A heat engine is a device that converts thermal energy into mechanical work. It operates by taking in heat from a high-temperature source, using that energy to perform work, and then releasing some waste heat to a lower temperature sink. Common examples of heat engines include internal combustion engines found in cars and steam engines used in trains. The efficiency of a heat engine depends on the temperature difference between the heat source and the heat sink. The greater the difference, the more efficient the engine can be. This principle is essential in designing engines that maximize performance while minimizing energy loss.