Peter Lynn ARCHIVE
235mm bore, 80mm stroke, 3.46 litre swept volume (16.4).
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). In practice, to keep the displacer seal closer to the cold end temperature, insulation is used only at the hot end- but displacer internal volume changes can still be substantially adiabatic. 16.2 ran (early 2018), Sbut barely- due to high friction, poor sealing and high internal compression (too little displacer volume).
16.2 was then re-built as 16.3 with a much longer unskirted displacer to increase its internal volume (and hence decrease its internal compression ratio). 16.3 started quickly, had low friction and overrevved unless the burner was turned down or the vent opened- all signs of a successful design- but always slowed and stopped after a few minutes when the adiabatic displacer’s sealing failed because of thermal distortion as it heated up.
In 2021, LSM 20 was built to further explore the adiabatic displacer concept. 16.3 was then completely rebuilt in 2023 as 16.4 with a simpler, lower friction way to get better air transfer- to improve on the high load and high friction mechanism used for 18.1.
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 improved this and is a delightful engine but cannot be run at high heat/loads because of potential piston seizing (steel on steel). Later that year, 18.1 was built with a variant of Whitworth’s quick return mechanism that enabled nearly all the expansion to occur before the working fluid (air in this case) moves to the cold space- but 18.1’s mechanical loads are too high to permit useful rpms. It has therefore remained uncertain if improved transfer has lifted performance.
16-4’s air transfer is nearly as ideal as 18.1’s but uses pivoted linkages only- no cams or rolling surfaces, so has lower loads and less friction. It’s not rev limited like 18.1 but it doesn’t yet have an alternative simple crank linkage option such as 18.1 has. This would provide an unequivocal test as to whether a quick return displacer improves output. 16.4’s heating is inadequate but in other respects it is an excellent engine; compact, with good sealing and low friction. Stroke needs to be reduced to 60mm to make it comparable with other LSMs- power would probably not decrease but would be at higher rpm (max power is a mainly a function of heat transfer area, which wouldn’t change).
PETER LYNN, ASHBURTON, NEW ZEALAND, to MAY 2025
