To build a thermal machine that can do the work by using heat, you need to create certain conditions.First of all, the heat engine to operate in a cyclic mode, where the number of consecutive thermodynamic processes to create a loop.The result of the cycle gas enclosed in a cylinder with a movable piston, does work.But one cycle for periodically operating the machine is small, it should perform the cycles repeatedly within a certain time.The total work performed during the specified time, in reality divided by the time gives another important concept - power.
In the middle of the XIX century created the first heat engines.They are the work, but spend more amount of heat produced by the combustion of fuel.It was then that theoretical physicists asked: "As a gas works in thermal machine?How to get the most work with the minimum of fuel? ยป
to perform analysis of the gas needed to enter the whole system of definitions and concepts.The collection of all definitions, and created a whole research area, called "Engineering Thermodynamics".In thermodynamics, was adopted by a number of assumptions, it does not detract from the main conclusions.Tools body - ephemeral gas (does not exist), which can be compressed to zero volume whose molecules do not interact.In the natural environment, there are only real gases, which have well-defined properties, indistinguishable from an ideal gas.
To view a model of the dynamics of the working fluid, have been offered the laws of thermodynamics, which describe the basic thermodynamic processes such as:
- isochoric process - a process that is carried out without changing the volume of the working fluid.Conditions isochoric process, v = const;
- isobaric process - a process that is carried out without changing the pressure in the working fluid.Conditions isobaric process, P = const;
- insulated (isothermal) process - a process that is performed while maintaining the temperature at a given level.Isothermal process, T = const;
- adiabatic process (adiabatic, so called modern heat engineering) - a process, performed in space without heat exchange with the environment.Conditions of adiabatic process, q = 0;
- polytropic process - it is the synthesis process, which describes all the thermodynamic processes mentioned above, as well as all other opportunities for engaging in a cylinder with a movable piston.
During the creation of the first heat engines were looking for a cycle in which you can get the highest efficiency (efficiency).Sadi Carnot, examining a set of thermodynamic processes, instinctively came to the development of its cycle, got its name - the Carnot cycle.It has consistently performed isothermal, then the adiabatic compression process.The working fluid after these processes has a margin of internal energy, but the cycle has not been completed, so the working fluid expands and performs isothermal expansion process.To complete the cycle and return to the original settings of the working fluid is performed adiabatic expansion process.
Carnot proved that efficiency in its cycle reaches a maximum and depends only on the temperature of the two isotherms.The higher the difference between them, the correspondingly higher thermal efficiency.Attempts to create a heat engine of the Carnot cycle and failed.It is an ideal cycle which can not be performed.But he proved the main principle of the second law of the impossibility of getting a job, equal to the costs of thermal energy.It made a number of definitions to the second (law) of thermodynamics, according to which Rudolf Clausius introduced the concept of entropy.The main conclusion of his research - the entropy is increasing, which leads to thermal "death."
most important achievement was the understanding of the Clausius adiabatic process, the entropy when performing the working fluid changes.Therefore, the adiabatic process Clausius - is s = const.Here s - is the entropy, which gives another name for the process is carried out without supply or removal of heat - isentropic process.The scientist was looking for such a cycle heat engine, which did not occur to an increase in entropy.But, unfortunately, this has failed to establish it.Therefore I deduced that the heat engine can not be created at all.
But not all researchers have been configured as pessimistic.They were looking for the actual cycles of heat engines.As a result of their search Nikolaus August Otto created his cycle heat engine, which is now being implemented in engines running on gasoline.Here, the process performed by the adiabatic compression of the working body and isochoric heat input (combustion at constant volume), then there are adiabatic expansion (work done in the working medium increasing in its volume), and isochoric heat removal.First internal combustion engine Otto used as combustible fuel gases.Many were later invented carburetors, which began to create benzovozdushnoy mixture of air and gasoline vapors and feeding them into the engine cylinder.
In Otto cycle fuel mixture is compressed, so the amount of compression it is relatively small - fuel mixture tends to detonate (explode upon reaching the critical pressure and temperature).Therefore, the work in the adiabatic compression process is relatively low.Here we have introduced another concept: the compression ratio - the ratio of the total volume to volume ratio.
Finding ways to increase the energy efficiency of fuel continued.Increased efficiency seen in increasing the compression ratio.Rudolf Diesel developed his cycle in which the supply of heat is carried out at constant pressure (in the isobaric process).It formed the basis for the cycle engines using diesel fuel (also known as diesel fuel).The Diesel cycle is not compressed fuel-air mixture and air.So they say the work is done in an adiabatic process.The temperature and pressure at the end of high compression, so the nozzle is carried out through the fuel injection.It is mixed with hot air, it forms a combustible mixture.It burns, it increases the internal energy of the working fluid.Further expansion of the gas is on the adiabatic, committed stroke.
attempt to implement the Diesel cycle in heat engines failed, so Gustav Trinkler created combined cycle Trinklera.And its use in current diesel engines.In the cycle of the heat supplied by Trinklera isochor, and then on the isobars.Only after this is done, adiabatic expansion process of the working fluid.
By analogy with the heat engine working piston and turbine.But their process of removing heat at the end of the useful adiabatic expansion of the gas is performed by isobars.For airplanes with turbine and turboprop adiabatic process is performed twice during compression and expansion.
To justify all the fundamental concepts of adiabatic process have been proposed formulas.It appears important quantity, called the adiabatic index.Its value for a diatomic gas (oxygen and nitrogen - are the main diatomic gases present in air) equal to 1.4.To calculate the adiabatic index used two interesting characteristics, namely isobaric and isochoric heat capacity of the working fluid.The ratio of k = Cp / Cv - is the adiabatic index.
Why theoretical cycle heat engines used adiabatic process?Actually polytropic process are performed, but because they occur at high speed, made to assume the absence of heat exchange with the environment.
90% of electricity is generated by thermal power plants.In them is used as the working fluid steam.It is obtained at boiling water.To increase the working capacity of steam, it is superheated.Then, the high pressure superheated steam is supplied to the steam turbine.It also committed an adiabatic expansion process steam.Turbine gets rolling, it is transmitted to the electric generator.That, in turn, generates electricity to consumers.Steam turbines operate on the Rankine cycle.Ideally efficiency is also associated with an increase in temperature and pressure steam.
As seen from the above, the adiabatic process is very common in the production of mechanical and electrical energy.
