It's how to transfer energy into vehicle motion. The energy's got to come from somewhere. In generators of Alternating-Current [AC] power, magnets or coils of wire spin around coils of wire or magnets. In that instance... how do you spin the magnets or coils of wire? That's where moving water like rivers can come in. There's also condensing steam, but how do you heat the water to make steam? That's where geo-thermal can come in, but lots of people like burning million-year old dead bodies [though some people like burning trash instead].

So... it's like the snake eating its own tail, trying to figure out from where you get the energy. You've got to do something to... blah blah... to then, eventually, turn that energy into vehicle motion.

Or go completely electric on your vehicle, and just worry about charging the batteries. That way you don't have to generate your power IN your vehicle [ignoring down-hill and breaking generation], but still have it "generated" energy at some main power-plant somewhere. But what's that power-plant? Nuclear, biomass, geotherm, hydro, wind, hot sex, solar, coal, fuel-cell...