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. This process is often described by the laws of thermodynamics, particularly the Carnot cycle, which outlines the most efficient way to convert heat into work. Common examples of heat engines include internal combustion engines found in cars and steam engines used in trains. These engines rely on the expansion of gases or steam to push pistons or turn turbines, ultimately generating motion. The efficiency of a heat engine is determined by how well it can convert heat into useful work while minimizing waste heat.