Slippery_pete:
It doesn't apply in a fixed volume piston engine (because the unswept volume determines the number of molecules you can get in there [there being the combustion chamber])
Wrong again. The "unswept volume"
is the combustion chamber (or at least part of it depending how semantic you want to be). It is the swept volume x the volumetric efficiency that determines the number of molecules you get in - not the "unswept volume".