Chris Hogg laid this down on his screen :
Because of the hotter
combustion temperatures, diesels emit more NOx, which seems to be the
main point of issue ATM.

Partially solved at low engine speeds by an EGR, problem is that the
EGR's on a diesel need to be regularly cleaned out and no manufacturer
has them on their service list to be cleaned. The net result is a
diesel engine with a choked up intake system.