In Back to the Future, the DeLorean time machine initially requires plutonium to generate its power.
Doc Brown equips the modified DMC DeLorean with a plutonium-fueled nuclear reactor. The reactor supplies the enormous electrical input needed by the flux capacitor and time circuits: 1.21 gigawatts. In the first film, Doc obtains the plutonium by tricking Libyan terrorists, who later attack him at Twin Pines Mall.
Plutonium powers the time-travel apparatus, not necessarily every ordinary function of the car. The DeLorean still has a conventional automobile engine, which explains why Marty can drive it after arriving in 1955 even though the machine has run out of plutonium. To return to 1985, he instead channels a lightning strike into the system.
Viewers sometimes answer “fusion” because the later Mr. Fusion Home Energy Reactor converts household waste into power. That device appears at the end of the first film after Doc returns from 2015, replacing the original plutonium setup for subsequent time travel. Thus plutonium is the correct initial fuel, while Mr. Fusion is the later upgrade.