Troubleshooting a Moriya fan variant

[David Morrow]

I have built 3 Moriya type stirling fans now - a regular size, a 1/2 size and a double sized. I have had difficulty in getting all three to run even after having spent countless hours remaking and refining parts. I suspect that I am the author of my own misfortune though. Not being one to leave a design alone, the one fundamental change that I made that I can see possibly causing the problem is that the displacer and power cylinders in my variants are about 50% farther apart, center to center, than the original and I suspect that the connecting passage between the cylinders is too long.

Does anyone have an informed opinion on the problem ? No guessing please.

[adem9438]

Have another look at pressurised Stirling Engines, both sides of the power piston are pressurised,

[Ian S C]

adem 9438, ?????
David, The length of the connecting passage is not the problem, you must have altered the crankshaft, you have made it accurately. Friction is the biggest problem, followed by leaks. I'll give it a bit more of a think, and see if I can come up with anything . Ian S C

[David Morrow]

adem 9438, ?????
David, The length of the connecting passage is not the problem, you must have altered the crankshaft, you have made it accurately. Friction is the biggest problem, followed by leaks. I'll give it a bit more of a think, and see if I can come up with anything . Ian S C

I spent a lot of time on the crankshaft to make sure that it runs true and I have it running on some very nice bearings. When I spin the crank with the power piston out, it spins very easily and very quietly.

[Ian S C]

David, that's good. Now what sort of fit is the power piston, With the cylinder detached, and held vertical, and the piston entered in the top, and let go, it should drop through, now if you do the same again, but hold your hand over the end, it should stop, or move very slowly. If that test works, your piston / cylinder is OK. What is the piston and cylinder made of? Ian S C

[David Morrow]

David, that's good. Now what sort of fit is the power piston, With the cylinder detached, and held vertical, and the piston entered in the top, and let go, it should drop through, now if you do the same again, but hold your hand over the end, it should stop, or move very slowly. If that test works, your piston / cylinder is OK. What is the piston and cylinder made of? Ian S C

The cylinder is brass and the piston is graphite. I've done the test that you suggest and it is / was fine ; lots of compression and an easy slide fit when the bottom end wasn't sealed. I say "was" because in my quest to make it all work, I reduced the piston down in size to the point that I have to make a new one but that's easily done. I also spent a lot of time ensuring that the power cylinder was straight and parallel - no bell-mouthed ends.

Last night I disassembled the crankshaft and have been drastically cutting down the weight of the webs. They are aluminum but are a bit large and maybe too heavy. I have seen other stirlings with similarly sized crank webs that seemed to run just fine. But, I figured this wouldn't hurt and if anything, they were just adding to the flywheel effect that needed more power to get moving. Not sure if my theory is correct or not but I've seen still others with very small or no crank webs that also run fine. What got me thinking along this line of thought was a photo of a rather rough looking fan made in Pakistan; its crank had no webs at all and looked like nothing more than a piece of bent heavy wire. It sometimes feels like the more we try to refine these things, the tougher it is to get them to run.

[David Morrow]

IT'S ALIVE !

I moved the power cylinder a bit closer to the displacer cylinder, lightened the crankshaft and made a new power piston. I think the power piston was the problem.


[youtube]http://www.youtube.com/watch?v=lv205LoTTJk[/youtube]

[Ian S C]

David, that looks good. I imagine that your camera is like mine, and exaggerates the sound. the motor seems reasonably balanced, as it does not move around the table. You might get a bit less load on the bearings by shortening the shaft at the fan end. You may find that you can actually get it to work with out the fan, a number of my motors will work without a fly wheel, fast, but with little torque.
Just a query, what is the wheel shaped bit on the side of the displacer cylinder, just behind the fan?
Ian S C

[David Morrow]

David, that looks good. I imagine that your camera is like mine, and exaggerates the sound. the motor seems reasonably balanced, as it does not move around the table. You might get a bit less load on the bearings by shortening the shaft at the fan end. You may find that you can actually get it to work with out the fan, a number of my motors will work without a fly wheel, fast, but with little torque.
Just a query, what is the wheel shaped bit on the side of the displacer cylinder, just behind the fan?
Ian S C

The little brass wheel is the pulley attached to a cooling fan for the cold end. I had the room and wanted to add a little more whimsey and visual interest but also with the hope that it would be functional as well. Once you get to about the 1:25 minute mark in the video, you can see that I've attached it to the crankshaft so it actually functions. That's one of the details that I'll spend some time with getting the alignment better.

Shortening the shaft is also on my list. Lots of stuff like that to tighten up and refine.

[theropod2]

Sweet!

Nice work.

R

[Ian S C]

The stove top fan in my gallery is designed so that the airflow is back toward the motor. I have got a bit of a wire cage around the fan, although I don't think it is really needed. The blades on the fan are wooden, and light weight. See if you can lighten the fan, you might lessen the noise (that's if the noise is as loud as in the video). Ian S C