Will diesel engines ever be added, or is it to much of a coding mess.
Diesels are a little bit available. Because the there is an Mechanical Injection 
But an real Diesel Injection is not here. 
As i Know Diesel is allwasy an direct Injection.
direct diesel injection in cars appeared in the 90s. before that there was indirect injection; in which injection and start of combustion were taking place in the cylinder head in someting called a prechamber.
these were inefficient systems wasting a lot of heat , but were easy to manufacture and did not require great pressure to work and as a bonus allowed high rpms ,6000-6500 rpms on heavily modded engines could be achieved. they were also much quieter than modern diesels, having a distinct rumbling sound , much like a 2 stroke petrol.