LTD Stirling Engine Development-Displacer Design

[Tom Booth]

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

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

The link isn't working for me. Get a "not found" error.

Anyway, I think your flexure spring dwell mechanism could have wide application. For example, in a more traditional Andy Ross type Stirling engine:

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Also, it might be a means of softening the noise generated by a Ringbom "Tapper" type engine.

https://youtu.be/xTShY1T5V5Q

From what I can tell from your first slow motion video, the engine appears, or gives the impression of having a remarkable slow steadiness to it's operation, like a double acting steam locomotive engine. A consistent push - pull action that results in good torque through the entire 360° rotation.

[VincentG]

https://photos.app.goo.gl/ZbqxHPNjJSkYFnAv7

It runs faster than ever. But now has good power at higher rpm and great low speed torque, with a consistent push as you say. I would be interested in buying another scale model gama and alpha type to see how the spring influences their operation. It can be set to be non contact I think, and just the stretching of the spring will delay the displacer or piston at the ends of it's travel, for higher speed engines.

[Tom Booth]

https://photos.app.goo.gl/ZbqxHPNjJSkYFnAv7

It runs faster than ever. But now has good power at higher rpm and great low speed torque, with a consistent push as you say.

Very impressive! Thanks!

I would be interested in buying another scale model gama and alpha type to see how the spring influences their operation.

That could be interesting.

It can be set to be non contact I think, and just the stretching of the spring will delay the displacer or piston at the ends of it's travel, for higher speed engines.

I can imagine how that might work, but then, is that the equivalent of the NASA engine?

nasa_stirling_flexure.gif (292.8 KiB) Viewed 8714 times

[VincentG]

I can imagine how that might work, but then, is that the equivalent of the NASA engine?

I was still thinking of a crank based control system. It was my understanding that NASA used that flexure mechanism as a form of dampening control and to control the displacer with a purely axial influence (aka: zero side loading) to avoid any metal to metal contact in their air bearing system.

Otherwise it seems to be a ringbom type control. I like the full contact displacer of the LTD with the spring control as it allows a forced timing event beyond just being pressure based.

[Tom Booth]

BTW, what's that little cone? Looks like a rubber cap covering a valve or something.

Sorry if you've already explained this, but looks like something new afaik.

Are you charging this thing with hydrogen or something?

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Oh,.. I think I know. That must be where you were attaching the pressure gauge. Covered up as not in use.

BTW #2

Is it OK with you if I take the liberty of uploading some (or all) of your demonstration videos to my YouTube channel? Maybe as a what do they call it - Playlist.

That type engine was my first Stirling, I naively hoped to generate power from the top of my wood stove, like in 2010 or something. It ran great for about 2 minutes with what seemed like quite a lot of power, then the clear plastic housing and styrofoam displacer started melting. So,...my quest to find replacement high temperature materials. I think I may have just about arrived at a solution accidently, with the Perlite and waterglass "paper" development.

Imagine your engine running on a 2000°C propane flame. A very hot wood fire can get as high as 1000°F. My goal is finding materials that can withstand a hydrogen flame which is about 200° hotter than propane.

[Tom Booth]

..... I like the full contact displacer of the LTD with the spring control as it allows a forced timing event beyond just being pressure based.

Me too...

I think being able to extend the dwell may surpass "free piston" capabilities in some circumstances, for a practical (and affordable) "workhorse" type engine. I can't imagine a NASA "free piston" engine driving a two man saw directly, but a scaled up version of your little LTD style engine with some real torque at low speed seems a real possibility to me at this point.

https://youtu.be/3WREE3xF2ko

[VincentG]

Tom, you can post any of my videos you'd like. I would just ask you also provide a link to my channel https://www.youtube.com/@The-Stirling-P ... t/featured

Your correct, the port is for pressure readings. Although it is also very useful for balancing the internal pressure for peak power. I have a source for laboratory grade helium that I do plan on testing with.

In the home/diy setting I think the slow speed operation of this style engine is key. I agree, at high temperature it would have VERY useful power.

My concern now is that in the real world, efficiency is the very opposite of the theoretical equation that says the higher delta t the better. That may be true, but only AFTER we can get some control over thermal bridging and effective heat transfer areas.

[Tom Booth]

...I agree, at high temperature it would have VERY useful power.

My concern now is that in the real world, efficiency is the very opposite of the theoretical equation that says the higher delta t the better. That may be true, but only AFTER we can get some control over thermal bridging and effective heat transfer areas.

True. The higher the heat, the more rapidly it can conduct and overheat the engine.

The conventional method is mostly to short circuit the heat into a water cooling jacket before it can get all the way over the "bridge", but I think something like a non-heat-conductive gap or other preventative that blocks, rather than conducts away and wastes the heat is much preferable where possible.

I've been experimenting with a coating of hollow glass microspheres on the bottom of the displacer, followed by a layer of aluminum foil. Basically trying to turn the bottom of the displacer into a very thin thermos or Dewar type heat reflective surface.

[KristofB1982]

https://photos.app.goo.gl/ZbqxHPNjJSkYFnAv7

It runs faster than ever. But now has good power at higher rpm and great low speed torque, with a consistent push as you say. I would be interested in buying another scale model gama and alpha type to see how the spring influences their operation. It can be set to be non contact I think, and just the stretching of the spring will delay the displacer or piston at the ends of it's travel, for higher speed engines.

This is quiet impressive.

[VincentG]

This is quiet impressive.
Top

Thanks!

I've been experimenting with a coating of hollow glass microspheres on the bottom of the displacer, followed by a layer of aluminum foil. Basically trying to turn the bottom of the displacer into a very thin thermos or Dewar type heat reflective surface.

This is exactly what I think needs to be done here. Keep us posted on the hollow glass. I was going to get some very thin copper foil for this. Some experimenting with my new thermal camera shows copper is excellent at reflecting IR heat.

[matt brown]

I was still thinking of a crank based control system. It was my understanding that NASA used that flexure mechanism as a form of dampening control and to control the displacer with a purely axial influence (aka: zero side loading) to avoid any metal to metal contact in their air bearing system.

Xlnt observation on NASA, but same for most commercial free piston designs where they all seem to be optimized for electrical output from linear generator (major pros & cons).

I like the full contact displacer of the LTD with the spring control as it allows a forced timing event beyond just being pressure based.

Can I paraphase: "I like a forced timing event" and I can't imagine any ICE with wishy-washy timing events. William Beale invented the FPSE 60 years ago, but I've never seen anyone mention springs in general until it popped up here.

[matt brown]

https://www.youtube.com/watch?v=Jbyf87MC4fY

Have you guys seen this suite of mechanical videos ? I've had a copy of "101 Mechanical Movements" forever, but this suite has motion (and color). BTW Vincent, an obscure issue I encountered with alpha schemes is driving vs driven parts (and I suspect Bumpkin is onto this). Ex: typical piston/slider-crank has crank driving piston during compression BUT has crank driven by piston during expansion (aka piston driving crank during expansion). What, who cares ???

When I first got into SE and became transfixed with phasing issues, a friend said no problem...simply use something similar an old world valve train arrangement where the camshaft is like the crank, the tappets are like walking beams, and the valves are like pistons. My friend's thinking was since an ICE camshaft...obviously...controls valve dwell then a 'similar' arrangement should be possible to control SE piston phasing. He was convinced that it must be possible until I pointed out the more obscure obvious: the camshaft is the driving part and the valves are the driven parts with no way for the valves (friend's 'pistons') to drive the camshaft (friend's 'crank').

So, Vincent, your displacer cam was only possible due to displacer 'floating' and always the driven part.

[Tom Booth]

[... William Beale invented the FPSE 60 years ago, but I've never seen anyone mention springs in general until it popped up here.

There has certainly been discussion of "springs" here Matt. You joined the forum when?

The elegance and simplicity of VincentG's design is brilliant IMO, and his ability to put his ideas into practice with actual prototypes in such short order is admirable.

Discussion, talk and theorizing on the subject however, is not new.

I posted this back in 2006 for example:

linkage.gif (9.53 KiB) Viewed 8669 times

Showing use of a "spring" to effect a "dwell".

Discussion here:

viewtopic.php?f=1&t=77&p=156

The NASA "spring" was not afaik intended for producing dwell. I think, now that I think about it more, without being attached to the crankshaft by some kind of connecting rod, the NASA flexure would not be pulled back against its own momentum to produce a dwell, as VincentG's arrangement would do (in theory).

This forum isn't a competition for "who thought of it first", I don't think. My 2006 spring/dwell idea was cumbersome at best and never implemented even as a prototype.

[VincentG]

William Beale invented the FPSE 60 years ago, but I've never seen anyone mention springs in general until it popped up here.

Just looked into this fella and wow he holds a lot of patents on SE's. Had a hand in the Ross yoke too.

So, Vincent, your displacer cam was only possible due to displacer 'floating' and always the driven part.

I have to disagree here. One of my first sketches and planned projects when I got into SE's was an alpha with a cam driven cold piston. With a roller follower in a groove(opening and closing cam profiles all in one) and a large enough base circle, I don't see why its not possible. When the displacer(of my LTD) "fell" down the ramp of the cam, it accelerated the engine. The ramp rate for an alpha is much more shallow than a gamma/beta would require as well IMO. Might add too much friction though. Either way, a spring is a better way to do this.

Good find on those videos.

[matt brown]

So, Vincent, your displacer cam was only possible due to displacer 'floating' and always the driven part.

I have to disagree here. One of my first sketches and planned projects when I got into SE's was an alpha with a cam driven cold piston. With a roller follower in a groove(opening and closing cam profiles all in one) and a large enough base circle, I don't see why its not possible. When the displacer (of my LTD) "fell" down the ramp of the cam, it accelerated the engine. The ramp rate for an alpha is much more shallow than a gamma/beta would require as well IMO. Might add too much friction though. Either way, a spring is a better way to do this.

Good find on those videos.

Only possible on a small...slow...model. I have this visual of a revolving cam track with a spinning roller lifting & falling while a crosshead bearing (of some type) above the cylinder carries an OFFSET load. 2 rollers & 2 cams would nix offset load, but further complicate everything. I agree, try to stay with a spring (or two).

You're doing great !!! ...(1) you're onto how important phasing is, even with a low end model (2) you're onto minimal vs maximal heat, just little bites of heat that the bugger can use (3) no worries about thermo mumbo-jumbo