The power piston on the SunnyTech LTD engine is considerably larger in diameter than any of my other model engines, low or high temperature.
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Model LTD max power effort
Re: Model LTD max power effort
BTW, I just noticed while trying to find a new piston of some kind to replace the broken one.
The power piston on the SunnyTech LTD engine is considerably larger in diameter than any of my other model engines, low or high temperature.
The power piston on the SunnyTech LTD engine is considerably larger in diameter than any of my other model engines, low or high temperature.
Re: Model LTD max power effort
This old "thermoacoustic" displacer cylinder is just about a perfect fit, if I can clean up the broken edges without breaking it even more.
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Re: Model LTD max power effort
Watching that magnetic LTD engine run faster and faster with more and more heat input, and at the same time the displacer motion becoming progressively less and less until barely moving at all, I'm wondering if there is not a point where the influence of the displacer becomes negligible or actually non-existent, and the engine has transitioned over into operating in "thermal lag" mode.
Re: Model LTD max power effort
I was almost wondering the same, but the volume ratio is so low... I imagine a load would change things quite a bit too.
Re: Model LTD max power effort
Low "volume ratio"?
That crossed my mind. The LTD seems to have this big displacer chamber, but the actual volume is largely taken up by the displacer.
The thermal lag/thermal acoustic has a long tube, but much of that is empty space. The volume of actual working fluid is perhaps not all that much different.
Re: Model LTD max power effort
Could be. Its definitely surprising how fast it spins with such minimal(seemingly) heat input as the displacer almost vibrates off the hot plate.
I will say that watching the slow motion, and slowed even further with youtube playback, the displacer lift is more advanced than I had thought. A stronger magnet would get a bit more advance and lift out of it too.
I will say that watching the slow motion, and slowed even further with youtube playback, the displacer lift is more advanced than I had thought. A stronger magnet would get a bit more advance and lift out of it too.
Re: Model LTD max power effort
I increased the stroke further, to 1.2", with provisions to go back to 1". I was worried the displacer springs were too stiff but with just .2" displacer crank throw, it's no problem at all and they provide much more pressure on the heat sinks than before. Have to wait overnight for the silicone sealant to cure. Next time it's apart, I'll machine proper o-ring grooves in the displacer cylinder. It's the only way to get a reliable seal with the thin bottom plate. I should probably make a stiffening ring even at that. Youtubers often make it look easy and quick to build stuff, but it's not lol. Not enough time in the day.
Any bets on if this thing will run? I have my doubts.
Dead volume: 5.2cc (neutral space not heated or cooled) I can reduce this substantially with some more time.
Swept displacer volume: 5.25cc
Power piston volume: 7.2cc
I'll let you guys figure the compression ratio/swept volume ratio mumbo jumbo.
https://youtu.be/qcPsh7TKhUY?si=dRtdvGZWRyrrQihD
Any bets on if this thing will run? I have my doubts.
Dead volume: 5.2cc (neutral space not heated or cooled) I can reduce this substantially with some more time.
Swept displacer volume: 5.25cc
Power piston volume: 7.2cc
I'll let you guys figure the compression ratio/swept volume ratio mumbo jumbo.
https://youtu.be/qcPsh7TKhUY?si=dRtdvGZWRyrrQihD
Re: Model LTD max power effort
Still using the original bottom plate? I know what you mean about how thin it is. The top plates on those engines seem quite solid but the bottom is almost flimsy. At 1000°F mine was warped but I pounded it back down flat with a sledgehammer.
Still having problems finding something to make a good seal that could take the heat. A stiffening ring seems like a good idea, the leaks seem to be between the bolts. A thicker bottom plate might lay flatter. More bolts.
May end up looking like this thing
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Re: Model LTD max power effort
Original bottom plate still. More bolts would definitely help.
Good news and bad news. Silicone must have been bad and never cured. Wouldn't hold anything over 5psi and quickly leaked down to 0. Resealed with better silicone to give it one more try another day before orings. It tried to run even on just boiling water but between the higher compression and then heat input I could hear the pressure leak out right at tdc.
The larger displacer chamber could handle some air leaks, but at this volume ratio I don't think there's much tolerance for leaks.
Good news is that the foam displacer with the cutoff wheel heat shield and silicone paint job held up to direct flame no problem. That means I can use a 12" chop saw wheel to protect the larger displacer on my 60l drum build.
Good news and bad news. Silicone must have been bad and never cured. Wouldn't hold anything over 5psi and quickly leaked down to 0. Resealed with better silicone to give it one more try another day before orings. It tried to run even on just boiling water but between the higher compression and then heat input I could hear the pressure leak out right at tdc.
The larger displacer chamber could handle some air leaks, but at this volume ratio I don't think there's much tolerance for leaks.
Good news is that the foam displacer with the cutoff wheel heat shield and silicone paint job held up to direct flame no problem. That means I can use a 12" chop saw wheel to protect the larger displacer on my 60l drum build.
Re: Model LTD max power effort
Just wanted to be clear. The "neutral space" is volume that is not in direct contact with the heat exchangers but ideally will be heated and cooled along with the rest of the gas. It's the area between the displacer and the chamber walls, as well as the cutout in the displacer to clear the power piston.Dead volume: 5.2cc (neutral space not heated or cooled) I can reduce this substantially with some more time.
Swept displacer volume: 5.25cc
Power piston volume: 7.2cc
Re: Model LTD max power effort
Engine now holds pressure much better. Surprisingly it ran over a tea candle with the power piston volume set to 5cc(1" stroke). So thats 5cc Pp to 10.5cc total displacer volume. Power was low and favored the cold stroke.
A couple observations. With such little air to heat, the displacer required almost no advance to run best.
The aluminum top plate is so effective at conducting heat that if left uncooled, it quickly warms to a point that the gradient goes away. But if using ice water, it's seems to suck out heat faster than it can go in. I ran into the reverse problem when months ago I used an aluminum bottom plate on the stock steel top plate.
Something I should have remembered, the thermal transfer rate of the hot and cold end need to be matched. That means the same surface area if similar materials are used, or more surface area on the end with less thermal conductivity(hot end).
I'll make an aluminum bottom plate and try again. I'm thinking that maybe at this small scale, there's so little air and its too responsive for a crank driven piston. I'll also try powering the displacer with a motor and running free piston to check max cycle rate. Another thing to try is open cycle(Matt will smile here) with piston port exhaust as I had done in the past. I'm not married to one particular cycle.
A couple observations. With such little air to heat, the displacer required almost no advance to run best.
The aluminum top plate is so effective at conducting heat that if left uncooled, it quickly warms to a point that the gradient goes away. But if using ice water, it's seems to suck out heat faster than it can go in. I ran into the reverse problem when months ago I used an aluminum bottom plate on the stock steel top plate.
Something I should have remembered, the thermal transfer rate of the hot and cold end need to be matched. That means the same surface area if similar materials are used, or more surface area on the end with less thermal conductivity(hot end).
I'll make an aluminum bottom plate and try again. I'm thinking that maybe at this small scale, there's so little air and its too responsive for a crank driven piston. I'll also try powering the displacer with a motor and running free piston to check max cycle rate. Another thing to try is open cycle(Matt will smile here) with piston port exhaust as I had done in the past. I'm not married to one particular cycle.
Re: Model LTD max power effort
The previously mentioned pressure test was performed using a rubber plug adapter in the cylinder to my mityvac hand pump. Ending up finding that the glass piston is worn out and leaks horribly. At these volume ratios, small displacements and low RPM, leaks like this are enough to completely disrupt the theory of going beyond the zero point. It's amazing it even ran like this. I'll have to try casting an epoxy piston or machine one from Delrin of much tighter tolerance. If that's not good enough, I can try casting silicone into a mini air brake chamber style diaphragm.
Re: Model LTD max power effort
Here's what I came up with for a quick piston prototype. It's meant to run with some oil to help sealing and hopefully a long service life aided by the cylinder staying cold.
The end of the piston is tapped for 1/8 npt pipe thread and a Delrin npt plug was made to thread in and allow fine tuning the OD of the end of the piston. Pipe thread is tapered, so the more the plug is threaded in, the larger the OD of the piston gets. With no expansion from the plug, the piston has around .001" clearance to the cylinder. Unfortunately my lathe is old and beat up so it doesn't really make a round cut. It's eccentric by about .001". I may try my mini lathe and see if that makes a rounder cut. The milled flats are just to hold the piston with a wrench and adjust the plug.
Two Delrin gapless style compression rings handle the majority of the sealing. These rings are very low tension, I'll make some higher tension rings and see how that goes. It has very little friction as of now, and the faster the piston moves the better they seal. Hopefully, as with any ring style seal, they will wear into shape a bit for a better seal over time.
The end of the piston is tapped for 1/8 npt pipe thread and a Delrin npt plug was made to thread in and allow fine tuning the OD of the end of the piston. Pipe thread is tapered, so the more the plug is threaded in, the larger the OD of the piston gets. With no expansion from the plug, the piston has around .001" clearance to the cylinder. Unfortunately my lathe is old and beat up so it doesn't really make a round cut. It's eccentric by about .001". I may try my mini lathe and see if that makes a rounder cut. The milled flats are just to hold the piston with a wrench and adjust the plug.
Two Delrin gapless style compression rings handle the majority of the sealing. These rings are very low tension, I'll make some higher tension rings and see how that goes. It has very little friction as of now, and the faster the piston moves the better they seal. Hopefully, as with any ring style seal, they will wear into shape a bit for a better seal over time.
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Re: Model LTD max power effort
Also trying an epoxy piston for comparison. With a good precision lathe, I think the expansion plug and no rings would be the best/easiest option while also allowing adjustment for wear over time.
The hot plate and displacer are getting a total redesign to hopefully balance the heat input and extraction.
The epoxy displacer chamber seemed to work remarkably well, holding a good temperature gradient from top to bottom instead of reaching a more uniform temperature like metal would tend to do.
The hot plate and displacer are getting a total redesign to hopefully balance the heat input and extraction.
The epoxy displacer chamber seemed to work remarkably well, holding a good temperature gradient from top to bottom instead of reaching a more uniform temperature like metal would tend to do.
Re: Model LTD max power effort
Here is the new epoxy engine. Displacer chamber free volume is 15cc, with the displacer sweeping 14cc due to the flow channels cut into the circumference. Still have to get a power cylinder and piston.
The main cylinder with grooves cut for large silicone orings
A top view of the hot plate which is set in the epoxy with high temperature silicone.
Bottom view of the hot plate. There is a provision to have a sealed hot fluid circulation system.
Bottom of assembled engine without the sealed cap on hot plate.
Assembled engine. Pressure tested up to 15psi but the displacer bushing leaks pretty bad at that point. I'll need to upgrade this with a lip seal.
Thermal image after being tested on boiling water and ice for 10 minutes. I couldn't be happier with this result. Thermal separation is excellent.
The main cylinder with grooves cut for large silicone orings
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