Design on Experiment LTD Stirling

Discussion on Stirling or "hot air" engines (all types)
Bigsteve
Posts: 1
Joined: Wed Nov 10, 2010 7:56 am

Design on Experiment LTD Stirling

Postby Bigsteve » Wed Nov 10, 2010 8:22 am

My apologies if this has been covered before in the forum... I couldn't find it with a search. I am building an LTD stirling, based off of one on Jan Ridders website. I was wondering if anyone has tried a design on experiment with LTD's in general to improve their efficiency (the required differential, power, or top speed)?

If not, from your own personal experiences, where am I going to get the most bang for my buck, so to speak?

Reducing friction (obviously)
Displacer volume (increase vs. decrease)
displacer bore
displacer stroke
anything else?

Longboy
Posts: 107
Joined: Thu Oct 29, 2009 11:17 pm
Location: Tucson, AZ

Re: Design on Experiment LTD Stirling

Postby Longboy » Wed Nov 10, 2010 12:20 pm

You can modify and observe the changes. The most "bang for your buck" would be an engine built to plan that runs. You can experiment with the basic construction plans in your choice of materials used, their sizes and weights, sealants and bearings, lubricants and heat input distribution and waste heat rejection . As far as increasing efficiency, how would you know? What instrumentation would you use and how can you record it and report it? More trouble than it is worth to graph efficientcys. I believe most hobbiests should think in terms of improvement over efficiency. Just come back to the basic goal........can I get my engine to run faster with less heat input? In LTD Stirling, this would seem to be the pinacle of a successful build! :mrgreen:

jimlarsen
Posts: 212
Joined: Sat Apr 17, 2010 11:04 am

Re: Design on Experiment LTD Stirling

Postby jimlarsen » Wed Nov 10, 2010 8:15 pm

Several years ago I was challenged by Brent Van Arsdell (he owns The American Stirling Co, stirlingengine.com) to build a low temperature differential Stirling engine that was efficient enough to run from the heat of your hand. My design had to be something that could be built without a machine shop. It became a 2 to 3 year study of ways to re-engineer the LTD design to reduce friction and simplify construction. I gave a lot of attention to friction reduction through both bearing design and engine configuration. I can tell you that when you are dealing with micro-horsepower you must do everything you can to eliminate even micro-friction.

I turned the main axle to a vertical orientation. The end of the axle is almost sharp and rides on a hard surface. The flywheel will coast for over two minutes with this low friction setup. I could not use graphite and glass for the drive piston in this design because I had a budget restriction as part of the challenge, so I used a thermoformed diaphragm. It offers almost no friction and has very little wasted motion.

Jan Ridder makes some beautiful engines, and are quite different from my hand built approach. I am not a machinist, so I don't have the ability to produce that type of engine.

http://www.youtube.com/16strings

jimlarsen
Posts: 212
Joined: Sat Apr 17, 2010 11:04 am

Re: Design on Experiment LTD Stirling

Postby jimlarsen » Mon Nov 15, 2010 8:11 am

My LTD engines wil run with a temperature differential of 9 degrees.

jimlarsen
Posts: 212
Joined: Sat Apr 17, 2010 11:04 am

Re: Design on Experiment LTD Stirling

Postby jimlarsen » Tue Nov 16, 2010 7:47 am

I am working on a scaled up LTD design, but not as big as the one in the video. My surfaces will each be 12" x 12". I am calling it the Toy Driver.

The reason you don't see lots of large LTD engines is an issue of horsepower. LTD engines don't produce much.

PKM
Posts: 17
Joined: Thu Sep 09, 2010 5:09 am

Re: Design on Experiment LTD Stirling

Postby PKM » Tue Nov 16, 2010 9:31 am

Bigsteve,

Of the factors you mentioned in your original post all but one of them seem to be design factors, not properties suitable for tweaking and iterative improvement.

Reducing friction (obviously)
Displacer volume (increase vs. decrease)
displacer bore
displacer stroke

The optimal displacer volume seems to be well specified on other posts on this forum- the consensus is 2/3 of the displacer cylinder volume. LTD engines seem to run best with a large bore compared to stroke, so make this ratio as large as you practically can. A long stroke isn't necessary for LTD engines as heat migrating from the hot end to the cold end isn't a major concern, and a long stroke means pushing the displacer further which wastes power, so a small stroke is ideal.

Power piston bore and stroke are less set in stone. The swept volume is important as it needs to match the change in volume of the contained air moving from the hot side to the cold side- too short a stroke wastes potential power, too long a stroke makes the engine try to run backwards at TDC and BDC. Once the power piston bore and stroke have been determined, one potential avenue for experimentation is the ratio of power piston stroke to displacer stroke. They tend to be the same on simple engines so that only one crank is needed, but there is no real reason for them to be the same.

Friction is, as you say, the killer for LTD engines, and there are as many ways of addressing that as there are moving parts in an engine. Large LTD engines seem to be a bit of a pariah that no-one takes seriously, so there may not be much established wisdom as to the best way to build them. Why not just build an engine you can easily tweak and see what results you can get?


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