Hot air engines

[paul]

Although this forum seems to be almost entirely steam-driven (so to speak) does anyone hereabouts have experience of hot air (Stirling) engines? I'm thinking of trying a very very simple one as my next project and wondered how forgiving they are for a novice.

I've found a few plans on the net including an interesting 'oldie' in the book mentioned in this thread www.green-trust.org/junkyardprojects/FreeIC&ECEngines/ModelBoilers.pdf

[SteveW]

Paul,

I managed to make and get one going at school years ago. The key thing is minimal drag/friction on every moving part.

There's a bunch of guys called the "Stirling Society" that have been running a stand at most MEXs for years now. One of them (Roy ??) has produced a comprehensive book on them at just over 30 quid, a must for the keen hot engine constructor.

[paul]

Thanks Steve - most of the ones on the net are made from tin cans but one or two seem more interesting, especially the 'Manson Engine...
www.stirlingengines.org.uk/work/cyc2.html

Don't fancy 'beating out' that dish shaped end though!

[paul]

I've been thinking about how to construct a slightly different version of the original Manson engine, probably on a slightly smaller scale and avoiding the 'dished' ends of the displacer/tube.

The only real issue is how to connect the cylinder to the displacement tube and the piston to the displacer. In the original article the author uses iron flanges into which each is soldered but I wonder if it might be easier/neater and more importantly lighter to turn the end caps of the displacer tube with a flange into which the cylinder can be soldered and do likewise for the displacer/piston.

Another issue is the displacer - should it be hollow for lightness or solid and what difference would that make to the transfer of heat from the hot to the cold ends?

Anyone who might have any thoughts on this can see my schematic here www.artefact-rescue.co.uk which will probably make more sense than this rambling post!

[havoc]

Well, we do have a few stirling engine makersin the club and the general idea is that they are rather hard to make. The fit of the cilinder and piston has to be flawless in order to get it working.

site with some plans: www.bekkoame.ne.jp/~khirata/english/make.htm and one with lots of links users.moscow.com/oiseming/lc_ant_p/lnk_stir.htm

One that is often made is the Low Temperature Difference engine. You will find lots of plans for that one on the net.

EDIT: found the links back to that one: www.steamengine.com.au/stirling/models/livesteam/index.html

[paul]

Thanks Havoc - I've already seen those links, I've been doing a lot of research the last few days! One reason I thought I'd scale things down a little is that it won't get too expensive if I have to remake parts (like the piston/cylinder as you say!). I've looked at the LTD engines but I'd really prefer something I could put a fire under!! LOL

[havoc]

If you have enough leaks around your piston, then you need a fire to get it going.

[paul]

[SteveW]

Paul,

The book I outlined by Roy Darlington included ideas for sourcing the various bit n'pieces for the more novel engines.

I've always found these things a draw and usually get chatting to the guys on the MEX Stirling stands. Items like glass syringes for the power cylinder and maybe the glass inner appropriately lapped or a lump of graphite for the piston. I've discovered an excellent source of high quality ball races in dead computer hard drives on the head actuator. Kitchen shops can be a good source of stainless steel tubes (think condiment dispensers) for the displacer cylinder because of is poor conductivity. The displacer piston needs to allow the air to pass around it as it cycled and it need to be of low thermal mass so as not to transport heat towards the cool end.

The HAE I made at school was a bit of tube with the ends capped for the displacer piston, a simple steel power cylinder reamed and lapped with an alloy power piston. There's also a lot of mystery on the ratios between the volumes of the various bits.

[paul]

Steve

Thanks again One suggestion I saw was to use a 'flask' type cup that comes with a shaped 'inner' and 'outer' that could be separated and used as displacer/tube.

According to the main source of info on the net (above) there's an optimum ratio of 46% of cylinder to displacer diameter (if I read that right!) but nothing I could see about volume. The hot air engine mentioned in the book from which the first url in this thread came has a wooden displacer with tin covering! Of course that's a traditional Stirling type rather than the Manson type. I might try that one instead - although it has more moving parts it's probably more tolerant of a few shortcomings in skill (and I have that in spades!).

[baggo]

Paul, have you seen Roy Darlington's design using a test tube and some marbles? Looks dead easy to make and unusual as well. I came across it recently when reading some back issues of ME during another sleepless night! I can find it again and send you a copy if you are interested,

John

[paul]

baggo said:

Paul, have you seen Roy Darlington's design using a test tube and some marbles? Looks dead easy to make and unusual as well. I came across it recently when reading some back issues of ME during another sleepless night! I can find it again and send you a copy if you are interested,

John

!!! No I haven't seen that John but I'd love to see a copy if you can get one to me (PM me for email details). Thanks mucho!