Stirling engines with an internal spring question

Discussion on Stirling or "hot air" engines (all types)
Wellington
Posts: 143
Joined: Mon Apr 18, 2016 4:02 am

Stirling engines with an internal spring question

Post by Wellington » Thu Jan 04, 2018 7:44 am

Hello again everyone. long time since my last post.

Is it possible to build a hot air engine that pushes a displacer forward with hot air expansion and then the rebounding displacer is done using a spring as well as the natural suction of the cooling hot end? (Thereby eliminating the need for a power piston or any kind or offset crankshaft timing).

Or a better description of what i'm thinking would be: a piston cylinder closed at both ends with displacer inside and a spring at the cold end. I realize this design idea would likely remove the definition of "stirling engine" and it would be considered some type of hot air engine. My question is would it work and what kind of performance/efficiency should i expect from using a spring instead of a power piston?

Any opinions greatly appreciated.

PS. Just another thought.....would it also work with twin pistons both heated at the same time moving the same crankshaft?

kind regards

Wellington

Alfista
Posts: 91
Joined: Thu Feb 18, 2016 9:14 pm

Re: Stirling engines with an internal spring question

Post by Alfista » Thu Jan 04, 2018 9:23 am

Wellington, it seems that you are describing the Ringbom or a free displacer or free piston engine; or am I missing something ? Perhaps I have not understood you.

You could look at the designs of Chipperfield, Martini or Beal. Senft has a nice book on miniature Ringbom engines which may be of interest to you. Not all engines of this type use springs but many do. Also, when bellows or diaphams are used, then it may also create the effect you describe.

Wellington
Posts: 143
Joined: Mon Apr 18, 2016 4:02 am

Re: Stirling engines with an internal spring question

Post by Wellington » Thu Jan 04, 2018 9:31 am

Alfista wrote:Wellington, it seems that you are describing the Ringbom.....
Im basically looking to build an engine that is as simple to build as possible yet be made of materials that are durabled and presentable enough for sale as a quality made product. I've been looking at linear free piston engines but have so far only found engines made from non durable materials. I'll tale a look at the ringbom engines. Thanks for the tip Alfista. Came across your youtube channel the other day. Very nice large engine you have built there.

regards

Wellington

Alfista
Posts: 91
Joined: Thu Feb 18, 2016 9:14 pm

Wagstir and Martini

Post by Alfista » Thu Jan 04, 2018 9:51 am

Wellington,

thank you very much for your compliment on the large Stirling engine !

I can suggest two very interesting designs : the Wagstir "Solarsimpleton" engine and the Martini "Stirling Cycle Amplifying Machine" aka the "Martini Displacer". These can be found in the Sier "Hot Air Caloric and Stirling Engines" book, Vol. One. These designs are both very intriguing and sophisticated. Both employ internal springs.

If I am not mistaken, the "metronome" engine is similar to what you have in mind and it is exceedingly simple to build.

Wellington
Posts: 143
Joined: Mon Apr 18, 2016 4:02 am

Re: Wagstir and Martini

Post by Wellington » Thu Jan 04, 2018 10:06 am

Alfista wrote:Wellington, I can suggest two very interesting designs : the Wagstir .......
cant find any info on google to the solar simpleton engine. Any links?. Might come visit you on youtube again soon.

kind regards

Wellington.

cbstirling2
Posts: 143
Joined: Sat Dec 17, 2016 9:35 pm

Re: Stirling engines with an internal spring question

Post by cbstirling2 » Thu Jan 04, 2018 11:44 am

Alfista, I just looked at YouTube.
Neat gamma engine I saw. Big question, I saw you are able to generate over 12 volts but what about amps?
A simple way to test for that is to hook your set up to another motor to act as a load. And then using two multimeters check for voltage as well as amps. Presto multiply those two numbers and you get Watts.
That's what I do to test my toy ones. I found that generating more volts and ignoring amps is fairly easy to do. You just change the gearing. Without a load, nothing is learned imho.
CBStirling2

Ian S C
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Location: New Zealand

Re: Stirling engines with an internal spring question

Post by Ian S C » Fri Jan 05, 2018 3:44 am

Wellington, the Ringbom motor is what you are thinking of, I'v built three of them, the reason that I use springs is to absorb the shock of the displacer hitting the end of the hot chamber, one of my early one punched the end out of the hot end. The displacer is moved by the differential pressure generated by a enlarged displacer rod, I,v not used any maths to calculate the size, but one I have has a 35 mm dia displacer, with a 12 mm die rod. I made a stove top fan with one of these motors, but it was TOO noisy, and TOO powerful(not something usually associated with these motors).
I also have a free piston motor with a Ringbom type displacer, this motor has a linear alternator of sorts, it will run a 6v transistor radio.
Ian S C
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Wellington
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Re: Stirling engines with an internal spring question

Post by Wellington » Fri Jan 05, 2018 7:42 am

Ian S C wrote:Wellington, the Ringbom motor is what you are thinking of.....
Hello again Ian, I think i want to do something even simpler than a ringbom engine to begin with. The thermo accoustic looks a simple engine. I think i will make the hot end from stainless steel and brass for the push rod. Do you think stainless steel will work for the hot end instead of borosillica glass syringes which are really quite expensive?.

regards

Wellington

Ian S C
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Re: Stirling engines with an internal spring question

Post by Ian S C » Sat Jan 06, 2018 2:45 am

Stainless steel is what I normally use, although I have used mild steel. The reason for using glass is so that the displacer can be seen. It would be possible to make a glass motor, big test tube for hot cap, smaller one for the displacer. A glass syringe for the power cylinder, with the glass plunger for the piston. No doubt someone will come up with an idea for a glass flywheel, but I think you would have to go to steel for the crankshaft.
Ian S C

Wellington
Posts: 143
Joined: Mon Apr 18, 2016 4:02 am

Re: Stirling engines with an internal spring question

Post by Wellington » Sun Jan 07, 2018 2:24 pm

Ian S C wrote:Stainless steel is what I normally use, although I have used mild steel.....
How do mild steel and stainless compare in performance? what are your experiences with the two? Also how does thick and thin effect performance?
Wellington

Ian S C
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Re: Stirling engines with an internal spring question

Post by Ian S C » Mon Jan 08, 2018 1:37 am

For displacers, I try to get somewhere about .010" to .015" thick, stainless is best because of it's resistance to heat conduction and corrosion. Mild steel eventually starts to droop in horizontal motors. If the metal is too thick, it causes increased friction through the extra weight. The long displacer acts as a regenerator, the reason why we want the minimum amount of heat conduction, and an ability to absorb, and give up heat. And a reason not to use aluminium, it has all the properties we don't want, these two were used in the free piston motor.

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Instead of steel springs for the free piston motor I use two magnets with the like poles facing each other(so they repel).
Ian S C

Wellington
Posts: 143
Joined: Mon Apr 18, 2016 4:02 am

Re: Stirling engines with an internal spring question

Post by Wellington » Mon Jan 08, 2018 11:16 am

Ian S C wrote: Instead of steel springs for the free piston motor I use two magnets with the like poles facing each other(so they repel).
Ian S C

hmmm, never realised you can use magnets in a free piston motor. That would be a lot simpler than finding springs with apropriate springyness. I guess the magnets would need to be protected from the heat so they dont lose any magnetism. Will probably go that direction as it seems simpler.
Thanks Ian.

Ian S C
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Re: Stirling engines with an internal spring question

Post by Ian S C » Tue Jan 09, 2018 2:52 am

The magnets I use came from an old micro wave oven, I just use magnets on the power piston, the displacer works as a Ringbom motor type and the springs on that act as bumpers to stop the displacer hitting the ends of the hot chamber. Here's another view of the power piston and magnets. The motor in its original form had springs, lots of springs, I did have it going quite well that way, then I saw a video of a motor made by a German guy using magnets.
Important, if you use magnets, either for a spring, or a linear alternator, or both as I have, the surrounding structure of the motor must be non magnetic. In this motor the power piston is cast iron in a bronze cylinder, the supporting structure is aluminium and brass. The exception is the iron cores in the coils.
Ian S C
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Wellington
Posts: 143
Joined: Mon Apr 18, 2016 4:02 am

Re: Stirling engines with an internal spring question

Post by Wellington » Tue Jan 09, 2018 11:30 am

Ian S C wrote: Important, if you use magnets, either for a spring, or a linear alternator, or both as I have, the surrounding structure of the motor must be non magnetic.
Looks like you are a seasoned veteran stirling engine builder. I'll remember the non magnetic tip. Thanks. I'm ready to start building now. will post a pic of my first engine soon i hope.
kind regards
Wellington

Wellington
Posts: 143
Joined: Mon Apr 18, 2016 4:02 am

Re: Stirling engines with an internal spring question

Post by Wellington » Tue Jan 16, 2018 6:53 am

Ian S C wrote:For displacers, I try to get somewhere about .010" to .015" thick,
will it still work almost as good if the stainless tube is 1 or 2mm thick?

regards
Wellington

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