Stirling won't run, need guidance

[EL34]

Hi all,
Just started reading about Stirlings recently and decided to build a small one on my CNC machine.

I decided not to build a tin can model first, I would just dive in and do something a bit nicer. Maybe should have done the tin can first.

Just last night, I had it completed enough to test it, but I still have to add more things, like heat sink fins on the cylinders and a wooden base.

Problem is, she no run.

I think I have the displacer cylinder set up wrong?

I was not sure of many things in the design stage, like.
Displacer cylinder:
How much cylinder air volume total,
how long piston should be in that total volume,
how much air gap around outside of piston?
Stroke length compared to power piston stroke length.
Cylinder volume ratio between displacer and power cylinders
etc.

Was looking for a nice design guide with some guidance spec's, but didn't find anything.

I think I have the timing correct. I did find a document that said that the power piston should be 90 degrees off from the displacer piston and I saw some nice animated CAD drawings that confirmed my timing setup.

The whole crank/flywheel/pistons assembly spins really nice and free, so binding is not the issue.

The displacer piston does not move when the cylinder tip is heated and spinning the flywheel does not get the thing jump started.

I set up some photos on my web site to look at the design.
Looking for some assistance from knowledgeable folks and possibly a nice document on designing a small stirling like the type I am building.
Thanks in advance for any help.
Doug


Here's my web page with some pic and info of the Stirling
http://www.el34world.com/Misc/Stirling/stirling1.htm



Edit: can't get images to display?

[Bernie The Bolt]

Hi Doug - you have suffered from the same problem as me in trying to find some ideal design sizes/ratios and I also went for a full metal engine with decent sizes. I'm no model maker or watchmaker!

From looking at your excellent pics I think you need to look at the size of the displacer cylinder. Seems a bit small to me, not much actual heated air volume. Also I suggest you dump that long ali plug in the end, it may hold heat but it also disipated heat away. Get your heat source directly on to the displacer cylinder.

I would like to add pics but not having a personal website seems to exclude it. Any suggestions?
Bernie B.

[EL34]

Hey thanks Bernie,
I came to the same conclusion last night about the displacer.

I am out in shop today making a new one with more air volume.
We will see how this one goes and go from there.

I made a new power piston yesterday and got my fit in the cylinder way better than the first one. It now makes a sucking noise when I add a little oil, and it moves freely, so we will se how it goes with the new power piston also.

I'll post the results when I get er fired up.
Thanks again.

[EL34]

Well the new displacer cylinder does not make it run either.

I have a feeling that I have too much air gap around the outside of the piston.

The hot air does not push the displacer piston because it can just bypass it so easlily.

My piston is about 10mm diameter and I would say that there is 3mm total air gap.

How loose should a displacer piston be in the cylinder?

[Bernie The Bolt]

My engine has about 3mm displacer clearance on a diameter of at least 100mm so I think you could be right in thinking it's a bit too loose on 10mm.
Don't give up... (I nearly did!)

[EL34]

Ahhhh, thanks so much Bernie. That gives me a bunch of info to go on.
So about 3% as opposed to what I have which is an ugly 30%

Inside diameter of your displacer cylinder.
Outside diameter of your piston.
Which you aleady answered, about 3%

Another bit of info would be helpful.
inside length of your displacer cylinder?
length of your piston?

Just rough guestimates are fine.

Gonna make a new displacer piston.
Tyring to figure out how large my new piston should be in relation to my cylinder.

[Bernie The Bolt]

I THINK the displacer length should be about 3 times the diameter and take up about 2/3 of the cylinder length. I'm really no expert on these sizes so treat the info as a rough guideline only!

The clearance should be very small in your case but it must actually clear.

Bernie

[EL34]

Ahh, great, thanks. Any info at all is good info to me at this point.

kinda suprised no one else here has offered any info.

It's rough learing something new without some experienced help from the web community.

so if I have a 10mm diameter piston,
I would shoot for a 30mm llength.

Based on 30mm length, the internal cylinder space would be 40mm.
so that would limit my stroke to about 10mm

am I calulating that correctly?

Thanks for the help Bernie

[Bernie The Bolt]

I think you are right - like me you already have a fixed stroke for the displacer so you can work it backwards from that point if my theory is correct. i.e the stroke is 1/3 of the length etc.
I'm wondering about the thermal conductivity of your brass cylinder and note that some of the smaller engines just use a pyrex test tube for this part. Let's try to avoid stainless for the moment shall we?

I'm also surprised that some tech boffin hasn't come up with some figures that shoot my advice to bits but I reckon between us and with a bit of trial and error we should end up with a couple of running engines one way or another!

I'm just a little envious of your CNC set up as I have to revert to manual turning and milling in my old fashioned workshop.

[EL34]

Hey Bernie.
I think the brass conducts heat too quickly. The supposed cool end of the cylinder is really hot, so there would not be a temperature difference.
I would like to do glass cylinders, I just used stuff that I already had on hand in my shop.

I built two cnc machines from scratch.
CNC'ing is another one of my 10 or so hobbies.
And now Sterling engines if I can figure out how to make them run. :???:

I also have a small hobby sized lathe that I kept from when I used to race slot cars (many many years ago). It has gotten lots of use on this project. It will do brass and aluminum, but it is not powerful enough to do steel.

A couple drill presses too.

Check out my CNC page if you would like to see the two machines I built and a bunch of the projects I have made with them.

http://www.el34world.com/Misc/Cnc/CNC0.htm

I still have, and am going to sell the first machine. I used the first machine to build the 2nd machine.

I may just build Boyd's tin can stirling, just to get the satisfaction that I made one run. Then go back to the other one.

[Bernie The Bolt]

Very impressive!

Way back in the '80's I made a batch of hand size hot air engines using CNC wherever possible.

I still have the drawings......

I think my email address is shown.

Cheers
Bernie

[Cartech]

I'll throw in my two cents here.. I have no experience building any type other than an LTD but I do own a commercial model Solar-1 Engine ( http://www.pmresearchinc.com/store/home.php?cat=5 ) It's specs are; Power Cylinder: 3/4 inch stroke by 3/8 bore. Piston length is approx 1 inch long with small ring groves cut in it and it's lubed with dry graphite, not oil. Displacer: 5/8 stroke by 5/8 bore inside a slighty (maybe 1 mm) larger cylinder. The hot cap is made of stainless steel and is an inch and 1/8ths long and the cold end is aluminum the same approx length. Total length together is about 2 and 1/2 inches long. I think the displacer piston is about an inch and 1/2 long. It's your basic 2/3rds full design meaning the displacer piston fills the displacer all but 1/3 of the air space. The displacer is aluminum and solid round stock, it isn't hollow at the bottom end in other words. Dead air space is minimal. (dead air is what's left at each end of each piston's stroke and in the connecting port between them. You want to keep this as small as possible.) You should be able to feel some compression yet the motor should spin several cycles with a quick flick. The specs above were taken by eyeball and a tape measure and should be pretty close. My model has no ball bearings but would run better if it did. I hope this helps.

[wbr62]

hello all

been browsing this site for for some time finally jumped in.

i looked at the photo of your engine,and aggree with the suggestions.

heres mine on a different part.

the aluminium block with the air transfer tube could be absorbing most of

the heat before it reaches the power piston.

the need for a regenerator is required for efficiency but only if the heat

can be retained

[EL34]

Oh man, that's a nice big clear picture of that solar-1. It makes several things very clear since I can see the detail so well.

I have been running into very small images on the web. People don't take big enough pictures. Especially since I run 23" monitors here at 1680 x 1050, The pictures are always tiny.

Thanks so much for that info cartech. I am going back to the drawing board with this info.

I think I am going to separate th two cylinders so that big aluminum block doe not trandfer heat.
I also need to make those heat sinks as I had planned.

I'll give this model another go. If I can't get it to work, I'll scavange the parts that can be re-used, like the fylwheel, etc.