Stirling Engine : Laura

[Andy HoldawayParticipant @andyholdaway]

Well done Brian, I admire your tenacity! That's spurred me on to have another go at mine.

Andy

[Brian JohnParticipant @brianjohn93961]

Jason : I tried that but less heat did not work. It still stops after about 4 minutes. I will try again later when we turn the air con on in this room and see if that helps.

PGK : my power piston is made from the supplied red brass or gunmetal as it is sometimes referred to. I have now located two sources of gunmetal bronze in Australia and I have ordered plenty for future engines. I still think it is a good idea to make the length of the power piston rod adjustable by cutting a thread on the end that fits into part 12 (connecting rod joint). The original plans call for this to be soldered. I am not one for soldering when there are other options available.

My problem could also be that as the things heat up, the O ring becomes soft and looses its seal. Bengs would not have designed it this way without considering this but I will keep this thought in mind.

[HopperParticipant @hopper]

That's great stuff Brian!

If you don't have one already, you might try a Viton o-ring for the glass tube seal. It will stand up to higher temps better than ordinary o-rings. Allied Bearings sells them.

[KettrinboyParticipant @kettrinboy]

Well done Brian , i knew this engine could run and no doubt you will have learnt a lot about what it takes to get a running hot air engine so the next one should be no problem.

regards Geoff

[Brian JohnParticipant @brianjohn93961]

Thanks Hopper, I will go there on Monday.

The air con has not helped. It still stops after four minutes. It could also be that as the engine heats up the D. cylinder cover expands and I loose my seal around the D. piston connecting rod. Those 3mm silver steel rods may still be required after all. They should arrive next week.

I still have a few things to try. I have also noticed that the engine must be oiled each time on all moving parts.

[pgk pgkParticipant @pgkpgk17461]

Again it may be a tolerance thing where the o-ring housing expands enough to reduce the ring compression. skimming a tiny bit off the face might help (or ruin what you have)

[Brian JohnParticipant @brianjohn93961]

Yes, I am reluctant to touch that just yet. I think the first thing to try is a cork or paper gasket between the D. cylinder and the cylinder holders. This will stop a lot of heat transferring to the power piston side and will also stop the D. cylinder cover from expanding as it gets hot.

If that does not work then I will have to wait for the 3mm silver steel rods to arrive.

Edited By Brian John on 26/02/2016 09:18:07

[Ian S CParticipant @iansc]

Congratulations Brian, persistence is the way to make things work. On my little Bohm motor the wick is barely showing out the top of the burner, this gives a flame about 12 mm high, a T Light candle might actually have a bigger flame. Yes a big flame will make it go faster, but get the flame as small as will allow the motor to run at a reasonable speed, it should run until it runs out of fuel, which is quite a while.

Ian S C

Edited By Ian S C on 26/02/2016 10:59:54

[HopperParticipant @hopper]

Quite often when a Stirling engine runs out of puff after a few minutes, it is because too much heat travels up into the cool end of the displacer cylinder. Hence the old ice-cube on the fins trick. A gasket between the flange that holds the glass tube in place and the cooling end of the cylinder is a good idea for this. Also it pays to make sure the glass tube is contacting only the rubber o-ring and is not touching the metal on the flange or cylinder. It might take some careful jiggling to get clearance all around. Perhaps a wrap of teflon plumbers tape around the glass tube might help?

I would also be a bit (very) suspicious of the solid ally displacer piston. This will absorb heat from the air in the hot end and conduct it down to the cool end and thus destroy the temperature differential that is essential to a Stirling.

Most model Stirlings use a hollow stainless steel displacer piston, because the stainless is a poor conducter of heat from one end to the other. Some even have a stainless disc inserted inside the hollow displacer about halfway along to further disrupt the flow of heat lengthways.

So you might consider making a hollow stainless steel displacer piston that is pushed onto an ally disc with a threaded hole in it to screw on to the connecting rod with a bit of high temp loctite.

Not sure of your displacer piston diameter, but AA rechargable batteries are one source of smaill diameter stainless cylinders with one end blanked off. Otherwise, you can buy small bits of stainless bar at Sharplift marine down at Portsmith and turn one up ffrom solid, boring out the center and then loctiting it to an end disc.

[Brian JohnParticipant @brianjohn93961]

Hopper : I agree with everything you have said. I have been thinking about this all night at work. One's mind can concentrate on other things when mopping the floor !

The glass tube has been in contact with the end of the brass cylinder. I will make a gasket for it to sit on.

I also think three gaskets should be made from high temp material (0.4 or 0.8mm ?) and placed between BOTH the D. cylinder (both sides) and the power cylinder (one side) thus separating everything from the cylinder holders. This would surely stop any heat transfer between the two cylinders.

Perhaps just plain cork gasket material would suffice ? This is designed to come in contact with hot engine parts in a car. The high temp material is for exhaust manifolds which is probably more than is required for a Stirling engine.

I am not sure as to the necessity of a stainless steel D. piston. Bengs have designed it with an Al piston so it should work with that. But it would be an interesting exercise to try and make one from stainless steel to see if this made a difference. The D. piston is 12.5mm diameter. I am not sure how my lathe would go machining stainless steel

I am working again tonight and sleeping most of tomorrow so I will have to wait until Monday to buy the Viton O ring from Allied bearings and the high temp gasket material from Autobarn or Supercheap

Edited By Brian John on 27/02/2016 07:05:20

Edited By Brian John on 27/02/2016 07:06:24

Edited By Brian John on 27/02/2016 08:12:25

[AjohnwParticipant @ajohnw51620]

I've seen suggestions to use Corian for thicker gaskets to help stop heat flow from one part of the engine to another. It generally comes in rather large pieces but if there is some one about who makes kitchen work surfaces they may have small bits about. I've no idea how well it machines but it is routed and drilled etc.

There is something else that might save people a lot of grief. A small geared motor that runs at circa 5 rpm. Substitute a pulley for the flywheel and run it without compression for a hour or two. Then maybe dismantle and look for rub marks. Then try heat.

Plumbers ptfe tape should be ok to keep the glass away from the metal but if it smokes stop – seems the fumes aren't good for people. They do a gas type in the uk that seems to be thicker than it used to be but it's still pretty thin.

Personally I would run any ball races up for a while too – maybe with my dremel via a polisher on the end.

You may be able to machines some grades of stainless Brian. 416 springs to mind.

John

–

Edited By Ajohnw on 27/02/2016 10:20:18

[Ian S CParticipant @iansc]

Ni metal hydride, NiCad, and alkaline batteries have steel cases, AA size is 50 mm long x 14.3 diameter.

Ian S C

[Brian JohnParticipant @brianjohn93961]

I woke up too late today to buy the gasket material but I have done a few experiments with the engine. I have to apply three or four drops of fine machine oil (sewing machine oil) to the power cylinder before it will start. I oil it from the open end where the piston connects to the piston rod. It will run for about 4 minutes before stopping. If I apply a few more drops of oil then it will run for another 4 minutes. It  will not then run again until the whole thing has completely cooled down.

I think there is a problem with heat transfer from the D. cylinder to the power cylinder  and this should be remedied by making the gaskets as discussed above. But this business with the oil in the power cylinder is a bit odd and indicates that there may be some other problem here.

 

Edited By Brian John on 28/02/2016 10:55:54

[JasonBModerator @jasonb]

A well oiled piston/cyliner will always work better than a dry ohe as the oil film between the two acts like a seal (piston ring)

[Brian JohnParticipant @brianjohn93961]

So would this indicate that I have still not got the correct fit between piston and cylinder ? I am not sure what else I could do other than hone the cylinder. This is a technique with which I am not familiar. I am sure this piston/cylinder would be more than good enough for a steam engine but Stirling engines seems to be quite particular !

Edited By Brian John on 28/02/2016 13:18:03

[pgk pgkParticipant @pgkpgk17461]

when the glass cylinder method wasn't working I switched to a bit of brass tubing. I did hone the inside using the hone I'd tried to use to hone the glass cylinder straight. this is where i asked some questions re method: **LINK**

having said that the PB piston i made isn't brilliant and I didn't have anything better than brasso to hand. the valve grinding paste i used on the glass would have ended up plugged into brass. And brasso is really too fine for a starting point.

I'm tempted to invest in some timesaver lapping compound but the set of 4 grades is a bit of an investment.

As per my build thread the reason for failure really came down to poor joints.. lapping those and paper gaskets (gaskets were your idea) have solved it.

[KettrinboyParticipant @kettrinboy]

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

[Brian JohnParticipant @brianjohn93961]

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

[pgk pgkParticipant @pgkpgk17461]

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)

[Ian S CParticipant @iansc]

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

[Brian JohnParticipant @brianjohn93961]

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.

[pgk pgkParticipant @pgkpgk17461]

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..

[Brian JohnParticipant @brianjohn93961]

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

[HowiParticipant @howi]

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.

[Brian JohnParticipant @brianjohn93961]

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.

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