Stirling Engine : Laura

I think the air con may be making a difference as the power cylinder is not getting hot now. It was 40 degrees inside the house today ! I have tried a 12.4mm D. piston instead of the recommended 12.5mm piston and I am getting the engine up to about 24 revolutions.....so close ! I am using lots of oil on all moving parts.

I wonder if I should adjust the work piston ? The original plans called for the piston rod to be soldered to the connecting rod joint and no adjustment could be made that way but I have made it adjustable by cutting a thread on the end. I think soldering parts on an engine like this is a pretty silly idea !

Edited By Brian John on 17/02/2016 12:01:33

Edited By Brian John on 17/02/2016 12:06:06

Hi Brian, sounds as you are nearly there, try adding more heat from say a blowlamp, your burner may not be giving it the heat it needs to sustain continual rotation.

I am a bit reluctant to do that at this point as it should not be necessary. But I will give it some thought. I would like to try the new bearings first.

I have set the work piston so that it is as close as possible to the cylinder head without actually touching. What is the correct setting for a work piston on a Stirling engine ?

Edited By Brian John on 17/02/2016 12:44:43

Having looked around Brian Stirling Engine design is a bit like black magic / suck it and see. If the power piston is fixed I'd just check it's to drawing and look for other problems. One aspect that will be important is the fit. Drop through but not when a fingers is over the other end. That needs a very high surface finish and a close fit - more like an air bearing than anything else. The phasing of the two "pistons" should be 90 degrees. They all seem to be apart from some academic research which is still close to that. Many types also only run in one direction. I'd guess that the direction of the phasing sets that.

You could fix a block of ice from the freezer to the power cylinder some how but I doubt if that is your problem if it's a small flame near the end of the test tube. The displacer getting too hot along it's length may mess things up.

On the flywheel bearings I'd expect the flywheel to spin for a very very long time spun up on it's own. Many minutes. I'm a bit at odds with no lubrication in them but going on some styles of beach casting reels it needs to be a very light oil and very very little of it. On the reels people wash them out in lighter fuel then add the oil but they are after a pretty precise level of braking. The stuff that comes in the bearings is no good for that. They also free up considerably after several casts. They run at very very high rpm for some of a cast. They could be "run in" on a lathe.

I'm still inclined to feel you should drive it from your lathe for a while. Say an elastic band round the flywheel and a bit of bar come pulley in the chuck to get I don't know say 100 rpm or so.

John

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The power piston is a good fit (drop through) and highly polished. I spent a lot of time getting that side correct so that I would not have to worry about it later.

I am 99% certain that the problem is the bearings. They never did seem very free running to start with. I was quite surprised. Perhaps I should have washed them out and re-oiled them before assembly. I may still do that tomorrow : kerosene or methylated spirits...I have no lighter fluid on hand ?

NOTE : the new bearings were posted from HK on the 2nd February so could arrive at any time. I would expect next week but who can tell when you live in Cairns...it be 10 days from Sydney !

Edited By Brian John on 17/02/2016 14:42:50

Lighter fuel is still pretty easy to get in the UK. We have £ / everything should be cheap shops. Main problem with it is getting rid of it after use. Best way is a very small dish of some sort as not much is needed and then setting it alight. There may be a bit of a bang come pop when the fumes ignite. Unleaded should do just as well. Small bottle of that may be available from fishing tackle shops for Colman pressure lamps. Meths should do some like acetone.

You could run the bearings up on your lathe. Turn something to hold them via the centre only. Run the lathe flat out and stop the outer from rotating. I'd use my finger being me but a pencil or bit of wood etc should be fine. Another way with an air line and bearings that are open is to hold the centre and blow at an angle onto the balls. Apprentices and others then drop them and they fly for amazing distances if they land on their edge.

I've been looking at run on the hand types. One mentions that when the flywheel is flicked round firmly on it's own it should run for several minutes. A very light flywheel. It use 2 683 stainless steel degreased and demagnetised bearings. 3mm bore, 7 odd and 2 thick. They show shields on one side in one somewhere. If the bearings have metal shields on both sides I've managed to get one out and put it back in the past, usually with care and a pin.

I found a good description of relubing a 7HT but they are after creating a certain amount of drag. Interesting way of getting oil in though.

http://hwee_surfcast.tripod.com/LubricatingBearings.htm

I've seen some designs where the bearings are held in recesses in each end of a short piece of bar. I'd hope that people would make and use a mandrel to locate things before forming the recess on both ends rather than just reverse in the 3 jaw. I think that as the bearings are usually a light press fit I'd machine straight through and use a spacer.

John

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Edited By Ajohnw on 17/02/2016 16:13:27

I don't like to be a nanny, but have mentioned this before-- Do not stick a finger in a bearing and blow with an air-line. Seen it done many a time and once saw it seize up. Bearing and finger went quite a long way.

Of course sticking a finger in the hole in the centre of a bearing would be a pretty stupid thing to do - that's why i said hold the centre.

John

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The lazy way to fush a bearing is with two cones...one with a hole down the centre that will fit on a syringe (canine nasal vaccine applicators are ideal) and the second cone to sit the bearing in ..often an eggcup will do. Force the flush solution down the centre and it has nowhere to go but past the shields, through the bearing and up.

It used to be suggested as a way of regreasing bearings but is daft for that.. no control over the grease amount and too much will bind them.

I used a mini blow torch today to add some extra heat but that made no difference. I want to try a slightly larger diameter D. piston (12.6mm) but I could not make one up as I had some problems with the lathe today.

I still think the main problem is the bearings and nothing much will happen until the new ones arrive.

I have also remembered that I removed the Hylomar sealant from the cylinder holders as it was very messy. I could have air leakage there but others have used sealant and still had no luck with their engines. I will leave things as they are for the moment.

Edited By Brian John on 18/02/2016 06:23:22

If you are getting 24rpm out of it you are doing well!!. Keep running it so it wears down all the high spots and frees up.

I would wash the bearings out with petrol or white spirit or thinners or whatever you have about the place. And then don't oil them. For this type of non-load bearing use they dont really need oil. If you feel you absolutely must put something on them, try some silicone spray or some non-gumming clock oil or gun oil.

And +1 on Ian SC's suggestion to loosen off the screws on the bearing caps, and on the bearing pedestal feet, and let it all find its own position as it runs. If your crank is ever so slightly out of alignment, this will allow it to move that little bit to reduce the misalignment.

And the ice cube on the displacer fins trick is a good one.

40 degrees in the house? It's ok to turn on the air con you know. Even the locals are whinging about this latest heat wave. I have to bale out of my tin shed when it hits 35. Sitting inside in air-con splendor at 30 degrees now. Luxury that is!

The trick with Hylomar is very little well spread Brian. They don't like leaks but there is one version that uses them.

I still wonder if too much heat is part of the problem because if the displacer gets too hot along it's length I feel they will have problems working. Perhaps it would be worth trying a candle like this one, or a much thinner wick.

It might make an interesting project for Brian at some point. Mix of hand work and finding some way of holding parts on the lathe to enable the use of a slitting saw. It's by J Jonkerman. There is a bit of milling as well but I suspect that could be don on a face plate really or a vertical slide of course. Plans are on the web.

John

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Brian you do NOT need a larger diameter diisplacement cylinder, there needs to be 2 or 3 mm space between it and the glass tube, air needs to freely move past the displacer, you might want to consider making the displacer smaller in length also. There needs to be a certain minimum volume of air in the system for efficiency. There needs to he sufficient air to expand and provide drive energy. You could make up a displacement cylinder from tightly wrapped wire wool without having to machine one. One of my engines uses this method for the displacer, like you I thought making it larger in diameter would make it work better, I can tell you that it does not.

With the wire wool displacer there should be minimal clearance, the idea is to use this as a moving regenerator, and the air should pass through the steel wool.

Ian S C

The design is probably basically correct so should work as is. It might be worth moving the displacer back away from the end of the tube if that is getting to hot.

They are curious things. The regenerator should be hotter at one end than the other, just like the displacer. The idea of the regenerator is to heat the cold air as it passes through it. I'm inclined to say really ????? but some sort of balance must be achievable.

This is a low temperature one and the gap around the displacer is very visible. It's made using stuff from Lidl.

Annoying. I had been wondering if they could be made like this - no power piston as such, just a piece of light rubber. Somebody now has done it. NASA seem to use a similar set up with what are referred to as free pistons. No con rod.

I substituted a whole new work cylinder and work piston assembly on the engine today. This piston was a slightly looser fit and more free running while still giving a decent seal. I am getting ten revolutions with no heat applied. But it still will not run.

One thing I did not notice on the youtube video was how slow the engine runs. It obviously has to be very freewheeling as there is not much force produced ; it is very unlike a steam engine.

I am hoping the new bearings will make a difference when they arrive (maybe next week) but I doubt that they will improve on the current 10 revolutions. I will try them out when they arrive though as I am running out of ideas.

I had intended to make a new D. cylinder cover (part 24) in case I am getting leakage but I am having second thoughts about this. The D. piston rod is 2.93mm and the centre hole was reamed using a 3H7 reamer so this should be as good as it gets. Anyway, I do not have a working tailstock on my lathe at the moment so I cannot make a new part.

I am still not too sure about sealing the cylinder holders (frames) with Hylomar. They are such a tight fit that even a small amount will ooze everywhere and perhaps block that connecting hole between the two cylinders. You just have no way of knowing once everything is tightened up what has happened in there.

One other thing I can try is to make gaskets for all the cylinder covers. I have the correct gasket paper so this might be worth doing. Why do steam engines need gaskets but Stirling engines do not : I would have thought a good seal is even more important on a Stirling engine ?

Edited By Brian John on 20/02/2016 06:41:36

A steam engine will be running at higher pressures so more chance of blowing through the junctions. If your design is anyhting like mine then the flanges of the cylinders are quite wide BUT o the design I'm following the recommendation is to seal with silicone sealant.. just a tiy amount biased towards the outer side of the flange and when i have undone them again then no sign of it getting inside - not that that has helped get mine running...(yet??)

Hi Brian , you say the displacer rod is 2.93mm dia and a 3mm h7 reamer will cut around 3.02 mm if your lucky so that would give a near 0.1 mm clearance between rod and bush , ive found 0.02-0.03 mm is the optimum clearance to get a seal and allow free movement of the rod , and thats in a 30mm long bush , i think yours is around 10mm long is it so you may be getting too much leakage at that point a piece of 3mm silver steel rod should be 2.99-3.00mm so perhaps remaking the displacer rod from that would bring the clearance back to something that should be good enough to work, really hope you get this thing running sounds like your close but if certain things are not right these engines just wont go., i would try a paper gasket between the brass frames as one blob of sealant squidging out will block the passage and you guessed it the motor will not run. lets hope these new bearings are an improvement

regards Geoff

Edited By Kettrinboy on 20/02/2016 09:18:22

I order 3mm stainless steel rod from China or HK and this is what I am sent : 2.93mm. I doubt that I will get any better result by ordering from another supplier. I have found that these things always seem to come out of one factory. It is meant for use in RC helicopters and cars.

From where can I purchase 3mm stainless steel or silver steel rod which is EXACTLY 3MM ?

No where is probably the answer to that Brian. Your best bet would be to buy 1/8" dia an very carefully polish it down. When it fits but is a bit stiff the best option would probably be to lap the two together with silvo/brasso etc but the diameter along the bar will have to be VERY consistent.

Then if you reamed in the lathe using a lubricant and at a slow speed immediately after drilling the hole things should work out. If I remember correctly from when Hopper was working on it your tailstock alignment is good. If not the reamer will produce a hole that is larger than it should be.

Only problem is that 1/8 dia might just turn out to be 50u bigger than 3mm. Stainless is usually a couple of thou down so that would leave it circa 100u over size.

Edit. There is an old trick for polishing bar I just remember and haven't actually tried. Books usually mention lead laps but it's possible to make them out of wood, pref a hard wood. 2 hinged pieces. Space the business end in this case say with 1 or 2 thickness's of photo copy paper. Drill a 1/8" hole centred on the gap, rub some lapping paste into it and then use it on the bar in the lathe moving it rapidly back an forth using finger pressure to to press it onto the work. At this sort of size there would be no need for the hinge really.

The same sort of thing can be done on holes using a dowel with a split in the end. There is a video  of this being done on rifle barrels by people dressed in ancient clothes. I reckon they have it wrong though, pushing the lap through rather than pulling but maybe the Brits made better rifle barrels. In this case, make the split etc and turn it to a suitable size then use.

John

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Edited By Ajohnw on 20/02/2016 11:05:39