What I like about this site is the potential to talk about all means to increase vehicle fuel economy. The limitations of diesel technology aren't limited by engine design they are limited by the realities of the refining process. As I understand it you can get more gallons of diesel from a barrel of sweet crude than other grades of crude. Sweet crude is the crude that will be the soonest to be in short supply. In other words what would you do with the surplus gasoline if cars were predominantly powered by diesel engines? I would strongly recommend the new book A THOUSAND BARRELS A SECOND to anyone that wants a realistic understanding of the realities of what fuels we have to work with in our quest to reduce the energy expended for vehicle transportation.