Stirling Engine : Laura

[AjohnwParticipant @ajohnw51620]

The air need to loose heat as it passes the displacer Brian as it's serving as a regenerator after a fashion.

More fin at the cold end of things should help as the commercial large ones sometimes use water cooling.

I did wonder about suggesting you added a fan.

You could soon find out if more cooling at the cold end would help. If it can run for 10min try applying a cold wet flannel to the power cylinder after say 7min or what ever.

John

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Edited By Ajohnw on 01/03/2016 11:40:40

[Neil WyattModerator @neilwyatt]

Apologies to Ken – I was checkings omething for him and forgot to log out.

Ten minutes is a long run for a small engine like that which is intended to be decorative rather than do a job of work.

Getting it to run longer will require effort to keep the cold end cool.

Personally I would be pleased with getting ten minutes, chalk it up as a success, and look for the next project.

Neil

[Brian JohnParticipant @brianjohn93961]

I removed the gasket from the power cylinder cover ; it runs better without it. The cylinder cover acts as a cooling part and if there is a high temp gasket between that and the cylinder then it blocks the heat from getting to the cover. It took me a while to figure out why it was only running for a few minutes then stopping. But it does show how sensitive it is to cooling. I think a bigger OD power cylinder with deeper cooling fins is worth trying.

Neil : Yes, I do regard this engine as a success but I think it could be made to run better and I am very curious if I can do it. It might also be worth honing the power cylinder bore so that would be something else new to try. I am still not sure how I would do that yet.

 

 

Edited By Brian John on 01/03/2016 14:03:25

[Brian JohnParticipant @brianjohn93961]

I made a new D. piston rod from the 3mm ground silver steel purchased from Minitech. The engine starts much more quickly now (almost immediately)…but it still only runs for 10 minutes.

I am in the process of make a larger OD (27mm) power cylinder with deeper cooling fins. After I have tried that I will call it quits. I am happy with the engine as it is but I am wondering if it can be made to run for a bit longer.

Edited By Brian John on 04/03/2016 06:47:55

Edited By Brian John on 04/03/2016 06:48:30

[pgk pgkParticipant @pgkpgk17461]

I'd have thought that the power cylinder isn't where you need cooling .. it's the cold cylinder cooling that would be better to improve. You can test the theory with keeping tissue damp on one then the other for evaporative cooling.

For an 'off the wall' idea I did wonder if power could be improved by pumping air into the system to pressurise it despite leaks – or just larger hot and cold cylinders/displacer though there must be a limiting factor there with displacer power to power piston power.

Anything that reduces friction has to help…lapping the power cylinder and piston should do so. I also looked (but couldn't find) some simple coating for the displacer rod – I've seen shrink tube in teflon somewhere. Perhaps a dry graphite spray?

(and i found all my missing diamond grits yesterday just after I'd finished cleaning, lubing and maintaining/relevelling my lathe after my last lapping attempts)

[Brian JohnParticipant @brianjohn93961]

I am also thinking of honing the power cylinder I am building now with an 11mm brake cylinder hone : The cylinder is 10mm in diameter.

**LINK**

Edited By Brian John on 04/03/2016 08:04:39

Edited By Brian John on 04/03/2016 08:09:31

[Ian S CParticipant @iansc]

Brian, once you get the motor going, leave the bore alone, and you will find that after an hour or two running the speed will increase, well that's what I find. My second engine showed that, I decided to run it until it wore out, it was run with a generator on it with an incandescent light rated at 3v and that got slowly brighter, after a day or so that one popped, so up to 6v and by the end of the week that one went too, so a 12v light is a good load. After 1000hrs the big end on the power piston was really worn (.020", so stop for a rebuild, it managed to cut the metal it's self with my mini power hacksaw. These things just get better as they go. This is the motor, it's not pretty, but it goes, and that is why I build them, and modify them.

Generally, after boring the cylinder in the lathe , I have a look, if there are machine marks I use a cylinder hone, then I get to work and lap the bore until the marks from honing disappear. For Stirling Engines I like the bore as smooth and shiny as possible.  The honing marks are left on with an IC motor, it helps the rings to bed in.

Ian S C

Edited By Ian S C on 04/03/2016 11:42:54

[Michael GilliganParticipant @michaelgilligan61133]

Posted by pgk pgk on 04/03/2016 07:49:59:

Anything that reduces friction has to help…lapping the power cylinder and piston should do so. I also looked (but couldn't find) some simple coating for the displacer rod – I've seen shrink tube in teflon somewhere. Perhaps a dry graphite spray?

.

I think you're onto something … although I am not aware of 'shrink tube' in teflon.

What does look interesting, is the range of [fixed size] tube available.

Methinks that a relatively long 'gland' of this material might do the trick, on a small engine.

MichaelG.

Edited By Michael Gilligan on 04/03/2016 12:54:49

[pgk pgkParticipant @pgkpgk17461]

I built a MinAir Spectra G r/c heli some years ago and included in the kit was a shrinkable tube to reduce friction between the tail push rod and guides… pretty sure it was marketed as teflon. MinAir has since gone bust although there is a German Co now producing helis in that name and supplying spares.

(I don't fly that heli much ..has a 2-stroke ported/modded petrol engine that weighs heavy. When i crash it the bill is usually several hundred pounds..and after crashing 3 times one season….)

[Michael GilliganParticipant @michaelgilligan61133]

Posted by pgk pgk on 04/03/2016 13:02:42:

I built a MinAir Spectra G r/c heli some years ago and included in the kit was a shrinkable tube to reduce friction between the tail push rod and guides… pretty sure it was marketed as teflon.

.

Perfectly happy to be wrong

It does seem odd though, that a [presumably Heat-] Shrinkable Tube could be made from such a temperature resistant material.

Please let us know if you find the data.

MichaelG.

.

Edit: … and wrong I clearly was.

Edited By Michael Gilligan on 04/03/2016 13:36:08

[pgk pgkParticipant @pgkpgk17461]

here:

**LINK**

[Michael GilliganParticipant @michaelgilligan61133]

Being proved wrong means I have learned something today.

MichaelG.

.

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

[Brian JohnParticipant @brianjohn93961]

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

[pgk pgkParticipant @pgkpgk17461]

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..

[Ian S CParticipant @iansc]

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

[HopperParticipant @hopper]

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.

[Swarf MakerParticipant @swarfmaker85383]

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.

[Michael GilliganParticipant @michaelgilligan61133]

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

**LINK**

MichaelG.

.

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

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

[AjohnwParticipant @ajohnw51620]

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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[Michael GilliganParticipant @michaelgilligan61133]

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.

.

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

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

[pgk pgkParticipant @pgkpgk17461]

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.

[Brian JohnParticipant @brianjohn93961]

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

[Brian JohnParticipant @brianjohn93961]

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.

[AjohnwParticipant @ajohnw51620]

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

–

[Brian JohnParticipant @brianjohn93961]

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

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