Newbie making a woodburner stove stirling fan

[screamineagle]

Hi Ian, that is a nice fan and would love to see your fan running as well. I'm not sure how you got it to work at 1:1.5. When trying that ratio, I had to have heat in the 600 F range to self sustain. My wood stove just doesn't get that hot.

[Ian S C]

When I built that one, I built another one as a Ringbom, it is nearer 2 : 1 ratio, the power piston has the same stroke and bore, but it's a bit noisy(free displacer), and it's too powerfull, and it sits on the stove, and the whole motor rotates with the gyroscopic effect of the rotating fan, I must try some more blades on the fan to absorb some of the power, also make them lighter, as the blades on that fan are sheet steel. It may be that the larger displacer is giving it more power, although a Ringbom automatically adjusts the ratio to suit the load and temperature. No pics of that one.
Ian S C

[screamineagle]

Hi, I did more messing around with my Stirling fan last night while on the wood burner. (Yes, my wife thinks that I have lost my mind.) I let the stove heat up to reach 425F on the top surface and achieved 270 rpm, the top of the piston area stayed right around 215F. While it was a fun test, I can't let my stove get that hot cause with the damper wide open it generates way to much heat for the house. I'm actually thinking about a second design using the basic design I made and modifying it to combine 2 into 1. I am thinking of making a two cylinder version (2 displacers, 2 pistons) configured into an inverted "V" with the displacers hitting TDC on opposite ends of the rotation. I would do this on the same crankshaft then geared up to spin a 16"-18" fan blade. I would make the gearing adjustable to optimize the speed to spin the fan. My goal would be to spin a larger blade at 500 rpm. I would think that a two cylinder version with a common crankshaft would wore more efficiently and smoother than increasing 1 piston/displacer several sizes up to spin a blade that big at a decent rpm. Has anyone ever tried this? I am open to comments and suggestions.

Ian, did you happen to find any video of your fan running? I am curious of the RPM you obtained and at what temperatures.

[Ian S C]

Sorry no vidios, and I have no thermometer, but max revs is about 500rpm, and best is about 400rpm, if the revs get too high the noise goes up, and the idea is silence, so it's a balance between the diameter , the number of blades, and the pitch. For just a casual indication I hang a bit of thread in the doorway across the room, and get an initial indication of the thrust by how far it gets blown.
The blades on that fan are made for easy pitch adjustment. The brown bit on the RH blade is were it got a bit over heated while standing stationary for an hour or three.
Ian S C

[PlaniTechic]

Hi Dave

I would go with the more Cylinder aproach too. Because if you upsize your engine that means the surfaces grow square to the size and the volumes to the 3rd power. So if you double the size of your engine you will have four times the surface and eight times the volumes and that means you're building a new engine and all the trouble starts over. Since you have still plenty of free space on your stove, I would suggest to just build another engine (or two ) like the one you already have and couple them together. The variable gearing is a good idea too, so you can find the "sweet spot" driving the fan blade at the given temperature.
A three cylinder inline with the axis of the fan at 90° to the crankshaft would be really cool

Plani

[screamineagle]

Hi, I have a question that I am thinking about and wondering if anyone know the answer before I actually make another fan for a test. So, the ratios that I use, 19.73:1, seems to work the best for me at my temperature range for the most amount of speed to the 10" fan blade. Keeping my piston volume and stroke the same, (.785 and 1" stroke) what if I changed my displacer stroke and matched my current volume? In other words, right now the displacer volume is 15.488 using a 1.5 inch stroke. What if I increased the displacer width to 4.4" while decreasing the height to maintain the 2/3 displacer to cylinder ratio and shortened the stroke to 1" keeping the same volume? I'm sure it would still work but my question is this. Do you think the overall speed will increase or decrease? I see many factors that will effect the outcome. The pros I believe would be a lighter displacer, a shorter stroke, less rotational mass. The cons I believe would be the top of the displacer would be physically closer to the woodburner making the heat differential much closer from top to bottom, a surface area that increases absorbing much more heat. This is the question I want to answer before designing the inverted V twin stirling fan.

Plani, Can you do a quick sketch on what a three cylinder inline with the axis of the fan at 90° to the crankshaft would look like? I can't picture it. If it looks pretty cool, I may opt for that over the inverted v twin.

BTW, this is very addicting once you have a little success. Of course being a guy, I want my next one bigger, fast, stronger, etc. Want to create hurricane force winds from the woodburner. lol

[PlaniTechic]

Hi Dave

I tried to make a sketch of what I was thinking when suggesting the triple cylinder Engine:

Sketch III.jpg (74.42 KiB) Viewed 5661 times

I hope you can get the idea. Just put three of your engines in a line. And since you mentioned an adjustable gear to drive the Fan, I was thinking why not put the Fan at 90° to the engines. So all three of them could get some cooling from the air which is drawn in by the Fan. How you devise the whole gearing (maybe some sort of belt and pulleys?) between the two shafts is up to you

If you make the displacer bigger in diameter and shorten its stroke, I would expect the engine to run somewhat faster. Shorter stroke makes for faster rpm in general and you also get some more surface on the hot and the cold end. Just make sure that there is enough cooling on the cold end and adjust the displacer so that it gets as close to top and bottom as possible without actually touching anywhere.
But as you already figured out, stirling engines are somewhat tricky to predict. So if you really want to know what happens you'll have to build one and try it out

Plani

[screamineagle]

Plani, that is awesome!!!! This may be far easier than my v twin thinking. I think I am going to try the shorter stroke experiment first to see which is faster. I wish I had some way of measuring power output like fractional horsepower.

[Ian S C]

You will virtually have to build three engines, ie three displacers, three power cylinders, it can be on one crankshaft, and if each displacer, and power cylinder are mounted as a V, only three cranks will be required. Well constructed, this could be a quite compact little power unit.
Ian S C

[PlaniTechic]

Hi Dave

I think I am going to try the shorter stroke experiment first to see which is faster.

Do that please, and let us know what you find out.

For power measurement you could devise some sort of Prony brake. Google will tell you how that one works... But you could also just stick your fan blade to the other engine and measure its speed. That will give you some indication of the improvement too. Just be aware that the power consumption of a fan also increases with the 3rd power to its speed. So if you want to double the rpm you will need 8 times the power at the shaft.

Stirling fans have been built many, but a three (or more) cylinder unit would be something new

Plani