Heat Engine

A heat engine is a system that converts thermal energy into mechanical or electrical work by moving a working substance, usually a gas or liquid, from a higher temperature state to a lower one. A heat source raises the working substance to its higher temperature, the substance does work as it transfers heat toward a colder sink, and part of that thermal energy is converted into usable work in the process. Not all of the heat becomes work, since some is lost to the surroundings and, in a mechanical engine, some is lost to friction and drag. The idea began with mechanical devices, but the term has since been extended to engines that produce electrical work as well.