The real reason diesel persists and gets developed is the refining is cheaper, it can be made from less expensive crude, it burns more efficiently, and it has more energy per gallon. 139,000 btu/gallon for diesel and 124,000 btu/gallon for gasoline. thats 12% more. The combustion process for diesel is more efficient and this is what Mazda is trying to take advantage of. Those efficiencies are 30% of gasoline is converted to motion vs 45% for diesel. Overall the diesel can return 135+% more mileage than a similar gasoline engine. Your diesel van can regularly return 26 mpg lightly loaded at reasonable speeds and a similar PM gasoline van will return 17. That is about 150% further on a gallon. Part of the difference is due to the 4 cylinder diesel competing with a 6 cyl gas van. Engines are pumps and the losses in pumping are proportional to the number of cylinders. 6 looses to 4.
Since gasoline is lighter (less dense) it does not require as much crude to make a gallon leveling the ratio of crude oil to miles somewhat.
Because the diesel is a lean burn technology it inherently produces more Oxides of Nitrogen (NOx) and those are what our newer emissions systems are controlling along with the carbon soot. On the other hand gasoline engines must be run at a perfect air/fuel ratio (Stoichiometric=14.6/1) but produce a lot of CO2 and hydrogen cyanide (HCN). The future control of HCN may make gasoline engines more trouble prone and less satisfactory. Where will it go? Some say diesel's dead, others say gasoline is approaching a wall of new regulation.