What are the potential source of energy for an engine?

Context: An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy.
Available energy sources include potential energy, heat energy, chemical energy, electric potential and nuclear energy. Many of these processes generate heat as an intermediate energy form, so heat engines have special importance. Some natural processes, such as atmospheric convection cells convert environmental heat into motion. Mechanical energy is of particular importance in transportation, but also plays a role in many industrial processes such as cutting, grinding, crushing, and mixing.

Answer: Potential sources of an engine includes potential energy, heat energy chemical energy, electrical energy and nuclear energy.