RE: Diesel combustion chamber shape.
I think having the charge well mixed is a good idea. My thinking has been that temperature needs to be more or less uniform throughout the combustion chamber, so that the charge fires all at the same time. On the other hand, I wonder if having a hot spot, similar to the glow plug in a glow plug engine, might be a good idea. This might give a more protracted and complete burn, starting at the hot spot and propagating from there.