Jim Larsen:"I have always been drawn to hobbies that spark my curiosity, problem solving, and creativity. For the past several years I have been designing and building small Stirling engines. I was challenged by an engineer to see if it would be possible to design a Stirling engine that would run from the heat of a hand, yet be something that could be built without a machine shop for under $30..."
"I have always been drawn to hobbies that spark my curiosity, problem solving, and creativity. For the past several years I have been designing and building small Stirling engines. I was challenged by an engineer to see if it would be possible to design a Stirling engine that would run from the heat of a hand, yet be something that could be built without a machine shop for under $30. I spent the next several years researching and experimenting with different designs. I had a lot of fun working out problems with friction and momentum while keeping things as simple and low tech as possible. The end result is what you see in the first book, “Three LTD (*) Engines You Can Build Without a Machine Shop”". (* Low Temperature Differential)
I threw out the idea of a conventional design and decided to re-think the LTD engine. My goal was to reduce friction and pressure leaks so that the engine would be able to run on a very low temperature differential. This was my approach:
- I eliminated the friction of the displacer shaft by eliminating the shaft and using magnets to move the displacer.
- This also eliminated the pressure leak caused by the displacer gland.
- I added vents to the pressure chamber so I could occasionally vent the pressure, or even replace the working gas with helium when I wanted to.
- I eliminated the need to lift the displacer by changing the orientation of the pressure chamber so that the displacer rocked back and forth, rather than being lifted.
- I simplified the drive mechanism so that there were fewer moving parts. Fewer moving parts equates to less friction.
- I created a horizontal flywheel that rides on a point, like a spinning top.
The end result, the first engine was a reciprocating engine that would run from the heat of my hand when the room temperature was below 70 F. I was able to measure a 10 degree temperature differential when running with air, and 8 or 9 degrees when I filled the motor with helium.
An unintended side effect from this design is the fact that this engine will adjust itself to accommodate for changes in temperature differential as it runs. Low temperature differentials cause a short swing of the drive mechanism to move the displacer. As the differential increases the magnetic drive allows the swing to increase. The magnetic drive works like a spring and stores and releases energy with each cycle.
The second engine was a modification of the same design, this time with the addition of a rotating flywheel. Adding the flywheel brings a more traditional look and feel to the engine. It also means that the travel of the magnetic drive is now in a fixed motion path and must be manually adjusted to match the operation conditions. I built in lots of adjustment points and this is fairly easy to do after you learn what to look for.
The third engine was designed in an effort to build a low temperature differential engine with as few moving parts as possible. The only moving parts on this engine are the displacer, the diaphragm, and the flywheel. It uses a magnetic drive so there is no pushrod for the displacer. This design uses a horizontal drive axle, which causes more friction than the vertical axle. As a result, this engine requires a higher temperature differential to operate, and will only run from the heat of my hand with the addition of a little ice on the cool side.”