Stirling Engine PE2

by dougconner, published

Stirling Engine PE2 by dougconner Nov 16, 2012

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This is a Stirling engine which is a type of heat engine. It needs about a 50 degF (28 degC) heat differential to run and cruises about 300 rpm unloaded.


All the plans and information to build this engine is now available. You can see lots of information about the engine with a video of it running at http://www.solarheatengines.com/2012/10/29/3d-printed-stirling-engine/

The zip file contains all 11 3D printed parts for the engine.
PDFs provide assembly drawings and parts drawings. A discussion of building these parts is available at http://www.solarheatengines.com/2012/11/21/3d-printed-stirling-engine-now-open-source/

An assembly and BOM PDF is now available. More assembly details are available at http://www.solarheatengines.com/2012/11/27/3d-printed-engine-bom-and-assembly-instructions/

I've posted an article on building the regenerator for the PE2 engine. If you build the engine you want to be sure to build one for good performance. I've included information for those wanting to learn more about Stirling engine regenerators too. http://www.solarheatengines.com/2012/12/18/a-regenerator-for-the-3d-printed-pe-2-stirling-engine/

I've posted an article on alternate designs for the power piston and the displacer. These new designs will not affect the engine performance, but may be simpler to build. The STL files are available in piston insert 3.zip and displacer3.zip. The article is at: http://www.solarheatengines.com/2012/12/31/new-power-piston-and-displacer-for-3d-printed-stirling-engine/

To build this engine you need a drill press to accurately drill out the rough 3D printed holes and for drilling the aluminum plates. Some simple lathe work is necessary to machine the power piston to fit in the brass tube.

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I have one of these built, I just haven't gotten it quite working yet, accidentally mislocated my pin hole on the power cylinder side for the correct stroke of the power cylinder. I am getting power out of the engine it just won't manage to get all the way through the cycle. I did it for a free project in my Second Semester Thermodynamics class, and I will be doing tests on it in my engine design class. I did end up making an all aluminum flywheel, and made the engineering drawings for the one I'm using, which I will post up when I get it running. I really think Doug did good quality work here. The design was easy to follow and there were no missing details. I did end up with a shortage of foam so I am using a new lighter weight model of displacer, but it's doing just fine.

Just a running question that maybe someone can answer, has anyone built one of these using Acetone smoothed ABS? I have the feeling that would vastly increase both the smoothness of operation, and the air tightness of the 3D printed material.

I found that a combination of sanding and acetone smoothing were helpful where the two large o-rings seal on the abs. I made a soft of paste by dissolving abs in the acetone and then painted it on the sealing surface. By alternating this with sanding using a fine sandpaper I was able to get a smooth surface and a good seal.

The first engine I had made on a Stratasys commercial printer and sealing wasn't a problem. I had to use this sealing technique on the second engine I had made on a lower-cost hobby-type printer.

What is the diameter of the fly wheel?

Great engineering, with only one flaw: working in imperial units is the way to in the US, not in the rest of the world. If I want to use this, I have to redesign it.

YES, you do. perhaps, in addition to giving away his design for free for everyone to use, he should redesign it for you so go don't have to do anything yourself?
Why shouldn't he use the materials available to him rather than use difficult to acquire metric parts.
I would have thought this was a great starting point for anyone wanting to convert it to metric, if not, go to eBay and buy the imperial parts you need.
for everything else, just multiply by 25.4 and you will have it in mm.
In England, we say "give them an inch and they'll take a mile" you'll have to convert that to metric yourself.

Wow! Great engineering Doug.

i'm printing one right now, but it feels so light. what's the infill ? and is it different for each part ?

Saw one of these in operation over an incadescent lamp at the Inside 3D Printing Conference in San Jose last week.

It looks like good printing resolution. I'm concerned the temperature limits might be lower than ABS. Definitely something to watch.

Great!! Thank you very much!!

If using slic3r scale by 2540% and the model will print at the correct dimensions.
1 in = 25.4 mm  

 Thanks, this will be helpful for people with that software.

 I also had to rotate several of the .stl files in order to fit them on the printer.  I print them and if they come out ok I'll upload them.

With 120F temperature delta, how large do you think this would have to be to generate 1kw? I have a 155F hot spring, and a cold creek. I would love to be able to generate some juice from it. I have about 220kw of heat potential, so even with a very low Carnot efficiency I should be able to get some usable electricity. Do you know what the Carnot of this engine is with 120F delta?

The answer to your question is you could probably make a 1kw engine using your hot and cold water sources--but it would be a big project. The Carnot limit for 155F and 35F is 20%, but it would be a real engineering triumph to make a system around 5%. If I was trying to build such a system I would hope to get 5% but anticipate around 2-3%. 

To give you an idea of the magnitude of the project, take a look at this article and video of a 1kw low-temperature differential Stirling engine. http://www.solarheatengines.com/2012/01/10/tamera-video-of-sunvention-sunpulse-engine/http://www.solarheatengines.co.... 
It uses a higher temperature ratio but is similar. 

With your source of hot and cold water efficiency wouldn't be as big of an issue (2% would work). I can tell you that there are other people that would be interested in such an engine if it was reliable and could pay for itself in a few years of operations.

Organic Rankine cycle turbine engines are are also used for generating power at your temperature levels. 

Is there a reason you used the foam with the displacer instead of just printing the whole thing?  Seems like you could print it with very low infill and have it be just as light.

I'd like to be able to print the displacer in one piece but I was not sure that I could make it both light and air-tight. For the displacer to work properly it cannot let air leak in or out of it except over long periods (tens of seconds or minutes). The large flat surfaces must also not flex when they are subjected to the +/- 0.5 psi pressure variations during operation. 

The other reason for using foam is that I have another version of the engine (all the printed parts are identical) but with more aluminum plates for internal heat transfer. This other version uses a displacer that is only 0.5 inches long instead of 1.0 inches. All I have to do is remove two of the foam plates. 

I don't have a 3D printer yet, but I'm hoping to get one soon. My engine was printed by Stratasys. This engine was my first 3D printed design. I knew very little about 3D printing and made what I felt was a conservative design that would work on the first pass. I'm still amazed it did. I'm also hooked on 3D printing. 

I have several parts on the engine I'd like to improve to simplify building the engine. I'm concerned about getting a 3D printer that can make accurate ABS parts. At the moment I can't really justify buying a Stratasys printer for $10K, but I could probably spend around $2K. My question is which one and will I be satisfied with the results? For example, Stratasys doesn't have a problem with overhangs, but as I understand it, many of the lower-end printers do. I also measured no detectable warping and the accuracy was excellent. 

looks like a good LTD sterling. make me wonder if a thermoacoustic type could be made in a simmiler way.

Interesting idea. I've never seen a LTD thermoacoustic engine. All the ones I've seen take a pretty hot flame. If you know of any LTD thermoacoustic engines, send me a link. 

... here is one "air the onset temperature is even below 30°C " http://www.aster-thermoacoustics.com/ "

Thanks for the link. I had no idea there were thermoacoustic projects at this level. Now I have to really understand what they are doing to see if I can design one. 

The STL files come in very small. What did you scale them as to print your model?

I designed this engine in inches. The hole for the displacer in the cylinder body should be 3.00 inches in diameter. Is that the problem?

I am a retired Australian engineer and am always looking for interesting projects. I will start to print the parts later today. 

Cool.  There's an animation explaining how this engine works here:

(forgive the shameless plug!).