I did not know where to include this thread since it is not a regular automotive question-answer type of thread starter. But I found the concept of the Duke engine so fascinating that I felt it should find a place in our forum. So mods, if you feel that it could be moved to a more appropriate section, I'll be relieved!
I came across this in the usual way, while browsing the net. And what caught my eye was the axial piston bit. As marine engineers, most of us are familiar with axial piston swash-plate type of variable-delivery pumps used for marine steering gear systems. That a similar (well, almost) principle could be used in reverse as a prime mover, proved irresistible. I am giving a link to the site (Duke Engines' incredibly compact, lightweight valveless axial engine - )
with cutaway videos showing the operating mechanism. Quite fascinating. One weakness I could see after a cursory view of the video (or apparent weakness, at least) appeared to be the matter of providing gas tight sealing where the pistons slides across the swash plate (or stationary head ring) from one port to the next. Rather like the problem faced in the Wankel rotary engine, where sliding vanes did not provide long-lasting seals. I look forward to hearing from members with their views.