Actually, I'll take it a step further. I'll go ahead and guess what this engineer's goal is:

To lean out the air/fuel mixture with an extremely fast pulse of high-pressure fuel in a high-compression chamber.

In a nutshell; shrink the displacement, cut back on fuel, add boost and fuel pressure. Enjoy the torque and economy.

Because that is where the big gains are with diesels. There is so much more energy packed into a gallon of diesel, that we are nowhere near the maximum efficiency of that process yet.

Now all of these features need to be fine-tuned to the Nth degree to reach ideal efficiency. That's just R&D. New materials, new software.