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Discussion on Stirling or "hot air" engines (all types)
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Saccotto
Posts: 3
Joined: Wed May 29, 2013 11:47 am

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Post by Saccotto » Wed May 29, 2013 12:26 pm

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Last edited by Saccotto on Sat Jul 06, 2013 10:11 am, edited 1 time in total.

Ian S C
Posts: 2221
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: HELP ME! MY GAMMA STIRLING DOESN'T WORK!!

Post by Ian S C » Thu May 30, 2013 4:04 am

Saccotto,I'll start with the displacer, ? you'v made it of magnesium, you shouldn't have that next to a fire, have you seen magnesium burn. Aside from that, how much gap is there, between the displacer, and the cylinder, 1.5 mm to 2 mm would be about right. The gland at the end, is it free sliding, but virtually air tight? What is the hot cap made of?
What is the power cylinder made of? The best metal for pistons is cast iron, I don't know how well magnesium will wear. The piston must be able to drop through the cylinder under its own weight, unlubricated, and if you block of the cylinder, the piston should stop sliding, but the fit should be enough to make it leak proof.
Crankshaft; with the con rods disconnected, give it a spin, it should take a few minutes to stop. How free are the bearings on the con rods, ball bearings are best but not essential.
A conventional displacer is a light weight hollow tube sealed at each end, steel is good, stainless steel is better. Aluminium is tempting, but conducts heat too fast from the hot end to the cold end, and doesn't stand the heat too well either. Your magnesium would conduct the heat in a similar fashion.

Saccotto
Posts: 3
Joined: Wed May 29, 2013 11:47 am

Re: HELP ME! MY GAMMA STIRLING DOESN'T WORK!!

Post by Saccotto » Thu May 30, 2013 4:51 am

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Last edited by Saccotto on Sat Jul 06, 2013 10:12 am, edited 1 time in total.

Ian S C
Posts: 2221
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: HELP ME! MY GAMMA STIRLING DOESN'T WORK!!

Post by Ian S C » Fri May 31, 2013 4:56 am

You will not get much with an aluminium hot cap, it conducts the heat from the hot end to the cold end too well, and anything more than a candle, or spirit burner will cause a melt down. Thin steel, if you can find an old vacuum cleaner, some of those have tube about the right thickness, its just a case of finding the right diameter of tube. I welded a cap on the end of a vacuum cleaner tube today to make a hot cap for my free piston motor.
Did you machine the magnesium ? If so, take a little bit of the dust/ swarfe, and put a match to it, do it outside, in a safe place. It used to be used in photo flash, and incendiary bombs. It's used in flares. Ian S C

Hopper
Posts: 54
Joined: Fri Feb 08, 2013 6:54 pm

Re: HELP ME! MY GAMMA STIRLING DOESN'T WORK!!

Post by Hopper » Mon Jun 03, 2013 2:18 am

And if you really want to have some fun, throw an old VW Bug magnesium gearbox housing on a campfire.

There are two common problems with first run on a Stirling: Friction and Sealing.
Friction must be so low that with the hot cap removed, you should be able to spin the flywheel by hand and it will keep going for a good number of revolutions after initial spin.
Friction sources to check: Displacer guide bush, power piston in bore, main flywheel bearing (use ball bearing with no grease and maybe a drop of sewing machine oil.) Alignment of the crankshaft, con rods, bores etc is critical too.

Sealing, check the displacer does not leak by putting it in near boiling water and looking for leaks. And put some soapy water around the displacer rod guide bush and put burner under hot end. Look for telltale bubbles. Check the hot cap gasket is not leaking too, and that power piston in bore is not sloppy.

Some times it is necessary to break-in the Stirling engine by connecting an electric motor etc to the flywheel to rotate it for a few hours to wear off the high spots and free it up.

Also, don't oil or grease any of the pistons or the displacer rod etc. It creates friction from the gumminess.

If that does not fix it, it could be your design, as mentioned in previous posts.
Stainless steel is the modern norm for the hot cap, with an insulating fibre washer between it and the cold end.

Your displacer cylinder looks like it maybe has too much cold end and not enough hot end. Common modern model practice is the length of the cylinder is about half hot end, half cold end. Older models used two-thirds hot end, one-third cold end.

Also it could be the ratio between the swept volume (stroke x piston diameter) of the displacer and power piston. Common rule of thumb is the SV of the displacer should be 1.5 times the SV of the power piston.
Yours, from a quick and rough calculation, is closer to 2 times. Which is ok, some models do run that high, (eg Bengs Modelbau) but might be something to look at. Easiest way to change it is increase either the diameter or stroke of the power piston a little.

I dont know about the use of magnesium or its heat properties or coefficient of friction but I am sure you could google it. With a brass power cylinder like you have, you could use a simple steel piston, or for less friction, cast iron.

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