Model LTD max power effort

[VincentG]

Figuratively. I'm certain it would work, but now I think it's just too big. Based on my new train of thought, I should only need a 12" inner diameter displacer chamber with a ~.25" stroke to run up to and over a 1000cc power cylinder. I spent a good amount of time thinking about heat exchanger surface area v. dead volume trade off and realized that the flat plate exchanger and corresponding metal foil on displacer is my go-to. Zero dead volume, simplicity, and I think more than enough surface area. Also, just as importantly, close and uniform contact with all of the gas. The flow of the gas from one side to the other I believe is critical, and one more feature that the basic LTD design got right.

[VincentG]

Had a thought that I could still try the 60l drum build as a large scale LTD and make use of the extra air volume.

Anyway here's a little update on the scale build. I've been tinkering with dimensions and fitment of various parts. Its loosely based on the original engine but really almost everything will be different. I don't think I'd make any forward progress at all without a 3d printer to make quick design changes and not spend a whole afternoon machining little parts.

This is the first version of a displacer connecting rod that has the spring dwell built in. It's adjustable for overall length to fine tune the timing. It also allows a bit stiffer spring to be used for the lifting spring than the return spring. That should come in handy with a heavier high temperature displacer. I'm slowly making progress on the other parts too.

2.4 displacer conrod.jpg (249.97 KiB) Viewed 14401 times

[matt brown]

Vincent, this is the best 'plunger' dwell mech I've seen !!! Most guys use single spring and hassle with progressive force across range. This dual spring arrangement reduces this progression 'chase' AND allows a 'neutral' position. It looks like the spring guy is using some ICE 'magic'...

[VincentG]

Thanks Matt I guess offroad suspension tuning carries over into hot air engines engines lol. Yea one standard coil spring seems too limited and finicky. The cool part is 3d prints should work even for larger engines. The forces should be pretty low on the displacer components.

Waiting on another ball bearing still but I think I've managed to eliminate the need for a separate crank throw for the displacer to simplify construction further.

[VincentG]

My bearing came in so I was able to print the new displacer drive system and connecting rod "big end". The 3d printed eccentric allows the crank shaft to pass clean though with no need for any crank throws, and easy timing changes. You can see that this bearing ID of 10mm is maxed out to get the .2" of crank stroked needed with the 3/16" shaft I'm using.

2.4 displacer cam rod end.jpg (94.68 KiB) Viewed 14150 times

[VincentG]

My plan is to make a throttling lever than will control engine speed by limiting the up travel of the displacer from the hot plate. By limiting displacer lift, speed should be easy to control.

[VincentG]

The new aluminum cold plate machined to accept the original glass cylinder.

aluminum cold plate.jpg (217.66 KiB) Viewed 13138 times

Three types of displacers for testing various heat sources. From left to right is a solid foam displacer, foam with a metal cutoff wheel on the hot side, and a solid concrete displacer. All are .6" in height.

3 displacers.jpg (309.83 KiB) Viewed 13138 times

[VincentG]

I had a lot of trouble getting small batches of concrete to cure to full strength, so I finally gave in and used epoxy to cast the new displacer housing. The mold was 3d printed, filled with epoxy, and then machined down to size.

Displacer stroke is only around .08", about the thickness of the nickel. I've decided to use a 1:1 swept volume ratio on the first go. So 5cc PP to ~5cc DP. Note that ideally this will NOT mean 2:1 (actual)compression ratio, but instead much less if heating and cooling are as effective as I hope, and also dependent on heater temperature.

Seeing the tight clearances in person and even I second guess whether or not it will even run...but I have to remember zeros change everything. Zero hot space when cooling, and zero cold space when at full displacer lift.

epoxy displacer housing.jpg (121.91 KiB) Viewed 8123 times

[Tom Booth]

...
Displacer stroke is only around .08", about the thickness of the nickel....

Such a slight movement of the displacer seems counterintuitive, but sometimes less is more.

I'm thinking of my magnetic lift LTD running at full tilt and the displacer barely lifting at all but just making a slight side to side dance. I doubt it was lifting as much as the width of a nickel.

In the timeframe of one cycle in a fast running engine, where is there more opportunity for more air than just what's near the surface to contact the hot plate anyway? Perhaps just wasted motion to lift any higher.

[VincentG]

I'm thinking of my magnetic lift LTD running at full tilt and the displacer barely lifting at all but just making a slight side to side dance. I doubt it was lifting as much as the width of a nickel.

That video is what gives me confidence that this will work.

[VincentG]

Tom, could you link to that video here? I couldn't find it this time.

[matt brown]

What's your current guess on lift duration (ICE buzz - lol). The 1:1 volume ratio is a good starting point, but unsure at this small scale.

[VincentG]

Going for around 180 degrees at first. From 30*BTDC to 30*BBDC. Roughly.

[Tom Booth]

Tom, could you link to that video here? I couldn't find it this time.

https://youtu.be/i9nz0vt7eQA?si=iwK510CTdx7ufx3S


Another extra slow motion:


https://youtu.be/F8P_g8Vwoz0?si=Z1Q9SHMu45CV6myV

The displacer does lift a little but not much if any higher than the gasket so it's difficult to even see under it when it makes a little hop.

The "gasket" is, or was, just a thin layer of silicone or some other kind of glue, so not all that thick. The gasket can be seen at the end of this video:

https://youtu.be/WCNsmE-Evbc?si=BsMbIQHogar_R7Xw

That was running directly on top of a 100 watt ceramic heat lamp.

With that extra high heat the engine ran even faster which had a different effect. The magnet was moving so fast that the displacer did not have time to drop down and kind of floated near the top, but again not moving down much this time.

Though, that may have been from air getting sucked in under the displacer due to the loose bottom? It kept running for a while even with the bottom quite loose.

https://youtu.be/WCNsmE-Evbc?si=BsMbIQHogar_R7Xw


With 100 watts continuous heat applied, an acrylic, non-heat conducting "cold" side blanketed with an aerogel blanket, I thought I had FINALLY succeeded in causing a heat bottleneck by blocking the "heat rejection". Yay!!! Sucesss!!!

But no, the gasket or glue or whatever melted and came loose. Apparently it just stopped running because the bottom was falling off.

BTW the knife was a little wet. I got it out of the sink, so had a little water on the blade.

Note the sizzle of steam when the wet knife touches the bottom plate of the engine. (1:54) That thing was hot hot hot!

[Tom Booth]

I had to use the knife to get the bottom off because trying to use my fingernail the metal plate was blistering my fingertips, too hot to touch. yet the top and sides of the engine did not feel hot at all.