Wednesday, April 05, 2006

Stirling Engines - Hard to Make?

I've been spending a great deal of time lately trying to build a model engine for an upcoming magazine article. To do so, I've had to re-read some old, old college textbooks.

The great thing about mechanical engineering as opposed to say, electrical engineering, is that so a lot of what you learned in college goes stays relevant. For example, I had need to pull out my 30 year old thermodynamics text book and the stuff in it is still, while not cutting edge, correct and useful.

Stirling engines are "external combustion" engines and unlike internal combustion engines, run on something called a Stirling cycle instead of an Otto cycle (The differences in combustion cycles are typically explained by mapping out the system's pressure and volume versus time. It's complicated.)

Anyway, Stirlings are actually more efficient and emit much less pollution, and here's the big thing-- they run on anything that produces heat; from gasoline to hydrogen to cow dung to ethanol .

The picture above is of a solar powered Stirling developed at Scandia National Labs.There was some interest in automotive Stirlings back in the 70's when Ford and GM outfitted some midsize cars with them, but they never went anywhere.

Now, I'm thinking that with hybrid technology, automakers could marry an electric motor to a Stirling, something like the way a Prius works. With a Stirling engine, we'd have a machine that doesn't pollute and is not dependant on foreign oil. That's all good.

Friends and readers, I'm trying to come up with a model Stirling engine that I can build in the basement without the need for a lathe and milling machine. I've tried a couple models so far, but I haven't been able to make it work well at all. One was a small engine made from a soup can, balsa wood, and balloon (a "beta" type stirling that uses a displacer piston) ; and the other was more elaborate with two pistons and a regenerator (an alpha type.)

If any reader is familiar with no-machining Stirlings, I'd appreciate hearing from you!


spacenookie said...

See Avoiding Engineering Ratholes , by Don Lancaster, for a not-so-positive perspective on sterling cycle engines.

Norwood Matt said...

Beautiful (machined - sorry) Stirling engine kits can be had from Böhm Stirling-Technik;

Check the explanation of the Stirling cycle at Animated Engines.

Have you looked into this kit from American Stirling Company?

Fred said...

Just for reference it is Sandia national labs.

sirsaidrich said...

I have just finished a workin model of the stirling engine cycle. Required some machined parts though. Most of it was built from old vcr parts though. This is a beta design and is going to be displayed to science students at the elementry level. Maybe I'll post a picture of it on my blog. It took a little to get the vaccum and power stroke to work but I found that the closer they are in pressure the better the engine runs. Keep trying.

william dutton said...

would love to build one of these.

Terry said...

Unless you want your life to be taken over ...keep away from these engines!
I built my first one 6
months ago.I'm on to mark 4 now and i can think of little else.
Your wife ( if you have one ) will forbid the mention of 'stirling'within her hearing and former friends will think you have gone a bit strange.
Don't do it, be strong.

Denis said...

I have read the article in Make and tried to build it
By the way, in the stirling cycle explanation on the page 92 shown the wrong direction of rotation.

callumM said...

Hey I have to agree with Terry here. i spent the last 6 months developing a software simulator for stirling engines. I lost many days and nights... that regenerator is a tricky beast! (Never quite got a heat flow model to work).