What is an adiabatic process?

To build a thermal machine that can perform work by using heat, it is necessary to create certain conditions. First of all, the heat engine must work in a cyclic mode, where a series of successive thermodynamic processes create a cycle. As a result of the cycle, the gas, enclosed in a cylinder with a movable piston, performs its work. But one cycle for a periodically operating machine is small, it has to perform the cycles time after time within a certain time. The total work done for a given time in reality, divided by time, gives one more important notion - power.

In the middle of the XIX century, the first thermal machines were created. They did the work, but spent a large amount of heat, obtained by burning fuel. It was then that theoretical physicists asked questions: "How does the gas work in a heat engine? How to get the most out of work with a minimum of fuel? "

To perform the gas analysis, it was necessary to introduce a whole system of definitions and concepts. The totality of all definitions and created a whole scientific direction, called: "Technical thermodynamics". In thermodynamics a number of assumptions have been adopted, which do not at all detract from the main conclusions. The working body is an ephemeral gas (not existing in nature), which can be compressed to zero volume, the molecules of which do not interact with each other. In the surrounding nature, there are only real gases, which have quite certain properties, which are distinguishable from an ideal gas.

To consider models of the dynamics of the working fluid, the laws of thermodynamics were proposed that describe the basic thermodynamic processes, such as:

• The isochoric process is a process that is performed without changing the volume of the working fluid. The condition of the isochoric process, v = const;
• The isobar process is a process that is performed without changing the pressure in the working body. The condition of the isobaric process, P = const;
• Isothermal (isothermal) process is a process that is performed while keeping the temperature at a given level. The condition of the isothermal process, T = const;
• The adiabatic process (adiabatic, as it is called modern heat engineering) is a process performed in space without the exchange of heat with the environment. The condition of the adiabatic process, q = 0;
• The polytropic process is the most generalized process that describes all the above-mentioned thermodynamic processes, as well as all the other possible ones for committing in a cylinder with a movable piston.

During the creation of the first heat engines, we searched for a cycle in which the highest efficiency (efficiency) can be obtained. Sadi Carnot, exploring the totality of thermodynamic processes, came to the development of his cycle, got his name - the Carnot cycle. It consistently performs the isothermal, then adiabatic compression process. The working body, after performing these processes, has a reserve of internal energy, but the cycle is not yet complete, so the working medium expands and performs the isothermal expansion process. To complete the cycle and return to the original parameters of the working body, the adiabatic expansion process is performed.

Carnot proved that the efficiency in its cycle reaches a maximum and depends only on the temperatures of the two isotherms. The higher the difference between them, the higher the thermal efficiency, respectively. Attempts to create a thermal machine in the Carnot cycle never came to fruition. This is an ideal cycle that can not be performed. But he proved the main principle of the second law of thermodynamics about the impossibility of obtaining a job equal to the costs of thermal energy. A number of definitions were formulated for the second law of thermodynamics, on the basis of which Rudolf Clausius introduced the concept of entropy. The main conclusion of his research is that the entropy is constantly increasing, which leads to a thermal "death".

The most important achievement of Clausius was the understanding of the essence of the adiabatic process, when it is executed, the entropy of the working substance does not change. Therefore, the Clausius adiabatic process is s = const. Here s is entropy, which gives one more name to a process performed without supply or removal of heat, an isentropic process. The scientist was engaged in the search for such a cycle of a thermal machine, where no increase in entropy would occur. But, unfortunately, he could not create such a thing. Therefore, he deduced that a thermal machine can not be created at all.

But not all researchers were so pessimistic. They were looking for real cycles for thermal machines. As a result of their search, Nikolaus August Otto created his own cycle of a heat engine, which is now being realized in gasoline engines. Here, the adiabatic process of compression of the working fluid and the isochoric heat input (combustion of fuel at a constant volume) are performed, then the expansion adiabat appears (work is performed by the working body in the process of increasing its volume) and isochoric heat removal. The first internal combustion engines in the Otto cycle used combustible gases as a fuel. Much later, carburettors were invented, which began to create benzene-air mixtures of air with gasoline vapors and feed them to the engine cylinder.

In the Otto cycle, a combustible mixture is compressed, so the compression value is relatively small - the combustible mixture tends to detonate (explode when critical pressures and temperatures are reached). Therefore, the work under the adiabatic compression process is relatively small. Here one more concept is introduced: the degree of compression is the ratio of the total volume to the volume of compression.

The search for ways to increase the energy efficiency of fuel continued. The increase in efficiency was seen in an increase in the degree of compression. Rudolf Diesel developed his cycle, in which the supply of heat is carried out at constant pressure (in the isobar process). Its cycle formed the basis for engines using diesel fuel (it is also called solar oil). In the Diesel cycle, not the combustible mixture but the air is compressed. Therefore, they say that work is done in the adiabatic process. The temperature and pressure at the end of the compression are high, so injectors are injected through the injectors. It mixes with hot air, forms a combustible mixture. It burns down, while the internal energy of the working fluid increases. Further, the expansion of the gas proceeds along the adiabat, and the working stroke is made.

An attempt to realize the cycle of Diesel in thermal machines failed, so Gustav Trinkler created the combined cycle of Trinkler. It is used in today's diesel engines. In the Trinkler cycle, heat is supplied by the isochore, and then by the isobar. Only then is the adiabatic expansion of the working medium carried out.

By analogy with reciprocating thermal machines, turbines operate. But in them the process of heat removal after the completion of a useful adiabatic expansion of the gas is carried out by the isobar. On airplanes with gas turbine and turboprop engines, the adiabatic process takes place twice: during compression and expansion.

To justify all the basic concepts of the adiabatic process, the calculation formulas were proposed. Here there appears an important quantity, called the adiabatic exponent. Its value for a diatomic gas (oxygen and nitrogen are the basic diatomic gases available in air) is 1.4. To calculate the adiabatic index, two more interesting characteristics are used, namely: isobaric and isochoric heat capacities of the working fluid. Their ratio k = Cp / Cv - is the adiabatic exponent.

Why is the adiabatic process used in theoretical cycles of thermal machines? In fact, polytropic processes are being performed, but due to the fact that they occur at high speed, it is generally assumed that there is no heat exchange with the environment.

90% of electricity is generated by thermal power plants. In them, water vapor is used as a working fluid. It is obtained by boiling water. To increase the working capacity of steam, it is overheated. Then, at high pressure, superheated steam is fed to a steam turbine. Here, too, an adiabatic process of vapor expansion takes place. The turbine receives rotation, it is transferred to an electric generator. That, in turn, generates electricity for consumers. Steam turbines operate on the Rankine cycle. Ideally, an increase in efficiency is also associated with an increase in the temperature and pressure of water vapor.

As can be seen from the foregoing, the adiabatic process is very common in the production of mechanical and electrical energies.