Stirlingengineforum.com

So I've been doing some research on Stirling engines and would like to make an attempt at building one. I some basic questions to get me started off. For my first build I would like to keep it somewhat simple. I am fairly skilled at machining but would like to use as many pre-existing components as possible. I'm looking at building a small (desktop size) horizontal alpha type. As it happens I have access to multiple sizes of pyrex test tubes and I have seen these used in a lot of engines. My first plan is to use a 25mm (dia) x 150mm (length) test tube as the hot side cylinder, and machining a brass piston. With that being said I was wondering what type of clearance values should I plan on using, and also what the relationship is between the hot side and cold side in terms of piston and cylinder sizing. Any thoughts, ideas, or criticisms would be greatly appreciated. I'd like to get a solid plan down for each of the cylinders and pistons and then go from there.

Hi. I m no expert, but have a look in this board the post says "recicled materials", the author uses glass cyringes for an alpha style engine. Would give you an idea to adapt dimensions to a slightly bigger. There are some articles that deal with scaling of designs, but i believe scaling of the order of you are describing should not be an issue. Dont know about brass-pyrex combination. Seems a lot of folks get good results with graphite-glass. I would check expansion ratios and weight to pick materials.
I wonder also since pyrex is cast (and not machined) do we know for certain it has a perfect inner surface?

I would not recommend an ALPHA motor for a first, a GAMMA type is simpler, not requiring two fitted pistons, one hot, and one cold. The GAMMA motor is the traditional Stirling type with a power piston, and a displacer, and is probably the most common of all the hot air engines.
Test tubes are rarely round, glass syringes are round, and make good cylinders. Myself, I use cast iron, or steel, with cast iron pistons, although one motor has a brass cylinder with an aluminium piston with a Teflon/PTFE skirt (no metal to metal contact). The ideal glass cylinder is one produced in the USA by Airpot, it has a Graphite piston that is fitted to it (they are for door closers), and if you live in USA you can E-Mail and get a sample. Test tubes are often used for the displacer, and displacer cylinder in the likes of a GAMMA motor.
The clearances on the piston is fairly critical, the dry piston should drop through the cylinder quite freely under its own weight, but stop when the end is blocked, and if you build the ALPHA motor you may have a bit of work getting the hot piston right, if the brass (or what ever you use) expands too much you will not get a second chance with a glass cylinder. I normally use stainless steel for the hot cap. Ian S C

Thanks guys. This is some good things to consider before I get going. Do you have any good info on sizing ratios between pistons?

Ok, so I found some info in other threads on here regarding sizing ratios, though any additional information would still be appreciated

With the traditional displacer type motor the ratio between the swept volume of the displacer to the power piston is 1.5 : 1. The next ratio is the length of the displacer, it should be about three times its diameter long.
If you go for the ALPHA motor, both pistons are the same size, and stroke, 1:1. On the hot piston is a cap called a Heylandt Crown, this part is built in a similar style to a displacer, best made of thin stainless steel, and as light as possible, this keeps the direct heat from the piston its self. Sorry I don't know the correct ratio diameter / hight, I made mine 1.5 X diameter high, 1:1 would be OK I think.
The fly wheel I find is best by experiment. Ian S C