Mileage is really interesting, of course, less good performance.
Engines are VM (Detroit Diesel) and are 3 cyl 1,5 liters, and 4 cyl. 2.o and
2.9 liters, with powers of 82, 113 and 163 HP.
Here in Italy new cars are for more than 50% with diesel engines because of
lower price of  fuel and better mileage, still I prefer gasoline ;-)
Almost all Hyundai car can be found diesel, except for Coup? (Tiburon), Atoz
and Sonata.
Difficult to say if Hyundai plans to sell diesel also in USA, I think not as
there is not such a difference in prices of fuel even if mileage is for sure
better with diesel.

Zotto