LSM 14 STIRLING CYCLE AIR ENGINE.
Unpressurised, 2.5litre, beta layout (piston and displacer in the same cylinder).
The 4th engine in a development series aimed at achieving 1 kw from a simple, compact, unpressurised air engine. Earlier prototypes: LSM11/2007, LSM12/2010, LSM13/2011.
For all Stirling cycle engines, heat transfer is a key design parameter.
Tube heat exchangers were used for LSM 11, but these restricted gas flow, and required a secondary seal for the displacer- net negatives for unpressurised engines of this size. Annular clearance heat exchangers worked well in LSM 12, while for LSM13, directing gas flow over hot and cold end discs was tried- but found to be marginal. LSM 14 therefore uses the simple LSM 12 style annular clearances. LSM's displacer is 7mm smaller in diameter than the cylinder.
For unpressurised Stirling cycle air engines (like LSM 14), that can only achieve modest specific output (power for size), eliminating as much friction as possible is also a dominant criteria.
A challenge with this series of engines has been to keep the actuating mechanism compact while ensuring that the displacer remains central in the hot end heat exchanger as it reciprocates. Because the hot (lower) end and the displacer skirt are heated to above 450 degrees Celsius, all mechanical guides must be in the upper (cold) zone. LSM 14's displacer is constrained axially by a sliding yoke on the top end of the displacer rod and the displacer rod's fit in the piston. Although the displacer skirt is 450mm below the piston when fully extended, it stays within the allowable +/-2mm (just!), even while subjected to thermal distortions.
LSM 14's first dynamometer run showed 300watts at 300rpm; significantly better than 11, 12 or 13, managed. Combustion has since been improved and I expect it's now putting out closer to 500watts (200watts/litre)- but still only 50% of the goal. 1kw from a single cylinder unpressurised air engine of 2.5 litre swept volume has never been achieved as far as I'm aware- but it's probably possible- and worth striving for. Hopefully, reliability, compactness, and simplicity will not be compromised during this process. And, a secondary goal for this series is to use only basic engineering machinery/skills, and commonly available materials.
Improvements can come from further optimisation of the main variables (phase angle, volume ratio, heat transfer areas and flow paths) and (relative to when unpressurised air engines last attracted significant development effort in the late 19th century), from the better materials (particularly stainless steel) and off-the-shelf components that are now available.
There is also now 100 more years of accumulating thermodynamic knowledge to draw on.
After the requisite additional prototypes, plans for a developed LSM series engine might become available at peterlynnhimself.com