Audi could probably win with a rubber band powered car if they wanted to, with the lack of competition in LMP1 right now.
I'd like to see something really creative, like the CNG hybrid Chrysler was working on a few years back.
Diesel is too dirty for real life use anyways. Particulates and NOx, you know, the stuff that causes smog. They're pretty good on CO2 but that's only important depending on your religious faith.