Scratch-built Stirling

[Groomengineering]

Hey Kevin there's more work to do, no time for sitting around wiggling pieces and making vroom vroom noises...... like I do... :big:

Looking good! There's just something about a Sterling...

Cheers

Jeff

 

[ksouers]

Thanks Jeff.

...sitting around wiggling pieces and making vroom vroom noises...... like I do... :big:

I call it part of the break-in process ;D


Ain't it amazing? Just heating and cooling a sealed volume of air can cause things to move. What's really amazing is that it happens so quickly.

 

[njl]

Ain't it amazing? Just heating and cooling a sealed volume of air can cause things to move. What's really amazing is that it happens so quickly.

Yep I find that totally amazing too. We have a model engineering show near Bristol each year and it usually has a good turnout from the sterling builders. Some of them go at quite a speed. I find them fascinating to watch. One day (soon I hope) I will have a play with building a sterling, your build thread is giving me inspiration.

Nick

 

[ksouers]

Go for it, Nick! It's been a fun build. Though for me this particular project has been more of an obsession.



A bit of an update.

I needed to get some dimensions by direct measurement, but the point to measure from didn't exist yet. So, I found a couple pieces in the "bits o" box, you know bits o this and bits o that. Just a couple pieces of aluminum. Any way, made my mock up part and fitted it. Got my measurement and made a quick and dirty con rod and rod end for the displacer and fitted them, checked out the range of motion. Everything was moving at it should, no binding or interference. My numbers were good to go.Then I thought to myself, "Self, you can use that." And so I did.

It was time for a test run. Made some quick and dirty gaskets from some card stock I had laying around. Assembled everything but didn't hook up to the flywheel just yet. Moved the piston in and out, the displacer also moved in and out. Good, no leaks, or at least very little.

Hooked up the con rods to the flywheel and left the plug out that seals the channel drilled between the cylinders. Everything moves freely, the flywheel makes a couple revolutions before stopping when turned by hand. Low friction, but of course, not real low. With everything plugged up it'll just barely turn over by hand due to compression.

I used a propane torch for heat. Of course, no joy in the running department. At this point it could be any of a number of things.

First up is friction, of course. I'm using plain bearings at the moment, I have ball bearings on order, should be here in a day or two.

Heat, there seems to be an inordinate amount of heat transferred to the frame. The hot cap is pretty short and it's possible the torch was heating everything up, but I'm also quite sure there was a lot of conduction from the hot cap to the frame. I'll add a deflector to help keep heat away from the cold side and frame. Also, some better gaskets to help reduce the conduction.

Timing. Yep, still need to work on the timing. I don't remember if I had the displacer leading the piston or not. I need to check that out, though I it seemed to have a preference for wanting to go one direction over the other.

Anyway, those are the first things I'm going to attack. I didn't expect it to run, but it would have been great if it ran for a few seconds. I did collect (subjective) data, which is what I really wanted to do, I consider the first test a bit of a success.

 

[njl]

Kevin, you could try putting some ice on the cold end and repeating your test, that would give you a bigger pressure difference which might be enough to overcome the friction or reduce the effect of heat leaking into the frame.

Nick

 

[mklotz]

I wasn't going to say anything but since you brought it up... the displacer chamber looks a bit short. You may want to take a look at these suggestions from James Rizzo...

http://www.homemodelenginemachinist.com/index.php?topic=5951.msg63174#msg63174

The chamber should have thin walls to minimize heat conduction and be made from a relatively poor conductor such as steel.

The displacer should be a close fit in the chamber. The displacer acts as a regenerator and a close fit means more turbulence and better heat transfer as the air moves past the displacer during the displacer stroke.

Piston and displacer should be ninety degrees out of phase with each other. Use that as a starting point and diddle after you get it running. If it won't run with that phasing, it probably won't run at all.

I'm sure you'll make it work. I'm looking forward to the video since I'm a real fan of Stirlings.

 

[Deanofid]

Kevin, I have some good insulator stuff here. Same stuff used in the last one I built.
It's sheet, 1/16" thick, non-asbestos ceramic. If you want some, send your address and I'll
put a piece in an envelope and send your way.
I have plenty of it.

Dean

 

[doc1955]

Moved the piston in and out, the displacer also moved in and out.

To me it sounds like your displacer is just a little to close fit. The air needs to move freely from one side of the displacer to the other. And if you move the power piston which I believe you are saying and the displacer moves then most likely it is much to close of a fit.

 

[ksouers]

Thanks Nick. I did contemplate dripping cool water on the cold end last night. I do plan to do that sometime in the future. I'd also like to make one with a water jacket, but that's for a later project.

Thanks for reminding me of that link, Marv. I had used Rizzo's "guidelines" when I started building this engine. Though it looks like the one I missed was the swept volume of the displacer, mine is 1:1 vs Rizzo's 1.5:1 with the power piston. I thought the hot cap looked a little short when I made it, but I've also seen successful engines running with small caps.

One of the things I wanted to play with was the length of the displacer cylinder, so looks like that is going to happen rather quickly. I will also have to adjust the stroke, but I've made allowances for that with over sized crank disks. The displacer is .010 smaller than the bore, so I've got a close but not tight fit.

I knew the pistons would be 90 deg out of phase, I just couldn't remember which was supposed to top out first, thus no idea which way to spin it to get it started. I tried both directions, one seemed more willing to run than the other. Or is that one way was less willing, since neither way worked?

Dean, thanks for the offer, I'll definitely keep it in mind. I have a couple ideas I want to try first. At the start I had planned to use some high-temp RTV between paper facings as the gasket, I still want to try that.

As stated at the outset this engine is for experimentation and I mean that in the most serious way. While I have ball bearings on the way, they won't be installed immediately. I want to try things that are maybe a little different. So even failures are a success as long as I learn something from it. The one epithet I wear proudly is "stubborn, hard-headed German" ;D

Thanks Doc. I plan to try many different things, even after the engine is running. Including different fits for the displacer.

 

[mklotz]

I checked my Stirling of that type and, when the piston is at TDC, the displacer is at mid-stroke and moving towards the cold end of the displacer chamber when the flywheel rotates in the "running" direction.

Ten-thousandths clearance (I presume that's 0.005" all around) seems a bit small but I'm not an authority. Give it a try and, if it doesn't run, try a bit smaller diameter displacer. However, I would try a larger displacer volume (longer displacer) first.

Good luck from another hard-headed German.

 

[doc1955]

I think .01 is not sufficient clearance for the displacer I'm thinking you'll need to open up the cylinder and give it about .03 clearance. Going with to close a fit will put more pressure that the motor will have to over come as it is trying to squeeze the air through a .005 gap.

 

[ksouers]

Thanks for checking the direction, Marv. Is that with the top of the flywheel running towards the back? That's the way I'm set up, the displacer is leading the power piston by ~90 degrees when the flywheel is rotated back and down.

I double checked the displacer diameter: .4877, the cylinder is .500 as checked against a ground shaft that is .4995


OK, next attempt with a 1/2 inch longer hot cap, 1/4 inch longer stroke on the displacer. Regardless of success or failure I'll also make a smaller diameter displacer.

Thanks for the suggestions, guys.

 

[NickG]

My 2p worth - as others have said and this backs up your remedial action, first 2 things I noticed were -

1. compression ratio is too high, there should be very little compression. 1:1 is good for efficiency but you'd need a lot of heat and good cooling. Generally, the bigger the ratio of swept volumes from displacer to power piston the less heat will be required. I did exactly the same on my first stirling, it was easier for me to increase the bore of my displacer. When doing so, I also lengthened the hot cap and displacer as you are thinking of to lessen or slow down heat transfer between hot and cold ends.

2. Clearance between displacer piston and cylinder is too small. If you move the power piston, the displacer should not move on its own, the air should displace around it. I think a ring bom type engine works by having a tighter fit and the air moving the displacer piston, but you don't want that, you want a clearance to minimise any loses whilst not being too large. My clearance was about 1/32" all around on a 1/2" dia displacer piston, so a lot more than yours. This very small clearance might account for some of your high compression.

3. the usual stuff, minimise friction, with the hot cap off, the thing should spin over for quite a few revolutions, I would say at least 10.

Hope everybodies input helps, sure you'll have it running soon with your proposed changes.

Cheers,

Nick

 

[ksouers]

Thanks NickG.

I remember your first stirling, going through similar tribulations. Seems like ages ago now. You followed that up with a very nice LTD.

 

[mklotz]

Thanks for checking the direction, Marv. Is that with the top of the flywheel running towards the back? That's the way I'm set up, the displacer is leading the power piston by ~90 degrees when the flywheel is rotated back and down.

Exactly.

 

[rleete]

Bringing this one back from the dead, as I've been bitten by the Stirling bug.

Whatever happened with this build? It seems just as the weather broke, the progress halted.

 

[ksouers]

Hi rleete,

Thanks for the interest ;D

It'll come back, I just haven't had time to hit the shop lately. About the time I stopped to wait out the heat I started a big project at work that should wrap up in a couple weeks. Then back in September I was given responsibility for 60+ Linux computers that belong to a sister company on the mid Atlantic coast. The machines and software are old and out of vendor support, so it's been a busy time to get everything back up to date. On top of it all, that data center is to be relocated about 100 miles further south, so it'll start all over again some time next year. These 18 hour days are starting to drag the old man down.

Fun times here in the midwest :

I'm hoping to get some shop time between Thanksgiving and Christmas. Maybe.

 

[Longboy]

.......I just read all 5 pages here on your build. I agree with others here that your displacer clearance to the hot cylinder is just way too tight. Closer is not better here in Stirling. I build with a displacer clearance of .040. In this case a source for friction is drag in the cyl. by the displacer. The engine may be pumping the air rather than shuttling air back and forth. Give it a look/see for scuff marks on your displacer. The gland bushing looks long enough in the photos but the clearance for the pushrod to pass thru can give the displacer some out of parallel sag traveling in the bore. Oh, tell the guys at work, "tahellwithit, I got engines to build"! Dave.