Use of graphite as a displacer?

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Use of graphite as a displacer?

Postby toolman313 » Sat Feb 07, 2009 1:48 pm

I have constructed a high temp Stirling engine that runs at about 580
rpm. The displacer I made is a stainless steel, thin wall cup with an
aluminum cap on the cold side. I used a graphite bushing to seal
around the shaft connecting the displacer with the flywheel outside
the housing. I was just thinking that a graphite displacer might work
better. Even if it's solid, it would be lighter in weight and
graphite is a great insulator, keeping the hot side hot and the cold
side cold. What do you think of this and does anyone have any other
suggestions?

here is a link to a video of it.
http://www.youtube.com/watch?v=V0fvaeg4lmQ
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Use of graphite as a displacer?

Postby stan.hornbaker » Sat Feb 07, 2009 2:28 pm

The reciprocating parts should be as light as possible, therefore make the carbon/graphite piston like a cup. The displacer could be made of thin stainless with a silver soldered end cap. Minimize friction at all sliding joints/surfaces,reduce the weight of and balance the rotating flywheels. No more weight than necessary to make a smooth running engine. Make the weight of piston and displacer as nearly equal as possible.

Nice machine work!
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Response to Use of graphite as a displacer?

Postby bptdude___2569 » Sat Feb 07, 2009 2:43 pm

I saw Bill got the first response in. He is the site expert, too. I saw your video. Very impressive. I study the theory a lot, but I envy people who can build such fine little machines.

As for graphite? I have seen a number of thermoacoustic Stirlings that only use graphite for some reason I have to assume it good. The displacer and power piston are always one-in-the-same on these. Power output is so low, I guess the graphite is strong enough, light enough, and self lubricates the cylinder.

Look up the thermoacoutics on the web, and you will find much about the preference on graphite.

Good luck, and keep those way cool videos coming!

- Joe
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Response to Use of graphite as a displacer?

Postby toolman313 » Sat Feb 07, 2009 3:04 pm

Thanks for the info. I was also considering using a weight on the flywheels to balance the power piston and displacer but am not sure if it will work. With the weight not really pulling down on the crank pin but being shifted back and forth, the flywheel would initially be out of balance until it started spinning.
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Response to Use of graphite as a displacer?

Postby schiz0preni » Sun Feb 08, 2009 4:38 am

hye Tim!!
for the exact mass and diameter of flywheel,you actually can measure it..do some homework on engine dynamics..i've been doing some calculation,but it's all theories,not yet to implement on my engine though..
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Response to Use of graphite as a displacer?

Postby joefish711 » Tue Jun 30, 2009 9:52 am

Beautiful machine. I'd considered using a graphite displacer, but a word of caution: Graphite is brittle and has low shear strength. In a high temp. engine, you'd need to be sure that the graphite will not fail under the rpms you're spinning. I haven't done the math yet, but it seems it would be simple to calculate what speed a graphite displacer could handle before failure. I just don't have the material constants.
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