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A heat engine, also known as a heat power machine, is a device that converts heat energy into mechanical work. It operates on the principle of the second law of thermodynamics, which states that heat naturally flows from a hotter to a colder body. A heat engine facilitates this process by creating a temperature difference between two bodies, allowing work to be extracted from the heat flow.

The most common type of heat engine is the internal combustion engine, which is used in automobiles. Other examples include steam engines, which were historically used for powering trains and ships, and gas turbines, which are used in aircraft and power plants. Heat engines can also be classified based on the working fluid they use, such as steam engines (water), gas turbines (air), and Stirling engines (air or other gases).

The efficiency of a heat engine is determined by the Carnot efficiency, which is the maximum theoretical efficiency that a heat engine can achieve. It is given by the formula (T_hot - T_cold) / T_hot, where T_hot is the temperature of the hot body and T_cold is the temperature of the cold body. In practice, the efficiency of heat engines is always less than the Carnot efficiency due to various losses, such as friction and heat transfer.

Heat engines play a crucial role in various industries, including transportation, power generation, and refrigeration. They are essential for converting the energy from fossil fuels, nuclear reactions, and other heat sources into useful work. However, they also contribute to environmental issues, such as greenhouse gas emissions and air pollution, which has led to the development of more efficient and cleaner heat engines, as well as alternative energy sources.