LTD to HTD Stirling engine converstion

[Tom Booth]

Very impressive Tom. I think with right gearing and generator to keep your engine in its powerband (I think around 60-100rpm) you'll really see some good output. I'd be more than happy to send you some 3d printed flexures and gears to experiment with.

Cool,

I think it would be a good idea to put the 3D print files up on Thingaverse or someplace for others as well.

I'd also be interested in the files as I do have a 3D printer I need to learn how to use.

Right now it would be great if I could just print out some new pulleys. (And maybe something to prevent piston slippage, not sure what though).

[Tom Booth]

I finally decided to super glue that thing in using superglue and baking soda. I figured that thing won't give me any more trouble.

Then as I was about to replace it back in the engine it slipped out of my fingers and hit the floor breaking to pieces.

Resize_20230515_185354_4915.jpg (193.94 KiB) Viewed 12852 times

It looks like there might be enough left to still be useable. It might actually be an improvement. I don't really see the point in having such unnecessarily long pistons with all that extra drag.

I may just grind off the jagged edges with the diamond bit Dremel tool and put it back in the engine. Maybe even cut some oil ring groves. That seemed to do wonders for a thermoacoustic engine with a similarly unusually long piston.

A blessing in disguise perhaps?

[Tom Booth]

I just stuck the piston back in as-is and gave it a whirl.

There was actually improved performance. All 20 LED's lit up nice and bright.

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The voltage under load was just 2 volts

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Unfortunately, or fortunately, this time the engine really blew out (or burned out) a seal by the time I was done recording, (not yet uploaded) and some smoke can be seen billowing out the side of the engine through the blown seals when turning the engine over by hand, after it quit running completely.

I only managed to hold the volt meter to the generator contacts for a couple seconds, fumbling around with propane torch, two meter probes and the phone, but, the 2 volts while under load is definite, as I was able to get a better reading with two hands while not recording.

I haven't measured the voltage output no-load as yet.

The engine, however, is out of commission and will need a rebuild, so that will have to wait.

Just as well. I think I've done all the preliminary testing I needed to do, if for no other reason, to convince myself there is any reason to pursue this further.

Really it was VincentG's successes with working with this type of engine that got me inspired to try it again after sitting on the shelf for the past ten years.

Thanks VincentG !

[VincentG]

You just made my day Tom, I'm glad I was able to inspire you. I bet with the right generator or just a step-up converter you could charge up some batteries over a wood stove after a rebuild. And that's all we're really after here anyway so really job well done!

You mentioned uploading files to thingiverse. I could do that, though my flexure spring is a bit finicky to get right. It would be neat to settle on a particular LTD model that is a bit more modular than this one. Then it would really be worth spending some time on designing CAD models for upgraded parts to increase power and efficiency.

[Tom Booth]

Video: the engine running with the improved, more secure and accidently on purpose drastically shortened piston.

This time all 20 LED's burned brightly and the engine ran quite well and steady, even without increasing the gear ratio any more.


https://youtu.be/RSZV11MNxjI


By the end of the video, though, it blew a gasket.

Considering it has withstood repeated heating in excess of 1000°F I think it did OK.

Any suggestions what I might use to replace the silicone gaskets that could withstand even higher temperatures?

I'm also considering replacing the bottom plate with titanium. Then if it were only heated in the middle, the heat might not conduct outward and the gaskets along the edge would not get so hot

Holding this titanium plate being heated by the torch I barely felt any increase in temperature just an inch away from where it was white hot.


https://youtu.be/6xDX2OvD09I

[VincentG]

Most impressive Tom. There really is alot of potential in these "models".

While you could search for super materials to withstand these higher temperatures, I think that's what has kept powerful SE's so elusive to the general public.

I was going to wait for the start of my next thread in the series of ltd engine development, this time about housing and heat exchanger design and integration, but I'll share my thoughts now.

I think what needs to be done is to not heat the bottom plate directly, in order to prevent heat leakage to the entire housing. The focus should be on heating the air as independently as possible. Instead, a secondary hot plate(or as advanced a heat sink as you'd like) should be mounted inside the engine just above, but isolated from the original bottom plate. The heat could be transfered in a number of ways, like flowing molten salt or just copper heat conducting pipes bonded to the internal exchanger.

Then, the only seal exposed to full heat will be a small bulkhead style fitting that lets the water tubing or heat pipe pass through the bottom plate. I'll upload a quick sketch later.

[Tom Booth]

Most impressive Tom. There really is alot of potential in these "models".

While you could search for super materials to withstand these higher temperatures, I think that's what has kept powerful SE's so elusive to the general public.

I was going to wait for the start of my next thread in the series of ltd engine development, this time about housing and heat exchanger design and integration, but I'll share my thoughts now.

I think what needs to be done is to not heat the bottom plate directly, in order to prevent heat leakage to the entire housing. The focus should be on heating the air as independently as possible. Instead, a secondary hot plate(or as advanced a heat sink as you'd like) should be mounted inside the engine just above, but isolated from the original bottom plate. The heat could be transfered in a number of ways, like flowing molten salt or just copper heat conducting pipes bonded to the internal exchanger.

Then, the only seal exposed to full heat will be a small bulkhead style fitting that lets the water tubing or heat pipe pass through the bottom plate. I'll upload a quick sketch later.

Looking forward to it.

I'm kind of imagining something like a heat conducting metal rivet (with very wide flanges) through the bottom of an otherwise non-heat conducting/insulating bottom plate. The central "heat pipe" could be completely encapsulated in refractory leaving the top and bottom flanges as the exterior to interior heat exchangers.

Not sure if that's quite exactly what you had in mind but something along those lines?

[VincentG]

For your HTD engine, exactly like that. With that description no sketch is needed.

[matt brown]

Holding this titanium plate being heated by the torch I barely felt any increase in temperature just an inch away from where it was white hot.


https://youtu.be/6xDX2OvD09I

Thanks for the hands on demo. I knew titanium was a poor conductor, but didn't know it's this 'bad'. There's a few startups in Hong Kong trying to perfect titanium thermos bottles and I'm planning to buy one to carry coffee to work. They're all struggling due to titanium is a bugger to work and weld. Besides xlnt insulation potential, the titanium doesn't taint the coffee like SS. The only downside is the $200 price tag, but still worth it for me.

[Tom Booth]

I thought this video was interesting in connection with the tiny little "generator"

https://youtu.be/U3U0tNRQfZA

The motors in the video are a little bigger but similar enough I think.

In other words, the little Stirling has to work pretty hard to get anything at all out of one of those little motors. Given an actual efficient generator the output could likely at least be doubled.

The little motors are more efficient as generators at very high RPM and, of course, that is not the case running from a little LTD engine.

[Tom Booth]

I wanted to point out that based on the testing in the above video, I tested the string of LED Christmas lights and they required a minimum of 3 volts to light up at all. (Two 1.5 flashlight batteries in series).

With the probable abysmal efficiency of the generator the high heat LTD style engine likely could have powered at least three times the electric load with a reasonably efficient generator.

[VincentG]

I was hoping you would get back to this to try and get some real shaft power figures. 1 watt per cc in an atmospheric engine would be a pretty awesome number to hit for now.

[Tom Booth]

I was hoping you would get back to this to try and get some real shaft power figures. 1 watt per cc in an atmospheric engine would be a pretty awesome number to hit for now.

The bottom silicon gasket I made for that engine eventually melted, with repeated testing at over 1000°F.

My main objective was just to prove that an LTD style or construction design is not limited to low temperatures. With heat resistant parts, I could get the same engine to run throughout the entire range of heat source, from a warm cup of water up to nearly 2000°F so I felt satisfied and haven't replaced the gasket.

It would likely be a relatively easy fix to replace the gasket, but how to "get some real shaft power figures" I hardly have a clue how to go about doing that.

[Fool]

Two Diy-Homemade dynamometer ideas, electrical and friction brake.

Electrical:
Hook the engine to a generator. Hook the generator to a load. Measure the volts and amps. VxA=Power.

Generator could be as easy as gluing a few magnets to a flywheel and winding your own scrap copper wire coils for the frame.

The load could be made with scrap resistors, or LEDs or even an electric motor if small enough. The resistors won't care what the voltage is. The others will have a minimum on voltage but measuring voltage and current would still be possible even if they are not on.

Measure voltage with a DVM set on AC, unless using a DC generator. Use a one ohm resistor in series to measure current. Some other options are possible here too. Different meters. Different values. Diode rectification. Etc.

Friction brake:
Wrap the axle with a soft heat resistant string or cable in the opposite direction it turns, so it tends to unwind and pick up the weight when running. Add a weight, very little. Torque will equal weight times shaft diameter divided by 2. W x dia/2=Torque. Torque x rpm = power. Some adjustment to those formulas will be necessary for unit conversion to horsepower or watts.

You could also use a spring scale on a torque arm connected to a pivoting brake shoe, to measure torque. The spring scale could be as simple as a rubber band, or several. Spring scale should be calibrated using weights before each run. Temperature will effect rubber springs more than meatal springs. The pivoting brake shoe could be as simple as a split wooden stick with an adjustment screw to tighten it down on the axle.

It's easy to build a rudimentary dynamometer and manually measure power from voltage, amps, or torque and rpm.

[VincentG]

Thank you for finding that generator video Tom, it's something I've been wanting to know.

Using .25x slow motion, I counted about 8 revolutions per second for your last video. That's about 480 rpm. I'm not sure what your pulley ratio is but it looks about 2.5 to 1, so maybe the generator was spinning around 1200 rpm which is very low on the efficiency scale of those I'd say. 480 rpm is also much faster than any LTD has been measured at peak power(though high temperature may change this), so it could probably make better power with quite a bit more load at lower rpm.

In any event, seems those little lights draw around .05amps each, so 20 lights is around 1 watt. Assuming the best efficiency numbers from the generator video above that means shaft power could have still been over 3 watts. The stock displacement of that engine is 1.6cc.

Considering how much better my engine ran after the mods, I'd speculate that charging a phone with these little engines is a much lower benchmark than anticipated. Thanks!