I'm genuinely surprised no one has talked about the traits/abilities diesel engines will have. Its time we change that.
I agree, a different variety of engine would be nice. But we dont want to make the game look too complex for new players. :)
So as I mentioned on Steam, the big question is where diesel engines fit in terms of advantages and drawbacks. They can't just be "like steam but better but more expensive".
Comparable performance to the coal equivalent, but they have in-module fuel tanks with a limited amount of fuel. Can't vary your fuel capacity the way you can with coal modules or share fuel with other modules, but you save on space and a little weight. Good for smaller ships, but heavy maintenance cost (compensates for lack of hauling jobs and balance).