Stirling Engine : Laura

again a reminder re temp differentials. Mine hasn't run indoors for the last week...took it back to the shed, wiped piston and cylinder.. it ran at a heck of a speed and gradually slowed down as the woodburner warmed the shed up. It stopped running at T-shirt temps..

1. PGK : my engine still will not run even with the air con working.

2. Ian : have you sometimes had the same problem with Stirling engines ie. run well for a few days and then stop working ?

3. What I am doing now : I have ordered a 10mm brake cylinder hone and I am making up a new power piston and cylinder. (Their phones were not working after hours and giving strange messages at the time but all okay now.) The hones are 10% oversize so there is no need to order an 11mm hone. Should honing oil be used in this process or will any light machine oil suffice ?

4. Are Teflon piston rings ever used in Stirling engines as they are sometimes are used in steam engines ? Who makes 10mm Teflon piston rings ?

Edited By Brian John on 11/03/2016 06:38:10

Brian, #2, yes, although mainly my one factory made one, the little bohm, I usually have to take the piston out, clean it and the bore with a bit of paper kitchen towel. Most of my own design seem ok. Have been having about zero success today trying to run my biggest Ringbom motor as a free piston motor, so I put the crankshaft back in, but I think there is a hole in the hot end of the hot cap, it is a repaired hot cap, the displacer of the Ringbom motor knocked the end out on about it's second run, I brazed it up, looks like it needs seeing to.

Ian S C

Brian, I still haven't got my engine to run, having walked away from it for a while, but have decided to try a couple of different things.

I seem to have excessive leakage where the displacer rod exits the cylinder, so have turned up a Teflon bearing sleeve which gives a really good seal with negligible friction. I will try it when new test tubes arrive (don't ask)! The biggest problem I seem to have is sealing the test tube joint. It was only after you posted that you could see movement in the flame that I checked mine - I had put it down to air movement off the flywheel! I can either get a leak or a cracked tube. I'll try some silicone around the O ring next.

I have also ordered some graphite to make a new power piston. The fit of mine seems pretty good as it is, but I reckon graphite could be better. The cylinder hone sounds like a good idea too. I'll let you know how I get on.

Andy

I had one of those cheap aldi box-sets of O rings and found some slightly thicker ones which sealed my displacer cylinder. This was a metric set but they also make imperial O ring sets and you might somehting a wee bit larger rather than using sealant (my cheap sealant didn't like getting cooked). Or perhaps even cram in 2 smaller ones?

At some stage i'll get back to making the power cylinder out of glass (as mine is supposed to be) but even the glass syringes i ordered aren't as consistent in 'circularity of bore' as I'd like.. so it's going to be back to honing glass. It took ages to clean all the carbon dust out of stuff so don't fancy that again right now.

I must admit I'm not looking forward to the mess off the graphite, but I think I would prefer it to trying to hone glass!

I tried some different O rings, viton, silicone and rubber in different thicknesses, but I think I will have to open out the seat where it sits as it's a bit tight (hence the broken test tubes). Worth having a look at the Aldi ones though.

I can't do anything until the replacement tubes arrive, but I think the biggest difference will come from the Teflon seal. Time will tell!

Andy

Remember that you need to get a high quality finish on the piston as well Brian. Personally I would have given a wooden lap a go. Some diy stores stock dowelling and then there are broom handles etc.

I think any sort of seal on the power cylinder will add too much friction. What would ideal would be the high degree of roundness and finish that unpressurised air bearings need. If it was that good though there might be problems with one part being at a different temperature to the other.

You might find some of this useful but it's a pity different grades aren't offered

**LINK**

It's also about in round pieces. I have some at 5um purely intended for fibre optics finishing and it polishes hss easily. I've found that a bit of slightly soapy water prevents it from clogging. A very light oil would too. Honing oil would probably be ok but it stinks. I'm not keen on breathing oil fumes. I had a bit of a problem using wd40 frequently on a lathe once. Maybe duck oil would be better.

In the UK Cousin's seems to be the best source for an A4 sample pack with various grades and types in it. Some of it can be stuck down.

A number of wood workers are using it to finish hone their tools.

John

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To be fair my first attempt at lapping glass was working out quite well despite using valve grinding paste. It was me cranking the lap open a touch more when nearly finished that cracked the work. Now I've found my diamond grits it should work out better.

Good, things are working again now. I have made a slightly larger diameter power cylinder (28mm) so the cooling fins are a bit deeper. I also found that the gasket I had made to sit between the power cylinder and the frames had deteriorated thus compromising the seal a bit. But I still have to add a few drops of machine oil to the cylinder before it will run. I tried to get a good a fit as I can but any tighter and the piston jams up. I suspect that the only way to improve on this and negate the need for oil in the cylinder is to hone the power cylinder. I will order a 10mm brake hone tomorrow.

Perhaps the finish I get inside the cylinder with my boring bar is not quite as smooth as it should be or maybe my boring technique is not good enough. It certainly looks okay. We will not know until I try honing the cylinder to see if this improves the seal between piston and cylinder.

I also experimented with moving the flame away from the glass tube thus using less heat : no, it needs the full flame to work. The engine will run for about 20 minutes but I have to add a drop or two of oil every 5 minutes.

**LINK**

Edited By Brian John on 15/03/2016 09:07:18

Edited By Brian John on 15/03/2016 09:08:24

This engine seems to be running well now. The gaskets I made to separate the D. cylinder from the cylinder frames do not seem to make much difference so I assume that the heat transfer is caused by the hot air moving from the D. cylinder to the power cylinder. The power cylinder still gets quite hot after 20 minutes.There is little to be done about that except perhaps making a stainless steel D. piston as per Hopper's suggestion. I have ordered some suitable stainless steel bar stock and I might give that a go when it arrives next week.

All in all, I am happy with things as they are and I will make a timber base to put the engine and the copper plate/burner on...if the rain ever stops so that I can varnish the timber !

20 minutes running sounds OK, about as much as you can hope for, short of moving to the antarctic  

I do not think making the D piston out of SS will make any difference and could make things worse I.e extra weight may cause more friction etc

Leave as is and move onto the next project.

Howard

I agree with Howard, if you do go to stainless the displacer must be hollow, the idea being is to get it as light as possible. My little Bohm motor gets too hot to handle after about 10 minutes, it has a hollow stainless displacer, and a stainless hot cap.

Ian S C

I can make it hollow and make it so that one quarter piece (piston rod end which is not hollow) fits inside the other three quarter piece (glass tube end which is hollow) then Loctite them together. Whether it is worth doing this is the big question ?

Edited By Brian John on 17/03/2016 10:03:28

Improvement : I have made a brass sleeve to fit inside the 8mm burner hole. The hole is now 4mm as per the plans. It runs for much longer on one fill (15 minutes...previous was 10 minutes) and it runs a bit slower. I cannot tell that the D. cylinder is any cooler than before but I have no way to check this other than to touch it !

I will drill and tap the cone to take a 3mm grub screw to hold the sleeve in place. I could use solder or JB Weld but the grub screw is easier if I need to make further changes.

**LINK**

Edited By Brian John on 19/03/2016 06:07:17

Over the time of running the temperature will stay about the same, by reducing the size of the flame, you have reduced the heat in put to the motor. The motor will take longer to heat up, but it's one of the experiments that I try, to see how little heat will still run the motor.

Ian S C

Pardon me being cynical, but isn't it amazing that on returning to drawing specifications things improve ?

I think Brian's actually got a good burner there, he can change wick sizes at will, it would be worth trying 3 mm now, he might get near half an hour running on that.

Ian S C

I have noticed that as soon as it starts to run out of meths and the flame gets a bit smaller than the engine slows down. I think the current 4mm burner is the minimum for this size engine.

I do like the changeable wick holders as I can use different size wicks for other things without having to make a whole new burner.

I am in the process of making up a wooden base with brass handles : the engine and the copper plate will all be mounted on that. In the mean time, I am running this engine every day for ten or twenty minutes to see how it wears. The glass tube is blowing off again after 5 or 10 minutes. I thought I had solved this problem with the Viton O ring but it is still happening. I have been thinking of making a double collar for the D. cylinder ie. a second collar with its own O ring to sit on top of the existing one. Any thoughts on that ? I am not sure what else I can try at this point.

The plans call for a 12.5mm D. piston but my glass tube keeps blowing so I performed and experiment. I had a 12.2mm aluminium piston I made many weeks ago ; it was a stuff up when I took off too much metal. I tried that in the engine today and it ran okay but a bit slower that usual. So I then reduced my current 12.5mm piston down to 12.4mm. This seems to work well without any reduction in speed or performance. So far, the glass tube is holding well.

Yes, I know : one should not deviate from the plans ! But Bengs do not manufacture the glass tube ; the internal dimensions of this tube may have changed slightly since the original plans were produced.