Project for Beta Stirling Engine

[patrickkang]

Hi guys, I am Patrick. I fabricate out a Beta type Stirling engine but unfortunately it is failed to operate. Hope for some helps and answer.

Dimension of parts:
diameter and height of power piston = 30mm and 20mm (material aluminium 6061)
diameter and height of displacer = 28mm and 50mm (material aluminium 6061)
height and diameter of cylinder= 30.02mm and 93mm (material aluminium 6061)
stroke for piston and displacer = 15mm
phase angle at crank = 90 degree

Regards and thanks,
Patrick

[Ian S C]

Patrick, Hi , happy new year.
Starting with the piston, and cylinder. I don't know if you have machinery, such as a lathe, but I'll start with the cylinder, the bore should be round, parallel, and quite well polished. Now make the piston, when you finish turning it down it should just about fit in the cylinder, then polish it down so that it slides in, and if you hold the cylinder vertical, the piston slides down under it's own weight, and if you hold your hand over the end, the piston stops sliding, or moves very slowly, you then have it fitting just right.
Displacer, I'm not doing the maths for you, The ratio of the swept volume for a high temperature motor is 1.5(displacer): 1 (for the power piston), you will often see this ratio quoted, it's the one used by Robert Stirling on his air motors. The displacer should be(about), three times longer than its diameter, and a side clearance of any where between .5 mm and 1.5 mm will be ok, and say 1.5 mm each end.
The most important thing is to eliminate friction, and use good bearings, if you can find them, use small ball bearing races on the crankshaft, and if possible on the con rods.
Don't dispare, and good luck. Ian S C

[Leo123]

diameter and height of displacer = 28mm and 50mm (material aluminium 6061)
height and diameter of cylinder= 30.02mm and 93mm (material aluminium 6061)
stroke for piston and displacer = 15mm

[patrickkang]

Hi Ian. Happy New Year to you too. Can I have your email so that I can send you the picture of my stirling engine. Please pm me your email. Hope you can figure out the problem that I encountered based on my drawings and pictures. Thank you.

Patrick

[Ian S C]

It is unwise to make the displacer from aluminium, stainless steel is best, although mild steel will do, and the displacer cylinder should also be of stainless steel, mild steel can be used here too, but should be a little thicker than if it were stainless. With stainless steel, .010" is a usable thickness for cylinders up to at least 2"/50 mm diameter. When aluminium is used in this situation, apart from not being able to withstand enough heat, it conducts heat too fast from the hot to cold end(not good).
As for the power piston/cylinder, aluminium should not be run together, the two surfaces tend to try and weld themselves together, friction is very high, you might get away with that combination if the metal was anodised. Best metal for pistons is cast iron, and this is best used in a cast iron cylinder, but steel is nearly as good, and quite adequate.
Ian S C
Just look me up under Members(top right)

[patrickkang]

I think I am totally wrong in the beginning of the design because I thought that aluminium is the best material for cylinder as it is light and it can conduct heat faster to heat sink . Juz wan to make sure... the cylinder in Stirling engine is only for cooling function right? The other function of displacer is regenerator right? Besides that, I also measure the weight of the displacer and piston. I found out that the displacer is so much weightier than piston. Is this also the factor of malfunction of my stirling engine. Sorry for my bad English. By the way, I am from Malaysia. Nice to meet you!

[Ian S C]

The important thing in the displacer is isolating the hot end, from the cold (as much as practical), stainless steel does not conduct heat all that fast, if you run your motor, with the hot cap a dull red heat for a while, then disassemble the motor, you will find the displacer will be discoloured, blue at the top, and fades to brown, yellow, until about one third down there is a plain metal colour, this gives an accurate temperature reading of the displacer,(look up tempering colours for steel, unless you already know this). The displacer cylinder can be made in two halves, hot end, stainless steel/ or mild steel, cold end, you can make this end from aluminium, and either turn fins on the outside, or as I did on my last motor, form a water jacket around the aluminium cold end.
The power cylinder could well be made of aluminium, but with a cast iron liner inside it. A cast iron piston can be made very light, the skirt of the piston shown in my gallery has a wall thickness of 1 mm, and the crown is 3 mm thick, that is a 30 mm diameter piston. Ian S C