The Carnot cycle determines the maximum possible efficiency for a heat engine between two temperatures. It is theoretical and cannot operate in physical systems, but establishes maximum efficiency for other engines. The cycle includes four processes and assumes no change in entropy. Efficiency can be calculated using the maximum and minimum temperatures.
The Carnot thermal cycle, more properly called the Carnot cycle, is an idealized thermodynamic cycle that is used to determine the maximum possible efficiency for a heat engine operating between two given temperatures. It is used for theoretical purposes, but cannot actually operate in physical systems. While, in theory, an engine could be built to operate near maximum efficiency, the heat transfer in the cycle is too slow to be a practical system. The main value of the Carnot cycle lies in establishing the maximum efficiency for other types of heat engines.
In constructing the Carnot thermal cycle, two assumptions are made, to give it the maximum possible efficiency: all processes are reversible and there is no change in entropy. A reversible process is one that can be returned to its original state without loss of energy. Entropy is the amount of energy in a system that is unavailable to do work. According to the second law of thermodynamics, the amount of entropy in a system must increase or stay the same as a process occurs. Neither of these assumptions can be satisfied under natural conditions, but they are useful for determining maximum efficiency.
Four processes repeat themselves in a thermal Carnot cycle. The first is an isothermal expansion. “Isothermal” means that the temperature remains the same throughout the process. Volume increases and pressure decreases during this and energy is added to the system.
The subsequent process is known as adiabatic expansion. In adiabatic processes, the system does not gain or lose heat. Temperature changes occur due to changes in pressure and volume. For this particular step, the pressure is decreased and the volume is increased, in order to decrease the temperature.
The third is an isothermal compression. Pressure increases and volume decreases during this process and energy is removed from the system. Finally, an adiabatic compression is performed to bring the system back to its original state. The pressure is increased and the volume is decreased to raise the temperature.
Due to the assumption that there is no change in entropy during the Carnot cycle, it could run indefinitely and retain the same amount of energy each time it returns to its original state. There is still some entropy in even this idealized system, however, which means it cannot be 100% efficient. The actual efficiency of a Carnot thermal cycle can be calculated using its maximum and minimum temperatures, on the absolute or Kelvin (K) temperature scale. In this equation, the minimum temperature is subtracted from the maximum and this number is then divided by the maximum temperature.
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