Stirling Engine : Laura

[roy entwistleParticipant @royentwistle24699]

Have you blocked any holes with the gaskets ?

[Brian JohnParticipant @brianjohn93961]

No holes are blocked. I might have another go at reboring the power cylinder tomorrow and making a new piston for it.

I am surprised at how hot the D. cylinder gets. I thought stainless steel was a poor conductor of heat ?

[Ian S CParticipant @iansc]

Brian, with the stainless hot cap you can take the temperature up to red heat, but I still don't like the solid aluminium displacer, this is waisting a lot of heat, just transferring it direct to the cold end. Thin walled steel /stainless is best, and you will find on dis- assembly after running that the hot end of the displacer will be quite blue, indicating the temperature it has got to.

Ian S C

[Brian JohnParticipant @brianjohn93961]

But it should still run shouldn't it ? After all the other engine ran with the aluminium displacer and the glass tube/cylinder.

[HowiParticipant @howi]

You need 3 things for a Stirling engine to work :-

Low friction – as it has already run that should be no problem

NO air leaks – nothing kills a Stirling like an air leak, it ran with the glass displacer so what has changed?

Temperature differential – try putting some ice on the cold end to increase the differential

 

My bet is item 2 an air leak

[Brian JohnParticipant @brianjohn93961]

I agree : my gut feeling is an air leak somewhere as I think it is a bit TOO free wheeling. There should be a bit more resistance felt when turning the flywheel over. I shall investigate the power piston/cylinder fit tomorrow.

But does a 13.6mm D. piston inside a 14.6mm D. cylinder sound about right ?

[HowiParticipant @howi]

Posted by Brian John on 08/05/2016 12:31:32:

I agree : my gut feeling is an air leak somewhere as I think it is a bit TOO free wheeling. There should be a bit more resistance felt when turning the flywheel over. I shall investigate the power piston/cylinder fit tomorrow.

But does a 13.6mm D. piston inside a 14.6mm D. cylinder sound about right ?

D piston and cylinder are not too important, the D piston is only moving the air from the hot end to the cold end and back. The volume of the D piston might be too big, the Stirling works on the principle of expanding and contracting of air in a closed space, as such volume of air plays a big part. A big displacer means less air!!!!

I would suggest it is not the power piston or cylinder at fault as they worked OK before your mod.

I think you need to look at the displacer cylinder assembly for air leaks as that is the part you have changed…….

Edited By Howi on 08/05/2016 17:40:38

[pgk pgkParticipant @pgkpgk17461]

Howi, i think this is a second engine rather than just a hot end change so very much deja vu

Without Ian's greater expertise I'd also speculate on the d piston material..easiest back to glass with a hotter flame on the cylinder. and back to leak checks with soapy water.

What might be interesting to try would be a slightly shorter glass displacer with a bundle of wire wool at it's rod end to make up the length and act as regenerator??

[HopperParticipant @hopper]

I'd get rid of that solid aluminium displacer piston. It might work with tthe glass outer cylinder, but stainless is going to be conducting more heat lengthways toward the cold end than glass, which combined with a solid block of ally for a piston sucking up heat rather well is not a good combination. Most common displacer material is stainless tubing with a stainless cap on the hot end and an ally plug on the other for the rod to screw into.

But certainly check for leaks first. You should feel some compression when turning the flywheel.

[AjohnwParticipant @ajohnw51620]

Hopper hit the nail on the head – the reason borosilicate test tube etc displacer tubes are popular is low heat conduction. From memory one reason stainless was attractive is because it has lower heat conduction values than other metals. Checking circa 1/2 mild steel etc values. I believe that is for real stainless as well, no free machining.

That's only half the story though. The power the engine has comes from temperature difference so how well the cold end radiates and keeps the end of the displacer cylinder cooler than the hot end counts as well. Same for the displacer. If it gets to the same temperature all along it isn't going to work. There must be a temperature difference. Aluminium isn't going to help that aspect. You might have done better to retain the glass tube and make a stainless displacer. Using aluminium with much bigger fins on the part that holds the displacer tube should help as well.

Last but not least. An engine of some size and design can only dissipate a certain amount of heat. That is what produces the temperature difference that gives the engine power. Some level of heat input will give maximum power. Increasing the heat input past that wont give more power. The temperature difference will more or less remain the same. This is also why models stop working after running for a while. The temperature eventually stabilises between the hot and cold parts and is so low that there isn't enough power to make the engine run any more.

John

–

[HopperParticipant @hopper]

Posted by Brian John on 08/05/2016 12:31:32:

 

But does a 13.6mm D. piston inside a 14.6mm D. cylinder sound about right ?

Yes that is about right for gap. Is your stainless cylinder the same ID as the glass tube, and the same length? Is the displacer piston the same dimensions as the original?

You also need to make sure the displacer piston length is such that there is only a small gap – say 1mm or so – between displacer and hot cap at that end of the stroke, and a similar small space between the other end of the displacer and the brass cylinder plate at the other end of the stroke. At the same time, the displacer piston is usually made so the length is equal to about two-thirds of the total length of the combined hot/cold cylinder.

For example, if the stroke of a hypothetical displacer is 1", the displacer piston would be 2" long and the combined hot and cold ends of the displacer cylinder would be 3" long (plus a few thou for clearance). That way, the air is either in the hottest third of the cylinder at one end, or the coldest third at the other, with minimal time in the middle third. This maximises the temperature differential, which is waht makes the engine work.

 

Edited By Hopper on 09/05/2016 11:05:00

[Ian S CParticipant @iansc]

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

[Brian JohnParticipant @brianjohn93961]

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.

[Brian JohnParticipant @brianjohn93961]

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

[Ian S CParticipant @iansc]

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

[Brian JohnParticipant @brianjohn93961]

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

[Brian JohnParticipant @brianjohn93961]

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

[Ian S CParticipant @iansc]

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

[Brian JohnParticipant @brianjohn93961]

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.

[HopperParticipant @hopper]

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.

[Brian JohnParticipant @brianjohn93961]

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.

[Ian S CParticipant @iansc]

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

[Brian JohnParticipant @brianjohn93961]

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

[Ian S CParticipant @iansc]

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

Ian S C

[Brian JohnParticipant @brianjohn93961]

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

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