Newbie making a woodburner stove stirling fan

[screamineagle]

Hi, this is my first post so I hope I don't get beat on too bad here. I have build 2 stove top stirling fans and neither one works very well. I am trying but am having issues. I would like to know if you all would be willing to answer a few questions so I can compare the dimension of what I have built to what you made. Can you tell me the:
piston length and width? My piston size is 1" wide by 1" high
displacer housing tube length and width? Mine is 2-1/16" by 4" steel tube
displacer length and width? Mine is 2-1/2"l by 1-7/8"w
total piston stroke length? Mine is 1"
total displacer stroke length? Mine is 1-1/2"
I know that the crank stroke will be 1/2 of the total stroke so I can figure that out. I am using fire resistant foam as the displacer to keep it light, I was wondering what you all used? Also I have made mine using all of the space with the displacer, should I be leaving a gap?
I thank you in advance for your time and any info you are willing to share,

Thank you,
Dave

[Ian S C]

With the design of crankshaft that you have the displacer should be 1" dia. The displacer should have a gap around it, 1/16" is a good start there. I use a hollow stainless steel displacer where possible, if not, I use steel, you want a wall thickness of .010" + or -, and it must be sealed to the inside of the displacer cylinder. A 1" dia displacer should be about 3" long. The hot cap should be stainless steel, but mild steel is ok, I'v got a stove top fan that uses a bit of chromed plated steel tube that onceapon a time was part of the hose system of a domestic vacuum cleaner for the hot end.
Ian S C ps Wellcome aboard

[screamineagle]

Hi. I guess I should have mentioned this part. I modeled the stove fan I was making after the Vulcan fan design and I like the way it looks and sits on the woodburner very nicely. I would like to keep my displacer housing size at the 2 1/16. While I really appreciate the advice on changing the displacer size I would much rather change out the crankshaft stroke length to whatever you guys think I should do so I can keep the design look. Sorry about that.

Dave

[Ian S C]

I remember seeing plans for a fan motor on the web, it used the glass from a Coleman gas lantern for the displacer chamber, must look it up again one day.

[screamineagle]

I have been reading a lot on displacer size and stroke on this site. Do you think that increasing the displacer length to 3" and cutting down the stroke to 1" on the displacer piston would make a difference? I guess what I am asking is what can happen if the is more than a 1-1.5 ratio between the volume of piston sweep to displacer sweep? The top of my woodburner is generally 300F, as high as 350F once in awhile.

[Ian S C]

At that temp a larger ratio would be ok I think, something like 1:2, or even 1:3, and some examples on the net are quite low profile, ie a short and fat displacer.
Ian S C

[screamineagle]

Finally!!!!! I got it to work right. Thank you for all the help.

You tube video
https://www.youtube.com/watch?v=Ycrq6UTtkN8

[screamineagle]

The piston to displacer volume ratio of 1:2 to 1:5 was driving me insane trying to make this application work correctly. Only when I made the displacer volume way bigger did I finally get this working in the right direction. The ratio I came up with is around 1:16 that worked in my heat range. The displacer is balsa wood with 4 1" holes drilled through packed with stainless steel wool. Hope this helps to anyone making one of these.

[screamineagle]

Made a few minor tweaks and sanded the piston ever so slightly to reduce friction. Now turning over 240 rpm

You tube video
https://www.youtube.com/watch?v=VpGtGc3ojSQ

[PlaniTechic]

Hi Dave

Congratulations to this great running engine
As I found, it's always a struggle to get everything just right but then it's very rewarding once the engine is running.
How did you seal the glass to the rest of the cylinder, especially on the hot side?


Plani

[screamineagle]

Hi, I used a quart paint can and cut it down to leave it 1" in height. This left a 1/8" gap all around the glass, which was a wine bottle that I cut, and filled that gap all the way down to the bottom with red RTV. I figured that would provide a whopper of a seal and allow flexibility for expansion/contraction as well as holding the heat at the bottom.

[screamineagle]

BTW, this is actually blowing a pretty good amount of air at 240 rpm with a 30 degree pitch 10" fan blade. Also the temperature at the base was 320F and the top of the glass around the piston was 140F so the cooling fins I made and the stainless steel wool in the displacer are really helping make a big temperature difference. The piston to displacer volume ratio is actually 19.73:1, I posted the wrong number earlier, sorry about that. It worked at 16:1 but works much better at the one I am using now.

[Ian S C]

Here's my stove top fan. Just another way of doing it, this has the ? normal ratio of 1.5 : 1, and the displacer dia X 3 in length. Skate board bearings in the main bearings, and the big end of the power con rod. Ian S C

[PlaniTechic]

Dave,
There's another question that I was wondering about: Did you ever run the engine before you installed the steel wool regenerators? I see that this is the way it's done with this type of engine, but I'm somewhat curious if it really helps or if the benefit of the regenerator is eaten away by the added dead volume
So, if you are willing to experiment, I would suggest to cover up the 1" holes on both sides (maybe with masking tape or the like) and run the engine again. It will then be obvious which design is better...

Ian,
That,s a very nice Fan you made. Do you happen to have a video of it running?


Plani

[screamineagle]

Hi, great question and glad you asked. The displacer as I mentioned is balsa wood. I ran it without the holes filled with stainless steel wool (solid balsa) initially and only managed about 60-70 rpm. Where it makes a difference is the temperature between the base and the top piston area. I only had a differential if about 80 degrees F. With the stainless steel added, the differential more than doubled and so did the rpm.