According to Wikipedia
Engine efficiency - Wikipedia, the free encyclopedia
thermal efficiency of a modern petrol engines is about 18 to 20%, meaning they convert 20% of total heat of petrol combustion into movement.
On the other hand, diesel engines have a thermal yield of about 40% .
From the best of my knowledge, the main reason isnīt for rpm or friction or even the fuel energy(but they count also), but for the thermodynamics curve of the diesel cycle. It works at higher pressure that needs a fuel that do not detonate at that high pressure.
Turbos are allowing constructors to make smaller and lighter engines with a lot more power, while other advances are allowing them to supress noise and vibration a lot.
It is common to see Jeeps with diesel engines that used to had 2500 cc that turned to 2200 cc producing more HP than their predecessors.
OldBeaver