Peter Lynn ARCHIVE
320mm bore, 32mm stroke, 2.5 litre swept volume.
Layout: Concentric piston-displacer type (beta), 90 degree phase, no regenerator.
Overall size: 0.4m high, 0.35m diameter, 25kg.
Fuel: gas (ring burner).
Heating: by gas ring burner through 320mm dia. concave stainless steel cylinder end.
Cooling: air cooling by centrifugal fan through 2.5mm wall concave aluminium piston,.
Displacer spun thin-wall concave ended stainless steel, 100mm long, internally insulated.
Displacer and piston control by balanced counter rotating cranks.
Sealing: Two 3mm wide by 4.5mm deep cast iron piston rings with Si rubber expanders.
Flywheel: geared to crankshafts 4.8/1, recessed into the piston concavity.
The aim was to get more swept volume into a smaller overall engine and to test:
Whether a very high bore/stroke ratio is practical?
If using a geared-up flywheel is worth the extra complexity?
What advantage there is from perfect balance?
How effective heat transfer by 'squirting' air against the cylinder ends is?
Output from unpressurised closed cycle engines is very sensitive to friction losses and high bore/stroke ratios offer a significant reduction in ring friction (though leakage is proportionally higher). This is because seal friction losses are inversely proportional to bore for constant swept volume. A carbon/graphite packing backed with an RTV expander was tried but had high friction - sealing was good when hot, leaked when cold. The CI rings in this engine have never sealed satisfactorily without using expanders (high friction), even after prolonged running in. More accurate rings with greater depth and spring are required.
The geared flywheel is highly advantageous; more compact and less expensive, but also more effective. When sealing well, 13.1 will run slower than 11.3 which has a flywheel 10 times the weight. The flywheel gearing is noisy because of backlash in its straight cut gears. This could be cured by using a toothed belt for example.
The balanced counter rotating crank shafts layout is a success because it is compact and provides excellent displacer guiding. That it’s almost vibration free is a useful bonus.
Heat transfer by 'squirting' the internal air mass over the hot and cold end surfaces has not been successful. Heat transfer is marginal- 13.1 loses revs after starting.
13.1 needs better heat transfer, better sealing (diaphragm?), less friction and quietening.
PETER LYNN, ASHBURTON, NEW ZEALAND, to MAY 2025
