View Full Version : Stirling Engine

Dan Olson
02-24-2004, 01:29 PM
Hello all,

I've gotten interested in building a Stirling Engine of my own design, but I'm having trouble finding any information as far as either rules of thumb or true formulas as far as figuring volumes, areas, etc. of the displacer and power piston and the relationship thereof. I'd like to make an air cooled gamma type, with a power piston bore of approximately .625" and a stroke of about 1", but as far as the displacer piston and cylinder go, I really haven't got much clue as to how they are sized. I know it would be much easier to start from someone else's plans, but I've already built a liquid cooled stirling of about the same size from plans, and I'd now like the satisfaction of building something from my own head. One thing I see on a lot of diagrams that I have been able to find is a regenerator, which I'm quite certain the engine I built didn't have (unless I fail to understand exactly what it is)... is this a requirement? I must confess that I designed and built an engine over the weekend and failed to get it to run, which is why I ask for more information... I used copper plumbing line and cap for the hot end, and made a finned aluminum cold end and used a pice of steel tubing as the displacer. I wonder if the copper has too much heat transfer and is warming the aluminum excessivley, not allowing me to keep a large enough difference in temperature between the ends. I also plumbed the power cylinder out of the junction between the hot and cold sides (a neutral zone if you will), but all the engines I see come from the cold side. I should have realized there was probably a reason, but it seemed to me that all we are looking for is pressure differences, and since the pressure is the same all over the displacer cylinder, one place is as good as another. Hopefully this question is appropriate here, since it is machining related, and I'd imagine at least someone here has some experience with these engines. Thanks for any help you can give me.


02-24-2004, 02:07 PM
See these links:





Dan Olson
02-25-2004, 04:34 PM
Thanks, Evan. Hopefully I can get something that works from those documents.

02-26-2004, 03:02 AM
It is a subject of great interest to me (what isn't?) I am looking for a design that will produce about 1/2 to 1 HP from propane or firewood. I haven't put much effort in this yet, it is on the back burner http://bbs.homeshopmachinist.net//smile.gif.

02-26-2004, 11:55 AM
I applaud your efforts at building a Stirling engine. I caution you, however, that designing and building a successful engine is not a trivial matter.

I also, have been interested in Stirling engines, and as much as I would like to start cutting metal and get something built, I know that the first step is to acquire an extensive knowledge of what has been accomplished in the past relating to building such an engine.

The problem as posed can be broken into two parts: the design, and the construction, of the engine.

Designing a Stirling engine is actually an engineering exercise, in particular, a thermodynamic engineering exercise. A Stirling engine is a particular type of heat engine, and as such, the laws of thermodynamics govern its operation. Therefore you will need to acquire at least a basic understanding of thermodynamic principles in order to design a working engine.

This is the point where, I believe, most people become discourage. Thoughts like "I don't know any thermodynamics" or "that's too hard" come to mind. But, given the proper resources and motivation I've seen that most people can learn the basics of thermodynamic principles.

The best place to begin is to read some books on the subject. For learning some thermodynamic principles I recommend a college intro physics text such as those by Tipler or by Sears, Zemansky, and Young. These are written at a level that is accessible to people with a high school math education. While many of the ideas appear difficult, with some perseverance you will gain an understanding of the basic ideas. More than that, you will gain a new and more accurate way of seeing the world.

Next I suggest the excellent book on heat engines: "The evolution of the heat engine", by Ivo Kolin. This book give a very readable description of various heat engines through history, yet also provides sufficient technical details of the thermodynamic principles behind the operation of these engines.

Modern Stirling engines are truly marvels of engineering. Practical engines for many uses have been built, but they typically are just not quite superior enough to the currently installed base of gas, diesel, and electric motors to justify change. A prime property of Stirling engines is their potentially high efficiency -- over 40%.

Lastly, if you really want to start cutting metal, I suggest building another Stirling engine from plans. I recently built one of the Gingery engines from the plan book "Build a two cylinder Stirling cycle engine", by Dave Gingery, Lindsaybks.com This is a good project, of the size that you are interested in. The plans are pretty good. There are some details missing, and some dimensions are either missing or wrong, but nothing that an inventive type couldn't quickly overcome.

Good luck

Dan Olson
02-26-2004, 01:01 PM
Thanks for the replies. The (interesting?) thing is that I'm currently studying to become a mechanical engineer, and therefore whether I like it or not, thermodynamics is going to play a rather large role in my life over the next few years. The actual motivating factor into my wanting to design/build an engine was a demonstration of a Stirling engine in my physics class last week. I'm afraid I still lack the education needed to design such an engine, but with the links Evan gave me and the help of the physics book, I may be able to come up with something. I must admit that I underestimated the design requirements a bit when I started. PV=nRT alone doesn't quite get you there http://bbs.homeshopmachinist.net//smile.gif Thanks a lot.

02-29-2004, 06:10 AM
Geoff Edwards, Manager of Applied Resolutions is an authority on sterlings and is working with a number of aussi schools to develop lessons on the sterling, including construction.

Geoff's home page is www.appliedresolution.com.au (http://www.appliedresolution.com.au)


02-29-2004, 07:57 PM
I hope you don't mind me tacking this onto your thread, but I thought this was sort of interesting and kinda related in a roundabout sort of way. Seen them for sale in Home Depot... http://www.caframo.com/ecofans.htm

03-01-2004, 12:09 AM
<font face="Verdana, Arial" size="2">Originally posted by Arcane:
...I thought this was sort of interesting and kinda related in a roundabout sort of way.http://www.caframo.com/ecofans.htm</font>

Amusing but hopelessly inefficient.

03-01-2004, 03:12 AM
That is interesting but has nothing to do with Stirling technology. It is a thermo electric generator running a small electric motor. If I put that on my wood stove when it is stoked right up it would fry.

03-01-2004, 04:34 AM
<font face="Verdana, Arial" size="2">Originally posted by Arcane:
I hope you don't mind me tacking this onto your thread, but I thought this was sort of interesting and kinda related in a roundabout sort of way.</font>

See also http://www.exair.com/spotcooling_products/cg_page.htm

An exercise in thermodynamics...

03-01-2004, 04:17 PM
<font face="Verdana, Arial" size="2">Originally posted by Evan:
I am looking for a design that will produce about 1/2 to 1 HP from propane or firewood.</font>


Scale up ?

03-01-2004, 04:47 PM
I have a file from a Japanese government site that was doing research into energy recovery. They built a sealed unit that produced electrical power from wasted heat. If you are interested in it email me and I will send it to you it is about 1Mb compressed. It is called the Eco-boy, they spent 500 million developing this and a few other sterling engines. One of them is a rotary type sterling engine (no pistons) but my details are not complete as it was not complete at the time either.

03-01-2004, 05:04 PM

One of the problems with sterling engines is that they don't scale at all well. They must be designed for the intended size, not a simple project either.