One of the European manufacturers, either SAAB or Volvo if I remember correctly, has a variable compression system that works by tilting the head under certain conditions. This allows the compression ratio to be variable within a certain range. As for variable displacement, that would be much harder to do. If you want to vary the stroke, the only way I can see is to change the length of the rods on the fly. That would be an engineering feat in and of itself. Now, let's say you were able to do that. You would need to take into account the piston to valve clearances, as well as how the compression ratio is going to be affected. Get all that to work together would be an incredibly daunting task. If you leave yourself too little room, your pistons will slap the valves. If you leave too much room, you're base c/r will be very low, which wouldn't be very good for efficiency or performance, really. All of this, on top of the fact that you'd need to engineer a way to alter rod length on the fly. I'm not saying it's impossible, but I doubt we'll see it any time soon, if ever. Oh, and putting some kind of magical stroke-adjusting gear on the camshaft won't work. The camshaft has no effect on the pistons' stroke. Interesting concept, but I don't think it'd be practical.