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Free Piston Stirling

11/13/2007 10:15 AM

Hello Everyone,

I have been Googling free piston Stirling engines lately and have found a good bit of information about them. However, what I'm looking for is how to calculate the mass of the pistons, spring constants, working pressures,......so that it might operate. I'm guessing equations of motion for a coupled harmonic oscillator would get me started in the right direction. Does anyone have any ideas?...Stirling Sam? NASA seems to have the best information so far...

And one more thing...how would you start it?...maybe a solenoid?...Perhaps it would self start if the pistons were tuned correctly and in the right position....

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#1

Re: Free Piston Stirling

11/13/2007 10:34 AM

Oops! Sorry Stirling Stan. I referred to you as Stirling Sam

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#2

Re: Free Piston Stirling

11/13/2007 10:41 AM

Every one I've ever held in my hand started by itself. I've only held one:The Physics department head has a model that works with as little heat difference as the difference in room temperature and a person's hand.

P. S. How about some links?

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#4
In reply to #2

Re: Free Piston Stirling

11/13/2007 11:11 AM
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#3

Re: Free Piston Stirling

11/13/2007 10:47 AM

This one's made of cardboard and has no springs.

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#7
In reply to #3

Re: Free Piston Stirling

11/14/2007 9:37 AM

Hi Horrible old Bat.

That is not a free piston stirling engine. Spencer.

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#8
In reply to #7

Re: Free Piston Stirling

11/14/2007 9:44 AM

Indeed. I was thinking the same thing myself.

I am actually thinking along the lines of building a prototype free piston machine that can produce electricity or refrigeration with thermal solar or waste heat. I have found some stuff about Schmidt analysis, but I was hoping for some info that would help illuminate the Schmidt analysis. I like to have the concept down before I start crunching numbers.

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#10
In reply to #8

Re: Free Piston Stirling

11/14/2007 10:45 AM

Hi Dag.

The invention of the free piston stirling engine was by Professor William Beale in the late 1950s. he was Professor of mechanical engineering at the University of Ohio.

You can get all the information you need in Graham Walkers book; Stirling Engines, ISBN 0-19-856209-8, in chapter 11 of the said book there is a whole load of information by W Beale on designing free piston stirling engines. There are equations and graphs pertaining to the sizes and weghts of both the power and the displacer pistons, plus gas spring forces.

I hope this information helps you in your quest. Spencer.

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#11
In reply to #10

Re: Free Piston Stirling

11/14/2007 10:49 AM

Awesome! This is precisely the kind of information I was looking for. Now to find the book!

Thanks Scapolie!

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#12
In reply to #11

Re: Free Piston Stirling

11/14/2007 12:02 PM

Hi Dag.

Here is another book that is a must: Stirling engines by G.T.Reader and C.Hooper, ISBN 0-419-12400-4. I knew Commander Reader when he was in the Royal Navy, he was an excellant officer. Spencer.

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#13
In reply to #12

Re: Free Piston Stirling

11/14/2007 12:14 PM

You wouldn't happen to know where to find the book by Walker, would you? I searched amazon and a few others and came up empty handed...

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#14
In reply to #13

Re: Free Piston Stirling

11/14/2007 12:54 PM

Hi Dag.

You could try the publishers; E & F.N.Spon, 733 Third Avenue, New York, 10017.

I bought all my books in the 1970s and 1980s, but when I started to build stirling engines I went to my nearest Museum of Science and Technolgy to measure all of their stirling engines. I have been designing and constructing stirling engines since the early 1970s, Some of my most powerfull engines have been put into a 24ft boat to try them out. The question of pressure is dependant on what powers you need?

Remember that the pressure of any gas that is in a contained (sealed) system will only be 2 and a half times it's original pressure when heated to any temperature liable to be met in any stirling system!!!

Because a piston in a stirling engine moves then the pressure will only increase by a factor of two. If you increase the pressure within the confined space to 10bar, then you will effectively gain only 10bar pressure of work on the pistons. eg. 20bar over 10bar.

If you have any problems then please contact me. Spencer.

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Anonymous Poster
#5

Re: Free Piston Stirling

11/14/2007 8:32 AM

Hi Buddy, Like you, I started by looking on internet for information. I selected the gamma type stirling engine because its mechanical parts are simpler to build. There is 4 parts in that type of motor: 1- air displacer, 2- power piston, 3- flyweel, 4- cam shaft. I succeed in building 3 motors yet. Here is my tips: a-try to respect displacer ratio of ~3:1 in a ~4:1 chamber. b-try to use anuminum material for better air exchange. I suceed with stainless steel but alunimum would be much better. c-use blue burner alcohol for food stuff; it will burn hotter and cleaner. d-the magical formula: PV=nRT or in this application: PV is proportionnal to T. e- In my opinion, forget about the pressure for your first models since pressure is very dangerous and will lead in far complex mechanical parts. f-my secret: displacer piston chamber is sealed with a teflon washer (on the cold side) who let displacer shaft travel very easy with little oil. Remember, don't give up, it tooks me 2 months for my first motor to 'run'. Gamma type need to be launch. Have fun! Doogster

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#6
In reply to #5

Re: Free Piston Stirling

11/14/2007 8:59 AM

Thanks for the suggestions. It's good to hear from someone who has traveled the path before me. Have you done anything with a free-piston or alpha configuration? I have a novel idea for an alpha configuration that I'd like to try before I delve into a free piston machine.

"try to respect displacer ratio of ~3:1 in a ~4:1 chamber."

Can you explain your ratio? I think I know what you're getting at, but I'd like to make sure.

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#9
In reply to #5

Re: Free Piston Stirling

11/14/2007 9:55 AM

Hi Guest.

Sealing the displacer piston with teflon rings is not your secret!!!

I have been designing and constructing stirling engines of all sizes and powers since 1972, and all my pistons are sealed with teflon rings. But for the best material for piston rings in stirling engines you can't beat one of the so-called filled teflon, this is teflon filled with various compounds such as iron oxide or molybdenum disulfide. This makes teflon a lot more durable and stiffer, the name of this teflon compound is " Rulon", depending on which type of rulon you need there are several, they are all denoted by a letter preceding the name rulon. For example: I use Rulon A or D.

Spencer.

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