\u003e\u003e Physics: The principle of operation of thermal motors. Efficiency coefficient (efficiency) of thermal engines
Internal energy reserves in the earth's crust and oceans can be considered practically unlimited. But to solve practical problems, it is still not enough to have energy reserves. You must still be able to drive the machine to the factors and plants, the means of transport, tractors, and other machines, rotate the rotors of electric current generators, etc. Humanity needs engines - devices capable of doing work. Most of the engines on Earth are heat engines. Thermal motors are devices that convert the internal fuel energy into mechanical.
Principles of action of thermal motors.In order for the engine to work, the pressure difference is needed on both sides of the engine piston or turbine blades. In all thermal motors, this pressure difference is achieved by increasing the temperature of the working fluid (gas) for hundreds or thousands of degrees compared to the ambient temperature. Such an increase in temperature occurs when fuel combustion.
One of the main parts of the engine is a gas-filled vessel with a movable piston. All thermal engines are a working fluid that makes work when expanding. Denote the initial temperature of the working fluid (gas) through T 1. This temperature in steam turbines or machines acquires pairs in a steam boiler. In internal combustion engines and gas turbines, the temperature increase occurs when combustion of the fuel inside the engine itself. Temperature T 1. Heater temperature. "
The role of the refrigerator.As the operation is performed, gas loses energy and is inevitably cooled to some temperature T 2.which is usually slightly higher than the ambient temperature. It is called temperature refrigerator. The refrigerator is the atmosphere or special devices for cooling and condensation of the spent steam - condencators. In the latter case, the temperature of the refrigerator can be slightly lower than the temperature of the atmosphere.
Thus, in the engine, the working body during expansion cannot give all its internal energy to performing work. Part of the heat is inevitably transmitted to the refrigerator (atmosphere) together with spent ferry or exhaust gases of internal combustion engines and gas turbines. This part of the internal energy is lost.
The thermal engine makes work due to the internal energy of the working fluid. Moreover, in this process, heat is transmitted from more hot bodies (heater) to a colder (refrigerator).
The circuit diagram of the thermal engine is shown in Figure 13.11.
The engine's working fluid gets from the heater when combustion of fuel. Number of heat Q 1.makes work A.'And transmits the refrigerator the amount of heat Q 2. .
Efficiency coefficient (efficiency) of the heat engine. In the possibility of the complete transformation of the internal energy of gas into the operation of thermal motors is due to the irreversibility of processes in nature. If the heat could spontaneously return from the refrigerator to the heater, then the internal energy could be fully turned into a useful operation using any thermal engine.
According to the law of energy conservation, the operation performed by the engine is equal to:
where Q 1. - the amount of heat obtained from the heater, and Q 2. - The amount of heat, given to the refrigerator.
The coefficient of useful (efficiency) of the thermal enginecall the attitude of work A'Made by the engine, to the amount of heat obtained from the heater:
Since all engines have a certain amount of heat transmitted to the refrigerator, then η<1.
The efficiency of the thermal engine is proportional to the difference in the temperature of the heater and the refrigerator. For T 1 -T 2\u003d 0 Engine can not work.
The maximum value of the efficiency of thermal motors. The laws of thermodynamics make it possible to calculate the maximum possible efficiency of the heat engine operating with a heater having a temperature T 1.and refrigerator with temperature T 2.. For the first time, this was done by the French engineer and scholar Sadi Carlo (1796-1832) in the work "Reflections on the driving force of fire and about cars capable of developing this power" (1824).
Carno came up with the perfect thermal machine with perfect gas as a working body. The ideal heat machine carno works on a cycle consisting of two isotherms and two adiabat. First, the vessel with the gas is in contact with the heater, gas is isothermally expanding, making positive operation at temperatures T 1, at the same time he gets the amount of heat Q 1..
The vessel is then thermally insulated, the gas continues to expand already adiabato, while its temperature drops to the temperature of the refrigerator T 2.. After that, the gas is brought into contact with the refrigerator, during isothermal compression it gives a refrigerator the amount of heat Q 2.shrinking to volume V 4.
Carno received the following expression for the efficiency of this machine:
As expected, the efficiency of the car carno is directly proportional to the difference in the absolute temperatures of the heater and the refrigerator.
The main importance of this formula is that any real thermal machine operating with a heater having a temperature T 1, and refrigerator with temperature T 2.can not have an efficiency exceeding the efficiency of the perfect heat machine.
Formula (13.19) gives the theoretical limit for the maximum value of the efficiency of thermal motors. It shows that the thermal motor is the more effective than the higher the temperature of the heater and below the temperature of the refrigerator. Only at a refrigerator temperature equal to absolute zero, η
=1.
But the temperature of the refrigerator almost can not be below the ambient temperature. You can increase the temperature of the heater. However, any material (solid) has limited heat resistance, or heat-resistance. When heated, it gradually loses its elastic properties, and at a sufficiently high temperature melts.
Now the main efforts of engineers are aimed at increasing the efficiency of engines by reducing the friction of their parts, losses of fuel due to its incomplete combustion, etc. The real opportunities to increase the efficiency here are still large. So, for the steam turbine, the initial and final temperatures of the pair are approximately as follows: T 1.≈800 K I. T 2.≈300 K. At these temperatures, the maximum value of the efficiency of the efficiency is:
Increasing the efficiency of thermal engines and approaching it to the maximum possible - the most important technical task.
Thermal motors make a job due to the difference in gas pressures on the surfaces of the pistons or turbine blades. This pressure difference is created using the temperature difference. The maximum possible efficiency is proportional to this temperature difference and inversely proportional to the absolute temperature of the heater.
The thermal motor can not work without a refrigerator, the role of which is usually played by the atmosphere.
???
1. What device is called the thermal engine?
2. What is the role of the heater, refrigerator and the working fluid in the thermal engine?
3. What is called the efficiency of the engine?
4. What is the maximum value of the efficiency of the thermal motor?
G. Y. Mikishev, B.B. Bukhovtsev, N.N.Sotsky, Physics 10
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The heat engine (machine) is a device that converts the internal energy of fuel into mechanical work, exchanging heating with the surrounding bodies. Most modern automotive, aircraft, ship and rocket engines are designed on the principles of the thermal engine. The work is made due to changes in the volume of the working substance, and the value of the efficiency of any type of engine is used, which is called the efficiency (efficiency).
How is the heat engine
From the point of view of thermodynamics (section of physics, which studies the patterns of mutual transformations of internal and mechanical energies and transmission of energy from one body to another), any thermal engine consists of a heater, refrigerator and a working fluid.
Fig. 1. Structural scheme of the thermal engine:.
The first mention of the prototype thermal machine belongs to the steam turbine, which was invented in ancient Rome (II century BC). True, the invention has not yet been widely used due to the lack of many auxiliary parts at the time. For example, it was not yet invented such a key element for the operation of any mechanism as the bearing.
The overall work scheme of any thermal machine looks like this:
- The heater has a temperature T 1 high enough to convey a large amount of heat Q 1. In most heat machines, heating is obtained during the combustion of the fuel mixture (oxygen fuel);
- The working fluorescence (pairs or gas) of the engine makes a useful work. BUT,for example, move the piston or rotate the turbine;
- The refrigerator absorbs part of the energy from the working fluid. Refrigerator temperature T 2< Т 1 . То есть, на совершение работы идет только часть теплоты Q 1 .
The heat machine (engine) must work continuously, so the working fluid should return to the initial state so that its temperature becomes equal to T 1. For the continuity of the process, the operation of the machine should occur cyclically, periodically repeated. To create a cyclic mechanism - return the working fluid (gas) to its original state - the refrigerator is needed to cool the gas during the compression process. The refrigerator can serve as an atmosphere (for internal combustion engines) or cold water (for steam turbines).
What is equal to the efficiency of the thermal engine
To determine the effectiveness of thermal engines, the French engineer-mechanic Sadi Carno in 1824. introduced the concept of the efficiency of the heat engine. The Greek letter η is used to designate the efficiency. The value of η is calculated using the thermal engine efficiency formula:
$$ η \u003d (A \\ OVER Q1) $$
Since $ a \u003d Q1 - Q2 $, then
$ η \u003d (1 - Q2 \\ OVER Q1) $
Since all engines part of the heat is given to the refrigerator, it is always η< 1 (меньше 100 процентов).
The maximum possible efficiency of the perfect thermal engine
As an ideal heat car, Sadi Karno offered a car with perfect gas as a working fluid. The ideal carno model works on a cycle (carno cycle), consisting of two isotherms and two adiabat.
Fig. 2. Carno cycle:.
Recall:
- Adiabatic process - It is a thermodynamic process occurring without heat exchange with the environment (Q \u003d 0);
- Isothermal process - This is a thermodynamic process occurring at a constant temperature. Since the perfect gas internal energy depends only on temperature, the amount of heat transmitted gas transmitted Q. It is completely on the performance of A (Q \u003d A) .
Sadi Carno proved that the maximum possible efficiency, which can be achieved by an ideal thermal engine is determined using the following formula:
$$ ηmax \u003d 1- (T2 \\ OVER T1) $$
The carno formula allows you to calculate the maximum possible efficiency of the thermal engine. The more the difference between the temperatures of the heater and the refrigerator, the greater the efficiency.
What are the real efficiency of different types of engines
From the above examples, it can be seen that the largest efficiency of the efficiency (40-50%) are internal combustion engines (in a diesel version) and jet engines on liquid fuel.
Fig. 3. efficiency of real thermal engines:.
What did we know?
So, we learned what the engine efficiency is. The magnitude of the efficiency of any thermal engine is always less than 100 percent. The larger the temperature difference of the heater T 1 and the refrigerator T 2, the greater the efficiency.
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The work performed by the engine is:
For the first time this process was considered by the French engineer and scientists N. L. S. Karno in 1824 in the book "Reflections on the driving force of fire and about cars that can develop this power."
The goal of research of carno was to find out the causes of the imperfection of thermal vehicles of that time (they had an efficiency of ≤ 5%) and the search for their improvement paths.
Carno cycle is the most effective of all possible. His efficiency is maximum.
The figure shows the thermodynamic process-cycles. In the process of isothermal expansion (1-2) at temperatures T. 1 , work is performed due to the change of the internal energy of the heater, i.e., due to the grade of the amount of heat Q.:
A. 12 = Q. 1 ,
Cooling gas in front of compression (3-4) occurs when adiabatic expansion (2-3). Change in inner energy ΔU. 23 with adiabatic process ( Q \u003d 0.) Fully converted to mechanical work:
A. 23 \u003d -Δu. 23 ,
The gas temperature as a result of adiabatic rash (2-3) decreases to the temperature of the refrigerator T. 2 < T. 1 . In process (3-4), gas isothermally compressed, having transferred the amount of heat to the refrigerator Q 2.:
A 34 \u003d Q 2,
The cycle is completed with the process of adiabatic compression (4-1), in which the gas heats up to temperature T 1.
The maximum value of the efficiency of thermal engines operating on the ideal gas, along the carno cycle:
.
The essence of the formula is expressed in proven FROM. Carno Theorem that the efficiency of any thermal engine may not exceed the efficiency of the carno cycle carried out at the same temperature of the heater and refrigerator.
The efficiency of the thermal engine. According to the law of energy conservation, the operation performed by the engine is equal to:
where - the heat obtained from the heater is heat, given to the refrigerator.
The efficiency of the thermal engine is called the ratio of the operation of the engine performed, to the amount of heat obtained from the heater:
Since all engines have a certain amount of heat transferred to the refrigerator, then in all cases
The maximum value of the efficiency of thermal motors. French engineer and scholar Karo (1796 1832) In the work "Reflections on the driving force of fire" (1824) put a goal: to find out however, the operation of the heat engine will be the most effective, i.e. under what conditions the engine will have a maximum efficiency.
Carno came up with the perfect thermal machine with perfect gas as a working body. It calculated the efficiency of this car operating with a temperature heater and temperature refrigerator
The main value of this formula is how the carno proved, relying on the second law of thermodynamics, which, any real heat carrier, working with a temperature heater and a temperature refrigerator cannot have a efficiency coefficient exceeding the efficiency of the perfect heat machine.
Formula (4.18) gives the theoretical limit for the maximum value of the efficiency of thermal motors. It shows that the thermal motor is the more effective than the higher the temperature of the heater and below the temperature of the refrigerator. Only at a refrigerator temperature equal to absolute zero,
But the temperature of the refrigerator almost can not be much lower than the ambient temperature. You can increase the temperature of the heater. However, any material (solid) has limited heat resistance, or heat-resistance. When heated, it gradually loses its elastic properties, and at a sufficiently high temperature melts.
Now the main efforts of engineers are aimed at increasing the efficiency of engines by reducing the friction of their parts, losses of fuel due to its incomplete combustion, etc. The real opportunities to increase the efficiency here are still large. So, for the steam turbine, the initial and final temperatures of the pair are approximately as follows: at these temperatures, the maximum efficiency of the efficiency is:
The actual value of the efficiency due to various kinds of energy losses is:
Increasing the efficiency of thermal motors, approaching it to the maximum possible - the most important technical task.
Thermal motors and nature protection. The widespread use of thermal engines in order to obtain energy convenient to use most compared to
all other types of production processes are associated with environmental impact.
According to the second law of thermodynamics, the production of electrical and mechanical energy in principle cannot be carried out without removal into the environment of significant amounts of heat. This cannot but lead to a gradual increase in the average temperature on Earth. Now the power consumption is about 1010 kW. When this power reaches that the average temperature will increase noticeable manner (about one degree). Further increase in temperature can create a threat to the melting of glaciers and a catastrophic increase in the level of the world's ocean.
But these are far from being exhausted by the negative consequences of the use of thermal motors. Fireings of thermal power plants, internal combustion engines of cars, etc. Continuously discarded to the atmosphere, harmful plants, animals and human substances: sulfur compounds (when combustion of coal), nitrogen oxides, hydrocarbons, carbon oxide (CO), etc. In this regard, cars are represented, the number of which grows threateningly, and the cleaning of waste gases is difficult. At nuclear power plants there is a problem of the burial of hazardous radioactive waste.
In addition, the use of steam turbines on power plants requires large areas under the ponds to cool the spent steam with an increase in power plant capacity sharply increases the need for water. In 1980, in our country, for these purposes, it was required near water, that is, about 35% of the water supply of all branches of the economy.
All this puts a number of serious problems with society. Along with the most important task of improving the efficiency of thermal engines, a number of environmental protection measures are required. It is necessary to increase the effectiveness of structures that impede the emission of harmful substances into the atmosphere; To achieve a more complete combustion of fuel in car engines. Already no longer allowed to operate cars with a high content of CO in exhaust gases. The possibility of creating electric vehicles capable of competing with ordinary, and the possibility of using fuel without harmful substances in waste gases, for example, in engines operating on a hydrogen mixture with oxygen are discharged.
It is advisable for saving area and water resources to construct entire complexes of power plants, primarily atomic, with a closed water supply cycle.
Another direction of the accompanying effort is to increase energy efficiency, the struggle for its savings.
The solution of the problems listed above is vital for humanity. And these problems with maximum success can
to be solved in a socialist society with the planned development of the economy across the country. But the organization of environmental protection requires efforts to the scale of the globe.
1. What processes are called irreversible? 2. Name the most typical irreversible processes. 3. Give examples of irreversible processes that are not mentioned in the text. 4. Word the second law of thermodynamics. 5. If the rivers flow reversed, would this violate the law of energy conservation? 6. What device is called a thermal engine? 7. What is the role of the heater, refrigerator and the working body of the thermal engine? 8. Why in thermal motors can not be used as an ocean's internal energy source? 9. What is called the efficiency of the thermal engine?
10. What is the maximum possible value of the efficiency of the thermal engine?
The efficiency of the thermal engine. According to the law of energy conservation, the operation performed by the engine is equal to:
where - the heat obtained from the heater is heat, given to the refrigerator.
The efficiency of the thermal engine is called the ratio of the operation of the engine performed, to the amount of heat obtained from the heater:
Since all engines have a certain amount of heat transferred to the refrigerator, then in all cases
The maximum value of the efficiency of thermal motors. French engineer and scholar Karo (1796 1832) In the work "Reflections on the driving force of fire" (1824) put a goal: to find out however, the operation of the heat engine will be the most effective, i.e. under what conditions the engine will have a maximum efficiency.
Carno came up with the perfect thermal machine with perfect gas as a working body. It calculated the efficiency of this car operating with a temperature heater and temperature refrigerator
The main value of this formula is how the carno proved, relying on the second law of thermodynamics, which, any real heat carrier, working with a temperature heater and a temperature refrigerator cannot have a efficiency coefficient exceeding the efficiency of the perfect heat machine.
Formula (4.18) gives the theoretical limit for the maximum value of the efficiency of thermal motors. It shows that the thermal motor is the more effective than the higher the temperature of the heater and below the temperature of the refrigerator. Only at a refrigerator temperature equal to absolute zero,
But the temperature of the refrigerator almost can not be much lower than the ambient temperature. You can increase the temperature of the heater. However, any material (solid) has limited heat resistance, or heat-resistance. When heated, it gradually loses its elastic properties, and at a sufficiently high temperature melts.
Now the main efforts of engineers are aimed at increasing the efficiency of engines by reducing the friction of their parts, losses of fuel due to its incomplete combustion, etc. The real opportunities to increase the efficiency here are still large. So, for the steam turbine, the initial and final temperatures of the pair are approximately as follows: at these temperatures, the maximum efficiency of the efficiency is:
The actual value of the efficiency due to various kinds of energy losses is:
Increasing the efficiency of thermal motors, approaching it to the maximum possible - the most important technical task.
Thermal motors and nature protection. The widespread use of thermal engines in order to obtain energy convenient to use most compared to
all other types of production processes are associated with environmental impact.
According to the second law of thermodynamics, the production of electrical and mechanical energy in principle cannot be carried out without removal into the environment of significant amounts of heat. This cannot but lead to a gradual increase in the average temperature on Earth. Now the power consumption is about 1010 kW. When this power reaches that the average temperature will increase noticeable manner (about one degree). Further increase in temperature can create a threat to the melting of glaciers and a catastrophic increase in the level of the world's ocean.
But these are far from being exhausted by the negative consequences of the use of thermal motors. Fireings of thermal power plants, internal combustion engines of cars, etc. Continuously discarded to the atmosphere, harmful plants, animals and human substances: sulfur compounds (when combustion of coal), nitrogen oxides, hydrocarbons, carbon oxide (CO), etc. In this regard, cars are represented, the number of which grows threateningly, and the cleaning of waste gases is difficult. At nuclear power plants there is a problem of the burial of hazardous radioactive waste.
In addition, the use of steam turbines on power plants requires large areas under the ponds to cool the spent steam with an increase in power plant capacity sharply increases the need for water. In 1980, in our country, for these purposes, it was required near water, that is, about 35% of the water supply of all branches of the economy.
All this puts a number of serious problems with society. Along with the most important task of improving the efficiency of thermal engines, a number of environmental protection measures are required. It is necessary to increase the effectiveness of structures that impede the emission of harmful substances into the atmosphere; To achieve a more complete combustion of fuel in car engines. Already no longer allowed to operate cars with a high content of CO in exhaust gases. The possibility of creating electric vehicles capable of competing with ordinary, and the possibility of using fuel without harmful substances in waste gases, for example, in engines operating on a hydrogen mixture with oxygen are discharged.
It is advisable for saving area and water resources to construct entire complexes of power plants, primarily atomic, with a closed water supply cycle.
Another direction of the accompanying effort is to increase energy efficiency, the struggle for its savings.
The solution of the problems listed above is vital for humanity. And these problems with maximum success can
to be solved in a socialist society with the planned development of the economy across the country. But the organization of environmental protection requires efforts to the scale of the globe.
1. What processes are called irreversible? 2. Name the most typical irreversible processes. 3. Give examples of irreversible processes that are not mentioned in the text. 4. Word the second law of thermodynamics. 5. If the rivers flow reversed, would this violate the law of energy conservation? 6. What device is called a thermal engine? 7. What is the role of the heater, refrigerator and the working body of the thermal engine? 8. Why in thermal motors can not be used as an ocean's internal energy source? 9. What is called the efficiency of the thermal engine?
10. What is the maximum possible value of the efficiency of the thermal engine?
