Jan Ridders Coffee Cup Stirling

[SillyOldDufferModerator @sillyoldduffer]

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.

Gas	Specific heat capacity at constnat volume (J kg-1K-1)	Specific heat capacity at constnat pressure (J kg-1K-1)
Air	993	714
Argon	524	314
Carbon dioxide	834	640
Carbon monoxide	748	1050
Helium	3157	5240
Hydrogen	10142	14300
Nitrogen	741	1040
Oxygen	652	913
Water vapour	–	2020

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

[AjohnwParticipant @ajohnw51620]

Hydrogen is at it's best or worse when it forms a stoichiometric mix with pure oxygen. In that state it's classed as super high explosive. So super in fact that it burns so fast it's of little practical use.

As a warning (but more for fun and disprove some ones theory ) about being careful with gassing lead acid batteries as the give of a perfect mix we connect to circa 50 yds of narrow bore plastic tube (3mm) to a cell and arranged to the tube into a series of letter. Set the whole thing up along with a camera, lit the mix and took a photo at the same time. The photo showed the whole tube lit up. I aught to remember the propagation speed but can't.

Bet you were all dying to know that.

In some sort of open inverted container filled with hydrogen and nothing else it just burns and goes pop. Some nut thought that a long narrow bore would prevent the flame from propagating as it would starve itself of oxygen as it burnt along the tube but forgot that it was mixed rather well at exactly the right ratio.

John

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[SillyOldDufferModerator @sillyoldduffer]

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

[Neil WyattModerator @neilwyatt]

When the hard metal is worn by the soft one it means bits of metal or grit are embedded in the soft metal. You may not have cleaned up well enough after using an abrasive at an earlier stage.

Neil

[Ian S CParticipant @iansc]

If you lapped the bronze cylinder, some of the grit may have embedded it's self in the bronze. I have one engine with a brass, and another with bronze cylinders, and I have kept away from lapping or honing these cylinders for this reason. I believe that you can use a grinding medium called Timesaver and over come the problem, there is a grade for bronze I think.

Ian S C

[AjohnwParticipant @ajohnw51620]

The corrosion is strange. The RTV gives of acetic acid which I believe is one of the things that has been used as a rust remover – vinegar. Never tried it and have my doubts but one problem with rust removal is how quickly it can crop up again if not oiled pretty quickly.

Vinegar can be used to clean brass – that was suggested on the brass burners on a stove we used to have. it's very effective but takes a while.

John

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[AjohnwParticipant @ajohnw51620]

Just a thought on a suitable lubricant. Sperm whale oil has very desirable characteristics for some lubrication tasks. Why is explained on the wiki. Now jojoba oil is used as it has even better high temp characteristics. That can be bought, lots on ebay. Doubt if it would work out one Ian's engines but it might help lower temperature ones. Also silicone based sewing machine oils.

The stirling engine book I have mentions the need to meticulously clean up. I think it suggests acetone. It also suggests running for a while and then cleaning again.

For lapping something inefficient that breaks down easily might be the best option.

John

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[SillyOldDufferModerator @sillyoldduffer]

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

[Danny M2ZParticipant @dannym2z]

A small ultrasonic cleaner (such as the Aldi version) is invaluable to remove lapping residue from pistons and cylinders.

It is amazing how much residue appears in the bottom of the container after a few minutes agitation in soapy water.

I use mine for model engine parts – even more vital if one has been using diamond lapping compounds.

* Danny M *

[AjohnwParticipant @ajohnw51620]

It might pay to be a bit careful with sewing machine oil and other silicone oils in some cases. Most silicone products revert if over heated = sand. Well that's one way of putting it.

John

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[Ian S CParticipant @iansc]

The best way of running a Stirling Engine is with no oil if possible, or as happens with one or two of my motors, they get a bit of oil splashed from else where and need the piston withdrawn, cleaned, and the bore cleaned. Kitchen paper towel does the job. My little Bohm (my only commercial motor) needs wiping out each time I run it.

[SillyOldDufferModerator @sillyoldduffer]

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

[pgk pgkParticipant @pgkpgk17461]

Off topic nostalgic post about ultrasonic cleaners..

Way back when we were one of the first to embrace ultrasonic dental scalers. For the teeth we cleaned under g/a generally we turned the gain up to max. It seemed a good idea to clean my cheapo timex wristwatch when it looked filthy… so i took it apart, dropped it into an alcohol solution and applied the probe. lots of lovely filthy muck started coming out of the works..followed rapildy by the numerals on the dial..and then the hands….

[Michael GilliganParticipant @michaelgilligan61133]

Great story, pgk

… But I thought Dentists wore Rolex

MichaelG.

[pgk pgkParticipant @pgkpgk17461]

Posted by Michael Gilligan on 15/04/2016 10:28:11:

Great story, pgk

… But I thought Dentists wore Rolex

MichaelG.

…not a human dentist or constrained to such a narrow field

..so sadly no Rolex either..

[AjohnwParticipant @ajohnw51620]

They can't wear their Rolex's at work Michael – it would make the customers wonder about their fees.

The molybdenum comment is a new one on me. I wonder how an element breaks down. Fact the auto industry don't like it because things do take a lot longer to wear out. The don't like people having the engine oil changed more frequently than the service schedule either for the same reason and generally wont change the stuff they put in the gearbox either for the same reason.

The usual comment about molybdenum was that it would poison engine sensors when they hardly had any – temp and oil pressure. It is true to say that if they could find something that would have problems they would be very likely to use it. On the other hand especially in relationship to grease they have no qualms at all about using it if they have failure during warantee problems.

John

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[SillyOldDufferModerator @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

[pgk pgkParticipant @pgkpgk17461]

I'd expect the small cleaners to work. I had a larger bath with dual transducers for surgical instrument cleaning but even with that one had to use appropriate solutions to loosen blood and tissue fragments and often dirty stuff took a couple of 25min cycles to really clean instrument joints out. they're not a magic instant thing at all (unless you stick a probe onto your watch)

I cleaned a small lacquered copper snuff box in mine once… left it too long and took the lacquer off. a function of time and power (and available finance). We're time rich with cheap low power stuff.

[AjohnwParticipant @ajohnw51620]

I find a drop of Teepol in them gets rid of most things. Teepol was just a laboratory glass cleaner but they seem to have extended the range but still do a glass cleaner. It even gets the gum out of carb jets and things like that.

A bit expensive maybe but only a little is needed

**LINK**

The bath I use has a heater in it as well.

John

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[Danny M2ZParticipant @dannym2z]

Posted by SillyOldDuffer on 15/04/2016 21:07:56:

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.

Some people swear by it for coating target bullets and claim that it reduces barrel fouling.

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.

When I purchased my small (1/2 litre) ultrasonic cleaner it was originally intended to be used for cleaning brass cartridge cases prior to re-loading. With a teaspoon of citric acid powder and a dash of washing-up liquid added to the water it did the job in 3 minutes. Before that I used a vibratory cleaner filled with crushed walnut shells – it took all night, walked around the floor and the next day much time was expended picking the walnut particles out of the primer flash holes. (Anyone want to buy a s/h Hornady Turbo Tumbler?)

When I was making a new piston and contra-piston for an M.E. Snipe model diesel engine (1.5cc) I lapped them as well as the hardened cylinder with 800 and 1200 grit lapping compound. The ultrasonic cleaner was put to use to quickly remove lapping compound so that the fit could be tested. Even after cleaning them with petrol and a toothbrush the ultrasonic cleaner found some lapping compound that the toothbrush had missed.

The size of your components (will they fit in the tank) seems to be the limiting factor and as for justifying the cost, #1 demonstration is to show SWMBO what a great job it does on her jewellery.

* Danny M *

[AjohnwParticipant @ajohnw51620]

Looking at new ones I vaguely recollect that some come without a drop in wire basket. I would be inclined to get one that does.

I borrowed mine and was told to not bring it back so that they could get a new one.

John

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[Ian S CParticipant @iansc]

One bearing that we used to lubricate with Moly grease was the tail wheel on Cessna, and Piper aircraft, and the warning I got was that you must not mix it with graphite grease, there is a good chance the bearing will burst as an extra ball appears in the race.

Ian S C

[AjohnwParticipant @ajohnw51620]

Many moons ago either the AA or the RAC treated a mini with it. They emptied the oil out of it and from memory drove it from the B'ham to London intending to see what happened at that point. As all seemed ok they drove it back again and then dismantled the engine. All ok apart from some slight signs of over heating.

I had a similar experience. I had been trying to by an E Type Jag that was pretty cheap really but a little out of my reach. It had done nearly 90,000 miles. There was a fuel crisis and I made a silly offer for the car and got it. It came with full service history which include adding moly to the engine oil. Some time after I bought the car I noticed a misfire, it was running on 5 cylinders. Not much of a problem on a light weight car with that much power so thought I would fix it over the weekend. Went out Friday night and on the way home the oil pressure dropped to zero and the engine seemed odder. Some pressure showed if I kept the revs up so I drove 20 odd miles like that. The following morning I wanted to go fishing so checked the oil, none so filled it up. Just as much went in as an oil change would need. Started the engine and noticed oil spewing out of the exhaust. It emptied it again in a couple of mins.

Took the cylinder head off as a start and found a huge hole in one of the pistons. I pleaded poverty at a Jag agents and got a new set of pistons and all bearings at trade. They used to grade pistons to bores and only the max size ones were available. Measuring up showed that I only just got away with using them. The max wear on the bores was a few thou ovality down the bore and the usual step at the top which the rings leave. It took seconds to round that off, Measuring up the various bearings I needn't have changed them at all.

Odd thing was that as usual when I bought a car at that time I used flushing oil on it, removed the sump and cleaned it all up. Filled with fresh oil, ran it for a while and then another oil change usually with flushing oil again. I didn't add moly or anything else to it. So personally I reckon that the claim that it does something to the surface of the metal is correct.

I "decoked" it at the same time and found it would really do 150mph after that. I reckon about 155. A bit short before.

John

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[Ian S CParticipant @iansc]

Getting oil correct is important. (OT) Back in the 1950s the RNZAF had Mk 3 Hastings transport aircraft, while the RAF had Mk1, and 2. One of our machines was passing through Singapore on it's way between NZ and UK, the RAF ground crew refuelled the aircraft, and filled the oil tanks, BUT they used the same non detergent oil as the Mk 1 &2 used, our machine with the more powerful engines used detergent oil. Next day and the aircraft took off, and at the end of the strip, after lift off, all the engines quit having blown all the oil out, I suppose someone got a slap on the wrist, and told to read instructions.

Now where were we, hot air engines, often better with no lub in ball races, although I don't do it, some flush out the bearing.

Ian S C

[SillyOldDufferModerator @sillyoldduffer]

I've always thought that if people were meant to fly we would have been born with feathers! I only know 4 flyers and they've all been in crashes, two of them bad ones. An uncle was in two "minor" incidents, no one hurt but the aircraft were written off both times. All of them due to pilot error, we humans can be very unreliable. Now I have to worry about the ground crew using the wrong oil as well!

Back to hot air engines, I tried flushing the first set of ball bearings used on the Coffee Cup engine with acetone and it initially helped. But only for a while. The bearings eventually failed, mainly due to sideways forces, but there was also a gummy residue in the race. I think this might have been the remains of the rubber seals.

Looking online later for replacements I found that these bearings come in a range of different types. Now I believe flushing may or may not work depending on the bearing you happen to have. There isn't a universal answer, you have to experiment.

I had a frustrating afternoon machining graphite for a new piston. It's the first time I have worked with graphite. Apart from being very messy it wasn't difficult to machine. Unfortunately the diameter of my first attempt was exactly 1mm too small. That's what I get for trusting my mental arithmetic.

My second attempt went very well until I broke it getting ready to part off. I bumped the cross-slide into the work when I meant to retract it and graphite's not very strong.

The third attempt is a near failure too. I took just a little too much off leaving the piston a tad loose in the cylinder. Although the engine works with an undersized piston I shall have to make another one. More mess.

The graphite piston seems to have solved my corrosion problem. Despite a strong smell of vinegar there is no sign of scratching or corrosion after a 40 minute run.

The engine spent the last couple of days sat on a radiator in the hope that the sealant would finally cure. It seems that completing the cure needs a higher temperature than my central heating system.

On the hot-plate at 40C there's still no trace of vinegar. But at 102C the engine reeks of it. I may try baking it in an oven.

I have, of course, got a new problem. How do you tap Graphite? Jan Ridder's design calls for an M3 tapped hole through the piston. I drilled a 2.5mm hole in the piston and tried to form the thread with an M3 tap in the usual way. At first the tap was reluctant to cut, then it started in squishy way, but most of the thread stripped just as the tap was about to finish. The flutes were clogged with powdered graphite.

Is there a preferred way to tap a soft material like graphite?

Thanks,

Dave

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