Stirling Engine : Laura

Hi Brian ,

whether oil is used on the power piston and cylinder depends on what materials are being used and how much clearance there is between piston and cylinder, on my similar engine which has an aluminium piston running at two thou clearance in a bronze cylinder i use contect duck oil as if I run it dry the piston would soon pick up after a few mins at high speed but using oil does cause a bit of drag on the cyl/piston slowing it down a bit , hard anodising the piston would perhaps allow it to run dry though , on all my other engines which use cast iron pistons in bronze or cast iron cylinders i run them dry as the graphite content in the cast iron seems to be enough to keep them from picking up and even after a 20 min run theres no marking on piston or cylinder , but you need a fine running clearance for them to seal and putting any oil in at all would slow the engines considerably , a cast iron piston in a cast iron cylinder is probably the ideal material selection for a cylinder and piston but a lot of other combinations also work well.

regards Geoff

Edited By Kettrinboy on 28/02/2016 19:15:07

Kettrinboy : I thought that the piston and cylinder should be two different materials ?

Today I purchased the Viton O rings, 0.4mm high temp gasket material and a rotary hole punch tool. I thought I would try the engine out with the Viton O rings first as this is easily done. The engine ran for 10 minutes before giving up the ghost so this is twice as good as before. (Thank you Hopper)

Tomorrow I will cut some gaskets that will hopefully confine the heat to the D. piston side thus keeping the power piston side cool.

NOTE : Did anybody see how long all those youtube videos of the Laura Stirling engine go for ? Are there any that are longer than 10 minutes ? Just a thought   .

Edited By Brian John on 29/02/2016 06:39:19

Edited By Brian John on 29/02/2016 06:40:15

Brian, keep in mind that this runs of temperature difference. It dosn;t matter how well you insulate the hot end form the cold end.. whilst you will delay the cold end warmig up it is inevitable because the displacer is mixing those heat zones about and there has to be some equilibration with the cold zone warming up to a new mean.

It;s been very noticeable to me over the last day that ambient room temp makes a huge difference in the motor running with just a few degs. This morning we have a severe frost and the living room was probably around 16C when i got up and the thing ran like the clappers on a bit of used tee-light. Once tyhe room warmed up to (guessing here) about 20C I had to use a brand new tee-light and the best flame position.

Last night when the frost started my wife had both central heating and the wood-burner running...women like it too hot.... I was down to a sweaty tee-shirt and the motor was struggling. A couple of drops of water on the cold fins and it ran like the clappers again. So we're talking room temp extremes of 16C cold to perhaps 24C when OH has cooked it up but that's enough to affect the cold side cooling ability. I've had it running for well over an hour (perhaps 2 hours) during yeterday afternoon but we were constantly going in and out and the house would have been around 18-19C with the draughts.

When you consider touching the hot side would take skin off.... I dunno 2-300C?? it;s quite bizarre that 2-3C ambient difference affects it so much. An eskimo engineer would have it easy (so long as he wasn't married)

Regarding cast iron, this is the only metal I know of that can safely be run on like metal, this gives the lowest friction of any common metals, next best is a cast iron piston in a steel cylinder.

The cylinder below is a bit of tube from inside a car shock absorber, the piston is made from a cast iron window weight.

Ian S C

PGK : Yes, I was thinking today that this engine has been designed to run at German room temperature which is VERY different from Cairns room temperature...especially during the summer !

Anyway, I will try the gaskets and see what happens.

Another thing I just thought to check...

I built my cold cylinder out of ally as cheaper than brass. Its thermal conductivity is higher than brass. Perhaps one from copper would be way better still..or longer cooling fins..or even copper cooling fins on a thinner ally cylinder..

I spent the afternoon cutting and fitting gaskets. I made two gaskets for the D. cylinder (front and rear of cylinder holders), two gaskets for the power cylinder (one at the cylinder holder end and the other at the cylinder head), and one gasket for the glass tube to sit on so it does not contact the D. cylinder. The gasket at the power cylinder head end was not necessary but I thought I would make one anyway just to make sure I had a good seal there.

Still the same : 10 minutes maximum ! The glass tube even blew off once so I have removed the gasket it was sitting on as it made no difference. I will try again tonight with the air con on but as somebody has already suggested, the problem is that the hot air transfers from the D. cylinder to the power cylinder. I think a larger power cylinder with deeper cooling fins might help...any thoughts on that ?

Things still to try :

1. 3mm silver steel D. piston rod. I am still waiting for these to arrive but I am starting to think/hope that this will solve my problem.

2. Stainless steel D. piston as per Hopper's suggestion so that it does not conduct as much heat. I am still looking for a source to supply the stainless steel of the correct grade.

Edited By Brian John on 01/03/2016 07:05:43

I do not think changing the d cylinder to SS will make any difference, hot air HAS to flow past it to get to the cool end, so it will warm up anyway. It is down to the material and cooling fins at the cold end to dissipate as much heat as possible to maintain the temperature differential for correct running.

Ken : I am starting to come to the same conclusion. I think this engine would run for much longer in a German winter (leave the window open) !

But I do wonder if a larger OD power cylinder with deeper cooling fins might make a difference ? The current cylinder is 22mm OD and the cooling fins are only 1mm deep. That is hardly enough to cool anything. For comparison, the D. cylinder is 30mm OD and the cooling fins are 5mm deep.

I have also seen many Stirling engines that incorporate a small fan to cool the power cylinder. I used to think that was a superfluous addition but now I can see why it may be needed on some engines.

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

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

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

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

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)

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

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

.

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

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....)

.

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

here:

**LINK**