Piston weight.

[sjdurgule]

I am thinking to make cast iron piston for beta kinematic engine of following specifications. If any suggestions / corrections please guide -

1. OD- 35 mm
2. Length - 35 mm
3. Displacer Rod Dia - 5mm
4. There will be no any seals.

Is this heavy piston will work for beta engine ? What will be the weight of the flywheel required ?

Satyavijay.

[Ian S C]

The piston does not need to be heavy, hollow out the piston so that the crown is about 3 mm thick, and the skirt about 1 mm thick, The 31 mm piston I have here at the moment weighs 30.7 g, but it has a thick crown, about 4 mm, the whole thing, con rod bearing gudgeon and pin, and a leathercup seal is 54.4 g because the con rod is a bit of 1/4" mild steel plate, I usually use aluminium, and this weight is proberbly why this engine jumps about a bit when it gets up speed, its a Ringbom stove top fan engine, and is too powerful for the job. If you look in my gallery you'll see a similar piston assembly, a little lighter, this is also in a stove top fan of a GAMMA type. The bush through the piston should not make much difference to the weight.
Just as a contrast the piston in my little beam engine weighs 2.4 g, and with the con rod attached 4.3 g, it has a diameter of 3/8", and is 1/2" long, the skirt is .5 mm thick. Ian S C

[sjdurgule]

Thanks Ian. I appreciate your helping nature to all.
Sir, what will be the appropriate length of piston for 35 mm OD (without seals) ?

Satyaviajy.

[Ian S C]

For a horizontal piston I would suggest 25 mm to 30 mm, and a vertical piston 15 to 20 mm, its got to be long enough to prevent rocking, but short enough to reduce the amount of friction, an reducing all points of friction is most important. The short piston is quite ok what ever possition the engine is, and mine are evolving over time.
I'm making a new piston because I thought I would lighten the one above, I got it down to 25.4 g, then I broke it. The biggest change will be when I lighten the con rod, and gudgeon block. Ian S C

[sjdurgule]

Thank Ian, for your detail information.

Satyavijay.

[Hopper]

Or you could try making a slipper piston, that is one that the piston skirt is cut away on the sides below the gudgeon pin but left intact on the load bearing front and back surfaces.

[Ian S C]

Your getting a bit fancy Hopper, I made a replacement piston today, it weighs 26.4 g, might try taking .5 mm off the crown tomorrow, and get down to 26 g, its ok with good fine grained cast iron going to thin piston skirts, but I'm using window weights, and they have some good iron, and patches of rather rough stuff. The piston on a atmospheric Stirling Engine does not have to be super strong, its not working with high pressures unlike a steam or IC engine, the idea is to get the parts as light as possible, I'll have a go at the con rod next. Ian S C

[sjdurgule]

Can we use aluminium (with cast iron cylinder) to reduce the weight of the piston ?

-Satyaviajy.

[Ian S C]

Aluminium is not the best material to use in a slideing bearing surface, it haas high friction, and can scuff against other metals, in a IC engine special alloys are used, but more important cast iron piston rings take the wear, plus the fact that there is pleanty of oil over everything in the engine. Ian S C

[Ian S C]

I reduced the con rod, and gudgion block from 54.4g to 30.6g, reassembled the motor, and sat it on the stove, it still bounces around, maybe not as much, the fan could do with balancing, but I think the lighter piston has helped, and the revs are up a bit, and will increase with use. Ian S C

[Ian S C]

There is one better than my light weight cast iron pistons, and that is graphite, these pistons heed an extreemly highly polished bore, and very accurate,ie parallel, and round, you may have read of the graphite pistons and glass cylinders, they are designed for closing doors, and if you live in USA you can get a free sample from the company that sells them. Ian S C