Sailplanes, for example, need no power source other than nature. But they're the exception.
Electric models use small motors, powered by battery cells.
Typically, they range in displacement from .049 cu. Glow Plug Glow engines cannot be operated with the same gasoline you'd get at a filling station pump. It contains methanol as the base, with varying amounts of nitromethane to increase the energy that the fuel can provide. Oil, pre-mixed into the fuel, lubricates and protects your tiny engine as it pounds out amazing power. When you get your new engine, first examine it carefully for any obvious defects.