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 is determined by how well it converts heat into work, which is influenced by the temperature difference between the heat source and sink. The Carnot cycle is a theoretical model that helps understand the maximum efficiency achievable by any heat engine operating between two temperatures.