It may have been the weight of the engine used in planes but,
DeltaHawk's business is based on designing a lightweight diesel aviation engine. Diesel aviation engines have been around since the 1920s and 1930s, but they were too heavy for most airplanes except World War II bombers, said Rip Edmundson, DeltaHawk information systems manager. Only in recent years have materials existed to allow the creation of a feasible light diesel engine, and that is the DeltaHawk quest. Less weight in the air means more mileage, other factors being constant.
But then it seems its gone full circle and there back now.