Oh, so that's how it works. See Fig. 2 of the patent. It's a lot like the Peugeot MCE-5, an earlier variable-displacement engine. The Nissan mechanism looks simpler and seems to take up less space inside the engine. Maybe that will make this workable. Variable-displacement isn't new, but previous attempts were mechanically clunky. It adds extra moving parts to the highest-load path in an engine - the piston to crankshaft connection. It has to deliver large benefits to justify the costs and problems that implies.

Most engines already have knock sensors. They're used to adjust fuel/air ratio and spark timing to just above/after where the engine starts to knock. A few engines adjust valve timing while running, which is also complex mechanically, but not as difficult as adjusting compression. Now there's another variable to tweak.

It may take a machine learning system to tune such an engine. All those interdependent variables to adjust make that a hard problem. Different values required based on engine speed, load, temperature, fuel consumption, and emissions outputs. All this is usually expressed as precomputed tables which are interpolated within the engine controller. Those tables have to be developed somehow.