Hot air and stirling engines

[Ian S CParticipant @iansc]

Hi Gordon, went into town (Christchurch) today, and visited the library, looking up hot air engines, looking at James G. Rizzos Stirling engine manual no.,2, on page 114 there is a diagram of a piston with a cup seal, but thats not how it goes in the cylinder, its pulled back so that you can see the disc on top holding it down, its got to be pushed forward and entered into the cylinder.

The temperature here today was up to 28 degC, getting to the stagewhen it will soon be too hot in the workshop.

In the UK Tango Drink cans are made of stainless steel (J.G. Rizzo) how do you distinguish them(other than name) from aluminium cans, both will be nonmagnetic. Does anyone know if there are any drink, or anything else in stainless cans in New Zealand. Ian S C

Edited By Ian S C on 03/12/2010 06:37:00

[Gordon WParticipant @gordonw]

No idea what drinks are available in NZ, our local tame nzer only mentions beer. Never had tango, so don’t know that either. But an idea would be to gently scrape the can with knife ,should give a good indication. Might be better to use car key etc. to save getting locked up. I,ve found good st/st stuff in all sorts of shapes and sizes in s/markets.

[Ian S CParticipant @iansc]

I’v given up at the moment, but last year I did a dayly round of our little town(pop under 2000) picking up discarded cans, I’v got over 100kg of cans waiting to go to the scrap dealer, and with all those cans not one stainless one, under a hedge I found an old steel beer can, must be 30 yr old at least. Next time I go to the supermarket I must take a magnet with me. Aluminium cans are useless, infact I must open up my tin can engine, last time I tried it ,it would’nt run, I suspect that the displace has collapsed, and it runs on a very small meths burner, although when it would’nt go I put a bigger one under it, to no avail.Theres only half a dozen screws to undo to find out whats wrong.Ian S C

[Ian S CParticipant @iansc]

   Just a wee note Gordon, if you (or anyone else) build Dyna or a similar motor, 8 to 10mm is a good size for bearings.  8mm is a good size price wise, skate boards etc use this size, and here in NZ I can get them for $NZ5 each (they are proberbly rubbish). In my latest motor I was going to use some ball races from an old  skateboard I found, but they had about .020″ free play, they do work, things wobble a bit.

   Been thinking aboutyour motor, with a 74mm bore, a close fitting, light weight  cast iron piston with oil grooves in the skirt should’nt need a cup seal.  I would keep away from the aluminium it tends to scuff on the bore. Cast iron piston skirts can be taken to very thin, I’v managed to get them to about 1mm thick, so they are proberbly similar or lighter than aluminium.  A hot air engine does’nt need the strength in its piston as an IC engine does.  Ian S C 

  

[Gordon WParticipant @gordonw]

Snowed in again, and b cold, but have been in shop, making the displacer actuator rod, all will be revealed one day! The ally of piston not in contact with cyl. wall, just the leather, like some air pistols. Am re-assessing the design as I go, and am probably too light, but never really expect it to work. If all is well then I might start spending money on it, like a very big lump of cast iron, and proper bearings.

[Ian S CParticipant @iansc]

Thinking further on your piston design Gordon, I just needs to be a disc to support the cup seal, with pleanty of clearance.  The only part that needs to touch the cylinder wall is the feathered edge, as pressure increasesthe seal is pressed against the cylinder.

                                                                Ian S C

[Ian S CParticipant @iansc]

Wounder if some ones got an idea, I’m running my latest motor, it’s designed to sit on a log burner, it has a fan in place of a flywheel.  As I don’t have a log burner(and its the middle of summer), I’v got it on the hot plate on my stove. The element is a 1200w one and the motor is running with the control half way between zero and one, on a scale of one to five, anyone guess a rough figure on how many watts its drawing?  Ian S C

[Murray TrickerParticipant @murraytricker65189]

I have been thinking, as the saying goes. To measure the heat going into the base of a HAE from a flat plate could be done approximately if you can measure the temperature difference across a plate inserted between the engine and the stove top.

If possible the top profile of the insert should match the engine base ie flat/flat and the same shape (round/round) hopefully.

My basic physics tells me that heat flow is given by:

 H=kA(T1-T2)/x (answer in Joules/second ie watts)

k= thermal conductivity of the insert

A= area of the insert

T1 is stove top temperature and T2 is junction temperature between the engine and insert.

x= insert thickness

 

Using pure aluminium (k=220) for the insert and an insert d=150mm I estimate 780 watts per mm insert thickness. I haven’t tried this out experimentally – am I right?

 

So I think the main thing you need to be able to do is measure temperature and have a known insert material. K is available in S.I. units for metals etc from http://www.engineersedge.com/properties_of_metals.htm

I’d like to know what people think of this.

[Ian S CParticipant @iansc]

    Murray, I’ll have to think a bit about using your formula, never been into more than basic physics.  f the control is linear( and it wont be), I imagine that the element is producing some where about 200 watts or so.  The base of the motor is about the same size as the element.

    The motor had its first try on a log burner this afternoon, but owing to the design of the fire , it was just not quite hot enough underneath, while the flue was a bit close at the top of the motor.  All’s not lost I’ll try some other types of log fire, and pellet burner.

     The element is not a flat plate, but a radiant one ie., a flat spiral that on high heat glows red.

      Things are looking good on this motor, I did an experiment last week, and attached magnets to the hub of the fan, an two coils behind the magnets and generated enough to light up two LEDs, maybe it could second as a battery charger, perhaps two NiCads. Ian S C

[Ian S CParticipant @iansc]

Last Tuesday got the latest motor going, its a gamma type, with the crankshaft vertical. It has a bevel gear in the middle of the shaft that drives the power take off shaft that has the flywheel mounted on it, this over all shape of motor makes it ideal for fitting in a boat. I was looking at Google earler in the day, and found a site from Japan by K. Hirata, and found that he has a motor almostly the same, except that his one is an Alfa, some thing about great minds thinking the same(theres a bit more too that, but we won’t go there). In October here in Darfield we have a model boat regatta, we have a good boating lake.

The power piston is 20mm dia x 12mm stroke, the displacer is 24mm dia x 11mm stroke. Because it has a plain mild steel displacer cylinder, it gets a small meths burner, I have tried it on LPG, and ran it red hot, but no advantage in speed, maybe a bit more power, but it would not last long at that heat, I’v killed a few engines that way.Ian S C

 

[Ian S CParticipant @iansc]

Mark, your initial question has just been answered, Andy Ross has put his book “Making Stirling Engines” on line for free down loading, I think you would need broadband, I’m not going to attempt it on dial up.

Took the boat with the hot air engine down to the Domain today and sailed it for an hour or so. Ian S C

[Richard ParsonsParticipant @richardparsons61721]

Long ago and far to the North of where I am now, I discovered an ‘Oil free lubricant’. It was used for in particular ‘Daisy wheel printers’. It is a spray pot of PTFE. Clean the surfaces (use a lint free paper/cloth). Spray on and leave to dry then lightly ‘buff’ the surfaces and away you go.
Try it it used to be cheap about £4 for several hundred doses.

Regards

Dick

[Ian S CParticipant @iansc]

Dick, that sounds interesting, I was thinking today about lubrication for the hot air engines, if you use CRC/ WD-40, its OK as far as viscocity goes, but as it dries it leaves a coating, I’d like a spray lub without residue. Diesel works, but I don’t know if its any good as a lubricant. Ian S C

[Keith LongParticipant @keithlong89920]

Diesel injection pumps rely on the lubricating properties of the fuel AFAIK, but it may not be up to use in a Stirling engine with a sensible power output,> I guess it will depend on the bearing loadings.

Keith

[John OlsenParticipant @johnolsen79199]

The idea of diesel oil inside a confined space with heated air fills me with foreboding.

 

Ian, the older type of simmerstat used a bimetallic strip heated by a coil or reistance wire when the contacts are open, much like an older style car indicator flasher. For all I know, newer ones may use solid state controls. (Although what could be more solid state than a bimetallic strip?) Anyway, I would not make any guesses as to how linear the control will be so other than saying somewhere between 0 and 1200 watts….

 

regards

John

[Ian S CParticipant @iansc]

Keith, I might keep on with the diesel, I’v got about 500ml, and it only takes about a drop every hour or so, and as the motor gets older it uses even less. The only time I get a bit of a flare up is if I spray some CRC on to a hot motor. Bearings OK they are ball races, only the piston on new engines needs a bit of lub, I tend to make them just a little tighter than they should be, an then run them in, so that the motor gradually gets faster and more powerful, usually over 10 to 20 hours of running, about 2000hrs is the time between rebuilds from expiriance with my early motors. Ian S C

[Mark Smith 3Participant @marksmith3]

Andy Ross has kindly allowed the down load of his wonderful book, Making Stirling Engines. It is available here http://StirlingBuilder.com. Thankyou Andy, it is fascinating reading.

Mark

[Mark Smith 3Participant @marksmith3]

Given the number of people who have read or are following this thread, I was a little surprised to note that the Survey in the latest issue, under the tile “Which of the following are you interested in reading about in Model Engineer”, does not mention Stirling or hotair engines.

Mark

[David Clark 13Participant @davidclark13]

Hi There

Hot air engine articles are welcome. I can’t include everything in a survey.

I know readers like hot air engines.

They are very popular.

regards david

 

[Mark Smith 3Participant @marksmith3]

Thankyou for your prompt reply, David. The great thing about stirling engines is simplicity. They can be constructed from junk or engineered to a high level of excellence, they are a suitable starting point for tyro engineers wanting to go on to greater things like locomotives and traction engines. Anyone wanting to see how to build these engines with any degree of skill only has to look at articles and books by people like Andy Ross and Roy Darlington to see what is possible with these fascinating machines.

Mark

Edited By Mark Smith 3 on 26/04/2011 09:49:25

[Ian S CParticipant @iansc]

Well Mark, I think we have arrived at the question you started this thread with! I spotted, then lost a reference to another free book down load, I’ll have to look for it again and see what its all about, the book is “Stirling Engine Design Manual”, by William R. Martini. Yes you can use hot air engines as a starting point to model ing other machines etc., or you can get hook like me with 15 motors now, and the ideas for another forming. I do make other things too, but mostly full size stuff. Ian S C

[Gordon WParticipant @gordonw]

To get back to lubrication of hot air engines, has anyone tried white spirit / turps substitute ? Just recalled using it for high speed rotating machines ,some years ago. Havn’t tried it myself yet.

[Ian S CParticipant @iansc]

Gordon, I,v tried kerosene, which is a few stages down the cracking scale from white spirts. Not sure of the basis of turps, I,d like to get some light grade synthetic oil to try, I wonder if I go to the local garage, I might be able to get a few drops from an empty container, although I’v heard that the best is the stuff used in gas turbine engines. Ian S C

[Mark Smith 3Participant @marksmith3]

I found sewing machine oil works well, after all they sound like sewing machines.

Mark

[Richard ParsonsParticipant @richardparsons61721]

I got my spray can from RS (usual disclaimer) years ago. I do not know if they still exist.

Rgds

Dick

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