LSM 16 STIRLING CYCLE AIR ENGINE

236mm bore, 64mm stroke, 2.8 litre swept volume

LSM 16 has been through many iterations.   

 16.1 (2016) was a beta with double skirted displacer and a water-cooled piston but was converted to 16.2 before completion.    

 In an alternative to alpha beta or gamma layouts, 16.2 used a displacer in two parts so that its length (and hence volume) can change as it reciprocates in a cylinder that’s closed at both ends (one end heated the other cooled).   I call this the “adiabatic displacer” layout, because provided there is reasonable insulation and at least moderate speed, volume changes inside the displacer are adiabatic (thermodynamically lossless).  16.2 ran (early 2018), but barely- due to high friction, poor sealing and high internal compression (too little displacer volume).

It was then improved as 16.3 (also adiabatic) which ran well.  In 2021, LSM 20 was built to further explore the adiabatic displacer concept.

 For direct crank-controlled Stirling engines of both alpha, beta and gamma styles, more than half the working fluid is in the cold space by the completion of the expansion stroke- which costs power and efficiency. The star linkage used for 15-4 in 2020 was aimed at improving working fluid transfer- and does, it’s a delightful engine (currently powering a rocking chair), but cannot be run at high heat/loads because of potential piston seizing (steel on steel). 

 18.1, was then made with a variant of Whitworth’s quick return mechanism that enabled nearly all the expansion to occur before the air working fluid moves to the cold space- but 18.1’s mechanical loads were too high to permit useful rpms.  It therefore remained uncertain if its improved transfer lifted performance. 

 16.3 was completely rebuilt in 2023 as 16.4 with transfer nearly as ideal as 18.1’s but using pivoted linkages only- no cams or rolling surfaces, so lower loads and less friction.   Provision for quick change to a simple linkage showed that there was no improvement.  This has led to a review of what optimal transfer should like, particularly the (ideally) constant volume pressure and temperature changes .  

16.4 was then rebuilt with a conventional linkage, a larger combustion chamber (the original had been limiting heating) and a dynamometer as 16.5 and is an excellent fully developed engine of the series.

 PETER LYNN, ASHBURTON, NEW ZEALAND, AUGUST 2025