Its name used to be Mware, so that's why you didn't find many information with the name "Diesel".
More compatible with modern system and it supports utf8 encoding.
Anyways, here's the tool supports unpacking/packing every game uses Mware.
How to use:
Unpacking: Drag & drop npk file to Npk3tool and choose the game's index to unpack.
Packing: Remain "[Unpack_name].npk~" folder, then drag & drop it to Npk3tool and choose game's index to pack.