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Hot air and stirling engines

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Ian S C23/12/2010 10:35:14
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7468 forum posts
230 photos
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 Tricker12/01/2011 02:50:58
10 forum posts
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 www.engineersedge.com/properties_of_metals.htm
I'd like to know what people think of this.
Ian S C12/01/2011 10:20:45
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7468 forum posts
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    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 C18/03/2011 12:42:11
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7468 forum posts
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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 C11/04/2011 13:18:50
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7468 forum posts
230 photos
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 Parsons11/04/2011 14:32:49
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645 forum posts
33 photos

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 C11/04/2011 15:11:40
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7468 forum posts
230 photos
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 Long11/04/2011 16:32:54
883 forum posts
11 photos

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 Olsen12/04/2011 07:17:58
1294 forum posts
108 photos
1 articles
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 C12/04/2011 14:15:23
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7468 forum posts
230 photos
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 325/04/2011 00:23:53
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175 forum posts
36 photos
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 326/04/2011 08:18:16
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175 forum posts
36 photos
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 126/04/2011 09:29:40
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3357 forum posts
112 photos
10 articles
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 326/04/2011 09:47:51
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175 forum posts
36 photos
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 C26/04/2011 12:14:54
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7468 forum posts
230 photos
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 W27/04/2011 10:33:48
2011 forum posts
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 C27/04/2011 13:24:32
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7468 forum posts
230 photos
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 327/04/2011 19:33:21
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175 forum posts
36 photos
I found sewing machine oil works well, after all they sound like sewing machines.
Mark
Richard Parsons28/04/2011 08:17:26
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645 forum posts
33 photos

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


Rgds

Dick
Trumpet / Flugel28/04/2011 10:57:20
8 forum posts
May I suggest trumpet valve oil as a lubricant? Ready available at most musical instrument shops, a bottle will cost around a fiver. It's a bit like sewing machine oil but perhaps a bit less viscose.
 
Hope this is of interest.
 
Peter.
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