Stirling Engine : Laura

After sorting out the problem with that airleak around the O ring I decided to restore the power piston to its original position ie. two turns closer to TDC. After I had done this, I put the flame on and began oiling all moving parts. I was only moving the flywheel very slowly and it started to run...........Success ......sort of !

On the first run, the glass tube blew off after about 60 seconds. This is a video of the second run ; the engine will only run for about 4 minutes before it stops. I suspect that, being another very hot day, too much heat is being transferred to the power cylinder.

Possible solutions are ;

1. Make a cork gasket so that the glass tube can seat up gainst this rather than being pressed firmly onto the end of the D. cylinder. This might interfere with the good seal I now have on the O ring.

2. Make a cork or paper gasket to fit between the base of the D. Cylinder and the cylinder holders (frames). I am leaning towards this idea at the moment.

Anyway, we are nearly there. I was quite shocked when it started to run by itself ...I was not expecting it at all. Tomorrow I may give it a go with a tealight candle before trying to make those gaskets.

**LINK**

Edited By Brian John on 26/02/2016 05:53:49

Congratulations, Brian

MichaelG.

Well done Brian you got there in the end. You could just move your burner nearer to the end of teh glass tube, this will put less heat into the engine and should also see it running a bit slower.

J

Well done. I said you'ld get yours working first

I wonder if part of the problem is the same as i hit with my temporary brass tube power cylinder... expansion from heat losing compression. The problem then being whether that is from heat through the brass or just the new internal temp when the air temps equalise. Cooling the power cylinder might show longer performance.

What is your power piston made from and does it have the same expansion coefficient?

I've also experienced the displacer cylinder glass coming adrift once the heat moves through..again i wonder if that's partly due to expansion of it's holder.. so less might be more (when it comes to heat) and the differential improved by colling the cold end.

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

Andy

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.

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.

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

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.

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)

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

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

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.

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

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

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

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

Ian S C

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

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)

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

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.