"Diesel fuel has a higher energy density than gasoline. On average, 1 gallon (3.8 L) of diesel fuel contains approximately 155x106 joules (147,000 BTU), while 1 gallon of gasoline contains 132x106 joules (125,000 BTU). This, combined with the improved efficiency of diesel engines, explains why diesel engines get better mileage than equivalent gasoline engines."
Greg is right about the btu by weight, not by volumn.

The diesel engine is more fuel efficient is that it can convert more its btu to motion-energy than gasoline engine.(35% v. 15%). Therefore
the gasoline engine for the same HP should be hotter than the diesel engine since the btu of a gas engine went into heat instead of
motion energy.
If I understand correctly, the key for higher btu converted to motion energy between these two type of engines is its compression
ratio. "Higher compression is more efficient"

Ken