Heat engines are devices that convert thermal energy into mechanical work. They operate on the principle of the second law of thermodynamics, which states that heat flows from a hot body to a cold body, and can be used to do work in the process. Heat engines are used in a variety of applications, including power plants, cars, and refrigeration systems. They are essential for converting heat energy into useful work in many industries and have been a focus of research for improving efficiency and reducing environmental impact. Some common types of heat engines include steam engines, internal combustion engines, and gas turbines.