Nasa "free piston" displacer in combination with crankshaft?

[Jagang]

I think I mistook your flywheel to be the boats propeller.

Anyway, looking forward to some photos of this build.

I think you'll only see the finished product, if it works, that is.
Don't have a lot of faith in that. (And that's probably why I find it hard to make photo's of every step.)
Might be some days before I continue to work on it.

[Jagang]

This project is parked right now.

I can't get a "kick" out of the can, so it's probably not a functioning power source at all.
Maybe I'll attempt again, some other time. For now, I might try a more conventional project.

[Tom Booth]

Thanks for sharing your thoughts and experience here. I think I must have at least half a dozen Stirling engine and other projects on the shelf myself at any one time. Some I can only get back to after months or even years, but it's really nice to know there is a place like this to come back to and pick up where I left off, if I ever do, or wake up in the middle of the night with some inspiration to share.

Sometimes I learn more from a project not working as expected.

What do you mean by "kick"?

Did you first try to get it running without the crankshaft, or went right into putting it all together?

These machines can be very finicky at times, especially "free piston" as there is nothing, no crank to force synchronization between piston and displacer and keep timing in sync. It depends more on some rather mysterious harmonic vibration that requires just the right balance of tension on the diaphragms, distance apart, temperature etc. that I'm not sure anyone fully understands. Or it could be any number of not do obvious mechanical problems.

I think anyone who has ever tried to build a Stirling engine has gone through the same thing. It very rarely ever works the first try.

[Jagang]

What do you mean by "kick"?

Did you first try to get it running without the crankshaft, or went right into putting it all together?

Yes, this.
It makes no sense to continue building the engine, when it's primary powr source doesn't even work, (while it should).

SoI failed at the "free piston"-part already.

[Tom Booth]

Well you posted a video from Blade and I've seen many many others, so we know it CAN work, or as you say should work.

Unfortunately this type of Stirling is not readily available to just pick one up somewhere. But, it might be possible to sort out what the problem might be.

[Jagang]

Maybe I should try without the recommended pinholes next time.
Or just stick to theories, and see if it gets picked up by someone else.

[Tom Booth]

I was wondering about whether there was really any need for pin holes. I know what you mean. I think. I thought it probably wasn't really necessary.

The engine without a flywheel I posted in this thread earlier, I deliberately sealed the balloon around the cylinder opening completely to see if the air would build up pressure. If that were really a problem the balloon should have gotten bigger and bigger to let out the expanding air, but that never happened.

Not exactly the same type of "free piston" but I think you might just be on to something. I personally suspect the pinholes could very well be a problem, you say holes plural?

I thought there was just one pinhole in the displacer, not any in the power diaphragm I haven't really examined that type of Stirling, infact, I don't know exactly what plan or recommendation you followed.

Normally in a Stirling, leaks of any kind are avoided, as the engine has to be completely sealed, so when I saw a video with someone poking holes in their diaphragm I was thinking - what? That doesn't seem right.

I thought maybe if the hole is really small and the diaphragm is not stretched much at all, the hole would not leak too much and the engine would still run, but if the diaphragm was stretched at all, then it could potentially become a real problem.

[Jagang]

Not exactly the same type of "free piston" but I think you might just be on to something. I personally suspect the pinholes could very well be a problem, you say holes plural?

Yup, one in the displacer, and one in the outer cylinder, so it can ajust to outside air pressure slowly. (which should be the average pressure inside as well.)

I thought there was just one pinhole in the displacer, not any in the power diaphragm I haven't really examined that type of Stirling, infact, I don't know exactly what plan or recommendation you followed.

It's basically recommended with any type of diaphram engine.
Solid pistons always leak SOME air, so they ajust naturally, but diaphram engines basically have a perfect seal.

[Jagang]

I think I'll try something more conventional first, and drop by this project (or something similar) later on.

[Tom Booth]

...holes plural?

Yup, one in the displacer, and one in the outer cylinder, so it can adjust to outside air pressure slowly. (which should be the average pressure inside as well.)

It's basically recommended with any type of diaphragm engine.
Solid pistons always leak SOME air, so they ajust naturally, but diaphram engines basically have a perfect seal.

I think this could really be a MAJOR problem. Can't say for sure, but having a PERFECT seal is a very very good thing. especially between the outer cylinder and the outside air. Not a good idea to have any kind of permanent hole or puncture. I've read or seen accounts here of people having engines that didn't run because of the tiniest, completely invisible leak that they only found by using compressed air and soap bubbles.

I think a much better solution would be to add a little valve that can be opened if necessary, to let a little air in or out, then completely closed for continued operation.

Like on this little diaphragm LTD.

Screenshot_20210509-162919.jpg (100.17 KiB) Viewed 7815 times

Any kind of little screw cap or plug that can be opened a little, if and when it might ever be necessary, but that can be tightly sealed during operation.

There is no need for equalization with the outside air when the engine is running, quite the contrary. It is the difference in pressure that makes it possible for the engine to operate.

[Jagang]

There is no need for equalization with the outside air when the engine is running, quite the contrary. It is the difference in pressure that makes it possible for the engine to operate.

A diaphram engine will probably stop running when the internal pressure moves above the atmospheric average.
A perfect seal is usually only beneficial if you have a seriously pressurized engine.

[Tom Booth]

There is no need for equalization with the outside air when the engine is running, quite the contrary. It is the difference in pressure that makes it possible for the engine to operate.

A diaphram engine will probably stop running when the internal pressure moves above the atmospheric average.
A perfect seal is usually only beneficial if you have a seriously pressurized engine.

Possibly, I'm just afraid that a hole in the diaphragm could potentially prevent the engine from running at all. There has to be a difference in pressure for the engine to start up.

A diaphram engine will probably stop running when the internal pressure moves above the atmospheric average

My experience and experiments, so far, indicate that this does not happen.

Pressure equalization may sometimes be necessary with something like a change in barometric pressure, a storm approaching, or taking the engine to a different elevation.

A very very tiny hole in the diaphragm MIGHT not be a big problem, but it can be difficult to assess, as a diaphragm can and does stretch, and how much pressure is escaping is difficult to say.

Think of a small puncture in a bicycle tire. It may be invisible to the eye, but you won't get anywhere on the bike.

[ccspring3021]

Hi,

BTW, can anyone explain why the NASA's stirling free displacer will run after the piston? I guess when the piston compress the gas, it will cause temperature rise, then the gas inside the displacer will expand, and vice versa.

If so, the system's working frequency is limited by the heat transfer speed.

[Tom Booth]

Hi,

BTW, can anyone explain why the NASA's stirling free displacer will run after the piston? I guess when the piston compress the gas, it will cause temperature rise, then the gas inside the displacer will expand, and vice versa.

If so, the system's working frequency is limited by the heat transfer speed.

I don't think the displacer moves much due to a change in temperature, it is more likely IMO, mostly responding directly to the change in pressure.

[ccspring3021]

Hi Tom,

I do not think the displacer changing respond to the pressure.
when the power piston is pressed down, the pressure in the volume should rise up.
If displacer is isolated with volume(both pressure and temperature), its interal pressure remain and it should be compressed, so it should lift up.
If displacer is not isolated, we see a small hole on the surface, its interal pressure should equal to the volume, so it should not change any more.
But we see it go down later, so the volume of displacer become bigger, it means its interal pressure become higher than the volume.