Originally posted by MrChad
Saab has a proto-type of such an engine.
The displacement varies by change cylinder head volume.
Basically the head pivots in relation to the block. When its up-- its at low Comp. Pivots down and is flat to the block like conventional head (high Comp.)
I've yet to fully understand how the head seals and doesn't leak if I did I'd likely be working at saab 
Anyway-GM-is exploring this option which means you have hope they will screw it up.
My current engine dreams are associated with a fuel injected weed wacker.........I hate smelling like 2cycle........
SAAB's system varies compression, not displacement. Changing the compression ratio does not affect displacement. The bore and stroke remain the same.