Jan Ridders Coffee Cup Stirling

Having built Jan Ridders Coffee Pot engine I am, surprise surprise, unable to get it to run.

There are no leaks and the engine spins freely. The piston passes the 'pop' and vacuum tests. That's the good news and I'm now focussing on the various defects. Of these I'm particularly suspicious of my displacer for two reasons but lack the experience to know if I'm right to worry or not.

Firstly the sheet of polystyrene carefully put aside to make the displacer has gone AWOL and I temporarily used cardboard instead. Is cardboard suitable for this purpose? (Cardboard doesn't feature as displacer material in any of the designs I've seen.)

Secondly, due to a cock-up making the upper plate, the displacer is smaller than it should be. The glide-bearing hole in the upper plate is off-centre and the diameter of the displacer is reduced to stop it fouling the cylinder wall. The effect is highlighted in red in the photo. How critical is the size of the displacer relative to the size of the cylinder?

It's not the cause of my problems but other builders of this engine might be interested in the second picture. It shows corrosion of the steel piston caused by acetic acid from the RTV I used to seal the displacer-cylinder. The corrosion is worse than the picture implies and it also greened up the exposed brass-work.

As usual I will be very grateful for all help and comments.

Regards,
Dave

Hello Dave,

I can't comment on the displacer, but I can say that the type of RTV used in fish aquariums doesn't give off the acid and won't rust/corrode things.

Pete

Thanks Peter,

I used "Pro Seal Blue RTV Silicone Instant Gasket".

It might be past it's best as I've had the tube in my garage for several years. Took a few days to set as well as smelling strongly of vinegar. The seal is good though so I'm not too unhappy.

As the blue colour of the sealant doesn't improve the look of the engine I may visit the fish section of my nearby garden centre for a replacement.

Dave

The displacer should have a clearance inside the cylinder, how much is debatable, but I would say 1mm in a 100mm should work. The depth of the displacer is relevant also, on your's it looks thin, also c/board corries will pass air. Can't see why board would not work but will need some engineering to stiffen up.

Many thanks Gordon.

My effective displacer gap is much bigger than 1mm in 100mm so that definitely needs fixing.

You are right that my displacer is thin: the cat food carton cardboard I used is only 1.8mm thick. I didn't realise that the depth mattered which may have been unwise given that the design calls for 4mm.

As it's easy to do I'll double the thickness and cut the displacer for a much closer fit. While I'm at it I'll seal the corrugations as well - airflow through them may be reducing the effective thickness of the displacer even more. Oh dear.

Fingers crossed!

Ta,

Dave

Keep posting, Dave. I'll be following the same trail later this summer!

Pete

Thicker cardboard will help, you say it spins freely, how many revs or seconds?, my ltd striking has a removable plug , with the plug removed it will spin over very freely  and for quite a while, put the plug back in and it does not spin for anything like the same time (air resistance slowing things down in sealed system).

What about timing, it should only run in one direction though that can be reversed by moving timing 180 deg.

How did you check for leaks? An air leak is a real killer.

Also look up regenerators, they do improve performance, but obviously get it going first.

Edited By Howi on 02/03/2016 17:33:01

The plan sizes are a 94mm dia x 4mm thick displacer in a 96mm bore piece of plastic tube. Displacer in balsa or polystyrene. The metal flanges on the displacer should be 1mm thick in aluminium. All hardly any weight at all.

It;s a low temperature engine so it wont be happy with increases in weight especially on the displacer.

I suspect that the silicone rubber tube for sealing he mentions might be the kind that is pushed over bare wire to insulate it but the green one used on the earth in mains wiring is probably over 2mm dia.

John

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Edited By Ajohnw on 02/03/2016 17:57:56

Ebay 26100958875 should be ok for the tubing. They do several sizes.

John

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Progress Report

I've fitted a double thickness (4mm) cardboard displacer with edges sealed with a smear of latex glue. Still not running in either direction. (Until Howi's advice I'd only tried spinning it clockwise.)

Some more facts:

I followed Jan's design fairly closely. My acrylic tube is 96mm inside diameter. The other components are to spec except that the upper and lower plates are 1/4" (6.25mm) thick rather than 5mm. The flywheel is 1/4" thick rather than 7mm and I increased the diameter to 106mm to compensate. (Design says 100mm diameter.) The piston is mild steel rather than graphite but that's allowed. I used RTV silicone instant gasket rather than cut rubber hose to make the seals. The plate fixing spacers are drilled through and secured with 2mm stainless nuts and bolts rather than being tapped M2 at the ends with an air gap between.

The various bearing points have been de-burred. The engine was spun by hand and then given about 30 minutes drive with an electric meccano motor for "Running In - Please Pass"

Mistakes:

Due to a measuring boob the lower plate groove is 7mm wide rather than 4mm. The extra width is inside the cylinder and the dead space is partly filled with RTV.

As previously confessed the hole in the upper plate for the glide bearing is not centred exactly. The new displacer runs closer to the cylinder wall but it must be a tad unbalanced as a result.

The flywheel wobbles slightly and needs to be shimmed. I'm not entirely happy that it's balanced properly.

Tests

From a flick the engine spins 9 to 11 times with the power piston disconnected. It spins 6 - 7 times when connected. Providing heat doesn't give more revolutions.

The upper and lower plate seals where tested by immersing the engine body in water. The upper plate brass work was tested by spinning with soapy water smeared around the bolts and seams. The displacer piston rod and piston where tested with sewing machine oil, now removed. As I've had the engine apart I will have to repeat the leak tests.

Visual check. The displacer doesn't seem to be fouling the side, top or bottom of the cylinder.

Feel check. Rotating the engine doesn't reveal any stickiness in the motion.

Next step will be to fix the flywheel wobble and recheck the balance. Unless I get more good advice that is!

Thanks again,

Dave

One test you could try apply some hot water to the base (hot) plate, spin the flywheel clockwise, then anticlockwise, one way should spin better/longer than the other. If you do not get this something is badly wrong somewhere, most likely to be air leak, ltd strilings will NOT tolerate air leaks, it needs to be a sealed system to work.

How are you checking for air leaks?

I used a removeable screw, replaced with an adaptor to connect to some silicon tubing, immerse the whole thing in water and blow into the tube. With it out of the water, blowing and sucking on the tubing should get the piston moving.

It would be best if the displacer was concentric, and if you can't get some polystyrene 4 mm thick, maybe 2 thinner bits, you could use balsa wood. You need to cut down the bulk of the metal in the disc in the centre of the displacer, on my 6" dia LTD motor the centre piece is an aluminium nut with a flange about 3/4" dia x .020" thick, the rod is a bit of bike spoke. I wonder if you could glue a bit of aluminium over the out of centre hole, and re-drill the hole in the centre, I would suggest that unless the instructions say other wise the displacer should have a gap of about 2 mm, if its too close it causes drag as the air moves back and forth.

Have you added weight to the flywheel to balance it against the weight of the displacer and piston. Test it with the bottom plate removed, Blue Tac is good for temporary weight, the balance has to be fairly good.

I,m not that fussed by Jan's idea of using steel for a piston, cast iron would be far better, but if you follow the instructions, the motor should go, but you might have to work through it as Brian and pgk pgk have on their motors.

What sort of bearings have you used, the main bearings should be ball races, take the shields of the sides of the bearings, and wash out the grease, don't oil them, that's it.  On my motor there are ball races on the big ends, and they should get the same treatment.

Ian S C

Edited By Ian S C on 03/03/2016 09:16:30

I had wondered about other ways of making very low weight displacers. Maybe make a disc of something easy to cut and light. Cut most of the interior away and cover with aluminium foil. I'd been wondering about using foam board but it's only about in one thickness as far as I know,

Sadly B&Q etc don't sell polystyrene foam cutters for a couple of quid now. The price is up eg but could be cheaper elsewhere

**LINK**

They are pretty easy to make - use stainless mig wire. There should be info about on the web. Model aircraft etc people cut wing formers from it.

Edit - on the other hand if some one hasn't anything suitable for feeding current into a short length of mig wire they would be better off buying some ni chrome wire. There is lots on ebay these days.

John

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Edited By Ajohnw on 03/03/2016 09:43:51

Edited By Ajohnw on 03/03/2016 10:30:45

If memery serves myfordboy's youtibe channel shows him cutting a polystyrene disc out using a soldering iron with a bit of wire attached to the tip and a very crude but effective jig on a even simpler version of this type engine.

And as i discovered they'll run a lot better in a cool room than a hot one

Laminate a few frozen pizza trays?

Hi Dave,

I am by no means an expert, but I have made one of these and it runs. I used available materials for mine, so I have a steel piston, silicone sealant on the top and bottom plates, and the spacers are made stainless, as I didn't have any plastic available. So those factors shouldn't stop it running.

I think your problem is the displacer.

As you have made the displacer piston thinner have you made corresponding adjustments to the length of the piston rod and or the connecting rod to bring the top face of the displacer piston as close as possible to the under side of the top plate at tdc.

Have you made the displacer cylinder length such that the displacer piston almost touches the bottom plate at bdc.

If you have too much dead space, then I think this would stop the engine running.

Other factors that will stop it running:

Air leaks, I tested mine by removing the power piston and sitting the engine in water covering just over the top plate. I sealed a bit of tube into the power cylinder with blu tak and blew down it, any leak is obvious from the bubbles. you don't need much of a leak to stop it working.

It helps if you achieve reasonable balance, I took the bottom plate off, then give it a spin, a bit of blu tak will do to get it fairly close.

Obviously the whole thing needs to be as free running as possible, but it seems like yours is ok, also the power piston needs to be a good fit, but it sounds as if you are happy that yours is ok.

If it were mine I would:

Take the displacer cylinder apart and check how close the diplacer comes to the top plate.

Put the tube on and check how close the underside of the diplacer comes to the bottom plate, obviously making allowance for the groove in the bottom plate.

These measurements need to be pretty small, probably 1mm or less. If not before doing any re engineering, get some better material for the displacer piston, either polystyrene or balsa.

If it is any consolation I was talking to Martin Gregory on the SMEE stand at ME exhibition, and when I told him I'd built one of these Jan Ridders engines and it ran, his comment was that i was the only person he'd come across that had made one work. Apparantly the proportions are not quite right, but I can't quite remember what is wrong. Martins own engine will run on the heat from your hand, the one I made from the Jan Ridders design needs a good cup of boiling water to work. But that aside it definately can be made to work. No disrespect mean to Jan Ridders he has done some great work designing engines and giving them away free.

Hope this helps

Good luck

Trevor

Polystyrene should be easy to get, the back of any shop, pub etc. usually have loads to get rid of. I burn it , it's the easiest way. For a one off just use a sharp knife, serrated kitchen knife works. Finish with sandpaper or similar. I mark out leaving 3 or 4 mm on size. Make a hole in center and glue a bit of dowel with wood glue. Clean up to size, then trim dowel and drill for the con. rod. That's just one way of course. balsa wood could be used, or any thing that is light and stiff.

Jan Ridders does have plans for a run on the hand model but it needs a temp difference of more than 10C to run like that which is somewhat dependent on the weather.

This model is a little different. It's intended to have a model aeroplane prop mounted on it. I'd suggest one with a low pitch.

From looking at a number of designs I don't think that JR's engines of this type are any different to others really. I'd say the main difference is the weight of components that have to move when the engine is running especially the ones claimed to run at very low temperature differences.

They are curious things. There is some rocket science type sums around. These make assumptions and with this sort of engine indicate that there may be 60 or 70 mW of energy available - not a lot, around 1/10,000 HP. The sums are bit of a joke at this sort of power level really. One thing for sure they will need very careful construction.

John

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I edited the post where I mentioned using mig wire for foam cutting - I wanted to make a 3m glider once so the wire needed to be 1m plus long for a 3 section wing and stainless mig wire gave a reasonable amount of resistance.

I powered it via a transformer with a light dimmer on the input side as it gave too many volts. It also allowed the temperature to be adjusted. Worked well but making a stiff frame for that length of wire was harder than expected mainly down to the dia of the wire.

Oh - I understand the reason that cheap foam cutters for ceiling tiles etc went was down to health concerns. Nasty  fumes so do blow them away. Myfordboys idea is neat but I wonder if he waits for the iron to cool down a bit.

John

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Edited By Ajohnw on 03/03/2016 11:48:46

You can buy small reels of ni-cr resistance wire from Maplin. I bought some for this purpose and made a wooden frame, I will post a picture later.

Mark