Regarding rafts of engine concepts, a vehicle with electric motors and an empty engine bay could offer a myriad of possibilities. Just add electricity.
Xoon mentioned free piston linear generators. A few of these, and whatever you want to feed them in the tank, and you can go do your thing. Additionally, if the vehicle were to take a hit, as long as some FPLGs remain operational, you do too (I think).
A group of students from Eindhoven built a formic acid fuelled range extender trolley to hang behind an electric city bus. If the contents of the trolley could be made to