Jan Ridders Coffee Cup Stirling

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

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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

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John,

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

MichaelG.

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

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.

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

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

Very interesting monitoring, S-O-D

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

MichaelG.

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...

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

John

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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

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

[email protected]

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

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.

My school math is out of date too and biased towards biology so we never got involved in calculus. You can probably track the area of each pulse so long as the Y axis is geater than zero by sampling and accumulating tiny triangles and the rectangles beneath the previous one...?

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Dave,

Others may contradict me [not an infrequent occurrence] but here's my take:

If the curves were mathematically precise, it would be simple [but not particularly informative] to calculate the 'area under'. ... BUT, in fact they are complicated and therefore ANY method of estimating the 'area under' will come down to square-counting; however clever it may appear, and however small those squares might be.

Have faith in what you are doing ... it's good !!

MichaelG.

Edited By Michael Gilligan on 26/03/2016 10:21:03

I think you will have to count squares too. Integrals need a function - wish you luck on that aspect.

I did do pure maths for a while but it didn't suite my brain. As wonderful as some aspects where it wasn't clear what they could be used for. From memory one way of seeing if a function could describe some results is difference tables. You might find details on the web. I'm pretty sure it wouldn't work in this case.

John

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Hi ,Have been interested in hot air engines for many years,tried to make them work with mixed results. There is a museum in Hereford , " The waterworks museum" . which has a large collection of full size engines,in steam sometimes. well worth a visit . Keep up the good work .

best wishes Maurice.

Count the squares- the smaller the square the more accurate it will be, I'm sure you know that. Any mathmatical formula just does the same thing but with tiny imaginary squares. Well done getting the diagrams in the first place, I'm looking to make a brake of some kind, there is lots on the net and am slowly getting my head round it.

Here's some Proney Brake photos, the motor is my latest flame licker. The beam on the brake is 5" long, with notches every 10th of an inch, the weight hanger weighs .250 oz, and the total weight of the weight in the photo is 1 oz.

Ian S C