Piston sealing

[sjdurgule]

Can we give teflon coat on piston for sealing purpose ? and whether it will work in 300-400 degree celcius ?

[Ian S C]

There is a teflon coating that can be put on metal used as a piston. Its called Xylan And it is in the form of a paint, and requires to be baked on. Using this would allow the use of aluminium for a piston.
The cold end of a Stirling Engine where the power piston is should be no were near 300to400*C, If its water cooled, it should not excede the boiling point of water by too much, if the cooling system is effective, it should be less than this. A cheaper solution is to use a cast iron piston in either a cast iron cylinder, or a steel cylinder. The ideal one for a model with quite a bit of power is a glass cylinder, with a graphite piston, in USA it is possible to contact the manufacturer, state which size, and get a free sample. Ian S C

[sjdurgule]

Thanks Ian for reply. I am preparing cast iron piston (now it is in workshop for turning) in mild steel cylinder as per your comments / remarks on this forum. But I want to know how much microns it should be less than cylinder ID & if I press the piston in cylinder (keeping other end of cylinder closed) how much time/seconds it should hold the inside air.

Thanks.

[Ian S C]

First make the cylinder, and lap the bore until it is like a mirror, then make the piston (its easier to get the piston to fit the bore than the other way round), the piston should slide through the cylinder under its own weight, and stop if you block off one end, and only move very slowly. Theres no exact measurement, and it will be different depending on the diameter, but its proberbly around .0005", but just polish the last wee bit, I use 1200 grit wet and dry paper, and for the cylinder I use brass polish, then toothpaste. Do the slide test with the piston/ cylinder dry, and cleaned of abrasive.
An example of a cast iron piston may be seen in my album, I should have photo'ed it up the other way, the skirt is 1 mm thick, and the crown (where the 3/8" UNF thread is) is 3 mm thick, the diameter is 35 mm. That is the piston from the stove top fan. Ian S C

[sjdurgule]

Thank you very much, Ian. Now I will prepare the cylinder (MS) and piston (CI) and I am thinking to grind the cylinder and piston. Then I will take sliding test and put the results on this forum.

[sjdurgule]

As per Ian I have prepared the MS cylinder of ID 25mm & one CI piston & one MS piston. Both cylinder and piston are grinded. Piston OD is kept less by 4 to 5 micron. CI piston is smooth as told by Ian. Pistons are free in the cylinder & if I press the the piston it resists and bounces back but if I kept pressing then it goes inside slowly. Then I carried out one test- I heated the one side of cylinder on kitchen gas with piston inside. I was expecting the piston would come out at least little distance but failed. Now I am very much anxious about success of my free piston stirling engine. If anybody have build the free piston stirling engine please give me guide lines for making it.

Thanks.

[Ian S C]

You did'nt say you were building a free piston engine, that alters things a little, the piston you have sounds very good. With a free piston engine you need the cylinder vertical, with the piston hanging down, and balanced on a spring(don't ask what size, thats different for each engine), or you can do as I have, use two magnets (ex microwave oven), with the like poles facing each other. Attach the spring/magnet to an extension attached to the piston, don't worry about weight, you may need a bit more. Next, the displacer, it works on a similar principal to a Ringbom motor, my design has the heated end at the top, and the displacer rod coming out the bottom, and again balanced on a spring, it may work up the other way, but the heating system would get in the way of the linear alternator under the power cylinder, its quite possible that a pair of magnets would work here too. These engines are more difficult to get going that a conventional one with a crankshaft, and flywheel. I got mine going, and was talking to Dr Don Clucas from the Whispertec Company, who designed and build the WhisperGen co-generation system, he said that in all his years of builing Stirling Engines he had never got one to go.
If you go the magnetic way, most of the motor MUST be built of non-magnetic materials, a castiron piston should run in a bronze, or non-magnetic stainless steel cylinder, or the motor will not move at all! Ian S C

[sjdurgule]

Thanks for your suggessions. I am thinking to use mechanical spring (for easy to adjust) for piston and gas spring for displacer. If I give more heat (500-550*c) then whether it is possible to run FPSE horizontally ?

[Ian S C]

Don't know about running side ways, there is more friction that way, and you don't want that. Ian S C

[sjdurgule]

Thanks Ian. After so many changes my free piston engine is physically ready and by pressing it's piston, displacer moves accordingly. I have used magnets for spring action. The engine is vertical and heater is at top and hence it is difficult to heat it. I tried with press iron to heat but it is not working. I think heat input is less than required.
How much minimum heat is to be requirted to work free piston engine ?

-Satyavijay.