Thanks, Grab. V interesting. I note from Ford's description of the Atkinson Cycle (your 3rd link) that they leave the inlet valve open late so that the compression stroke pushes some of the charge back into the inlet manifold. Wouldn't the same effect be more efficiently gained by closing it early, rather than late? I'd have thought that with the elimination of reverse-flows in the inlet system you'd get a slight efficiency tweak and also an engine capable of revving higher. But all my i/c-engine experience is 30 years old now (and most of it was on diesels then) so what would I know?

Anyway, I'm grateful for the info you've given.