An Engine is a thermal device that converts heat energy into mechanical energy. Mechanical energy is principally derived in the form of torque on the output shaft of the engine and is utilized for necessary driving works.

Energy input to an engine is ‘heat’. Heat so used by the engine is derived from different sources, such as: (i) Solid fuel, (ii) Liquid fuel, (iii) Gaseous fuel, (iv) Nuclear fuel. Energy input system of the engine ensures efficient release of heat from the fuel. For the case a chemical fuel (solid/liquid/gaseous), combustion is the process that is to be carried out to liberate heat from fuel through an exothermic reaction. For nuclear fuel, a nuclear reaction is to be carried out in a nuclear reactor so that energy is liberated from atoms through nuclear chain reaction in which ‘nuclear binding’ energy is released as a result of re-arrangement of atomic particles.