system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are Diesel engines more fuel efficient than gas/petrol engines?
gpt
A few reasons:

Diesel contains more energy per mass unit than gasoline, meaning that you have to burn less fuel to get the same amount of power out of the engine.

Diesel engines run with a much higher compression ratio than gasoline engines.  This increases the expansion space during the ignition/combustion part of the engine cycle allowing more energy to be extracted from the fuel burn.