The Carnot cycle is the theoretical basis for the design and operation of all internal combustion engines

Among all cyclic thermodynamic processes, the Carnot cycle has a special theoretical significance and practical application. Often it is called unsurpassed, great, ideal, etc. And to many it seems to be something mysterious and incomprehensible. However, if it is true to place all the accents, then all the simplicity, genius and beauty of this invention, which was discovered by the French scientist and engineer Sadi Carnot, will instantly open. And it becomes clear that there is nothing supernatural in the proposed process, but only the most effective use of certain laws of nature.

So what is really the most famous and mysterious Carnot cycle? It can be defined as a quasi-static process based on bringing the thermodynamic system into thermal contact with a pair of thermostat tanks having constant and stable temperature values. It is assumed that the temperature of the first (heater) is higher than that of the second (refrigerator). Carnot cycle is that first the thermodynamic system, initially having a certain thermal value, comes into contact with the heater. Then, by an infinitely slow decrease in pressure, a quasi-static expansion is induced in it, accompanied by the borrowing of heat from the heater and resistance to external pressure.

After that, the system is isolated, which again causes quasi-static expansion along the adiabat until its temperature reaches the same parameters of the refrigerator. With this type of expansion the thermodynamic system also performs some work of resistance to external pressure. In this state, the system is brought into contact with the refrigerator, while by continuously increasing the pressure it is compressed to a certain point, whereupon it subsequently gives the second reservoir the heat energy borrowed from the heater. The Carnot cycle is unique in that it is not accompanied by any loss of heat. Theoretically, such a scheme can be called a perpetual motion machine. This is all because the thermal efficiency of the Carnot cycle, which depends solely on the temperature of a pair of tanks, will always be as high as possible. However, so far no one has managed to create a machine whose thermal efficiency would exceed thirty percent of the allowed by the cyclic process of Sadi Carnot.

And this process is called ideal, because it is much better than other cycles that can turn heat into useful work. On the other hand, because of the difficulties in organizing and carrying out isothermal processes, its application in real engines is extremely difficult. For maximum efficiency of heat transfer, such a machine must be completely isolated from the external environment, which in fact is practically impossible.

The reverse Carnot cycle underlies the principle of the heat pump, which, unlike a refrigeration machine, should give as much energy as possible to a hot object, for example a heating system. Some of the heat is borrowed from the external environment, which has a lower temperature, the rest of the required energy is released when performing mechanical work, for example, a compressor.