Stirling Engine : Laura

Being proved wrong means I have learned something today.

MichaelG.

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Here is another source, with good data and a offer of free samples.

So what size of heat shrink tubing would you use for a 3mm silver steel rod....4mm ?

A difficult one.. it looks like there are differing proportions of shrinkage types 4:1 shrinkage 2:1 shrinkage etc.. and I couldn;t find obvious data on final wall thickness.

The pice I used building my heli ended up (guesstimate) around 1/4 to 1/2 mm thick wall. You'ld likely have to experiment making up a few different rod thicknesses and shrink tubing them..

For Teflon bushes, I make my own, at first I used to carve them out of a bit of carbon impregnated Teflon rod, that was ok, but lots of swarfe, I kept all of that. There was a bit in ME, late 1980s/early 90s about Teflon bushes. A mould is needed, this gets packed with the swarfe, and compressed, the mould is made from steel. Next heat the mould until its blue, allow to cool, push the contents out.

The one in the photo is 1/2" bore, 5/8" OD for the displacer rod on a Ringbom motor (free displacer)

Ian S C

Many Stirling engines have no fins at all on the power cylinder. Cooling is done by the fins on the displacer cold end. You might improve displacer cylinder cooling by making an aluminium cold end with larger diameter fins. Also make the fins thinner than the gap between the fins. This allows air to circulate more into the fin root area, where temperature is highest and therefore heat transfer rate will be highest. Look at the fins on a motorbike engine for example.

Also you might think about putting a baffle plate between the flame and the cold end to stop radiant heat from the flame getting into the cold end fins and end cover. A piece of shiny stainless steel sheet, or a sample swatch of that roofing insulation foil could do the job. Just cut a hole in it and stick it over the glass tube.

I once thought that the taper on the fins of an air cooled engine were the result of a drafting angle for the castings.

However, there is some experimental science that shows that a tapered section fin conducts heat away from the base of the fin more rapidly than a fin of constant cross-section.

This paper is about heatsinks for electronics, not engines, but it's quite useful.

**LINK**

MichaelG.

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A two page summary, without the maths, would be handy ... any offers ?

Edited By Michael Gilligan on 05/03/2016 12:53:17

There is a photo and rough description of a wooden lap here Brian

**LINK**

They can be improved. Start with some dowelling. Fit a long thin woodscrew in the end with say 1/2 of it in the dowel via drilling a hole a little bit bigger than the core diameter of the screw. Turn the od to match the bore noting the length in the link. Put some shallow spaced out grooves in it along the length say every 10mm. Saw 3 slots along it's length. These need to go well past the turned dia might help to remove the screw first. Fit the screw with the lap in the lathe and trim up the size. Add the abrasive and use as shown in the photo. The screw can be used to expand it. Use a slow speed. The abrasive could be brasso, silvo, tooth paste etc.

If the dowel wont turn well use a file to size it aiming for a nice even diameter. You could try the brake cylinder hones but I don't think they would leave that good a finish.

Another technique that might work to initially polish that I have used when porting cylinder heads to make cars go faster without anyone knowing is a slit in the end of a piece of bar held in a die grinder. All I was doing was decoking them. Just stick a piece of a suitable grade of emery cloth in the slit. Poke it in the hole and start the die grinder. You could mount the piece of bar in the lathe chuck and run it flat out. The way I did that on ports meant that I was working on one side of the bar. A suitable length of emery cloth might be persuaded to work on 2 sides with the bar on centre in the toolpost. That might help things stay round.

I have a feeling Jason mentioned something similar using flap wheels but they may be too coarse.

John

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I"ve just found an old Cranfield paper on cylinder finning, which may be of interest.

**LINK**

One important point though: the dissipation from fins is [presumably] 'an absolute' and therefore designs cannot be scaled. ... This suggests that we should probably be making use of the lessons learned in cooling Electronics, rather than full-size Engines.

MichaelG.

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Edit: This, from DH Chaddock no less, is also worth a look.

Edited By Michael Gilligan on 05/03/2016 14:37:20

Re thinning fins to the tip... with 3 ways cooling can happen it's a reasonable presumption that just as ionisation always acts fartherest from a fine pointed object than along it's sides.. that radiant heat would act similarly. That and allowing more air between fins at the tip where they require less support.

I suggested earlier that a copper cold cylinder or copper fins applied to oen should work better too.

A day of mixed blessings ! I broke another M2 tap ...this time it was the 1st tap ; last time it was the 3rd tap. I know what I did wrong on both occasions. I still have no Alum but I might abandon this power cylinder anyway as I think it is a waste of time making a larger cylinder.

My lathe tailstock continues to give problems. It seized up again and now it wobbles around so I cannot do much with the lathe at the moment until I get this sorted out.

I did some experimenting with the power piston trying to find the optimum setting ( best distance from top of piston to end of cylinder at TDC). I have found this to be 4.3mm. This is a much bigger distance than I thought it would be. What determines this distance ? I thought it should be as close to the cylinder cover as possible but it is nowhere near this.

Once I found the correct position for the cylinder I squirted a few drops of machine oil in the cylinder before replacing the cylinder cover. Off it went in about 3 seconds...it hardly had time to heat up ! It ran for 10 minutes before the glass tube blew off. I quickly put on the spare glass tube and it ran for another 15 minutes...so 25 minutes in total. I think this proves that heat transfer from the D. cylinder to the power cylinder is not the problem. I did not have the power piston set correctly. I also think the piston should be a tighter fit in the cylinder ; it should not need oil in the cylinder to run well.

When I get my lathe working properly again I might try to make a better fitting power piston.

Edited By Brian John on 06/03/2016 09:38:14

The engine stopped working today. I have spent four hours trying to get it going but no luck at all. It is not a friction problem, that much I know. Any suggestions would be welcome.

By moving the power piston back you have lowered the compression ratio which will tend to make it run at lower temperatures with less output power.

Out of interest I saw some where or the other this engine running with a spirit lamp with a rather small wick. As the whole thing getting hot causes problems there isn't much point in using a flame any bigger than needed.

If you have put oil in it cleaning it all out thoroughly might get it running again. It might have found it's way into the port and the displacer area.

John

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Yes, the oil did find its way there. I have cleaned it all out now but it still will not run. The oil was needed in the power piston or it would not start. I think that this indicates that I did not have a tight enough fit between piston and cylinder. I may have to make another power cylinder and piston to get things going again.

I did make the wick and its hole larger than I should have... I am still in ''steam engine mode'' !

Edited By Brian John on 08/03/2016 13:35:40

With a Stirling, if it's not friction it's a leak. You are probably on the right track with power piston being a bit loose. With a good neat fit there, you should be able to get her to run with a much smaller flame, which will cut down the heat moving to the cool end.

It seemed like a really good fit between cylinder and piston but I decided to make another piston anyway and hone the present cylinder. I honed the cylinder by placing it in the lathe chuck and then wrapping some 1500 grit wet and dry around a pencil. Bad idea....it did not work as well as I thought it would.

So I broke out the spare piston and cylinder. You might remember that I made two or three of just about every part . This new combination seemed to be a really good fit : I place the cylinder on my thigh to seal it off at that end and then push the piston in out. I get a good springy feel and ''push back'' so surely that is a good seal ?

Still no good.. It just will not run. I am starting to think that the Hylomar seal between the cylinder frames may have been compromised in some way. I might have another go at making a paper gasket for this. My last effort was not good.

Where can I buy an 11mm brake cylinder hone like this :

**LINK**

Edited By Brian John on 09/03/2016 09:25:06

Looks like there is a phone number on the website to me Brian.

John

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Yes, but when I called a recorded message requested an access code so perhaps they do not sell to the general public. I should have mentioned that before. I will send them an email instead.

Edited By Brian John on 09/03/2016 10:30:00

Brian, you blew the test tube off the displacer the other day, perhaps when it was replaced you got a leak there. The wick hole in the burner only needs to be 3 mm to 4 mm diameter, and the wick not too tight, and sticking up probably 2 mm at the most. Ian S C

I can adjust the flame by lowering the wick and I can also move the flame away from the glass tube to lower the heat but none of this is working. I have two glass tubes : both of them worked before but neither of them will work now so I do not think that is the problem.

I am sure it is down to the fit of the power piston in its cylinder. I am back to the stage where I was last week. It was almost running but not quite. It was not until I put a few drops of oil behind the power piston that it started to go. But that is not working now. I think I will have to make another power cylinder but I really need a brake hone to get it really smooth. I will make some phone calls tomorrow : somebody must have those flex hones in Australia