To build a heat engine that can do work by using heat, it is necessary to create certain conditions. First of all, a heat engine must operate 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 does work. But one cycle is not enough for a periodically operating machine; it must perform cycles over and over again for a certain time. The total work done during a given time in reality, divided by time, gives another important concept - power.
In the middle of the 19th century, the first heat engines were created. They did work, but expended a large amount of heat obtained from the combustion of fuel. It was then that theoretical physicists asked themselves questions: “How does gas work in a heat engine? How to get maximum performance with minimum fuel use?”
To perform an analysis of gas work, it was necessary to introduce a whole system of definitions and concepts. The totality of all definitions created a whole scientific direction, which received title: "Technical thermodynamics". In thermodynamics, a number of assumptions have been made that in no way detract from the main conclusions. The working fluid 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 nature, there are only real gases that have well-defined properties that are different from an ideal gas.
To consider models of the dynamics of the working fluid, the laws of thermodynamics were proposed, describing the main thermodynamic processes, such as:
- isochoric process is a process that is performed without changing the volume of the working fluid. Isochoric process condition, v=const;
- isobaric process is a process that is performed without changing the pressure in the working fluid. Isobaric process condition, P=const;
- isothermal (isothermal) process is a process that is performed while maintaining the temperature at a given level. Isothermal process condition, T=const;
- adiabatic process (adiabatic, as modern heat engineers call it) is a process performed in space without heat exchange with the environment. Adiabatic process condition, q=0;
- polytropic process is the most generalized process that describes all the above thermodynamic processes, as well as all the others possible to be performed in a cylinder with a movable piston.
During the creation of the first heat engines, they were looking for a cycle in which you can get the highest efficiency(efficiency). Sadi Carnot, exploring the totality of thermodynamic processes, on a whim came to the development of his own cycle, which received his name - the Carnot cycle. It sequentially performs an isothermal, then an adiabatic compression process. The working fluid after performing these processes has a reserve of internal energy, but the cycle is not yet completed, so the working fluid expands and performs an isothermal expansion process. To complete the cycle and return to the original parameters of the working fluid, an adiabatic expansion process is performed.
Carnot proved that the efficiency in his cycle reaches a maximum and depends only on the temperatures of the two isotherms. The higher the difference between them, the correspondingly higher thermal efficiency. Attempts to create a heat engine according to the Carnot cycle have not been successful. This is an ideal cycle that cannot be fulfilled. But he proved the main principle of the second law of thermodynamics about the impossibility of obtaining work equal to the cost of thermal energy. A number of definitions were formulated for the second law (law) of thermodynamics, on the basis of which Rudolf Clausius introduced the concept of entropy. The main conclusion of his research is that entropy is constantly increasing, which leads to thermal "death".
The most important achievement of Clausius was the understanding of the essence of the adiabatic process, when it is performed, the entropy of the working fluid does not change. Therefore, according to Clausius, the adiabatic process is s=const. Here s is the entropy, which gives another name to the process performed without the supply or removal of heat, the isentropic process. The scientist was looking forsuch a cycle of a heat engine where there would be no increase in entropy. But, unfortunately, he failed to do so. Therefore, he deduced that a heat engine cannot be created at all.
But not all researchers were so pessimistic. They were looking for real cycles for heat engines. As a result of their research, Nikolaus August Otto created his own heat engine cycle, which is now implemented in gasoline engines. Here, the adiabatic process of compression of the working fluid and isochoric heat supply (combustion of fuel at a constant volume) are performed, then the adiabatic expansion appears (work is done by the working fluid in the process of increasing its volume) and isochoric heat removal. The first internal combustion engines of the Otto cycle used combustible gases as fuel. Much later, carburetors were invented, which began to create gasoline-air mixtures of air with gasoline vapors and supply them to the engine cylinder.
In the Otto cycle, the combustible mixture is compressed, so its compression is relatively small - the combustible mixture tends to detonate (explode when critical pressures and temperatures are reached). Therefore, the work during the adiabatic compression process is relatively small. Another concept is introduced here: the compression ratio is the ratio of the total volume to the compression volume.
The search for ways to improve fuel energy efficiency continued. An increase in efficiency was seen in an increase in the compression ratio. Rudolf Diesel developed his own cycle in which heat is suppliedat constant pressure (in isobaric process). His cycle formed the basis of engines using diesel fuel (it is also called diesel fuel). The Diesel cycle does not compress the combustible mixture, but air. Therefore, work is said to be done in an adiabatic process. The temperature and pressure at the end of compression are high, so fuel is injected through the injectors. It mixes with hot air, forms a combustible mixture. It burns out, while the internal energy of the working fluid increases. Further, the expansion of the gas goes along the adiabatic, a working stroke is made.
The attempt to implement the Diesel cycle in heat engines failed, so Gustav Trinkler created the combined Trinkler cycle. It is used in today's diesel engines. In the Trinkler cycle, heat is supplied along the isochore and then along the isobar. Only after that, the adiabatic process of expansion of the working fluid is performed.
By analogy with reciprocating heat engines, turbine engines also work. But in them, the process of heat removal after the completion of the useful adiabatic expansion of the gas is carried out along the isobar. On aircraft with gas turbine and turboprop engines, the adiabatic process occurs twice: during compression and expansion.
To substantiate all the fundamental concepts of the adiabatic process, calculation formulas were proposed. An important quantity appears here, called the adiabatic exponent. Its value for a diatomic gas (oxygen and nitrogen are the main diatomic gases present in the air) is 1.4. To calculatethe adiabatic exponent, two more interesting characteristics are used, namely: the 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 the theoretical cycles of heat engines? In fact, polytropic processes are performed, but due to the fact that they occur at a high speed, it is customary to assume that there is no heat exchange with the environment.
90% of electricity is generated by thermal power plants. They use water vapor as the working fluid. It is obtained by boiling water. To increase the working potential of steam, it is superheated. The superheated steam is then fed at high pressure to a steam turbine. The adiabatic process of steam expansion also takes place here. 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, the increase in efficiency is also associated with an increase in the temperature and pressure of the water vapor.
As can be seen from the above, the adiabatic process is very common in the production of mechanical and electrical energy.
