Phoelix

  'Rose from the ashes and keeps on walking'

  When the, low friction, loaded PTFE materials first became available to amateurs it was decided to try them out for the piston of a small hot air engine.  The material I had was a very small piece of 'Rulon', a plastic with both a lower coefficient of expansion (c of e) and lower friction than ordinary PTFE, and this was made into a 'cup washer' seal - similar to a leather cycle pump washer.
   The rest of the piston was made of unloaded PTFE and to overcome its high c of e the skirt was split and bound with brass wire before turning it to fit the cylinder.
   The cylinder was drawn brass tube and to assess the durability of this unlikely combination of materials it was decided to give it an extended life test.  So the remainder of the engine is conventional and kept as simple as possible with the exception of the electric heater which was wound on the hot end of the air chamber to permit long periods of unattended running and the mechanically operated snifter which can be seen just above the displacer rod bush.
   To provide cross-head loading on the piston the engine needed to drive a load and several small electric motors were tried as dynamos.  Unfortunately these wore out very quickly and were replaced by a cycle dyno' and this was loaded by a 3.5v  0.3a bulb.
   Testing could now begin and, following a disastrous melt down due to the wrong tapping on an oversized transformer and requiring a complete rebuild of the air chamber assembly, the engine was left running for long periods.
   One very noticeable effect during this time was the affect that ambient temperature had on performance.  Much of the running was during the Summer months and in the afternoons the speed dropped to reduce the voltage on the lamp to 2.5v.  but by 1 AM the following morning it could be seen at full brilliance, lighting up my workshop at the bottom of the garden.
   Many bulbs (and one dynamo) were replaced before I decided to figure out the cost of running the electric heater - and the testing stopped abruptly!
   The engine had logged a total of nearly 7000 hours, sometimes running for several weeks non-stop.   All bearings were completely worn out (the big end had been adjusted several times) but the piston seal was still 100% and, apart from the dreadful racket, it ran (and still runs) as well as ever.

  Note:-  PTFE = polytetrafluoroethylene = Teflon (?)

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