Member postings for SillyOldDuffer

I couldn't find much on the web to suggest that there's any kind of problem with Molybdenum: instead it seems to be a jolly good thing.

The lubricant is Molybdenum Disulphide rather than the element and it's a solid with properties akin to Graphite or PTFE. The only negative comment I found was "Thermal stability in non oxidizing environments is acceptable to 1100C (2012 °F), but in air it may be reduced to a range of 350 to 400 °C (662 to 752 °F)." That doesn't sound too bad.

Either I'm misremembering or maybe there was some sort of FUD campaign back in the day.

Does anyone know if small ultrasonic cleaners are any good for 'our' purposes? They seem to be aimed more at jewelry than the filth I have in mind.

No workshop progress today apart from keeping the engine toasty warm in the hope of getting rid of the vinegar.

Cheers,

Dave

Sorry for the diversion Eric and Martin, though it was fun.

I will of course be watching the programme thanks to Eric's helpful post.

Ta,

Dave

I've been interrupted by domestics again! After re-polishing and carefully cleaning both cylinder and piston I reassembled the engine and it went an absolute treat with no oil. Unfortunately after 35 minutes it began to scrape again before literally grinding to a halt in about 10 minutes.

Inspecting the piston revealed a new coat of rust on the head and the first 1mm of the piston body. A careful sniff of the hot engine revealed a trace of vinegar.

I now think I had two problems: grit causing severe scratching coupled with rust that gradually gums up the piston whilst attacking the fine polish of the cylinder.

Careful cleaning seems to have stopped the scratching but not the corrosion. A Q-tip wiped around the cylinder after a run comes out an unhealthy greeny/brown/yellow colour. The source of the vinegar can only be the RTV Instant Gasket I used to seal the engine. It is still curing. I suppose it cures much faster in a full-size engine where there is much more heat. Also, any acetic acid vapour that gets inside the works of a much larger machine will be swept away before it can do any damage.

I'm going to leave the engine with the piston out warming on a radiator over the weekend. I hope that will finish the cure. Though the graphite has been delivered I probably won't have time to make and fit a graphite piston until next week.

Danny's suggestion of an ultrasonic cleaner is a good idea: I think Maplins sell them next time I'm out that way.

John's point about Silicone and heat is worth checking too. I vaguely recall being warned off Molybdenum Engine Oil years ago on the grounds that when it wears out it degrades into two components: one's an abrasive and the other's an acid. Don't panic - I might be remembering that completely wrong and it was 40 years ago!

Smart Bohm engine Ian and thanks also for the information that it needs regular cleaning. As a beginner I find it enormously useful to know what to expect. Otherwise I fix things that ain't bust and accept shortcomings as being normal when they are actually faults to be overcome.

Cheers and thanks again,

Dave

Edited By SillyOldDuffer on 14/04/2016 22:00:59

Not quite everything Mark!

Though I admit almost all steam engines are drop-dead sexy!

Cheers,

Dave

Greetings and welcome Rod,

Nice oscillator and congratulations with your successful publication of a picture.

Have lots of fun with your new lathe, I'm in love with mine!

Cheers,

Dave

Thanks again chaps, I think Neil and Ian S C are correct. Good news as I might not have twigged to it and might have been a dirty boy again. I distinctly remember washing the piston carefully, but don't recall doing the same to the cylinder - it may only have got a wipe through with some tissue. Attention to detail is not one of my strong points.

This afternoon I re-polished both cylinder and piston and this time carefully washed both of them. Interestingly the first wash of the cylinder (in paraffin) didn't get all the grit out. A second wash did and then I rinsed and polished both with unperfumed Nail Varnish Remover, which is slightly diluted Acetone. I shall try the engine again tomorrow.

Never heard of Jojoba Oil before, might be worth a try. Using Sperm Whale Oil would ruin my green credentials even if I could catch one! Sewing Machine Oil is good stuff though.

Noggins End metals have sent me an email confirming dispatch of the graphite rod needed to make a graphite piston. With luck one of the solutions in hand should give me a reliable engine so I can start measuring again.

Cheers,

Dave

Sam,

Scary stuff, I believe you. But why didn't you report it at the time? Why don't you report it now? Do you really have direct knowledge that your employer or employees were or are bribing officials? If so please do something about it, these are serious offences.

You may be aware that the 2010 Bribery Act beefed up the law in this area. In addition to simplifying and clarifying previous law relating to offering and receiving bribes, it introduced "A new offence applicable to a commercial organisation that fails to prevent a bribe being paid by a person associated with it with the intention of obtaining or retaining business or an advantage in the conduct of business for the organisation. "? And "Even if the bribe is paid by another company to which you are contractually linked, your company may still face prosecution." The act sets a maximum 10 year prison sentence for individuals but company fines are unlimited. As I said, serious stuff.

Is there more bad-practice in the trade than just bribery? The building industry has above average H&S issues, and as discussed elsewhere in this thread, a range of other problems. Is it usual in your line of business to look the other way and do nothing whenever something illegal happens? If so, it might explain a lot.

Golly gosh Model Engineering covers a wide range of interests and issues! And I only came online to look something up.

Regards,

Dave

Memory's a funny thing. John's description of his fun and games with hydrogen reminded me of a school lesson where we punched a small hole in the base of a largish tin can and discarded the lid. The can was inverted and a thumb placed over the hole. Then the can was filled with hydrogen and placed bottom down on a laboratory tripod. With a lighted match at the ready thumbs were removed and the gas lit at the small hole. It was difficult to see the flame I remember. Anyway after 30 seconds or so enough air mixed with the hydrogen to cause a very satisfying bang. Most of the cans hit the ceiling which was about 15 feet up. Teenage boys, hydrogen, naked flames, no safety gear of any kind - what could possibly go wrong?

Anyway, the real purpose of this post is to ask advice about pistons!

I've decided to replace the EN1B piston in the pictures with one made from graphite but would like to know if anybody recognises what's going wrong here and how to fix it.

First problem is the orange pitting on the piston head in the picture. (The flaky ring is the innocent remains of some silicone. The blurry yellow thing in the centre is the out-of focus brass thread of the connecting rod clevis fork that lives inside the piston. )

My guess is that it's due to moist air causing alternating condensation and evaporation as the engine operates. But it seems very severe for an engine that works on hot air, usually only about 90C. It might be something coming off the RTV sealant but if so it's odourless. Any suggestions please?

Second problem is that the steel piston (13mm dia by 10mm long) is being scratched by the brass (or possibly bronze) cylinder. The two were carefully polished to a tight fit and then lapped together.

In operation the brass cylinder seems to retain its polish whilst the piston gets scratched: it takes 30-40 minutes to get into the state shown in the picture. The worst scratch is shown. The very fine scratch lines running around the circumference of the piston were made with 2500 grit emery paper used to touch up the piston after an earlier run. These lines weren't evident on the piston when it was new - it had a better polish. I followed the advice to run the engine without lubrication.

I'd be very grateful for suggestions as to what is causing this. I think the piston and cylinder are being made the usual way but am I doing something silly?

Thanks,

Dave

Fine body of men, when was the last time you had to bribe a British public servant!

Excessive employment of civil servants is a popular misconception. About 400,000 civil service jobs have gone since 1975. These people weren't doing nothing, their work was transferred to the private sector and it is still being paid for by the taxpayer.

Privatisation often worked well, but there are plenty of examples were services were disrupted, standards dropped, and costs increased.

If a real civil servant is giving you a hard time remember they don't make the rules: they are obliged to implement government policy, even if it's stupid.

Regards,

Dave

My poor old mum believes everything she reads in the Daily Mail and every week my sister and I spend about half an hour each trying to calm her down. The Mail is a jolly good read if you like a good sprinkling of overheated reports about health issues, crime, lefties, bad neighbours, or the misdoings of young people, minority groups, Europe, China, and foreigners. The spoof headline "Rumanian Supermarket Trolleys Cause Cancer" is a parody of the Mail style, but I feel it sums them up quite neatly.

Sam's defence fails if you substitute "Pravda" or Der Sturmer" for the Daily Mail. Thus "Pravda has the country's highest circulation. So something it does must suit the population. Or do those that berate Pravda think that the entire population has less sense than them?" Yes, especially during Stalin's time.

I don't think anyone should be embarrassed by enjoying the Daily Mail provided they read it critically. Many other papers and periodicals have agendas too. If you really want to know what's going on you have to wade through all of them.

Cheers,

Dave

Thinking about it in reverse indicates that high pressures would be beneficial - an open to atmosphere Stirling Engine couldn't operate in a vacuum.

I can't think why a leak would be beneficial, but then as confessed earlier I don't quite understand how the engine works. The only advantage I can think of is that leaks would tend to keep the cold end cooler. But it would also mean that the engine was losing pressure. It makes my head hurt!

Putting the whole engine in a container and pressurising it to a few bar wouldn't be difficult. I'm thinking domestic pressure cooker drilled to accept a nozzle and inflating it with a car tyre pump. It would be safe because they're professionally designed pressure vessels with a safety valve.

It should also be possible to try different gases. Of course it's all been done before . I found this table on the web showing that Hydrogen, Helium and Air have the highest specific heats of the gases. So at the same temperature they store more heat than other gases.

If I could guarantee that no oxygen could ever mix with the hydrogen a trial wouldn't be dangerous at all. It's a big "if" though! Nonetheless I shall order Igor to will take the risk next time we have a thunderstorm. The Hindenburg was full of hydrogen and that went well didn't it.

Back in the real world a bigger objection is that I'd have to hire a hydrogen cylinder, regulator and hoses. That's pretty expensive for a one-off experiment inspired by idle curiosity.

I found a British Made HSS M2 tap that I don't remember buying and it had no problem whatever tapping into a 1.7mm hole in mild steel. Job done.

Cheers,

Dave

Edited By SillyOldDuffer on 10/04/2016 17:05:22

Don't tell the foreman but I discovered this morning that the bloke who drilled the pillars for an M2 tap used a 1.5 mm drill thinking it was 1.6 and he didn't bother to check. No wonder the taps were a bit stiff, doh! When the replacement taps arrive I'll try 1.7 clearance as John suggests: no doubt plenty strong enough for this application.

Ian: do you know what the advantage of pressurising a Stirling engine is? Is it just that you can get more heat into a working fluid when it's more dense? Also, do you have a suggestion for what the IHP of a Coffee Cup engine might be. Anything I calculate is highly accident prone and it helps me very much to know roughly what the right answer is in advance! I wasn't too unhappy with 0.5W IHP from a 10 to 15W input, and of course the power that actually reaches the flywheel will be very much lower again.

The results you get from an 18W mug warmer and boiling water are very interesting. In comparison my engine starts at about 80C and it works noticeably better when the room temperature is low, say 12C. Central heating and warm days are very unhelpful.

My engine doesn't speed up as expected with increased temperature. I've had the lower plate up to 105C only to unexpectedly see the engine speed up as it cooled off after the test. It works best at about 88C with the top-plate 50-60C lower.

I'm pretty sure these odd results are because my engine still has faults, not least that the piston rubs when the engine gets hot. At one point it was chuffing like a baby steam engine and that's definitely not right. I ordered some graphite rod today in the hope of fixing that problem.

Back to the workshop! At least I'm getting my money's worth out of all that tooling, until I break it that is...

Cheers,

Dave

Edited By SillyOldDuffer on 10/04/2016 13:30:29

Very many thanks for the encouragement, Michael and John. I need some TLC at the moment because the engine has stopped working again!

I suspect that the power piston and cylinder bind when the engine is hot: certainly the polished piston ends up marred by fine scratches after 30 minutes or so. It also gets rusty on the inside face.

Originally I put the corrosion down to acetic acid from the sealant, but I'm still getting rust even though the sealant is long since cured and there's no smell. Possibly moisture in the air is repeatedly condensing and evaporating on the piston as the pressure alternates.

In the circumstances a graphite piston sounds like a very good idea. Thank goodness I can keep the cost of this endeavour secret!

John's suggestion that the engine might work better if pressurised is interesting: I shall have to do some more research. I don't really understand how the engine works either. On one level I think I get it, but it's not difficult to upset my mental apple-cart. For example I can't explain why the piston and displacer operate 90 degrees out of phase rather than any other angle.

Michael's suggestion of using plastic strip in the Mk2 dynamometer feels right too. I have some strip that might do if only I can find it. I have to get the engine working reliably before making a start on that though.

I made some changes to the program I wrote to analyse the data. Unfortunately the code that produces indicator diagrams has gone gaga and everything I've done to fix it has made the problem worse. Any computer experts reading this won't need to ask if I took a copy of the last working version: the professionals know that only wimps take backups.

On the other hand I think my earlier question asking how to calculating the area of an indicator diagram was based on a misunderstanding, so that's a bit of accidental progress. The pioneers needed to do it that way because their indicator diagrams were produced mechanically. I have the advantage of modern technology that captures many actual pressure readings throughout the cycle. It's easy to calculate the average pressure when you have actual numbers: I don't need to draw an indicator diagram to obtain the information needed to calculate Indicated Horse Power. IHP is derived from average pressure, piston face area, and the stroke length. My unchecked calculation using this approach suggests that the IHP of the Coffee Cup Stirling is a little under half a watt, which might even be about right!

Indicator diagrams are still worth producing because the shape and consistency of the curves say much about the health, or otherwise, of the engine.

In the workshop I snapped two M2 taps in improved pillars for the engine due to clumsy over-enthusiaism.  One of them jammed in the pillar so I will be following advice from the thread that recommended Alum as a way of dissolving the tap,

Cheers,
Dave

Edited By SillyOldDuffer on 09/04/2016 21:53:18

Edited By SillyOldDuffer on 09/04/2016 21:55:40

Isn't it annoying when domestic issues interfere with important things like hobbies!

After various unexpected delays I was finally able to complete a "proof of concept" dynamometer made out of Meccano and use it today to get a reasonable result from a test Meccano electric motor.

The electric motor is considerably more powerful than a Coffee Cup Stirling engine but I think I'm on the right track: it does seem possible to make a sensitive dynamometer using the torsion bar principle. (The technique is normally used at the other end of the power range to measure the beefy Shaft Horse Power delivered by a ship's engine.)

Here's the set-up:

The Arduino measures the angular displacement between the flywheel and the brake as well as the rpm of the motor. The connecting cable isn't shown but in operation data is sent to a PC for analysis through the Arduino's USB port.

The torsion bar is calibrated by attaching a horizontal balanced arm across the brake wheel and measuring how far it is deflected by slowly adding known weights to a bag suspended from the end of the arm. I got this nice straight line graph for Torque = mass x distance.

With the Arduino switched on and the PC collecting data, I laid a weighted nylon cord across the end pulley of the brake. Work is done by the motor in trying to lift the weight and in the brake's "bearings". (There's a lot of friction because Meccano doesn't have proper bearings.)

The work twists the torsion bar and because the Arduino can detect how much the brake is out of phase with the flywheel, I can determine how much the torsion bar is twisting.

After loading the data into a spreadsheet, I crunched about 100 readings to find the average deflection at average rpm. It was 1.8 degrees of twist at 365 rpm. From the calibration graph this corresponds to a torque of 0.00678 kg/m

And: Power = Torque * 2 pi * N / 60 (where N is rpm)

If I got the calculation right (oh dear!), the measured output is about 0.25W

As the input to the motor was about 1W (1.75V at 550mA), we have a motor efficiency of about 25%. This is in the right ball park for a small DC motor. Below is a graph of Torgue, Efficiency, Volts and Amps as provided by the maker of a real motor. It seems that 25% efficiency isn't unusual.

There's still a long way to go with this. Interested readers will have noticed that the readings come from the extreme sensitivity end of the Meccano dynamometer. The accuracy must be very suspect. Therefore a torsion bar set-up to measure a Coffee Cup engine will need to be better engineered: I don't think I can get the necessary improvement in sensitivity from standard meccano parts.

In the hope of getting more consistent operation from the engine I've also built an "electric hot-plate" to heat the Coffee-cup engine. Actually it's a 25W 22ohm wire-wound resistor mounted in a hole gouged into half a reflective insulating brick. It's much more controllable than a mug of boiling water.

20W is sufficient to reach operating temperature reasonably quickly and 10-15W keeps the engine going. My engine is still unreliable and it may well be possible to do better than this.

As I was advised earlier in the thread, I had to replace the ball bearings with a quality type. Cheap ones just about worked but reducing friction by removing their dust-guards eventually caused the bearings to stiffen due to unconstrained up and down thrust from the crankshaft. I don't think it's good when miniature bearings start making crunchy noises!

The engine runs noticeably faster with better bearings but I still have a friction problem. It may be that the steel piston is binding in the brass cylinder due to unequal expansion as the engine warms up. I'm suspicious that the odd bit of "brass" I used to make the cylinder is actually bronze. Would bronze be bad news in a cylinder?

Cheers,

Dave

Edited By SillyOldDuffer on 07/04/2016 16:36:48

Thanks for the comments from Messrs pgk pgk and John.

John first, I'm not doing particularly doing this to improve the engine (though that would be good), Rather I'm experimenting with modern technology and 19th century physics to see if I can determine the engine's efficiency.

I've always been interested in the principles behind engine design. Answering questions like how much heat is needed to produce a given power output or how big should a flywheel be tickle my fancy. The work is only original in so far as I'm applying it to a very low-powered engine using my equally low-powered brain!

I looked up Schmidt after you helpfully mentioned him in an earlier post. It seems that in the 1870s Professor Schmidt studied a Hot Air Engine made by Lehmann and used the analysis to derive a general theory of Stirling Engines. I'm sure Schmidt knew how to calculate cylinder power because the method was decades old by then - it is one of James Watt's genius ideas. The limitation in Schmidt's work is that his calculations assume that the piston and displacer movements are truly sinusoidal, which in a real engine they never are. Nonetheless, Schmidt seems to be the best starting point for an engine design.

I've had reasonable success producing the Indicator Diagrams needed to the calculate power being developed inside the engine. Counting squares will give a result.

The next step is to try and measure the actual power output available at the flywheel. The theory of Dynamometers is well known and the mechanics quite simple. Unfortunately, a Dynamometer sensitive enough to work with the Coffee Cup Stirling may well be beyond my skills. I'm enjoying the challenge of finding that out at the moment!

Which brings me to pgk pgk's remarks. Yes, I fear you are right. My proposal would add significant friction to the system before any braking is applied. If it works at all it will read low by an unknown amount. And the energy used to start the engine has to be considered too. Oh dear, it's time for another think!

I spent the afternoon making a model torsion Dynamometer out of Meccano. It's a proof of concept to test the software I haven't written yet so no problem there then! Assuming I get the code to work, it will be interesting to see if this crude Dynamometer gives a reasonable power output measurement from my Meccano electric motor. Whether or not the method can be successfully applied to the Coffee Cup engine is now moot thanks to pgk pgk's input.

Back to the drawing board,

Cheers,

Dave

Very good indeed, though it does remind me of my mother-in-law...

I spent the afternoon making and finishing a new power piston. The engine is working again and although it's not on top form I hope it will improve with use. Then I can start measuring again.

I find using a lathe to be most therapeutic. It helps me think about stuff too, almost sub-consciously.

As a result of my metal mangling cogitations I'm warming to the idea of trying to make a very sensitive dynamometer based on measuring the twist in a springy strip. The strip (something like the inside of a wiper blade) would connect the shaft of the engine to a second wheel. The second wheel would run inside a trough. Adding water or some other viscous fluid to the trough would provide a very light brake, one that should be easy to control. Magnets are an interesting possibility too, thanks pgk. Electromagnets would make it possible for the Arduino to apply braking just short of a stall.

Sorry about the crudity of the drawing, but the apparatus would be something like this:

I optimistically guess that the main problem would be friction in the bearings of the second wheel. As Ian says it doesn't take much to stop these little engines dead.

Nice kit on the Magtrol website. I'm frightened to look at their prices though!

Cheers,

Dave

Hi Daniel, been there done that!

I agree with Phil P. I have the same cross-vice as you and it doesn't do a good job for me either. It's particularly bad when going straight in with a twist drill.

The method I settled on is to hold the work in a machine vice that's relatively loosely clamped to the drill table. The vice is held sufficient to stop it jumping upwards but it can still move sideways.

A Centre Drill is used to make a pilot hole before switching to the twist drill. Then the point of the twist drill is carefully positioned over the pilot hole. When drilling starts, I allow the loosely clamped vice to centre the work under the drill. It may not be the best method but it works well enough for me.

I think the problem with cross-vices is that they are too rigid to let the work centre, but not rigid enough to keep the drill perpendicular.

Cheers,

Dave

Hi Ian,

Mucho thanks for your Prony Brake photos. A picture paints a thousand words and I can certainly see me making something like that.

My own thoughts haven't reached the practical stage yet. Something like this:

Or perhaps a torsion bar made from a metal strip stolen from a windscreen wiper blade? The strip would link the flywheel axle to a rotating load. The flywheel and load would both have magnets aligned at tdc when stopped. It wouldn't be too difficult to derive the torque from the time difference between tdc detected at the flywheel and tdc detected at the load when the engine is working to turn the load by twisting the strip.

Setting up an Arduino as a kind of stopwatch to do the job seems straightforward compared with the delicate precision needed of a mechanical brake intended to measure very low power outputs.

You'll appreciate I suggest these ideas with all the confidence of a man who hasn't thought any of it through properly, let alone actually tried to do it! The real world can be so cruel...

Cheers,

Dave

Thanks to everyone who advised on the area problem. The examples of integral calculus I found being used to solve similar "area under a curve" problems are way out of my league. Now I don't think I have a practical alternative to counting squares. And it's always a good idea to "Keep It Simple Stupid".

Thanks also to Maurice for the pointer to the Hereford museum: I shall go when the weather's better.

I'll probably contact Jan Ridders when my results are a bit more reliable. At the moment the engine is temperamental as is my experimental software. And I'm still picking your brains!

Next picture is the indicator diagram from the last time the engine worked. You've guessed it - I've since broken it again!

The picture shows the diagrams of about 20 superimposed cycles. You can see that a few cycles have large areas, but that about half degenerate into a much smaller loop. Not good - the engine is defective.

Given that all the other seals are OK, my interpretation is that the power piston is leaking. This is entirely possible because I've had to polish the piston and cylinder several times to remove corrosion. (I think the corrosion is being caused by acetic acid fumes released by the sealant as it cures, made worse by heat. )

Although the piston still just about passes the 'pop' test, the fit is noticeably looser than it was when new. Making a new piston is my next priority.

Cheers,

Dave

Edited By SillyOldDuffer on 27/03/2016 21:28:53