A heat engine is defined as any engine that uses heat to perform work. As such, the physics underlying the operation of a heat engine is a direct consequence of the first law of thermodynamics. Heat engines take heat from a higher temperature heat source and transform it into work and a release of heat in the lower temperature surroundings, Heat engines can also convert internal energy (heat) into mechanical energy.
A classical description of a heat engine is provided by the Carnot cycle, named after its inventor Nicolas-L(onard-Sadi Carnot, who first designed it in 1824. The Carnot heat engine is only used as a standard of heat engine performance. It operates as a reversible process and consists of a gas confined by a piston moving back and forth in a cylinder. The process has four steps. First, the gas is heated and it expands, moving the piston up. This is called isothermal expansion to the higher temperature. The second step is adiabatic expansion of the gas to lower its temperature, followed by the third step, isothermal contraction, cooling the piston so that it will move back down. Finally, adiabatic contraction of the gas with warming to the higher temperature maintains a cycle of heating and cooling to displace the piston up and down. It is the motion of the piston that allows useful work to be done and the efficiency of a heat engine describes how efficient the engine is at turning heat into work. Two temperatures are then required to run a heat engine; at one temperature the system is heated, at the other temperature it is cooled.
The refrigerator and the air conditioner are examples of heat engines that run in reverse because they take heat from a lower temperature source through work performed by a compressor, and transfer it to higher temperature surroundings.
To be efficient, a heat engine must operate using a reversible process, that is, a process after which the thermodynamic system and its surroundings are returned to the state they were in before the process started. Theoretically, the most efficient heat engine is the Carnot cycle, although the Carnot cycle is not used for practical applications because of engineering difficulties that cannot be overcome.
An example of a thermodynamic cycle used to design practical heat engines is the Otto cycle (isentropic compression, reversible constant volume heating, isentropic expansion, reversible constant volume cooling) of which the gas engine is the most well-known application. Other examples of thermodynamic cycles used in heat engine design include the Diesel cycle (isentropic compression, reversible constant pressure heating, isentropic expansion, reversible constant volume cooling), the Brayton cycle (isentropic compression, isobaric heat supply, isentropic expansion, isobaric heat output) used for closed-cycle gas turbines in many industrial applications, the Stirling cycle (isochoric heating, isothermal expansion, isochoric cooling, isothermal compression), and the Clausius-Rankine cycle (isentropic expansion, isobaric heat rejection, isentropic compression, isobaric heat supply), which defines the operating principle of steam engines and turbines. Aside from their different operating principles and applications, these cycles also differ in their relative efficiency.
By dealing with thermodynamic quantities such as heat and work, the heat engine is also used to formulate the second law of thermodynamics in various ways. For example, the second law states that heat flows naturally from regions of higher temperature to regions of lower temperature and not the other way. Another form of the second law derived from the heat engine states it is impossible to build a heat engine that can use all the heat from a higher temperature surrounding it and turn it entirely into work. Accordingly, it is impossible to make a heat engine with a 100% efficiency.
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