Jan Ridders Coffee Cup Stirling

[SillyOldDufferModerator @sillyoldduffer]

Posted by pgk pgk on 05/03/2016 16:48:29:

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

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

Ta,

Dave

[Ian S CParticipant @iansc]

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 05/03/2016 17:14:39:

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…

.

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

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 05/03/2016 17:14:39:

I wish I was good at maths…

.

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

As the man says : Read this first

**LINK**

MichaelG.

[SillyOldDufferModerator @sillyoldduffer]

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!

[AjohnwParticipant @ajohnw51620]

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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[pgk pgkParticipant @pgkpgk17461]

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…….

[SillyOldDufferModerator @sillyoldduffer]

Posted by pgk pgk on 06/03/2016 18:40:06:

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!

[Michael GilliganParticipant @michaelgilligan61133]

This paper may be of interest:

**LINK**

MichaelG.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Michael Gilligan on 08/03/2016 13:47:57:

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

[Neil WyattModerator @neilwyatt]

Posted by SillyOldDuffer on 08/03/2016 16:01:09:

Like me they don't have a way to measure pressure inside the cylinder.

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

[AjohnwParticipant @ajohnw51620]

The best way to measure the pressure easily is probably as per model engineer – a manometer with very short narrow bore pipe connecting to it but what ever is connected is going to mess up the compression ratio on a LTD engine.

Interesting that Michael's link calc's that in a different way. I don't think their engine makes much sense either and a gas burner on one side plus water on the other – LTD ???

I'd guess if people bought the rather expensive book there would be little of practical use in it other than an engine design.

John

–

[SillyOldDufferModerator @sillyoldduffer]

Posted by Neil Wyatt on 08/03/2016 16:58:44:

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

Farnell don't have them in stock at the moment so I've ordered one from Amazon. Interfacing the sensor to an Arduino is straightforward as described HERE by Alexandru Csete. (Fans of Software Defined Radio will know of Alexandru) Thanks for identifying it Neil!

I wondered about a manometer as suggested by John but couldn't think of a way of capturing data from it. Also I'm not sure a manometer as understood by me would respond fast enough.

I'm happy to stay with theory at the moment because the engine still isn't running…

Cheers,

Dave

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ajohnw on 08/03/2016 18:16:58:

…

Interesting that Michael's link calc's that in a different way. I don't think their engine makes much sense either and a gas burner on one side plus water on the other – LTD ???

…

.

John,

I've only skimmed it, but this might be more to your taste.

MichaelG.

[SillyOldDufferModerator @sillyoldduffer]

I like the look of MichaelG's latest link – only 257 pages of light reading.

Message for beginners! Don't be too discouraged if things don't always go well with this engine. My last update was that the thing had stopped working after a series of improvements. It took me all today to get it going again and I was starting to think that the <expletive deleted> was a write-off. Nothing I did helped.

The initial stoppage was probably a simple leak after correcting the flywheel but then I got myself into a "fix one thing break another" cycle. Stopping all leaks, making sure that the displacer never fouls the cylinder and minimising friction in the mechanism are all critical. I had to reassemble the engine 4 times before finally hitting the sweet spot. Then it ran so effortlessly it was hard to understand that there had ever been a problem.

Next step requires postie to deliver the bits I ordered to instrument the engine.

Dave

[SillyOldDufferModerator @sillyoldduffer]

Early days but I've got an Arduino Uno capturing Room Temperature, Lower & Top Plate Temperatures, RPM and Cylinder pressure from my Coffee Cup Stirling. The results are "Quite Interesting", perhaps due to faults in the engine, the sensors, my programming, and a flimsy understanding of thermodynamics.

This graph shows that the engine RPM fluctuates excessively. I think it means there are still some rough spots in the various bearings and sliding surfaces.

The cylinder pressures are much less sinusoidal than I expected. This may be because the pressure sensor is insensitive and/or the Arduino isn't fast enough. Depending on RPM I'm getting 20 to 50 pressure samples per revolution.

Next graph is the closest I could get to an Indicator diagram. It's an Open Office net graph showing cylinder pressure as it varies during a single rotation. The graph reads anti-clockwise. Power piston BDC is at sample number 1. This example shows pressure variations inside the displacer cylinder recorded during revolution 539 when the engine was turning at 81 rpm.

At 81 rpm the Arduino was fast enough to take 48 pressure readings during this particular revolution. Whilst a bug means the pressure is in "Dave Units" rather than kPa as intended, the shape and scale of the graph are believed to be correct.

At start, when the temperature difference between upper and lower plates is at a maximum, the net graph shows high pressure for about half a cycle. Later in the run, as in this example, high pressure is only available for just over a quarter of a rotation. I put this down to less heat being available for work because the upper plate has warmed up – the engine depends on the temperature difference between the lower plate (hot) and the upper plate (cold). Or perhaps something else is going on!

My first attempt to calculate the power of the engine from pressure data and power cylinder dimensions came up with just over 0.5W per stroke, but I spotted an error. A second attempt at applying the formula suggests about 8 mW per stroke but that feels too low. (When correct, the calculation reveals the power being developed inside the cylinder, which is much more than that available at the flywheel. Making a brake dynamometer to measure the actual output power to determine the efficiency of the engine will be another challenge!)

I have to admit that my mathematical skills are highly suspect. Never mind, perhaps tomorrow will bring enlightenment. It's very humbling when I remember that pioneers like Watt managed to understand this stuff from scratch.

[charadamParticipant @charadam]

SillyOldDuffer – "the thing had stopped working after a series of improvements" – is that the same as "it was working, so we fixed it"?

 

Edited By charadam on 15/03/2016 23:02:36

[Ian S CParticipant @iansc]

The electronic metering(?) of your wee motor is marvellous, wish I could do that. Spent the best part of this afternoon doing tests on one of my Ringbom motors, a high temperature machine. From what I worked out, it was producing 1.9 Watts at 370 rpm. I use a little Prony brake, and I modified it today so that it bears on my little 500gm digital scales, in the past it had a weight hanger and weights, and I now have a non contact laser tachometer, instead of a home made mechanical one. While fiddling I was thinking how could I measure the torque on my LTD motor, I'll be interested to see how you solve the problem. However good the bearings etc, you only will get a fraction of the power at the crankshaft, the power is going to be in mWs.

Ian S C

[Michael GilliganParticipant @michaelgilligan61133]

Very interesting monitoring, S-O-D

I hope there's an MEW article or two in there!

MichaelG.

[SillyOldDufferModerator @sillyoldduffer]

Posted by charadam on 15/03/2016 23:01:36:

SillyOldDuffer – "the thing had stopped working after a series of improvements" – is that the same as "it was working, so we fixed it"?

Edited By charadam on 15/03/2016 23:02:36

Worse than that Charadam – it was working really well until I fixed it. At the moment it's fixed good and proper, as in dead as a Dodo…

[AjohnwParticipant @ajohnw51620]

It doesn't want to let up it's secrets Dave.

John

–

[SillyOldDufferModerator @sillyoldduffer]

Ian SC. Can you tell me more about your Prony Brake please? I have no practical experience of such things and am keen to know more.

I was toying with the idea of turning the engine's flywheel into a pulley and looping a bit of string around it with some sort of strain gauge on each end to make a simple brake. That was until I found out how much strain gauges cost! I hope there are cheaper alternatives out there somewhere but I haven't found any yet.

Now I'm wondering about making some sort of spring-balance to replace each strain gauge. As the balances will have to be lightweight and sensitive I'm concerned my clunky mechanical skills won't be up to the delicacy needed. Calibrating them adequately might be fun too.

I've not had much hobby time last few days. As I write the engine is in bits. My modelling clay sealing system works well at first but leaks after the clay has heated and cooled a few times. Before reassembling with silicone seals I want to replace the steel pillars with insulators. It's obvious a lot of unwanted heats passes through the pillars to warm the top plate undesirably.

You would not believe the trouble I'm having trying to make simple insulating pillars out of a plastic coat-hanger. I thought it would be a quick easy job. Turning and drilling plastic seems to be much harder than doing the same job in metal. In the end I gave up in rage and frustration.

I had a career in computing and have messed with electronics and amateur radio since I was a schoolboy. Being able to use my other skills in this hobby is nice but I often get into deep water. There's an awful lot I don't know and I make an awful lot of mistakes with the practical work!

Michael: not sure I'm up to writing an article. I'm skating on thin ice at the moment!

Cheers,

Dave

 

Edited By SillyOldDuffer on 16/03/2016 20:31:40

[Neil WyattModerator @neilwyatt]

S.O.D. email me your spreadsheet – or at least the data for Cycle 539.

i think I can convert your data into a proper indicator diagram.

Especially if you can let me know where TDC is!

Might take me until the weekend – i have other spreadhsheets calling…

Neil

neil@mytimemedia.com

[SillyOldDufferModerator @sillyoldduffer]

Still having lots of problems getting the engine to run reliably. Having discovered that I had cylinders leaks, I experimented with a number of different materials some of which took a long time to dry. Very time consuming! Rejected for varioius reasons were HiTack Glue, Silicone Bathroom Sealant, Plumbers Mate, Blue Tack, and gaskets made from cardboard or sliced silicone piping.

RTV Blue Instant Gasket turned out to be the best. What annoyed me about this was that I used Blue RTV originally with no luck! This is because I used an old tube which must have gone bad with age: though unopened it might well be 15 years old. Serves me right for being mean.

The old RTV barely sets and gives off copious acetic acid fumes. New RTV rubberises in about an hour and doesn't smell half as bad.

When assembled and tuned correctly the engine runs well. Unfortunately each time I improve the engine I break it!

Anyway, some more "quite interesting" graphs. First an attempt at a Pressure Volume aka Indicator Diagram.

In this sample of six overlaid diagrams you can see that the engine is not performing consistently. I don't need an Indicator diagram to tell me that!

More useful is this one:

The down-stroke of each cycle shows a consistent glitch just before the pressure bottoms out. Careful inspection of the displacer shows that it tilts slightly after gently tapping a small blob of lower plate sealant. I think this explains the glitch.

Anyway, here's the question. The area outlined by the curves drawn in the Indicator Diagram reveal how much power the engine develops. Is anyone able to explain to a dunce like me how to calculate this area from the data used to produce the graph? At the moment I can guesstimate it from the number of grid squares enclosed by the curves, but it would be more satisfying to have a numerical solution. My program is written in Python3 which has good support for scientific and engineering number crunching. Unfortunately my mathematical skills aren't good enough for me to even identify a suitable function or algorithm in the available modules. Should have paid more attention at school but it's too late now!

Cheers,

Dave

[Ian S CParticipant @iansc]

I would suggest that you send the diagrams, and info to Jan Ridders, its probably something he never thought of anyone doing with one of his engines, and I think he would value the contact.

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