LTD Stirling Engine Development-Displacer Design

[VincentG]

Hello all,

This will be a continuation of my thread here viewtopic.php?f=1&t=5506 about improving the performance of an LTD engine. I am so interested in the LTD design because it has potential to be powered from indirect fire, solar, geothermal and extreme cold/hot weather even.

I've since realized there are multiple specific problems that need to be addressed. My goal here is to keep discussion limited to a specific cycle/part of the engine at a time in an attempt to make the information easier to sift through and well formatted.

Tom Booth is off to a great start with his ceramic foam idea. My theory on displacer design has evolved a lot in the short time I've been working on the SE. I initially thought the displacer could have elements that were heated by the hot plate to increase surface area. While I still think this may work, in testing it has been a problem to keep the displacer from becoming heat soaked and destroying the operation of the engine. My latest displacer design has these goals in mind.

• Light weight

• Rigid structure

• Low thermal conductivity

• Low thermal mass

• Heat resistant bottom surface

• As always, ease of manufacture

This is the latest 3d printed frame for the displacer. It is quite a light and stiff structure.

high eff. displacer top.jpg (299.96 KiB) Viewed 7126 times

The bottom has been lapped flat on my granite surface plate. This will ensure a good seal around the hot plate gasket to prevent any leakage of heat while in the "off" position.

high eff. displacer bottom.jpg (281.61 KiB) Viewed 7126 times

The first level of insulation is a radiant barrier. In this case its aluminum can. I think a heavy foil would be better due to less thermal mass.

high eff. displacer radiant shield.jpg (314.96 KiB) Viewed 7126 times

The insulative fill should I think be dense enough to not increase dead space, but as highly insulative and light as possible. Here I am using fiberglass, though I will replace this with a stiffer material like rockwool. The soft material should rest against the cold plate to prevent heat from being lost to it.

high eff. displacer insulation.jpg (296.79 KiB) Viewed 7126 times

[VincentG]

Instead of a separate thread, I think we can also cover displacer movement and timing in this one, as well as the function of the displacer, aside from shuttling air back and forth. This includes actively using the displacer to shut off the heat source as much as possible, and the aerodynamic design features that may be added.

For anyone who has followed my initial LTD development thread, the cam operated displacer system was overall a success, but had many shortcomings, including increased friction and greatly improved complexity. The timing control and dwell periods were well defined and easy to manipulate, and the speed of the displacer was reasonably fast, with full movement in around 30 degrees of crank rotation.

Can we achieve the same or better operation with a simplified design I asked myself. After mulling it over all week, I think the answer is yes. And I think the answer might be a novel approach.

With the very simple addition of a spring, all of the timing control, dwell, speed and more is accomplished with greatly reduced friction over the cam drive. For testing the little LTD I have 3d printed a flexure based spring, but in a larger version any coil spring of the correct weight will do.

The spring is mounted mid height in the displacer. The height of the displacer is set to a certain percentage more than the height of the displacer cylinder minus the stroke of the displacer crank. The percentage over that number is what determines the dwell and timing of the displacer. The taller the displacer, the longer the dwell time, and the more retarded the timing is. The shorter the displacer, the shorter the dwell time and the more advanced the timing.

The timing can be swayed toward the hot or cold side by adjusting the center position of the spring. The only real disadvantage from the cam system is you cannot change one timing event without effecting the other. I think that is ok, considering the friction reduction.

The one added benefit is, the displacer now has down pressure(in addition to gravity) to better seal against the hot plate gasket. This should allow some hot air pressure to build up, and then "pop" up the displacer even faster when spring pressure is relieved.

The flexure based spring.

displacer spring.jpg (286.12 KiB) Viewed 7110 times

A slow motion video showing displacer movement.
https://photos.app.goo.gl/7n4QGBDEB5j4WdgW9

[VincentG]

I should have added that the spring in larger engines would be more accessible and easier to implement into the connecting rod. The connecting should be of adjustable length to fine tune the timing.

[Tom Booth]

While reading your description, I was thinking you were talking about a spring to counterbalance the weight of the displacer, then realized it was for timing/dwell. Quite ingenious!

[VincentG]

Thank you Tom, its amazingly effective and provides even more rapid lift of the displacer than the cam system without the friction. In fact, its the only reason the engine is running so well with my failed attempts at making a displacer.

A diy displacer is proving to be more difficult than I had imagined. Any attempt at a solid plastic structure quickly gets heat soaked. I've only been successful with an open plastic ring that contains Styrofoam pieces in the 3 voids. And even at that, its not as good as the factory foam displacer. The engine does not have a cold power stroke without the original displacer installed.

Next I will try a full foam displacer with a thermally decoupled high temp bottom plate.

[VincentG]

At this scale at least, it seems foam is king. I have fashioned a new displacer from xps home insulation board. It machines nicely. This one has an air channel cut into the top to allow hot air to flow to the piston while the displacer is pressed against the cold plate.

It is even lighter than the original foam displacer, and larger for a higher compression ratio. Combined with the new spring based displacer drive system, the engine has never produced so much net power. This slow-motion video show how rapidly the displacer is lifted, and you can see the piston accelerate right from BDC. The banging noise is the impact of the expanding air on the crankshaft and connecting rod. https://photos.app.goo.gl/fEbhjy95Yei1MVcEA

Keeping the displacer cool is of the utmost importance. In hindsight, it's no wonder considering the amount of surface area and the fact it is traveling directly through the working gas. Next, as I said I will focus on thermally decoupling a heat resistant bottom surface(unknown material yet) from the foam displacer. By doing this, in addition to higher efficiency, it may be possible to keep a foam based upper displacer even at higher temperatures. At larger scale, the foam board remains extremely rigid and light weight.

I'm encouraged by the fact this engine is still only operating at about 1psi of pressure difference. Even at these temperatures, there is much more to be gained.

[matt brown]

I should have added that the spring in larger engines would be more accessible and easier to implement into the connecting rod. The connecting should be of adjustable length to fine tune the timing.

Kudos for adding a spring !!! At the rate you're progressing, you're going to stumble across a few more ways where dwell (aka wait state/s) can be manipulated way beyond conv'l thinking (more than a one trick pony).

[VincentG]

Matt, what kind of trinkets can I offer you to make a post detailing all the tricks up your sleeve, to save me(and others) alot of time.

[matt brown]

Per previous comms, I spent years (decades) trying to scheme a kinetic solution to resolve the out-of-phase issues that plague typical SE. This chase focused on just alpha schemes where I was trying to find a simple solution that had discreet processes ('clean') and was a tough nut to crack. I put aside all the isothermal and regen issues, and focused solely on the kinetic issue/s. OK, 'mission accomplished' but it took me forever to then resolve some further legacy issues. Yep, I was blinded sighted by solely focusing on mechanical issues until they were resolved. Oddly, a very sharp physicist also stumbled across 'my' solution, but backed away due to these 'legacy' issues, and left it for me whether or not to pursue. So many old timers have died off that I've become motivated to pick up the torch again while I can.

Overall, the ECE trick appears to be gaming dwell, but differently for different cycles. Thus, no single trick works for all cycles, but the dwell mindset will expedite the chase. I still see the issue as mainly mechanical, not thermal, a kinda timing issue like playing musical chairs.

My main solutions are Stirling cycle (comm'l & DIY) for various reasons, but my main interest is in simple Otto solution. However, there appears to be some wiggle room for some low end Brayton, Ericsson, and oddballs. I can't say more due to potential complications.

[Bumpkin]

Vincent, for light-weight high-temp insulation you might try carbon felt welding blanket. A downside for your purpose is that it’s permeable so it would add dead space. Actually I bought mine in hopes it might breath well enough to use as a displacer. (It doesn’t.) Anyway it’s a neat material; light, soft, flexible, and it seems fairly durable.

Matt, decoupling the timing reminds me of a notion I had long ago about using springs in the rods of an Alpha, or likewise using a spring between the crank throws. It was inspired by that LTD “dipping can” Alpha engine. It didn’t seem like an equal piston displacement 90 degree Alpha should work at such a low difference, then I realized the timing was decoupled by the rise and fall of the water in the cylinders. viewtopic.php?f=1&t=5234&p=16540&hilit= ... ing#p16540

Bumpkin

[Tom Booth]

.

... This slow-motion video ...

How slowed down is that?

Can you post a video of this at normal speed?

[VincentG]

Bumpkin, I'll look into it. It sounds like it would work well for lining the interior of the displacer cylinder as well. Some permeability is a benefit I think as it usually means lower thermal mass.

[VincentG]

Tom that video is 240fps and it is 8x slower. The first 4 seconds are full speed. I'll upload a longer video when I get home.

[VincentG]

Here is a full speed run. It's only 30fps so it might be skipping a bit.

https://photos.app.goo.gl/JEBHT2jo14LPVyhP

[matt brown]

Matt, decoupling the timing reminds me of a notion I had long ago about using springs in the rods of an Alpha, or likewise using a spring between the crank throws.

Bumpkin

Hey Bumpkin, "a spring between the crank throws" nearly killed me...very creative !!! I'd never thought of that, but this clearly demonstrates where one can go with a dwell mindset. The problem I find with most unconventional ideas is that their performance envelope is small. However, innovation is the name of the game.