Stirling Engines

I first discovered the Stirling Engine at a summer crafts and historical fair here in New England. Off to one side was a collection of machinists who were displaying the products of their labor. Among the expected single-cylinder hit-and-miss engines was a low temperature differential (LTD) Stirling engine that was running off of the heat of the sun. This particular engine was sitting on an ice cube, which I later learned was not such an amazing achievement. I've since purchased an engine that will run off the neat of the sun, or off the heat of your hand.

Seeing an engine run, no matter how slowly, off of the heat of your hand seems so close to magic that I was fascinated. In the end, I decided to try to design and build my own. The original fantasy was that I would be able to put my engine on my wood stove, in the winter, and have it do real work "for free", since the wood stove would have to be running in any event. In the end, I discovered that building such a powerful engine would be very difficult, if not impossible, but by then, I had learned quite a lot.

I started out by writing some Stirling Engine simulators, to better understand what was actually going on. You can find those simulators on my software web page.

Stirling engines were invented in the 1800s by an English minister, Robert Stirling. This was in the days before the internal combustion (gasoline) engine, before the diesel engine, even before the steam engine. Stirling engines are external combustion engines, where a heat source is applied to one side of the engine, while the other side is cooled, either with air or with a water jacket.

The principle of the Stirling engine is fairly simple. Consider a sealed coffee can in which there is a sponge that takes up about 2/3 of the height of the can, and which fits loosely in the can horizontally (i.e., it is not sealed, like a piston). Imagine a candle under the can, heating the bottom, and a piece of ice on the top, cooling the top.

When the sponge is on the bottom of the can, the air rushes up to the top, where the ice cools the air, and the pressure in the engine decreases. If the sponge is shoved up to the top of the can, the air rushes down to the bottom, where it is heated by the candle, and the pressure in the engine increases. Thus, if you can make the sponge move up and down, the pressure in the engine will increase and decrease. If you attach a piston to the engine, the piston will move out and in. You can then use the piston to both move the sponge and get power out of the engine.

I started out creating an engine from scratch. It never worked, but the pictures are interesting.

Luckily, I then stumbled across a local guy, Ralph Lemnah, who had worked as a machinist at IBM. Ralph's energy, creativity, and experience were key in allowing me to acquire the right tools and learn how to use them.

The first engine we tried to make was made using a PVC pipe: we called it Ugly One.

Our second engine actually worked! It is a somewhat unusual and counter-intuitive design, called a "ringbom" engine. Changes in the internal pressure of the engine not only drive the power piston, but also move a piston that moves the displacer.

My first working Stirling engine, a ringbom designed like Coleman One.

My second working Stirling engine, a “gamma” design using Coleman One concepts.

Please see my Software pages for various free Stirling Engine simulation programs.

I just discovered a web site that is full of novel Stirling engines, both as kits and fully assembled. Check them out if you are interested!