Peter Lynn ARCHIVE
Combined piston/displacer ‘Adiabatic” type.
Displacer diameter 203mm, length change 76mm, 2.46 litre swept volume.
Like 16.3, 20.1 is fundamentally different to Stirling engines of the alpha, beta or gamma layouts. Instead of using two pistons (alpha) or a piston and a displacer (beta and gamma), with crank angles at around 90 degrees to each other, they use a displacer that changes length, in a cylinder closed off at both ends. This variable volume displacer contains air that is compressed and expanded adiabatically (a reversible zero loss cycle) each stroke. In practice, depending on revs and how effectively the internal space is insulated, there will be some losses in this repeated expansion and compression, but it’s tolerable. After 30minutes reciprocating 20.1’s displacer outside the cylinder, there is minimal temperature increase (by touch).
A disadvantage of this layout in its simplest form is that the displacer seal is subject to higher temperatures than if running on an externally cooled surface.
Advantages of the adiabatic displacer system are that it’s simpler (no piston), more compact, easier to pressurise, excludes dust, has less dead space, and shorter flow paths. It also has a higher nominal compression ratio than either beta or gamma layouts.
A successor to 16.3, 20.1 incorporates various improvements:
*The displacer has a tent shaped upper surface to distribute oil evenly to the rings.
*The two piston rings are in the top part of the displacer running in a cylinder attached to the lower part, the reverse of 16-3. This is to improve cooling (does it though?), minimise dead space, and to reduce the amount of lubricating oil making its way to the hot end. It has the disadvantage of making the lower part of the displacer heavier (7kg).
*Water jackets are constructed to avoid the need for a cooling water pump (16.3 had one). To this end, for assembly there’s a joint at the top of the regenerator section.
*For better air transfer, the lower part of the displacer has a longer stroke than the upper and the average phase angle between them is 78 degrees (58 degrees for 16.3).
20.1 took 251 hours to make and another month for development but still has thermal distortion in the piston/displacer, which also plagued 16.3. It runs great for a few minutes then slows and stops until it cools down; then it’s off again. The cylinder goes out of round by 3 or 4 mm causing the rings to lose contact over part of their circumference and leak badly. I’m looking for a solution to this.
A minor challenge has been to develop a ring burner of the barbecue type that provides adequate heating. LSM series engines that have useful power (12.3 in a mobility scooter and 14.1 in a launch) have venturi type tube burners using a gas jet at bottle pressure to induct air. These easily get the hot ends to bright yellow and are wind resistant, but very noisy. 20.1’s ring burner is not yet providing equivalent heating- but is very quiet.
PETER LYNN, ASHBURTON, NEW ZEALAND, to May 2025
