LSM 13 Unpressurised Stirling Cycle Air Engine
320mm bore, 32mm stroke, 2,57litre swept volume,
Overall size; 0.4m high, 0.35m diameter, 25kg.
Layout; Concentric piston-displacer type (beta), 90 degree phase angle.
Fuel; gas (ring burner).
Heating; Through the 320mm dia, 2mm wall concave stainless steel end cap.
Cooling; through the 2.5mm wall concave aluminium piston, air cooling by centrifugal fan.
Displacer; Spun thin-wall concave ended stainless steel, 100mm long, internally insulated.
Regenerator; None (not relevant to what this engine was built to test).
Displacer and piston Control by balanced counter rotating cranks.
Sealing; 2/3mm wide by 4.5mm deep cast iron piston rings, expander backed.
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 also 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 leakages become proportionally higher). This is because seal friction losses are proportional to bore X's stroke whereas swept volume is proportional to bore squared X's stroke-. LSM 13 shows conclusively that bore/stroke of 10/1 is practical, even with conventional rings.
The geared flywheel is highly advantageous; more compact and less expensive, but also more effective. LSM 12 will run slower than LSM11 which has a flywheel 10 times the weight, and 20 the size ( by volume). The flywheel gearing for LSM13 is noisy because of backlash in it's straight cut gears, but this appears to be solvable in future (toothed belt?).
The balanced counter rotating crank shafts layout is an undoubted success also; much more compact and almost vibration free.
*Heat transfer by 'squirting' the internal air mass over the hot end cap and the convex aluminium piston has NOT been successful. There are various signs of poor heat transfer but the definitive one is that, for the same swept volume, LSM 13 develops only 1/3rd the power of LSM 12, and only 1/4 of LSM 14's.