Scratch-built Stirling

[ksouers]

Well, it’s been a couple months since completing the Elmer’s #6 so I guess it’s time for another project. I had been considering Elmer’s mine engine since before completing #6, but all the Stirling projects going on stirred up some old burning embers.

I’ve been deeply interested in Stirlings since I first started reading about them in Popular Science and Popular Mechanics as a youngster in the early 70’s. The Stirling was going to change the world, they said. We all know how that turned out; still, it’s a very interesting engine.

The magazine articles never went into the history of the engine, just talking up all the “revolutionary” research and development that was going on made it sound like it was some new engineering principle that had escaped from the darkest corners of Caltech or MIT. I was quite surprised years later to find out that it predates the Otto and Diesel cycle engines.

My first foray into the world of Stirling was about 20 years ago when I made a tin can model I had found plans for while “surfing the pre-internet” (library!). It was a wooden framed affair with a couple coffee cans for cylinders and soup cans for pistons with water to seal them. It was an Alpha type with one can being hot and the other cold, a fabricated copper pipe plumbed them together. I never did get it to work right; I think I got at most 2 revolutions before it would stop.

Now I venture into the world of Stirlings again, not armed with some plans from a faded magazine of days gone by. No, not this time. This time its scratch built. My own design. I don’t have any plans or drawing made, just some numbers scratched out on a pad and the worst (incomplete) C-o-C you’ve ever seen. It makes a 7 year old’s stick figure Hangman’s drawing look like it was done by da Vinci.

It’s time to give credit where credit is due. There is nothing new under the Sun. I’m relying on lots of people who have gone before me. In that vein I’ve borrowed heavily from the designs of Terry Coss. His designs are straight forward and to the point yet quite elegant in motion and function in a modernistic style. Fear not, Cedge, your Victorian art is quite safe.

Now, on with the show…

I started with the hot cap using the classic rule of 3. The displacer cylinder is 3 times the diameter, the displacer piston length is 2/3 the length of the cylinder. The swept volume of the power cylinder is equal to the swept volume of the displacer, 1/3 the volume of the displacer cylinder. Everybody got that? There’s a test afterward.

The cylinder inside diameter is 1/2 inch, as that’s the largest reamer I have. The hot cap is 12L14 steel, as that’s what I had on hand, with a length (inside) of 3/4 inch, 1/2 the cylinder length. The cold side will also be 3/4 inch long. I have several of those aluminum blocks with the hole already drilled in them, now I know what to do with them. The hole is .400 inches, the block is about 1.5 inches on a side and 3 inches long, give or take.

It was a little warm in the shop so that’s as far as I got.

I’m not fond of sweat dripping off my nose and onto the machinery and glasses. The “air conditioning” won’t arrive until October, so this is likely to be a very slow build.

Thanks for stopping in. I hope the next installment isn’t too far away.

 

[zeeprogrammer]

Excellent Kevin.

Can I take the test pass/fail? How about probation? Fail?

Looking forward to the build.

 

[Deanofid]

Thanks for the invite to your new build, Kevin! It seems like certain engine types go in bunches. I guess it's
the Rev. Stirling's turn.
Man! I don't blame you for leaving the shop with that kind of heat indoors. Sweat will rust machines almost
as fast as blood. Keep your nose drippin's off the lathe ways!

; )

Dean

 

[ksouers]

Thanks Zee. Everybody passes, there are no failures here just problems that haven't been solved yet ;D
And we are ALL on probation... :

It seems like certain engine types go in bunches. I guess it's the Rev. Stirling's turn.

Thanks Dean. Yeah, I've noticed that, too. We had a spate of mine engines, then it was hit-n-miss. Now it's Stirlings. Not that I mind, I like them all and it's really interesting to watch the way everybody finds a unique solution to their particular part they're making.

The machine was already well oiled when I started, but wiped down and oiled again before I turned off the lights.

 

[njl]

Looking forward to this build Kevin. Stirlings hold a fascination for me too, one day when my skills improve enough I hope to building one. Hope you manage to combat the heat and get some building done.

Nick

 

[Stan]

All the reading and experimenting I have done with Stirling and Stirling type engines is that the hot cap has to of a material with poor heat conduction. In the conventional Stirling the object is to reduce the amount of heat transferred to the displacer cylinder.

A lot of engines use glass for the hot end and for metal hot ends it seems like stainless steel is preferred. I have one with 660 bronze which is so-so, with a thick gasket and isolated clamp between the hot end and the displacer.

 

[cfellows]

Stan, you've probably read all the same material I have, but I'll relay what meager knowledge I have from reading.

The sides of the displacer piston and the displacer cylinder should be as thin as you can make them to cut down on heat conductance along the metal. Also, you want the displacer piston to transfer as much of the air as possible from the hot end to the cold and vice versa, so the clearance between the displacer and cylinder should be minimal without creating drag. Steel or stainless are good candidates for both the displacer and the cylinder. Finally, make sure you have adequate cooling capacity at the cold end of the displacer. These were tidbits I got from Rudy Kouhoupt at GEARS a few years before his death.

Chuck

 

[Stan]

Finally, make sure you have adequate cooling capacity at the cold end of the displacer.

That is what I was referring to with the gasket isolation between the hot cap and the displacer cylinder. If the hot cap is a good heat conductor, heat travels from the hot end to the junction with with the displacer cylinder passing the heat into the cold end.

I have millions of brain cells dying daily, but I think I remember Kevin built a Jan Ridder lamina and posted all his grief on this board. He is not a beginner with Stirling type engines.

 

[arnoldb]

Kevin, I've happily taken a seat in the peanut gallery ;D - I'm really looking forward to this!

Regards, Arnold

 

[ksouers]

Nick, thanks for the support and welcome.

Stan, thanks for dropping by. Yes, stainless is better but that does not preclude using other steels, too. I don't have any stainless at the moment big enough to use, so I used what I had on hand. This will work well. Optimal would be a ceramic, but I'm afraid I'm fresh out of space shuttle re-entry tiles ;D
The hot end will be insulated from the cold end as much as possible. This engine is mostly being built for experimentation, I want to play with it, not just watch it run. At some point I'll use other materials and compare the performance. As the build progresses you'll see some of the flexibility built into it. I've been toying with the idea of making a water jacket for the cold end, that may come later.

I have millions of brain cells dying daily, but I think I remember Kevin built a Jan Ridder lamina and posted all his grief on this board. He is not a beginner with Stirling type engines.

Right name but the wrong Kevin. That is KustomKB in BC. The only Stirling I've built was the afore mentioned tin can version many years ago. And I am a beginner with Stirlings, but I'm hoping this build will change that greatly.

Chuck, welcome, thanks for the support. And for passing along Rudy's wisdom.

Arnold, thanks. Put your feet up, get comfy and enjoy the show ;D

 

[ksouers]

Got a little more done before the heat chased me out of the shop.

I finished the cold end of the displacer cylinder using one of the pre-drilled blocks of aluminum I picked up some time ago. I have no idea what grade it is but I don’t think it’s 6061. It machines quite well.

Since I had to use the 4-jaw the work was done on the C2. Got it centered up on the existing hole then opened it up and reamed it to .500 inch. This job really should have been done on a larger lathe but I don’t have a 4-jaw chuck for the 9x20. I didn’t have much room to have a lot of cut-off tool sticking out of the holder so the fins aren’t as deep as the should be, but ali is a good heat sink and the cold side is really oversized so it should work out.

A relief was bored in one end for the hot cap then cut from the parent stock on a band saw, stuck back in the 4-jaw and faced the cut end.

I have a rod salvaged from an old printer that is exactly .500 diameter and it is a very close fit for the bores. The hot cap has a very satisfying “pop” when it’s pulled out. So, I can use it for a gauge when I start building the power cylinder and piston.

Thanks for stopping by.

 

[Maryak]

Kevin,

You must have the patience of a saint to have cut those very nice fins into square stock on the lathe. :bow: :bow:

Best Regards
Bob

 

[arnoldb]

Very nice fins Kevin :bow:

Golly man, the hole in your workpiece is 1/2" (12mm for me :big , so those fins and gaps must be about .04" What did you use as a "parting tool" ???

Regards, Arnold

 

[Deanofid]

Good progress, Kevin.
My, what thin fins you have... Good work on those!

Dean

 

[ksouers]

Bob, thanks for your support. Actually, I'm not very patient at all. If you look close you'll see a bent fin where my lack of patience got the best of me. Before I even finished the first groove I was thinking "Ya know, you really should have gone for the water jacket."

Arnold, very good eye! You must have just had them calibrated! I used a standard parting blade .040 thick, the fins are .050 thick. I just plunged a groove then offset .090 for the next one.

Thanks Dean. I almost made them .030 but thought that looked a little too thin and was afraid of tearing them off. If I were to do it again I'd round the fin area first like you did.

Thanks everyone for the support.

 

[ksouers]

Another short, hot day in the shop. Sorry about being a little short on pictures. It’s just basic lathe turning, nothing spectacular about it. And it’s just been too dang hot in the shop.

The power cylinder is 12L14 steel, 1.125 inches long.

I still need to do some finish work on the piston before I cut it from the parent stock, just drill and slot for the wrist pin. The power piston is 6061 aluminum and will be .625 inches long.

That should be about it for most of the lathe work. I still have the flywheel to do and there might be a couple small pieces to do later on.

Unfortunately the big mill is down for repairs. The phase shifter dealy wampus that turns house current into 3 phase threw craps a couple weeks ago, so that needs to be replaced in order to work on the plates that will hold the cylinders and drill the mounting holes.

Thanks for stopping by.

 

[doc1955]

I can relate to the temp in the shop. I haven't turned a spindle in a week just to hot here too plus the humidity is up.
You have done some good looking work I need to get going on the Stirling fans I'm working on.
I stayed with square fins also hopping th gain all the cooling surface I can I know it's not much of a gain but it should help. Keep up the good work!

 

[ksouers]

Thanks for the support, Doc. We've had the humidity too. With 3 major rivers running through here, plus a couple minor ones, there is no shortage of humidity. The heat must be getting to me. I was reviewing your build thread and when I realized there was still something I needed to do.



Well, I spoke too soon about this being the last of the major lathe work. It appears that “old-timers” has set in, I had completely forgotten about making the displacer piston.

So, back into the shop I go and start making the can. I cut off a piece of 1/2 inch 6061 and chucked it up in a collet and just cleaned it up taking off about 15 thou. I drilled it 29/64, that actually bowed out the sides slightly so I took a couple spring passes to clean it up.

The walls are really thin, I don’t know if they will hold up or not. We’ll see. After all, the whole project is an experiment so I don’t mind making parts over if they don’t work.

 

[SAM in LA]

Kevin,

Nice work.

I live on the Gulf coast with all of the heat and humidity one can stand.

You did give me an idea that may help me deal with it. :idea:

Perhaps I will have cooling fins cut into my round body. :big:

SAM

 

[GailInNM]

Nice work Kevin.
I think that you may have a problem with the 6061 piston in he 12L14 cylinder. They have to fit so close on a Stirling that the difference in the coefficient of expansion may cause it to bind when it is up to operating temperature. Most Stirling engines use power cylinders and mating pistons with similar coefficients of expansion because of this.
Gail in NM