LSM 14 STIRLING CYCLE AIR ENGINE.

Unpressurised, 190mm bore, 93mm stroke, 2.64 litre, beta layout (piston and displacer concentric in the same cylinder).  

The 4th engine in a development series aimed at getting 1kw from a simple, compact, 2.5 litre unpressurised air engine- aspirational or impossible? 

Heat transfer is a main constraint for all Stirling cycle engines.  LSM 11 uses tube heat exchangers, but these restrict gas flows and require a secondary seal for the displacer- net negatives for unpressurised engines of this size.  Annular gap heat exchangers work well in LSM 12, while for LSM 13, directing gas flow over hot and cold end discs was tried- but found to be marginal (insufficient area or a flow problem?).  14.1 has cylindrical heat transfer surfaces with an annular gap for air flow.  Its displacer has 3.5mm radial clearance.

For unpressurised Stirling cycle air engines like 14.1 that can only achieve modest specific output (power for size), reducing friction is crucial- ring tension needs to be kept to a minimum (while not compromising sealing) for example.

It’s difficult 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.  14.1’s displacer is constrained axially by a sliding yoke on the top end of the displacer rod and by 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 when subjected to thermal distortions.

14.1’s first dynamometer run showed 300watts at 300rpm; significantly better than earlier LSM’s (though 12.3 and some others are now up with it).  Having now thoroughly run-in (which improves sealing and reduces friction) and with a better burner I expect it's getting 400watts.  That it pushes Piwakawaka along at 5knots (320rpm) supports this.  This is still less than 50% of the original high goal but is ahead of any other unpressurised air engine of this size that I’m aware of.  Hopefully, further gains will be possible without compromising reliability, compactness, or simplicity.  A secondary goal for this series is to use only basic engineering machinery/skills, and commonly available materials.

Components and materials (particularly stainless steel) are now available that were unknown when unpressurised air engines last attracted significant attention in the late 19th century, an advantage not yet fully exploited.  Improvements can also come from better combustion and further optimisation of phase angle, volume ratio, heat transfer areas and flow paths- and from a 100 more years of accumulating thermodynamic knowledge.