Jan Ridders Coffee Cup Stirling

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

MichaelG.

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

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

Ian S C

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.

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Executive Summary ^^^

MichaelG.

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

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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

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

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...

I feel a wave of brutality coming over me. Nothing to lose by ripping them apart.

Ta,

Dave

Dave, that's the way, rip them out (carefully), my worry was that you were using plain bearings, these will work if you get every thing spot on.

Ian S C

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It's interesting to see that Senft's books, and his articles in Model Engineer, are referenced by NASA.

MichaelG.

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I've just found an on-line simulator, which looks promising

As the man says : Read this first

**LINK**

MichaelG.

No progress today. I made a joke about my cold being "man-flu" and it promptly turned nasty on me. Hot drinks, Night Nurse, TV, whining and a little light computing are all I've managed. Just when the project was getting interesting too!

I've been having that problem too Dave, Turned into a head cold too. Still not gone but I may feel like making my mostly cardboard low temp engine shortly.

Thanks for that link Michael - reading what it does cheered me up immensely. Sounds like that one will do as would be expected too. Not that the engine that results will.

John

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I did discover the definitive cold & flu cure as a young man and it has served me well ever since. take 1 bottle of ribena and bring to the boil. Add 1 bottle of rum and slip slowly until it's all drunk. When you wake the symptoms will be gone..although you may be left with a wee bit of a headache....

The sterilising effects of alcohol are well recognised.......

I have a very similar cold cure recipe. Same as yours except that after boiling the Ribena you throw it away and add an extra bottle of rum.

One thing I dislike about ageing is the way illnesses seem to bite harder, last longer and become alcohol resistant.

On the engine front, I had no trouble removing the shields from the ball bearings. Doing so resulted in a considerable improvement in RPM even though the flywheel was badly adjusted after reassembly of the engine.

The flywheel urgently needs a few minutes TLC on a lathe but I felt too seedy to risk a cold workshop. Instead I went online and ordered some 18B20 temperature sensors. I want to try monitoring engine temperatures and found that it's easy to interface several 18B20 sensors to an Arduino and then capture their data with a PC. It will be possible to get the Arduino to measure RPM at the same time. By making an electric heater I will be able to control the temperature of the lower plate.

Cheers,

Dave

I'm not doing this in the expectation of finding anything new, it's more the technical challenge and general interest!

This paper may be of interest:

**LINK**

MichaelG.

It is indeed, many thanks for the link Michael. It's very much along the lines I was thinking. I like the paper because it's not too academic or mathematical. Like me they don't have a way to measure pressure inside the cylinder.

The paper includes the sentence "The very low-thermal efficiency may be caused by engine part machining problems encountered during the manufacture of this engine. This causes the engine to have some misalignments and higher friction." Me too! I've just reassembled the engine with the benefit of all the latest improvements and now it won't run at all! I'm going backwards, ho hum.

The most likely cause of this outright failure is a leak. I'm still using plasticene as a sealant: it works well enough but has to be repacked carefully when the engine is reassembled.

Time for another rebuild, now where's that miniature spanner gone?

Thanks,

Dave

You could fit a pressure sensor directly in a hole in the system

cpc.farnell.com/1/1/86346-freescale-semiconductor-mpx4115ap-sensor-abs-press-16-7-psi-867b-6.html

There are smaller versions.

Neil