeneration, getting 5% without. Or 5% with and 1% without. With "free heat", efficiency doesn't seem important, but imagine getting five times more electricity from the same engine? 20 Watts without vs 100 Watts with. Build one with and without to see which has more kick.
The lack of regeneration is probably the main reason rotary displacers have not replaced their sliding counterparts.
Rotary expanders, and compressors too, suffer from an inability of either being heated or cooled, and worse if both are done. The rotor will heat on one side and cool on the other making heat travel straight through to the cold side. Please keep trying to solve those problems.
It might help to think like a heat particle. You go into an engine big and hot. Make the gas big and hot. Be expanded, cooled and rejected smaller. The bigger you go in, plus, the smaller you come out, the more the heat energy goes into work. If you short circuit the gas, no pressure, no work. If you can be saved from going out and put back in before more comes in, great.
Bigger hotter higher pressure in, more energy entering.
Smaller cooler lower pressure out, less energy discarded.
The more heat saved and reused the less needs to come in and go out. Savings on both sides.
Heat goes, in during expansion, out during compression. Work comes out.
Opposite for a refrigerator. Work goes in.
I know everyone here knows that. Sorry. I get off onto a tangent. I find this fascinating and helpful. Thanks.
Re: I'm planning to build a Stirling generator to work off heat from our kitchen stove.
I still feel like I'm learning the finer details every day. And it's all extremely interesting to me.
My idea for a regenerator on the vane engine would be to have a coolant ring between the hot and the cold side. This acts as a much needed insulator but also a regenerator if you look at the water flow in it. Because the engine is rotary it has one side that has a "heating to cooling" transition and one is a "cooling to heating" transition.
So you pump the water in at the "heating to cooling" side, when the water is coolest and the working fluid needs the extra cooling, and then pump it around to other side where the working fluid needs the warming from the already heated water.
That's what a regenerator does, but when I look at it this way it would only means I'm changing the timing a bit.
Which led me to believe the regenerator does a similar thing in other engines.
My idea for a regenerator on the vane engine would be to have a coolant ring between the hot and the cold side. This acts as a much needed insulator but also a regenerator if you look at the water flow in it. Because the engine is rotary it has one side that has a "heating to cooling" transition and one is a "cooling to heating" transition.
So you pump the water in at the "heating to cooling" side, when the water is coolest and the working fluid needs the extra cooling, and then pump it around to other side where the working fluid needs the warming from the already heated water.
That's what a regenerator does, but when I look at it this way it would only means I'm changing the timing a bit.
Which led me to believe the regenerator does a similar thing in other engines.
Re: I'm planning to build a Stirling generator to work off heat from our kitchen stove.
I'll also post the research I was talking about. Interesting read.
https://scholar.lib.vt.edu/ejournals/JO ... oster.html
https://scholar.lib.vt.edu/ejournals/JO ... oster.html
Re: I'm planning to build a Stirling generator to work off heat from our kitchen stove.
Sorry for spamming, but I thought I'd add this too.
If I apply a very crude ratio of heat exchange surface area to displacement it seems the rotary engine doesn't scale very well. It makes the displacement "ring" around the rotor thinner the bigger you go.
I don't know if this is a good way to calculate, but I use it to get an idea for the sizes of everything.
A 200mm diameter cylinder would need a 190mm diameter rotor. That doesn't look right to me hehe.
Also dividing the rotor with 4 vanes compared to 2 would lower the displacement: working piston ratio.
Just some findings from my maths. As I mentioned before, I'm not the best at those because the numbers start dancing in my head. I hope I've done something that makes sense haha.
If I apply a very crude ratio of heat exchange surface area to displacement it seems the rotary engine doesn't scale very well. It makes the displacement "ring" around the rotor thinner the bigger you go.
I don't know if this is a good way to calculate, but I use it to get an idea for the sizes of everything.
A 200mm diameter cylinder would need a 190mm diameter rotor. That doesn't look right to me hehe.
Also dividing the rotor with 4 vanes compared to 2 would lower the displacement: working piston ratio.
Just some findings from my maths. As I mentioned before, I'm not the best at those because the numbers start dancing in my head. I hope I've done something that makes sense haha.
Re: I'm planning to build a Stirling generator to work off heat from our kitchen stove.
I seem to remember seeing some rotary displacer designs that thermally coupled the opposing zones between the hot and cold zones for a degree of regeneration, with an external thermal bridge of conductive metal. As I see it, the whole chunk would have to run at about the same average temperature between the hot and cold sides, rather than having the gradient of a traditional back and forth regenerator. Dunno how effective it is, it might even diminish the last half of the otherwise natural hot-to-cold and cold-to-hot chamber transitions between the hot and cold sides. Anyway, I think it would be minimal affect for the effort.
The 4/1 ratio of displacer to power piston actually means about a 1/4 ratio of expansion/contraction, so (if I’m understanding your operating premise) if your rotor is 4 mm from the chamber at the end of the expansion zone it would be about 3mm at the end of the contraction zone. I don’t know how deep the vanes need to ride in the rotor to work smoothly, but if they were thin enough overall, might they be able to flex and ride the minimal thermal distortion of the chamber? I’ve never seen vanes that long and really don’t know anything about them.
Efficiency-wise, I think there’s a big thermal shuttling problem with the rotor and it would need to be as smooth and non-conductive as possible. Since it’s a smaller diameter than the chamber it of course has less surface area, which makes me wonder about maximizing that ratio. Power-wise, simple things are always the hardest and if you can make something this simple run at all I’d call it a winner.
Bumpkin
The 4/1 ratio of displacer to power piston actually means about a 1/4 ratio of expansion/contraction, so (if I’m understanding your operating premise) if your rotor is 4 mm from the chamber at the end of the expansion zone it would be about 3mm at the end of the contraction zone. I don’t know how deep the vanes need to ride in the rotor to work smoothly, but if they were thin enough overall, might they be able to flex and ride the minimal thermal distortion of the chamber? I’ve never seen vanes that long and really don’t know anything about them.
Efficiency-wise, I think there’s a big thermal shuttling problem with the rotor and it would need to be as smooth and non-conductive as possible. Since it’s a smaller diameter than the chamber it of course has less surface area, which makes me wonder about maximizing that ratio. Power-wise, simple things are always the hardest and if you can make something this simple run at all I’d call it a winner.
Bumpkin
Re: I'm planning to build a Stirling generator to work off heat from our kitchen stove.
Coupling the opposing zones is actually quite a nice idea worth examining. I suppose you'll always get some leakage between the zones, especially if you have two vanes and both zones are reaching opposite extremes. I looked at that as a bit of an efficiency loss, but in a closed cylinder this might just act as a regenerator.
I agree that the rotor would have to be made out of an insulating material. That makes it all the more challenging. These materials are usually not very good with high heat.
I agree that the rotor would have to be made out of an insulating material. That makes it all the more challenging. These materials are usually not very good with high heat.
Re: I'm planning to build a Stirling generator to work off heat from our kitchen stove.
https://www.researchgate.net/figure/Det ... _276918619
I just found this and it shakes up everything again haha. This seems to solve all the issues I have with my rotary design.
Although I don't fully understand it all yet.
I just found this and it shakes up everything again haha. This seems to solve all the issues I have with my rotary design.
Although I don't fully understand it all yet.
Re: I'm planning to build a Stirling generator to work off heat from our kitchen stove.
The thing I keep fighting with in my head is the regenerator.
It seems counter intuitive to me that at the moment you needs the air to be at its biggest to push the working piston, you start cooling it a little before it goes into the working piston cylinder. Thus shrinking it and loosing a little power.
The timing of it seems off.
It still feels to me like the regenerator is merely a shift in timing between the heating and cooling cycle. And that the cycle would benefit more from an actual shift in that timing with a more sudden change in temperature.
Maybe the limitation is usually mechanical? With crankshaft and piston setups I mean.
Or am I just understanding the basics completely wrong?
It seems counter intuitive to me that at the moment you needs the air to be at its biggest to push the working piston, you start cooling it a little before it goes into the working piston cylinder. Thus shrinking it and loosing a little power.
The timing of it seems off.
It still feels to me like the regenerator is merely a shift in timing between the heating and cooling cycle. And that the cycle would benefit more from an actual shift in that timing with a more sudden change in temperature.
Maybe the limitation is usually mechanical? With crankshaft and piston setups I mean.
Or am I just understanding the basics completely wrong?
Re: I'm planning to build a Stirling generator to work off heat from our kitchen stove.
Had a few brews tonight so maybe this is a bit too honest, but I was about to post something rude before I finally saw what smarter folks would obviously see; the second diagram of the hot rotor isn’t idiotic because it squishes a volume down to nothing; It’s idiotic because it doesn’t show a flow path to the cold rotor. Once understood; OK, so I think with the matching rotating regenerator that could work. And a little imaginereeing might even make it practical. I’d still say that if you can’t figure a way to radiantly heat and cool opposing sides of the regenerator you might be better off without its dead space, but kudos for exploring alternatives while so many others are trying to be the definition of insanity.
Bumpkin
Bumpkin
Re: I'm planning to build a Stirling generator to work off heat from our kitchen stove.
Yeah I don't think I agree with their regenerator idea either. They rely on opposing cycles to heat and cool each other, but that would require very thin metal in between. And then still I don't see how that's better than just some steel wool in a hollow space.
As I understand their design it has a big pool of fluid on the cold side. A way of transporting that to the hot side to create work. But on the hot side they connect a few chambers together with some tubing as a heat exchanger.
This got me thinking again. What if you would have pool of fluid on the hot side as well?
Then you'd need a way to pump it around. In one motion bring the hot air to the cold side but also scoop up some cold air and bring it to the hot side.
Enter the rotary vane motor.
With an eliptical cylinder like this and the right placement of the entry and exit, this could work.
I assume the side that pumps up the cold air would have to be a little smaller than the hot side.
Still feel like I'm missing something.
As I understand their design it has a big pool of fluid on the cold side. A way of transporting that to the hot side to create work. But on the hot side they connect a few chambers together with some tubing as a heat exchanger.
This got me thinking again. What if you would have pool of fluid on the hot side as well?
Then you'd need a way to pump it around. In one motion bring the hot air to the cold side but also scoop up some cold air and bring it to the hot side.
Enter the rotary vane motor.
With an eliptical cylinder like this and the right placement of the entry and exit, this could work.
I assume the side that pumps up the cold air would have to be a little smaller than the hot side.
Still feel like I'm missing something.
Re: I'm planning to build a Stirling generator to work off heat from our kitchen stove.
On that note, I reluctantly let someone borrow one of my LTD model Stirling engines the other day. I mentioned to be careful not to heat it too hot for too long as some parts were just plastic and styrofoam; only intended for running on a cup of coffee.
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