An Inverted ‘Ross Yoke' Watercooled Stirling Engine
Model Engineer|4635
As with all my hot air engines they are cheap to build, mostly from scrap

This engine was built during the horrendous snows of the 2018 winter months in my garden workshop (photo 1). As with all my hot air engines they are cheap to build, mostly from scrap. This is my 5th attempt and can be seen running on YouTube along with my other large Stirling engines, running in the workshop or at shows. The only bought in casting is the 9 inch flywheel casting from eBay. It was theoretically cheap, except for the heating costs of keeping a large workshop and me alive - but only just!

I wanted a slow, low compression, educational engine that was different with lots to see. I stumbled across a working animated ‘Ross Yoke’ crank design on YouTube and this fascinated me. I couldn’t leave it alone. That meant it would also fascinate others - it ticked all the boxes – so I zoomed it to the scale I wanted and printed out several copies. This led to some research into the finer points - as always, these things are far more complex than they first appear. The mathematics are horrendous, frightening and eventually were to come back and bite me for getting it wrong.

Eventually, after many sketches, I came up with this layout. It has many good points – it’s inherently strong and stable, has built in water cooling tanks (always a major problem, a serious must have), no water jacket casting needed, compact, interesting and quite lightweight, by my standards, very visible and hypnotic the seeds were sown for yet another unusual engine that I promised not to make.

The Ross Yoke crank is attributed to the hot air super star Andy Ross, who has published books on the subject along with his many advanced Stirling engines - good reading.

The beauty of this design is that the connecting rod remains near vertical thereby putting no side load on the piston, causing no friction or wear. The downside is that there are lots of linkages to rattle around (all needing ball races) and poor balance. The rattle is eliminated with accurate workmanship.

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