LSM 11 Hot Air/Stirling Cycle Engine - Built 2008


LSM 11 running at the Engineerium, 2009
LSM 11 running at the Engineerium, 2009
First prototype motor for "Piwakawaka" (replica 19th century gentleman's fantail launch).
Unpressurised; 158mm bore, 128mm piston stroke, 2,51 litre swept volume.
Overall size; 0.7m high, 0.25m diameter, plus side crank and flywheel, 80kg.
Layout; Concentric piston-displacer type (beta), 90 degree phase angle.
Fuel; gas or by burning books (esp. Mills and Boons, but religious tracts not exempted).
Heating; 30/170mm x 8mm dia. SS tubes.
Cooling: 30/170mm x 8mm dia. copper tubes.
Regenerator: SS gauze.
Displacer control by straight line linkage.
Sealing; two 3mm wide by 3mm deep conventional s.g. cast iron piston rings.
Target output; 1kw at 500rpm.
Achieved output; 0.4kw at 300rpm.

This was a first attempt to make a hot air engine able to match the steam engines that are conventionally used to power restored and replica 19th century motor launches.
Because open mechanisms are an arguably essential aesthetic feature of these launches, pressurisation wasn't used because it requires the various linkage mechanisms to be hidden.
Pressurisation: elevating the pressure of the working fluid, (air in this case) enables Stirling cycle engines to develop specific output (power for size) comparable with steam and IC.

For ship propulsion, historically, air engines have made a serious challenge only once.
In 1853. "The Ericsson" was launched in New York to great fanfare. It was 80m long, 2200 tons, and powered by Ericsson's "caloric" air engine. It's four working cylinders, each of 4.3m bore and 1.85m stroke, developed a claimed 400kw (but probably only half of this) at 15rpm. Unfortunately it's performance couldn't match steam ships of the day and, when raised after sinking in a storm off Sandy Hook (Chesapeake Bay near Baltimore) in 1854, was refitted with steam engines. Ericsson went on to build the successful ironclad "Monitor" for Union forces during the American civil war.
Philips (Eindhoven, Netherlands)resurrected the Stirling engine concept in 1938, and in the 1960's, some of their licensees investigated whether, with high temperature materials and pressurised hydrogen as the working fluid, a challenge to(the by then highly developed) Diesel engines for commercial shipping could succeed. Nothing came of this except a niche market for non-nuclear submarines: United Stirling (Malmo,Sweden) developed Stirling engines that can operate at combustion and exhaust pressures exceeding 20 bar as an alternative to batteries when running submerged.


Piwakawaka under Stirling power
Piwakawaka under Stirling power
But back to Piwakawaka: There was also another agenda in this development; a 15kw generator unit burning the rough methane available from effluent settling ponds (chicken, cow, human etc ). IC engines can do this job when the gas is scrubbed and filtered (which costs) and at large scale, gas turbines work well, but at the typical pig farm/chicken farm/dairy shed/rural town scale, these existing solutions aren't commercially feasible.
To this end, the LSM 11 design was developed scalably; that is, so that it could be made bigger without proportionally losing output . This is the reason for using tube heat exchangers rather than the annular gap form that was commonly used in 19th century air engines like the Heinrici and Ryder styles and that has been used for LSM12
But, when using tube heat exchangers, in addition to the usual piston seal, it's necessary for the displacer to have a seal as well, creating extra friction. Fortunately, this extra frictional loss increases as the square of dimension, while swept volume (and power output, providing speed stays constant) increases with the cube of dimension, so larger engines of this design would not be effected as much by this as LSM11 is.
LSM11 probably hasn't achieved its target output because the cross sectional area of it's tubes (8.5 sq.cm total) restrict gas flow, limiting rpm (and therefore power). Using larger diameter tubes would be likely to cure this without other unacceptable consequences.
However, there is no compelling reason why swept volume can't be increased to achieve required output. Even if a 15kw engine/generator requires 2m x 2m of floor, this will not limit dairy shed applications. For 19th century steam launches, any possible size of Stirling engine is sure to be smaller than a boiler/steam engine combination.

Efficiency, or lack of, isn't so easily dealt to however:
Historically, small unpressurised air engines typically achieved overall thermal/mechanical efficiencies of just a few percent. LSM 11 as presented here is probably managing just 2% or 3%. Compared to petrol engines (approaching 30%) and Diesels (above 40%) this is not good enough. Stirling engines can attain excellent efficiency though- theoretically better even than diesels- and at least one unpressurised engine (Robert Stirling's huge Dundas Foundry engine of 1843), seems likely to have achieved 18%. Size is the key; mechanical losses decrease as engines get bigger.

LSM 11 is now honourably retired: It's had 3 different heaters (1 solid fuel, 2 gas), 3 different linkage systems, 2 pistons, 3 displacers and 100's of hours of testing.
Peter Lynn, Ashburton, New Zealand, November 2009.

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