thermal efficiency engine. According to the law of conservation of energy, the work done by the engine is:

where is the heat received from the heater, is the heat given to the refrigerator.

Coefficient useful action heat engine is the ratio of the work done by the engine to the amount of heat received from the heater:

Since in all engines a certain amount of heat is transferred to the refrigerator, in all cases

Maximum value thermal efficiency engines. The French engineer and scientist Sadi Carnot (1796 1832) in his work “Reflection on the driving force of fire” (1824) set the goal: to find out under what conditions the operation of a heat engine would be most efficient, that is, under what conditions the engine would have maximum efficiency.

Carnot came up with an ideal heat engine with an ideal gas as the working fluid. He calculated the efficiency of this machine operating with a temperature heater and a temperature refrigerator

The main significance of this formula is that, as Carnot proved, based on the second law of thermodynamics, that any real heat engine operating with a temperature heater and a temperature refrigerator cannot have an efficiency exceeding the efficiency of an ideal heat engine.

Formula (4.18) gives the theoretical limit for the maximum efficiency of heat engines. It shows that the heat engine is more efficient, the higher the temperature of the heater and the lower the temperature of the refrigerator. Only when the temperature of the refrigerator is equal to absolute zero,

But the temperature of the refrigerator practically cannot 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 melts at a sufficiently high temperature.

Now the main efforts of engineers are aimed at increasing Engine efficiency by reducing the friction of their parts, fuel losses due to its incomplete combustion, etc. The real opportunities for increasing the efficiency here are still large. So, for a steam turbine, the initial and final steam temperatures are approximately as follows: At these temperatures, the maximum efficiency value is:

The actual value of the efficiency due to various kinds of energy losses is equal to:

Increasing the efficiency of heat engines, bringing it closer to the maximum possible is the most important technical challenge.

Thermal engines and nature conservation. The widespread use of heat engines in order to obtain energy that is convenient for use to the greatest extent, in comparison with

all other types of production processes are associated with environmental impacts.

According to the second law of thermodynamics, the production of electrical and mechanical energy, in principle, cannot be carried out without significant amounts of heat being removed to the environment. 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 is reached, the average temperature will rise in a noticeable way (by about one degree). A further rise in temperature could pose a threat of melting glaciers and a catastrophic rise in global sea levels.

But this is far from exhausted. Negative consequences application of heat engines. Furnaces of thermal power plants, engines internal combustion cars, etc. continuously emit substances harmful to plants, animals and humans into the atmosphere: sulfur compounds (during the combustion of coal), nitrogen oxides, hydrocarbons, carbon monoxide (CO), etc. Cars, the number which is threateningly growing, and the purification of exhaust gases is difficult. Nuclear power plants face the problem of hazardous radioactive waste disposal.

In addition, the use of steam turbines at power plants requires large areas for ponds to cool the exhaust steam. With an increase in the capacity of power plants, the need for water increases sharply. In 1980, about 35% of the water supply of all sectors of the economy was required for these purposes in our country.

All this poses a number of serious problems for society. Along with the most important task of increasing the efficiency of heat engines, it is necessary to carry out a number of measures to protect the environment. It is necessary to improve the efficiency of structures that prevent the emission of harmful substances into the atmosphere; to achieve more complete combustion of fuel in automotive engines. Already, cars with a high content of CO in the exhaust gases are not allowed to operate. The possibility of creating electric vehicles that can compete with conventional ones and the possibility of using fuel without harmful substances in exhaust gases, for example, in engines running on a mixture of hydrogen and oxygen, are discussed.

In order to save space and water resources, it is expedient to build entire complexes of power plants, primarily nuclear ones, with a closed water supply cycle.

Another direction of the efforts being made is to increase the efficiency of energy use, the struggle for its savings.

Solving the problems listed above is vital for humanity. And these problems with maximum success can

be solved in a socialist society with a planned development of the economy on a national scale. But the organization of environmental protection requires efforts on a global scale.

1. What processes are called irreversible? 2. Name the most typical irreversible processes. 3. Give examples of irreversible processes not mentioned in the text. 4. Formulate the second law of thermodynamics. 5. If the rivers flowed backwards, would this mean a violation of the law of conservation of energy? 6. What device is called a heat engine? 7. What is the role of the heater, refrigerator and working fluid of a heat engine? 8. Why is it impossible to use the internal energy of the ocean as an energy source in heat engines? 9. What is called the efficiency of a heat engine?

10. What is the maximum possible value of the efficiency of a heat engine?

Since ancient times, people have tried to convert energy into mechanical work. They converted the kinetic energy of the wind, the potential energy of water, etc. Starting from the 18th century, machines began to appear that convert the internal energy of fuel into work. Such machines worked thanks to heat engines.

A heat engine is a device that converts thermal energy into mechanical work due to expansion (most often gases) from high temperature.

Any heat engines have components:

• A heating element. A body with a high temperature relative to the environment.
• working body. Since expansion provides the job, this element must expand well. As a rule, gas or steam is used.
• cooler. Body with low temperature.

The working fluid receives thermal energy from the heater. As a result, it begins to expand and do work. In order for the system to perform work again, it must be returned to its original state. Therefore, the working fluid is cooled, that is, excess thermal energy is, as it were, discharged into the cooling element. And the system comes to its original state, then the process repeats again.

Efficiency calculation

To calculate the efficiency, we introduce the following notation:

Q 1 - The amount of heat received from the heating element

A’– Work done by the working body

Q 2 - The amount of heat received by the working fluid from the cooler

In the process of cooling, the body transfers heat, so Q 2< 0.

The operation of such a device is a cyclic process. This means that after doing full cycle, the internal energy will return to its original state. Then, according to the first law of thermodynamics, the work done by the working fluid will be equal to the difference between the amount of heat received from the heater and the heat received from the cooler:

Q 2 is a negative value, so it is taken modulo

Efficiency is expressed as the ratio of useful work to the total work that the system has performed. In this case, full work will be equal to the amount of heat that is spent on heating the working fluid. All expended energy is expressed through Q 1 .

Therefore, the efficiency factor is defined as.

>>Physics: The principle of operation of heat engines. Coefficient of performance (COP) of heat engines

The reserves of internal energy in the earth's crust and oceans can be considered practically unlimited. But to solve practical problems, having energy reserves is still not enough. It is also necessary to be able to use energy to set in motion machine tools in factories, means of transport, tractors and other machines, rotate the rotors of electric current generators, etc. Mankind needs engines - devices capable of doing work. Most of the engines on Earth are heat engines. Heat engines are devices that convert the internal energy of fuel into mechanical energy.
Principles of operation of heat engines. In order for the engine to do work, a pressure difference is needed on both sides of the engine piston or turbine blades. In all heat engines, this pressure difference is achieved by increasing the temperature of the working fluid (gas) by hundreds or thousands of degrees compared to the ambient temperature. This increase in temperature occurs during the combustion of fuel.
One of the main parts of the engine is a gas-filled vessel with a movable piston. The working fluid in all heat engines is a gas that does work during expansion. Let us denote the initial temperature of the working fluid (gas) through T1. This temperature in steam turbines or machines is acquired by steam in a steam boiler. in internal combustion engines and gas turbines temperature rise occurs when fuel is burned inside the engine itself. Temperature T1 heater temperature."
The role of the refrigerator As work is done, the gas loses energy and inevitably cools to a certain temperature. T2, which is usually slightly higher than the ambient temperature. They call her refrigerator temperature. The refrigerator is the atmosphere or special devices for cooling and condensing exhaust steam - capacitors. In the latter case, the temperature of the refrigerator may be slightly below the temperature of the atmosphere.
Thus, in the engine, the working fluid during expansion cannot give all its internal energy to do work. Part of the heat is inevitably transferred to the cooler (atmosphere) along with exhaust steam or exhaust gases from internal combustion engines and gas turbines. This part of the internal energy is lost.
A heat engine performs work due to the internal energy of the working fluid. Moreover, in this process, heat is transferred from hotter bodies (heater) to colder ones (refrigerator).
circuit diagram heat engine is shown in Figure 13.11.
The working body of the engine receives from the heater during the combustion of fuel the amount of heat Q1 does the job A´ and transfers the amount of heat to the refrigerator Q2 .
Coefficient of performance (COP) of a heat engine.The impossibility of complete conversion of the internal energy of gas into the work of heat engines is due to the irreversibility of processes in nature. If heat could spontaneously return from the refrigerator to the heater, then the internal energy could be completely converted into useful work using any heat engine.
According to the law of conservation of energy, the work done by the engine is:

where Q1 is the amount of heat received from the heater, and Q2- the amount of heat given to the refrigerator.
Coefficient of performance (COP) of a heat engine called the work relation A´ performed by the engine to the amount of heat received from the heater:

Since in all engines some amount of heat is transferred to the refrigerator, then η<1.
The efficiency of a heat engine is proportional to the temperature difference between the heater and the cooler. At T1-T2=0 motor cannot run.
The maximum value of the efficiency of heat engines. The laws of thermodynamics make it possible to calculate the maximum possible efficiency of a heat engine operating with a heater having a temperature T1, and a refrigerator with a temperature T2. This was first done by the French engineer and scientist Sadi Carnot (1796-1832) in his work “Reflections on the driving force of fire and on machines capable of developing this force” (1824).
Carnot came up with an ideal heat engine with an ideal gas as the working fluid. An ideal Carnot heat engine operates on a cycle consisting of two isotherms and two adiabats. First, a vessel with a gas is brought into contact with a heater, the gas expands isothermally, doing positive work, at a temperature T1, while it receives the amount of heat Q1.
Then the vessel is thermally insulated, the gas continues to expand already adiabatically, while its temperature decreases to the temperature of the refrigerator T2. After that, the gas is brought into contact with the refrigerator, under isothermal compression, it gives the refrigerator the amount of heat Q2, shrinking to volume V 4 . Then the vessel is thermally insulated again, the gas is compressed adiabatically to a volume V 1 and returns to its original state.
Carnot obtained the following expression for the efficiency of this machine:

As expected, the efficiency of the Carnot machine is directly proportional to the difference between the absolute temperatures of the heater and cooler.
The main meaning of this formula is that any real heat engine operating with a heater having a temperature T1, and refrigerator with temperature T2, cannot have an efficiency exceeding the efficiency of an ideal heat engine.

Formula (13.19) gives the theoretical limit for the maximum value of the efficiency of heat engines. It shows that the heat engine is more efficient, the higher the temperature of the heater and the lower the temperature of the refrigerator. Only when the temperature of the refrigerator is equal to absolute zero, η =1.
But the temperature of the refrigerator practically cannot be 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 melts at a sufficiently high temperature.
Now the main efforts of engineers are aimed at increasing the efficiency of engines by reducing the friction of their parts, fuel losses due to its incomplete combustion, etc. The real opportunities for increasing the efficiency here are still large. So, for a steam turbine, the initial and final steam temperatures are approximately as follows: T1≈800 K and T2≈300 K. At these temperatures, the maximum value of the efficiency is:

The actual value of the efficiency due to various kinds of energy losses is approximately 40%. Diesel engines have the maximum efficiency - about 44%.
Increasing the efficiency of heat engines and bringing it closer to the maximum possible is the most important technical challenge.
Heat engines do work due to the difference in gas pressure on the surfaces of pistons or turbine blades. This pressure difference is generated by the temperature difference. The maximum possible efficiency is proportional to this temperature difference and inversely proportional to the absolute temperature of the heater.
A heat engine cannot operate without a refrigerator, the role of which is usually played by the atmosphere.

???
1. What device is called a heat engine?
2. What is the role of the heater, cooler and working fluid in a heat engine?
3. What is called the efficiency of the engine?
4. What is the maximum value of the efficiency of a heat engine?

G.Ya.Myakishev, B.B.Bukhovtsev, N.N.Sotsky, Physics Grade 10

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The operation of many types of machines is characterized by such an important indicator as the efficiency of a heat engine. Every year, engineers strive to create more advanced equipment, which, with less, would give the maximum result from its use.

Heat engine device

Before understanding what it is, it is necessary to understand how this mechanism works. Without knowing the principles of its action, it is impossible to find out the essence of this indicator. A heat engine is a device that does work by using internal energy. Any heat engine that turns into a mechanical one uses the thermal expansion of substances with increasing temperature. In solid-state engines, it is possible not only to change the volume of matter, but also the shape of the body. The operation of such an engine is subject to the laws of thermodynamics.

Operating principle

In order to understand how a heat engine works, it is necessary to consider the basics of its design. For the operation of the device, two bodies are needed: hot (heater) and cold (refrigerator, cooler). The principle of operation of heat engines (the efficiency of heat engines) depends on their type. Often, the steam condenser acts as a refrigerator, and any type of fuel that burns in the furnace acts as a heater. The efficiency of an ideal heat engine is found by the following formula:

Efficiency = (Theating - Tcold.) / Theating. x 100%.

At the same time, the efficiency of a real engine can never exceed the value obtained according to this formula. Also, this indicator will never exceed the above value. To increase the efficiency, most often increase the temperature of the heater and reduce the temperature of the refrigerator. Both of these processes will be limited by the actual operating conditions of the equipment.

During the operation of a heat engine, work is done, as the gas begins to lose energy and cools to a certain temperature. The latter is usually a few degrees above the surrounding atmosphere. This is the refrigerator temperature. Such a special device is designed for cooling with subsequent condensation of the exhaust steam. Where condensers are present, the temperature of the refrigerator is sometimes lower than the ambient temperature.

In a heat engine, the body, when heated and expanded, is not able to give all its internal energy to do work. Some of the heat will be transferred to the refrigerator along with or steam. This part of the thermal is inevitably lost. During the combustion of fuel, the working fluid receives a certain amount of heat Q 1 from the heater. At the same time, it still does work A, during which it transfers part of the thermal energy to the refrigerator: Q 2

Efficiency characterizes the efficiency of the engine in the field of energy conversion and transmission. This indicator is often measured as a percentage. Efficiency formula:

η*A/Qx100%, where Q is the expended energy, A is useful work.

Based on the law of conservation of energy, we can conclude that the efficiency will always be less than unity. In other words, there will never be more useful work than the energy expended on it.

Engine efficiency is the ratio of useful work to the energy supplied by the heater. It can be represented as the following formula:

η \u003d (Q 1 -Q 2) / Q 1, where Q 1 is the heat received from the heater, and Q 2 is given to the refrigerator.

Heat engine operation

The work done by a heat engine is calculated by the following formula:

A = |Q H | - |Q X |, where A is work, Q H is the amount of heat received from the heater, Q X is the amount of heat given to the cooler.

|Q H | - |Q X |)/|Q H | = 1 - |Q X |/|Q H |

It is equal to the ratio of the work done by the engine to the amount of heat received. Part of the thermal energy is lost during this transfer.

Carnot engine

The maximum efficiency of a heat engine is noted for the Carnot device. This is due to the fact that in this system it depends only on the absolute temperature of the heater (Тн) and cooler (Тх). The efficiency of a heat engine operating on is determined by the following formula:

(Tn - Tx) / Tn = - Tx - Tn.

The laws of thermodynamics made it possible to calculate the maximum efficiency that is possible. For the first time this indicator was calculated by the French scientist and engineer Sadi Carnot. He invented a heat engine that ran on ideal gas. It works on a cycle of 2 isotherms and 2 adiabats. The principle of its operation is quite simple: a heater contact is brought to the vessel with gas, as a result of which the working fluid expands isothermally. At the same time, it functions and receives a certain amount of heat. After the vessel is thermally insulated. Despite this, the gas continues to expand, but already adiabatically (without heat exchange with the environment). At this time, its temperature drops to the refrigerator. At this moment, the gas is in contact with the refrigerator, as a result of which it gives it a certain amount of heat during isometric compression. Then the vessel is thermally insulated again. In this case, the gas is adiabatically compressed to its original volume and state.

Varieties

Nowadays, there are many types of heat engines that operate on different principles and on different fuels. They all have their own efficiency. These include the following:

An internal combustion engine (piston), which is a mechanism where part of the chemical energy of the burning fuel is converted into mechanical energy. Such devices can be gas and liquid. There are 2-stroke and 4-stroke engines. They may have a continuous duty cycle. According to the method of preparing a mixture of fuel, such engines are carburetor (with external mixture formation) and diesel (with internal). According to the types of energy converter, they are divided into piston, jet, turbine, combined. The efficiency of such machines does not exceed 0.5.

Stirling engine - a device in which the working fluid is in a closed space. It is a kind of external combustion engine. The principle of its operation is based on periodic cooling/heating of the body with the production of energy due to a change in its volume. This is one of the most efficient engines.

Turbine (rotary) engine with external combustion of fuel. Such installations are most often found in thermal power plants.

Turbine (rotary) internal combustion engines are used at thermal power plants in peak mode. Not as common as others.

A turboprop engine generates some of the thrust due to the propeller. The rest comes from exhaust gases. Its design is a rotary engine on the shaft of which a propeller is mounted.

Other types of heat engines

Rocket, turbojet and which receive thrust due to the return of exhaust gases.

Solid state engines use a solid body as fuel. When working, it is not its volume that changes, but its shape. During operation of the equipment, an extremely small temperature difference is used.

How can you increase efficiency

Is it possible to increase the efficiency of a heat engine? The answer must be sought in thermodynamics. It studies the mutual transformations of different types of energy. It has been established that all available mechanical, etc., is impossible. At the same time, their conversion into thermal energy occurs without any restrictions. This is possible due to the fact that the nature of thermal energy is based on the disordered (chaotic) movement of particles.

The more the body heats up, the faster the molecules that make it up will move. Particle motion will become even more erratic. Along with this, everyone knows that order can be easily turned into chaos, which is very difficult to order.