The toroidal "cylinder" might be a bit tricky to machine. Getting lubrication into (and more importantly, out of) the space on each piston between each set of piston rings will be, shall we say, "interesting". Somewhere in this engine, there needs to be a rotary seal between the "cylinder" and whatever is transmitting the action to the pistons. Historically, the mechanisms for transmitting the oscillating rotary motion to the output shaft on engines like this have looked rather weak. Spark plug can't be in the center of the combustion chamber.
I saw some guy with his hand in it while it was running. Looked pretty amateurish to me. The guy in the white coat and safety glasses was standing well back.
