Cummins already has a straight 6 diesel that passes all emissions tests for USA without using uria. It's not rocket science ( well for the most of it) prototype diesel electric hybrid sedans were built around 2001 that got 80mpg. It's such a no brainer...from 2010 to 2012 the car market here went up almost 3% but the diesel sales went up 25%.
Imagine a Diesel hybrid with 1000cc 3 or 4 cyl Mercedes engine like the one in the Smart car but designed specifically for a hybrid . It would probably get close to 100mpg as the diesel Smart already gets between 70 and 80 mpg.