Member postings for 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

+1

Things to avoid in my experience, at least when first starting out:

• DIY store metals. Aluminium rod so soft and sticky that it tears rather than cuts. "Gritty" ferrous metals.
• Stainless steels. Many of these get harder and harder as you try to cut them
• Electrical conduit and gas pipe, my examples all have a tough outer skin
• Copper and lead - too soft
• Most of the plastics found in the home
• Odd bits of scrap. Unless you know what It is, it might well be horrible.

By all means experiment but don't get discouraged if your new lathe struggles to put a good finish on something you found in a hedge! I was gob-smacked when I first turned free-cutting mild-steel because it was so much nicer to work with than the rotary drier and other junk metal I started with!

The metal rods out of old computer printers and scanners seem to be good stuff.

Good luck,

Dave

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

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...

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.

I just voted for Diane by renewing my subscription!

I don't buy the magazine because it perfectly matches my tastes, I buy it because I am generally interested in what others like me are doing. I like people who make or know about ram-jets, clocks, accessories, engines, boats, radio control, fixing stuff, restoration, aircraft, industry best practice and all things similar. I like the history of technology and I enjoy reading about plans, methods, tools, materials, performance, tribulations, exhibitions and club news.

Not all editions are equally interesting to me, nor do I expect them to be. And there's always a risk that I might drift away as interests change. Like others I have a small collection of ME dating from 1919. The old magazines reflect what the majority were contributing at the time: in 1919 it was Wireless! In the 1940s model boats, internal combustion engines and model aircraft were much more popular than they are today. Then there was a time when the pages of ME were dominated by traction engines. At the time these shifts in emphasis pleased many and thoroughly annoyed others. One thing has never changed - guilty or not it's the poor old editor who takes the flak.

Cheers,

Dave

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

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

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

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!

Sure thing Les. Good point.

Apologies if what I said came over in any way as being critical of Duncan. Not my intention at all - I respect him.

I was trying to pass on some info about a subject where I have some experience. No offence intended.

Regards,

Dave

There isn't a simple single answer. You will have to read the small-print and ask the right questions. The various shades of buying and selling "Second-hand" might be OK but you can't just assume it. If the salesman tells you his software is an asset ask him directly what that means in your circumstances.

Owning software can get complicated. A simple example: often exactly the same software is licensed differently depending on the sort of user you are. For example student and professional users often pay wildly different rates for exactly the same thing. But it's very unlikely that the terms and conditions will allow the student to profit by reselling his copy of the software at the professional rate. Or even allow the student to use his software for a business purpose. Or allow a business to buy a student license.

You don't say if you are hobbyist. It makes a difference to what's meant by an "Asset". For example, a business 'asset' might have accounting and tax advantages for the commercial owner that won't apply to hobby ownership. Also a business might need a multi-user solution. This requirement introduces another raft of license considerations. Choosing the right multi-user option can make a big difference to the cost. If serious money is involved and you're a business you should buy in consultancy.

Cheers,

Dave

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!

My understanding of Microwave Oven Transformers derives from an interest in building Tesla Coils. Tesla coils require a source of high-voltage high-current power, ideally more than 100mA at 6000V. The voltage can be obtained by connecting the high-voltage outputs of two or more MOTs in series. There is a high risk of insulation failure. For this and other reasons you have to be very careful using MOTs in a Tesla Coil. Perhaps the best advice is don't.

Individual transformers are also very dangerous. In normal operation in an oven their output will be at least 0.5A at 2000V. Although the shock alone could easily be fatal, being killed outright might be the least painful option. This is because the 3 or 4 kW available at the output will have no problem striking an arc into flesh and turning you into an electric fire. Burning will continue until someone switches you off.

An MOT could be used for degaussing only by rewinding the secondary with a few turns of very thick wire. 3000 amps at a volt or less would be good. It will still get very hot though!

Ships become magnetised when the steel they are being made of is struck repeatedly in alignment with the earth's magnetic field. The same mechanism probably magnetised Ken's worktop. Rotating the top through 180 degrees and beating it again might undo the effect. Alternatively after using a compass to establish which way round the top is magnetised, stroking it with a strong permanent magnet in the opposite sense might counteract the unwanted magnetism.

Heat also destroys magnetism but you would need to make the whole top very hot. Not sure that's practical!

Cheers,

Dave

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

Ta,

Dave

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...

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

I did an ambiguity! I was trying to say "In the US a tube is what the UK would call a valve, short for thermionic valve."

I'm easily confused:

Q. Which way does a clock turn?

A. It turns anti-clockwise if you are the clock.

Now we know what Neil's fiendish plan is. He's created a three-way paradox out of the conventional notion that objects correspond to words and images. I think it's part of a plot to explain away any dimensional errors in MEW drawings as being experiments in surrealism.

I may be a little over-medicated at the moment.

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)