The Ram 1500 EcoDiesel is this truck's obvious target, currently the only light-duty pickup with a diesel. Ram went the V6 diesel route; Ford could do the same or possibly aim more toward the tweener Nissan Titan XD, which has a big Cummins diesel V8. Our guess is the smaller engine with fewer cylinders, however, since that provides the fuel economy boost that an F-150 owner is likely to want, letting those who want a Blue Oval truck with big diesel power go to an F-250 Super Duty. Ram's EcoDiesel hits 19 mpg city and 27 highway, so numbers along those lines are a decent bet.
Previous intel on the future diesel F-150 suggests it could make use of a diesel V6 from Jaguar Land Rover. In the new Range Rover and Range Rover Sport Td6 models, that 3.0-liter engine makes 254 horsepower and 443 pound-feet of torque. EPA fuel economy in those SUVs stands at 22 mpg city and 29 highway; sticking it in a heavier pickup with worse aero would likely drop those numbers a bit, depending on gearing.
Judging by the doneness of this prototype, the diesel-packing F-150 could come as early as the 2017 model year. That would coincide with a likely mid-cycle refresh, which explains the swirly tape on this truck's bedsides.