Over the years at PRIME and GEARS, I kept seeing a cute little Stirling engine-powered
fan. It never failed to amaze me. Its minuscule 3/8-inch diameter piston
drove a five-inch fan that spun like gangbusters. Its ancestry left no doubt;
it was patterned after
Dr. James Senft's Moriya.
Dick Remington built the cute little gem. Figuring it needed a name, he and Dr. Senft put their heads together and decided to call it the Pre-Moriya.
Then, in 2005 at the Arizona Flywheelers show at Cottonwood our paths crossed, again. Dick was there with his extraordinary exhibit of hot air-powered fans, as well as his Pre-Moryia.
That did it. I decided right then and there I was going to have to build one, too; so, I did. Here are some pictures of it.
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After machining the major components, it came time to put them together
for a trial fit. So far, so good.
It runs! On the very first try the little rascal took off like a
scared jack-rabbit. The flame won't stay lit outside in the breeze,
so that is why it is not running, here.
At this stage of the game it was a good time to try out the optical tachometer that son Tad gave me for Christmas. Mini-Mo revs over 1,100 RPM with the fan as a load! Wow! The little critter really gets with the program!
Of course, it helps to have rolling contact bearings on the crankshaft and on the big end of the power piston connecting rod. They were salvaged from junked electronic equipment.
About half of the materials were salvaged. The aluminum for the finned cold section was a remnant from the 2X Scale Moriya . The blades were made out of scraps of brass discarded from a trophy shop.
With the fan guard finished, the job is nearly done. All it needs,now,
is a chimney.
This close-up shot shows one major departure from Dr. Senft's design.
The crankshaft throws are disks instead of square stock. It was a
practical consideration. It was far easier to get squared up on a
disk than on a tiny piece of key stock.
The fan "spider," however, is vintage Moriya.
The finishing touch: A chimney for the alcohol burner.
I borrowed a page from Dick Remington's notebook. The glass started out as a 25-mm borosilicate glass test tube.
Hint: If you want to try this, forget about using a glass cutter. I clamped the handpiece of a Dremel tool flex shaft in my quick-change boring bar holder and used an abrasive cut-off wheel to cut the glass.
For this quick-and-dirty job I grabbed the first thing that was handy to keep silica dust out of the air (and my lungs): kerosene. Applied with a 10¢ acid brush, it worked just fine.
Where is the wick, you ask? When not in use I keep it capped with this neat little reject from an ammunition factory.
If hot air engines interest you, here is a bit more information about
Blade diameter, 5.0"
Power piston diameter, 0.375"
Displacer diameter, 0.5"
Power piston stroke, 0.600"
Displacer stroke, 0.600"
Overall height, 8-3/4"
It runs quietly and smoothly. It has a critical RPM where it shakes
quite a bit, but it is at a very narrow band far below operating speed.
It's barely noticeable when it starts up; more so as it is coasting
down when the fire is put out. At speed, it is very smooth and
virtually silent (if resting on a mouse pad).
Basically, I took all Dr. Senft's dimensions for his Moriya and multiplied them by 0.5. Of course, there are many exceptions. For instance, I stretched out the height of the cold section to add a few fins; and, I rounded up the fin width to an even 1.0-inch. Half-scale would have been 0.875" and the center hole would have been 0.4375" from the edge. It was a whole lot simpler to use 1.0 and 0.5. :-)
The design is found in Steam and Stirling Engines, Volume 1. It first appeared in Live Steam magazine back in the '70s, give or take.
The dimensions of Jerry Howell's mini-fan are very close to those of the half-scale Moriya, so I borrowed bits and pieces from his drawings. Some of Dr. Senft's things didn't scale down very well, such as the crank throws and the big-end bearing for the displacer connecting rod.
The burner is just a piece of copper pipe with caps turned out of a scrap of bronze (salvaged from an Eaton 2-speed rear end). I used a short length of 3/16" brass hobby store tubing as a wick holder. The wick started out as quarter-inch, but it made too much flame, so I pulled half the yarn out of it. When stretched out, lengthwise, the outer weave easily adjusts to the smaller size.
The down-sized wick almost makes more fire than I want. Someday, perhaps, I'll order some 3/16" wick and pull some of the yarn out of it.
With the current wick it exceeds 1,100 RPM with the fan acting as the load. It produces a surprisingly strong breeze. I didn't measure the speed with the quarter-inch wick.
That much reciprocating motion creates a noticeable buzz when it's resting on a hard surface, so that's why I run it on a mouse pad. Then, it is virtually silent.
One of Mini-Mo's biggest surprises comes after extinguishing the burner. On most occasions it will continue to run on the residual heat in the hot cap for another one minute and twenty seconds!
As I finish each one of my projects I say, "This one is my favorite." The Mini-Mo, however, will not get knocked off this pedestal quite so easily.
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