Jan Ridders Coffee Cup Stirling

[SillyOldDufferModerator @sillyoldduffer]

I've just finished the evening with a smirk on my face. The engine runs! It looks a right mess but it's going. Special thanks to Trevor, Ian SC, Howi and Gordon with apologies to anyone I forgot : there was overlapping good advice.

Problems fixed were:

1. Displacer replaced with 4mm balsa and carefully fitted to the cylinder with a gap of 1 or 2mm. Adjusted to nearly hit the lower plate on the downstroke.
2. Plasticine type modelling clay works as a sealant, at least as a temporary expedient. Wallace & Grommit taught me everything I know about making gadgets. Slight leakage was fixed by smearing extra clay into the outside gap between upper & lower plates and the acrylic tube cylinder.
3. The flywheel was more carefully re-balanced with blobs of modelling clay in lieu of blu-tac. The engine went without doing this but the improvement was obvious. More needs to be done to the flywheel.

My coffee-cup is a vacuum type that holds heat much longer than an porcelain mug. At the moment the engine only runs on near boiling water. It goes for about 10 minutes. Don't ask how I know that it is still unpleasantly hot when spilled onto one's gentleman parts. I suffer for my art.

Before steps 2 and 3 it was obvious from the feel of the engine that it was in a better state. With heat applied It span much more freely in one direction than the other. In the forward direction a flick would result in up to 20 seconds of rotation, a significant improvement. Although the engine slowed down quickly I noticed that it was rolling gently past tdc rather more than could be explained by energy stored in the flywheel.

After fixing the minor leaks I spun the engine several times each time re-flicking it impatiently before allowing it to stop. After a fortuitous distraction I returned to find the engine still ticking over gently. It runs much more slowly than I expected – a stately 80 rpm dropping to about 45 rpm before stalling. Is this normal? I seem to remember seeing similar engines at exhibitions turning a lot faster. There will be no danger of it taking off when I add a propeller, ho, ho.

I shall have another go at it tomorrow. The flywheel isn't properly true and I think the balance can be improved. After more "running in" it needs serious cosmetic attention.

Thanks again,

Dave

[Ian S CParticipant @iansc]

Congratulations Dave, it's virtually impossible for it not to go, if you build it right, but it's the little bits that stop them going. According to JR it should do about 300 rpm on a cup of boiling water. If in future anyone has problems with one of his engines, you can E-mail him from his web site, don't forget this site as well.

Ian S C

Edited By Ian S C on 04/03/2016 09:39:52

[Trev67Participant @trev67]

Congratulations Dave,

well done for persisting with it. A few finishing touches and it's a very attractive little engine. It's amazinging how much interest you'll get if you demo it to people, they all think it's magic.

Trevor

[Trev67Participant @trev67]

Congratulations Dave,

well done for persisting with it. A few finishing touches and it's a very attractive little engine. It's amazinging how much interest you'll get if you demo it to people, they all think it's magic.

Trevor

[SillyOldDufferModerator @sillyoldduffer]

Thanks for the congratulations: your encouragement puts the icing on my cake! I get a real buzz when something I made myself works. Much preferred to the slough of despond I fall into each time a mistake adds to my scrap bin.

Today slow progress because I have a stinking cold and getting chilled in my 6C workshop doesn't feel like a good idea.

Experimentation after small tweaks reveals that the engine still only runs slowly for just over 9 minutes from a boiling start . There may be a problem with my seals because modelling clay softens with heat. Methinks a small leak may be developing as the top plate warms up. Consequently I've ordered some silicone tubing as suggested by AJohnW. The top plate warms up more than I like so I may blacken it and replace the steel pillars with something less conductive.

I found an article on the engine by Jan Ridders in the ME archive this morning. It's in #4329, July 4 2008. Unfortunately the free plan isn't in the digital magazine – must have been a pull-out.

Many thanks for the hint that Jan can be emailed and that he has a website. (My earlier research didn't find it for some reason.)

Cheers,

Dave (atchoo)

[pgk pgkParticipant @pgkpgk17461]

But did you ever find the polystyrene ?

I spent over an hour searching for my diamond honing dusts last week… came across them yesterday neatly placed with files rather than abrasive papers, wire wools, and scotchbrite, honing paste or stones….

[SillyOldDufferModerator @sillyoldduffer]

Nope pgk pgk. The elusive polystyrene must have transferred to an alternative dimension to join an Axe, Travelling Steady, Stopwatch, Camera Charger, yesterday's new bottle of Meths and my only copy of Machinery's Handbook. Not all is lost however: I know exactly where to find the Haynes manuals for all the old bangers owned by my family since 1970. I shall never need any of them again.

Edited By SillyOldDuffer on 04/03/2016 14:16:53

[AjohnwParticipant @ajohnw51620]

Don't talk to me about not being able to find things. While tidying up I have noticed that a number of things are missing. Some for so long now I've started wondering if I ever had them. Others because I have tidied them into a different place – bit like car keys usually kept in one place for a long time put them some where different and they become invisible as a result.

JR's drawing looks like he intends the displacer to have even top and bottom spacing at each end of the stroke to me. Just visually. I haven't worked it out. It might be worth doing that and adjusting either way if possible. There are arguments either way in respect to the space at the bottom.

John

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[SillyOldDufferModerator @sillyoldduffer]

Tidying up is fatal. As far as I'm concerned it renders things invisible. And it's so annoying when stuff I lost by tidying turns up out to be in plain sight – a month or two later! Scanning with a torch CSI style whilst constantly repeating the name of the missing item helps but I'm pretty hopeless. Forgetful bloke with machine tools might make a good sitcom.

Great minds think alike. I fiddled with the displacer just as you suggest this afternoon. It didn't make much difference but that may be because a bigger problem is masking the effect.

Nonetheless I was pleased with today's improvements. Although the engine still won't go faster than 80 rpm I increased the run time per boil from 6 minutes to 26 minutes with more careful sealing, balancing of the flywheel and "running in". The next big thing is the flywheel. It wobbles because the axle hole is oval at one end. Not sure how that happened but I admit to being the only suspect. It's a workshop job to fix and I'm not up to it until recovered from an exceptionally severe case of man-flu.

Dave

[AjohnwParticipant @ajohnw51620]

It's all down to our brain filling things we often see automatically Dave. Not an age problem so if keys are always kept on a side table and moved to the mantle piece there's a good chance we wont see them if we look quickly. Feeling irritable seems to make it worse.

LOL When it's the car keys I usually ask the wife to look. She doesn't drive so sees them immediately.

On these low temperature ones I want to try and come up with my own design. JR outlines some rules on these and others do to in more or less the same way. So I have scrounged every design I can lay my hands on and done a few simple sums to see if the "rules" make sense – not really just very very loosely. So loosely it's a bit of a nonsense. So I'm intending to make the stroke of the power cylinder adjustable but wanted to use a Ross yoke as it would be nicer to watch – if it works.

John

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[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ajohnw on 04/03/2016 20:46:51:

… wanted to use a Ross yoke as it would be nicer to watch – if it works.

.

+1 for that, John

**LINK**

MichaelG.

[SillyOldDufferModerator @sillyoldduffer]

The "sums" and design aspects of these engines has started to intrigue me too. I've been wondering if there is any practical way of measuring what goes on inside a small low temperature cylinder. James Watt's indicator provided information that helped enormously with heat engine theory and practice. Something similar for a small Stirling engine looks to be well out of my league though.

The Ross Yoke is a new one on me and led me to find another interesting website. Fascinating! It needs 8 bearings though – yet more friction for a model to overcome.

Regards,

Dave (still snivelling)

[AjohnwParticipant @ajohnw51620]

I was thinking in terms of one like this

**LINK**

This link is pretty good for explaining what should happen but it can't be that perfect in practice

**LINK**

There are some ideas of relative sizes from a design perspective on JR site. A bloke called Schmidt was the first to analyse them – complicated but I wouldn't assume they the sums work – they wont. Easier. There is a link to a simulator off the stirlingengine.org site probably based on Schmidt. If say all sizes are left the same but the diameter of the power piston or it's stroke is increased as would be expected from the theory more power is generated. JR explains the problem with that when they are working at low temperatures. The higher the compression ratio the more power that can be produced. The compression ratio is simply the ratio of the volume change with the power piston at each end of it's stroke. All of the low temperature ones I have seen come out very close to 1 It seems bigger models that take more heat can be as high as 1.1 to 1.2 – according to a book I bought years ago.

There are a couple of video's on the Phillips engine on youtube. They seem to be the only people who have really improved them. A long time ago as well. The other sort that people like NASA use which are more recent are called pistonless. There are loads of video's on making those on youtube out of tin cans. Just search for Stirling engines.

Last edit. Millions have been spent on them measuring everything imaginable – doesn't seem to have helped. There are some thesis about as well. Looks like some one has tried to use finite element analysis even accounting for uneven air mixing and friction just from the air moving about.

John

–

 

Edited By Ajohnw on 04/03/2016 23:40:31

Edited By Ajohnw on 04/03/2016 23:44:40

Edited By Ajohnw on 04/03/2016 23:45:14

[Michael GilliganParticipant @michaelgilligan61133]

John,

That Kontax Ross-Yoke is delightful … thanks for the Vimeo link.

The assembly instructions for the Kontax kit are exemplary !!

MichaelG.

Edited By Michael Gilligan on 05/03/2016 08:29:02

[Neil WyattModerator @neilwyatt]

Posted by SillyOldDuffer on 04/03/2016 21:08:30:

The "sums" and design aspects of these engines has started to intrigue me too. I've been wondering if there is any practical way of measuring what goes on inside a small low temperature cylinder. James Watt's indicator provided information that helped enormously with heat engine theory and practice. Something similar for a small Stirling engine looks to be well out of my league though.

I have a book on the history of heat engines, and it's got some indicator diagrams in it.

Quite a variety of shapes.

Neil

[Gordon WParticipant @gordonw]

Study all the info . you can find, calculate relative volumes of power piston and displacer, write it all down, study some more and try to make sense of it. That way madness lies. There are two things to remember- the engine is driven by expansion of the working medium.- The temps. are absolute ie add measured temp to 274 9 deg. centigrade. And apologies to all who already know this.

[Michael GilliganParticipant @michaelgilligan61133]

Good advice, Gordon … but I think the favoured number is 273.15

MichaelG.

[pgk pgkParticipant @pgkpgk17461]

What happened to -273C ?

looks like 2 of us posted at the same time

Edited By pgk pgk on 05/03/2016 10:02:32

[Ian S CParticipant @iansc]

Dave, what sort of bearings are you using for the crankshaft?

Ian S C

[Gordon WParticipant @gordonw]

Mike and pg you are of course almost correct, I tried your figure but the motor does not get any faster. What I really meant was that don't think you are doubling the temp. if you go from 20deg. to 40.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Gordon W on 05/03/2016 09:52:14:

The temps. are absolute

. And apologies to all who already know this.

.

Executive Summary ^^^

MichaelG.

[AjohnwParticipant @ajohnw51620]

JR I think and a French man have measured temperatures to see what differential their engines can work at and probably made ad hoc changes to see what happens. That seems to be the approach who ever does it sometimes creating a simulation model as a result. Schmidt effectively assumes perfect conservation of energy and no losses at all. This also assumes perfect gas temperature behaviour.

I only looked around to see if there were any really meaningful factors that could be used to design one not really expecting to find any because of the nature of the beast. The only interesting aspect really is compression ratio and the temperature differential they are expected to work at. My way of looking at that is that the compression is real and if the energy available from the temperature changes can't exceed the energy needed to perform the compression it wont work. The other factors are friction and mass of the moving parts. Also the colder part not heating up, that's not to difficult to achieve on very low temperature ones – the air is a big hear sink. The rest could only be optimised by ad hoc changes so no point in complicated sums.

John

–

[SillyOldDufferModerator @sillyoldduffer]

Posted by Ian S C on 05/03/2016 10:02:42:

Dave, what sort of bearings are you using for the crankshaft?

Ian S C

They are 623ZZ 3x10x4 Miniature Model Rubber Sealed Metal Shielded Metric Ball Bearings, £3.99 for 10. I tried to degrease them with dry cleaning fluid but wihout any improvement. They are, as it says on the tin, shielded. Now I've noticed they are rubber sealed, I'm worrying that dissolved rubber is gumming up the works!

The disconnected crankshaft and flywheel spin for about 35 seconds – not good I think.

Dave

[pgk pgkParticipant @pgkpgk17461]

rubber sealed bearings have to be the worst option..more drag on the seals than with the shielded ones and Jan Ridder recommends taking the shields off those anyway.

I've removed shields by popping a small hole in them to use with a pointed tool 'pry bar'. I'd expect the rubber seals would come off easy enough too

[SillyOldDufferModerator @sillyoldduffer]

A book like An Introduction to Low Temperature Differential Stirling Engines felt like it would be a good read until I saw the price! This NASA Report is free though. I wish I was good at maths…

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