David Morrow
Well-Known Member
- Joined
- Sep 8, 2008
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I'm just finishing a Moriya Stirling and, before moving on to an IC engine, I wanted to try another Stirling. I found a site where a 50% scale Moriya was built by Orrin B. Iseminger.
http://users.moscow.com/oiseming/lc_ant_p/pic_Prj8.htm
He gave a few dimensions on his web site for his scaled down bore and stroke. ( I use his piston size as the bore dimension although that is not strictly correct for the displacer as there is supposed to be a small air space around the outside. But that's close enough for this discussion ).
While out for a walk tonight, I realized that following a strict 50% scaling of the original bore & stroke would end up with a displacement figure well under 50% of the original. I think that you really need to determine the desired scaled down displacement and work backwards to arrive at the correctly scaled bore and stroke.
The original plan dimensions for the power cylinder are as follows :
Bore : 0.750"
Stoke : 1.000"
I used a formula for displacement of Pi x Radius squared x stroke =
In Excel I used a formula of 3.14159265 x ((.750 *.500)^2) x 1.000 = .4418 cubic inch.
Mr. Iseminger uses the following dimensions for his 50% scale engine :
Bore : .375" ( which is 50% x the original bore )
Stroke : .600" ( which is 60% x the original stroke )
That results in a displacement, using my formula, of 0.0663 cubic inches which is only 15% of the original displacement.
My question is, why would Mr Iseminger's engine run so well; he was most pleased with the result. Or, is my math way out of whack ?
If I start with a scaled down displacement of 0.2209" and keeping the same ratio of bore to stroke, I calculate that I will need the following :
Bore : 0.5950"
Stroke : 0.7933"
http://users.moscow.com/oiseming/lc_ant_p/pic_Prj8.htm
He gave a few dimensions on his web site for his scaled down bore and stroke. ( I use his piston size as the bore dimension although that is not strictly correct for the displacer as there is supposed to be a small air space around the outside. But that's close enough for this discussion ).
While out for a walk tonight, I realized that following a strict 50% scaling of the original bore & stroke would end up with a displacement figure well under 50% of the original. I think that you really need to determine the desired scaled down displacement and work backwards to arrive at the correctly scaled bore and stroke.
The original plan dimensions for the power cylinder are as follows :
Bore : 0.750"
Stoke : 1.000"
I used a formula for displacement of Pi x Radius squared x stroke =
In Excel I used a formula of 3.14159265 x ((.750 *.500)^2) x 1.000 = .4418 cubic inch.
Mr. Iseminger uses the following dimensions for his 50% scale engine :
Bore : .375" ( which is 50% x the original bore )
Stroke : .600" ( which is 60% x the original stroke )
That results in a displacement, using my formula, of 0.0663 cubic inches which is only 15% of the original displacement.
My question is, why would Mr Iseminger's engine run so well; he was most pleased with the result. Or, is my math way out of whack ?
If I start with a scaled down displacement of 0.2209" and keeping the same ratio of bore to stroke, I calculate that I will need the following :
Bore : 0.5950"
Stroke : 0.7933"