Stirling Engine : Laura

Hopper, I make my displacers three times the diameter long, the volume of the space is 1.5 times the volume of the power cylinder swept area. On dis-assembly after running it is normal to find that the displacer is coloured by heat, usually a bit blackish going to blue, and the colour fading out about a third of the way down.

Brian, I suspect that the clearance is on the narrow side, .5 mm gap, are you sure the displacer does not touch the hot cap.

Ian S C

I have been without the internet all day until now so forgive the late reply (thank you Telstra!).

Much mucking about today with still no success. I have read all the above just now and there is a good chance that this engine will never work as it conducts too much heat from one side to the other too quickly. However, if the glass tube version will run for well over 20 minutes then this version should at least run for 5 minutes...would you agree ? Also, I have put gaskets between the hot cap and the D. cylinder and the cylinder holders to slow down the heat transfer.

I have tried to keep all dimensions the same as the original glass version but I had to go with the stainless steel tube from Bunnings as the hot cap so things had to be modified slightly to accommodate this. See above for D. piston dimensions. Other than that everything is the same.

I have just realised that I did not enlarge the D. cylinder by an extra 1.6mm to allow for the larger piston diameter. I will do that tomorrow. That is plan A.

Ian : no the piston is not rubbing against the hot cap ; I am sure I would feel that. It is also a good close fit to the end of the hot cap. I may knock another 0.1mm of the diameter of the piston if nothing else works. That is plan B.

Plan C is to make the D. piston out of something other than aluminium.

NOTE ; the inside of the stainless steel tube is brown after all the heat today...almost like rust ! I am starting to suspect that it may not be stainless steel after all even though it was labelled as such. Perhaps stainless on the outside and something else inside ? It was stainless steel tubing from the laundry section of the hardware store.

Well, no wonder it would not run ! I set up the D. cylinder in the lathe chuck in preparation for boring as per above plan A and that is when I saw a fine hairline crack at the base of one of the cooling fins ...where it meets the recess to take the hot cap. The original plans call for 5mm deep cooling fins but I did not modify that and I should have. The recess to take the hot cap went almost all the way through...but not quite, hence the very fine hairline crack which I had not seen until now. The crack is too small to show up using my camera. I did think of repairing it using silver solder but I am not sure that would work very well.

I am in the process of making another D. cylinder but this time the cooling fins are only 3mm deep so there is plenty of brass left to make the recess for the hot cap. It is almost done ; I just have to drill and tap for the M2 screws now.

I might buy some high temp gasket material for the hot cap. I don't think what I am using now is high temp material. I think that would make a big difference.

What grade of stainless steel could I machine a D. piston out of ? The chances of finding some stainless steel tube of the correct diameter and making it from that are pretty slim.

Edited By Brian John on 10/05/2016 08:42:33

Edited By Brian John on 10/05/2016 08:43:03

Edited By Brian John on 10/05/2016 08:46:04

You might have to make your own tube. For your lathe I would suggest you try mild steel, work out the diameter you want, and bore out a bit of bar( a bit thicker than the final diameter) to the diameter minus the wall thickness, if you are clever you can bore a blind hole. Now take another bit of steel, and take it down to the bore of the displacer, and drill a small hole down the length of the mandrel, to let the air out as you put the displacer on with a drop of Loctite nut lock, now you can turn the outside of the displacer to size, Heat the displacer, when it changes colour you should be able to get it off. The wall thickness would be from .010" - .015"/.25 mm / .3 mm, a little more wouldn't matter. I would make it with about 1 mm clearance/ 2 mm smaller diameter than the hot cap.

Tough luck about the brass bit with the crack, we all make mistakes, but you might find a use for it on something else.

Ian S C

I made another D. cylinder but still no luck. I feel I am close : it wants to run. Spinning it in the wrong direction when heat is applied and it will only turn about 4 revolutions but spinning it in the correct direction and it will spin for about 20 revolutions. It must be close to running !

I also replaced my slightly wobbly cast iron flywheel with a similar size brass flywheel from PM Research. Tomorrow I will reduce the D. piston by 0.1mm. 

The second photo shows the old D. cylinder. I am going to try to save it.....no reason ! I have drilled and tapped through to the second cooling fin ; the M2 screw will hold it in the correct position. I will run a fillet of silver solder around the inside edge of the cooling fin where it has opened up. I will apply a drop of oil to the M2 screws to keep the solder out of the tapped holes.

Edited By Brian John on 11/05/2016 11:28:29

I have tested the engine to see how air tight it is. I have taken off the hot cap and removed the piston. I put some 3mm rod inside the D.cylinder cover, leaving it free to move then placed my hand over the open end while turning the flywheel. Note that I have now replaced the brass wheel with the original cast iron flywheel (just a hunch). I can feel the suction on my hand and the 3mm rod moves back and forwards so we do have a sealed system.

I am confident that once I reduce the diameter of the D. piston then it will run. It feels very close !

If that fails then I will look at making a stainless steel D. piston but that depends on what I can get my hands on.

NOTE : I think that the ''stainless steel'' tube I purchased is only steel with some sort of stainless coating. I doubt very much that it is 100% stainless steel. The inside is now covered with some type of brown oxidation. Does that sound like stainless steel ?

http://www.bunnings.com.au/award-16-x-1000mm-stainless-steel-tube_p4825496

Edited By Brian John on 11/05/2016 13:42:13

Edited By Brian John on 11/05/2016 13:43:04

Edited By Brian John on 11/05/2016 13:46:15

Edited By Brian John on 11/05/2016 13:56:35

The displacer for my smallest motor was made of a bit of rebar from a building site, it's s**t to turn and get a good finish, but it's not seen so what the heck.

This is the wee motor, the hot cap is the steel case of a ZZ size NiCad battery.

Ian S C

So close now : it will run for about 12 seconds before it stops ! I am gradually reducing the diameter of the D piston. I started off at 13.5mm and now I am down to 13.1mm. I will keep going down in 0.1mm increments until it runs.

I went to Tonkin steel today to see what they had in stainless steel. It is all 316 grade.

round bar : 15.88mm and 12.7mm

Tube : 15.9mm OD X 1.6mm wall and 19 OD X 1.6mm wall

The wall of the stainless steel tubing is a bit thick for this purpose but perhaps I can turn it down in the lathe.

Brian, Sharplift down on Redden St, Portsmith, are a lot easier to deal with on short pieces and offcuts of stainless steel. All good marine grade stuff, no rubbish. Tonkins always seem to want to sell only 3.2 metre lengths etc.

But as Ian mentioned above, even a hollow mild steel displacer is going to have better thermal properties than solid ally for a displacer piston.

Thanks. I will give them a call tomorrow.

Do you think I am wasting my time with this aluminium displacer ? If I can get it to run at all then I will look at replacing it later with something better but I really think it should run for 5 or 10 minutes at least.

If you get a bit of stainless with a smooth bore, not a great lump of weld down the seam, you can make a mandrel, and take 1 mm or so off/ 2mm off the diameter. My first attempt at turning SS was with 316, the bloke at the warehouse said I'd have a hard job, but it went OK except for great streamers of swarf, it's not free cutting. Here's the result, a bit bigger at 30 mm bore,  with an internal thread cut at the open end.

Ian S C

Edited By Ian S C on 12/05/2016 14:15:53

So it cuts like aluminium ie. comes off in ribbons/streamers ?

316 does for me, but be positive with the cutting, if you let the tool rub it will work harden,

Ian S C

I am down to 12.9mm on the D piston : it will run for about 30 seconds. I think the main problem is that so called ''stainless steel'' tube I bought from the hardware store ; it is not stainless steel at all...perhaps just a thin coating of stainless. If that had been genuine stainless steel then this might have worked. Too much heat is being transferred down the tube. One option may be to block the heat using some high temp. exhaust manifold gasket material between the hot cap and the D cylinder. At the moment I am only using gasket paper and it is not doing much at all.

So I will have to make another hot cap and perhaps a new D. Piston. I bought some proper stainless steel tubing today from Sharplift as per Hoppers suggestion. It is much cheaper there as the minimum length is 0.5 metre. I bought 12.8mm OD and 15.8mm OD ; the wall thickness is 2mm which is very thick so something will have to be done there. I am not sure how my boring tool will go on this material.

The 12.8mm tube should be okay for a D piston ; perhaps I could just make a cap for each end from brass and solder them in place...maybe ! I still have options.

Edited By Brian John on 13/05/2016 12:07:13

Brian John Why bore it make a mandrel and reduce the outside Use the inner bore as is as long as it's smooth

Sounds like it is close, but if the displacer cylinder is not stainless you will get too much heat at the cold end. The air needs to go from hot to cold to hot etc, it is the temperature differential that drives a Stirling.

I bought some exhaust manifold gasket material today but that idea went nowhere. How do you cut this stuff ! I hammered away it it with hole punch for 10 minutes and got nowhere. Even a sharp scalpel blade is useless. Anyway I gave up that idea and moved on to making a stainless steel D piston from the 12.8mm OD stainless steel tube.

Initially I was going to make the two ends to the piston from brass and then silver solder them in place but then I had this idea (how many times do we hear that !). I have put an M3 thread in the'' hot'' end , the piston rod is threaded so that it screws into this after passing all the way through the brass fitting at the ''cold'' end. That brass bush is held in place with an M3 grub screw. Looks good but will it work...I will give it a go later tonight.

The stainless steel is tough stuff and I comepletely destroyed my M2 spotting drill. Ideally the tube  would have a thinner wall : 1mm would be good but for now I have stuck with the original 2mm wall. I am not sure how this stuff would go with a boring bar. I might give that a go tomorrow if I make another piston. I was able to clean up the ends in the lathe with no problem so perhaps machining it is easier than drilling into it. Even tapping it was no problem with plenty of tap magic being used.

Edited By Brian John on 14/05/2016 08:22:39

Edited By Brian John on 14/05/2016 08:25:10

There are all sorts of stainless steels Brian. When Ian uses a tube out of a nicad battery he will be using a ductile grade which will be more or less the same as a free machining grade. These are much easier to machine than the "true" stainless steels. It's pretty easy to tell the difference with a magnet. The easier to handle grades are slightly magnetic. Treated with a bit of respect you shouldn't have much trouble turning them.

John

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The case from the NiCad is just used "as is". I needed some tube for a job the other day, a bit of towel rail tube was the right size, I needed about an inch off it, so I decided to use the parting tool as it was all set up, went in about .010" and the shiney outer surface stopped while the rest kept turning, there was a chrome plated brass tube fitted over a steel tube. All is not always what it seems.

Ian S C

I caught out much the same way. Bought a bundle of what I thought were 1/4" brass rods only to find that they were steel with a thin brass outer shell and brass finials.

Rog