What is the efficiency of heat engines. Thermal engine. Efficiency of a heat engine. Example of problem solving
Modern realities involve the widespread operation of heat engines. Numerous attempts to replace them with electric motors have so far failed. The problems associated with the accumulation of electricity in autonomous systems are solved with great difficulty.
Still relevant are the problems of technology for the manufacture of electric power accumulators, taking into account their long-term use. Speed characteristics electric vehicles are far from those of cars on engines internal combustion.
The first steps towards the creation of hybrid engines can significantly reduce harmful emissions in megacities, solving environmental problems.
A bit of history
The possibility of converting steam energy into motion energy was known in antiquity. 130 BC: Philosopher Heron of Alexandria presented to the audience a steam toy - aeolipil. A sphere filled with steam began to rotate under the action of jets emanating from it. This prototype of modern steam turbines did not find application in those days.
For many years and centuries, the development of the philosopher was considered only a fun toy. In 1629, the Italian D. Branchi created an active turbine. Steam set in motion a disk equipped with blades.
From that moment began the rapid development steam engines.
heat engine
The conversion of fuel into energy for the movement of parts of machines and mechanisms is used in heat engines.
The main parts of machines: a heater (a system for obtaining energy from outside), a working fluid (performs a useful action), a refrigerator.
The heater is designed to ensure that the working fluid has accumulated a sufficient supply of internal energy to perform useful work. The refrigerator removes excess energy.
The main characteristic of efficiency is called the efficiency of heat engines. This value shows what part of the energy spent on heating is spent on doing useful work. The higher the efficiency, the more profitable the operation of the machine, but this value cannot exceed 100%.
Efficiency calculation
Let the heater acquire from outside the energy equal to Q 1 . The working fluid did work A, while the energy given to the refrigerator was Q 2 .
Based on the definition, we calculate the efficiency:
η= A / Q 1 . We take into account that A \u003d Q 1 - Q 2.
From here, the efficiency of the heat engine, the formula of which has the form η = (Q 1 - Q 2) / Q 1 = 1 - Q 2 / Q 1, allows us to draw the following conclusions:
- Efficiency cannot exceed 1 (or 100%);
- to maximize this value, either an increase in the energy received from the heater or a decrease in the energy given to the refrigerator is necessary;
- an increase in the energy of the heater is achieved by changing the quality of the fuel;
- reducing the energy given to the refrigerator, allow you to achieve design features engines.
Ideal heat engine
Is it possible to create such an engine, the efficiency of which would be maximum (ideally, equal to 100%)? The French theoretical physicist and talented engineer Sadi Carnot tried to find the answer to this question. In 1824, his theoretical calculations about the processes occurring in gases were made public.
The main idea behind perfect car, we can consider carrying out reversible processes with an ideal gas. We start with the expansion of the gas isothermally at a temperature T 1 . The amount of heat required for this is Q 1. After the gas expands without heat exchange. Having reached the temperature T 2, the gas is compressed isothermally, transferring the energy Q 2 to the refrigerator. The return of the gas to its original state is adiabatic.
Ideal efficiency heat engine Carnot, when accurately calculated, is equal to the ratio of the temperature difference between the heating and cooling devices to the temperature that the heater has. It looks like this: η=(T 1 - T 2)/ T 1.
The possible efficiency of a heat engine, the formula of which is: η= 1 - T 2 / T 1 , depends only on the temperature of the heater and cooler and cannot be more than 100%.
Moreover, this ratio allows us to prove that the efficiency of heat engines can be equal to unity only when the refrigerator reaches temperatures. As you know, this value is unattainable.
Carnot's theoretical calculations make it possible to determine the maximum efficiency of a heat engine of any design.
The theorem proved by Carnot is as follows. An arbitrary heat engine under no circumstances is capable of having a coefficient of efficiency greater than the similar value of the efficiency of an ideal heat engine.
Example of problem solving
Example 1 What is the efficiency of an ideal heat engine if the heater temperature is 800°C and the refrigerator temperature is 500°C lower?
T 1 \u003d 800 o C \u003d 1073 K, ∆T \u003d 500 o C \u003d 500 K, η -?
By definition: η=(T 1 - T 2)/ T 1.
We are not given the temperature of the refrigerator, but ∆T = (T 1 - T 2), from here:
η \u003d ∆T / T 1 \u003d 500 K / 1073 K \u003d 0.46.
Answer: efficiency = 46%.
Example 2 Determine the efficiency of an ideal heat engine if 650 J of useful work is performed due to the acquired one kilojoule of heater energy. What is the temperature of the heat engine heater if the coolant temperature is 400 K?
Q 1 \u003d 1 kJ \u003d 1000 J, A \u003d 650 J, T 2 \u003d 400 K, η -?, T 1 \u003d?
In this problem, we are talking about a thermal installation, the efficiency of which can be calculated by the formula:
To determine the temperature of the heater, we use the formula for the efficiency of an ideal heat engine:
η \u003d (T 1 - T 2) / T 1 \u003d 1 - T 2 / T 1.
After performing mathematical transformations, we get:
T 1 \u003d T 2 / (1- η).
T 1 \u003d T 2 / (1- A / Q 1).
Let's calculate:
η= 650 J / 1000 J = 0.65.
T 1 \u003d 400 K / (1- 650 J / 1000 J) \u003d 1142.8 K.
Answer: η \u003d 65%, T 1 \u003d 1142.8 K.
Real conditions
The ideal heat engine is designed with ideal processes in mind. Work is done only in isothermal processes, its value is defined as the area bounded by the Carnot cycle graph.
In fact, it is impossible to create conditions for the process of changing the state of a gas without accompanying changes in temperature. There are no materials that would exclude heat exchange with surrounding objects. The adiabatic process is no longer possible. In the case of heat transfer, the temperature of the gas must necessarily change.
The efficiency of heat engines created in real conditions, differ significantly from the efficiency of ideal engines. Note that the processes in real engines occurs so rapidly that the variation in the internal thermal energy of the working substance in the process of changing its volume cannot be compensated by the inflow of heat from the heater and return to the cooler.
Other heat engines
Real engines operate on different cycles:
- Otto cycle: the process at a constant volume changes adiabatically, creating a closed cycle;
- Diesel cycle: isobar, adiabat, isochor, adiabat;
- the process occurring at constant pressure is replaced by an adiabatic one, closing the cycle.
Create equilibrium processes in real engines (to bring them closer to ideal ones) under conditions modern technology does not seem possible. The efficiency of heat engines is much lower, even taking into account the same temperature conditions, as in an ideal thermal installation.
But you should not reduce the role of the efficiency calculation formula, since it is it that becomes the starting point in the process of working to increase the efficiency of real engines.
Ways to change efficiency
When comparing ideal and real heat engines, it is worth noting that the temperature of the refrigerator of the latter cannot be any. Usually the atmosphere is considered to be a refrigerator. The temperature of the atmosphere can be taken only in approximate calculations. Experience shows that the temperature of the coolant is equal to the temperature of the exhaust gases in the engines, as is the case in internal combustion engines (abbreviated internal combustion engines).
ICE is the most common heat engine in our world. The efficiency of a heat engine in this case depends on the temperature created by the burning fuel. A significant difference between an internal combustion engine and steam engines is the merging of the functions of the heater and the working fluid of the device into air-fuel mixture. Burning, the mixture creates pressure on the moving parts of the engine.
An increase in the temperature of the working gases is achieved by significantly changing the properties of the fuel. Unfortunately, it is not possible to do this indefinitely. Any material from which the combustion chamber of an engine is made has its own melting point. The heat resistance of such materials is the main characteristic of the engine, as well as the ability to significantly affect the efficiency.
Motor efficiency values
If we consider the temperature of the working steam at the inlet of which is 800 K, and the exhaust gas is 300 K, then the efficiency of this machine is 62%. In reality, this value does not exceed 40%. Such a decrease occurs due to heat losses during heating of the turbine casing.
The highest value of internal combustion does not exceed 44%. Increasing this value is a matter of the near future. Changing the properties of materials, fuels is a problem that the best minds of mankind are working on.
« Physics - Grade 10 "
What is a thermodynamic system and what parameters characterize its state.
State the first and second laws of thermodynamics.
It was the creation of the theory of heat engines that led to the formulation of the second law of thermodynamics.
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- These are devices that convert the internal energy of the fuel into mechanical work.
The principle 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 working body(gas) hundreds or thousands of degrees above 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's denote the initial temperature of the working fluid (gas) through T 1 . 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. The temperature T 1 is called heater temperature.
The role of the refrigerator
As work is done, the gas loses energy and inevitably cools to a certain temperature T 2 , which is usually somewhat 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 lower than the ambient temperature.
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 of the fuel 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 13.13.
The working fluid of the engine receives from the heater during the combustion of fuel the amount of heat Q 1, does work A "and transfers the amount of heat to the refrigerator Q2< Q 1 .
In order for the engine to work continuously, it is necessary to return the working fluid to its initial state, at which the temperature of the working fluid is equal to T 1 . It follows from this that the operation of the engine occurs according to periodically repeating closed processes, or, as they say, according to a cycle.
Cycle is a series of processes, as a result of which the system returns to its initial state.
Coefficient of performance (COP) of a heat engine.
The impossibility of complete conversion of the internal energy of the 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. The second law of thermodynamics can be formulated as follows:
Second law of thermodynamics:
impossible to create perpetual motion machine of the second kind, which would completely convert heat into mechanical work.
According to the law of conservation of energy, the work done by the engine is:
A" \u003d Q 1 - | Q 2 |, (13.15)
where Q 1 - the amount of heat received from the heater, and Q2 - the amount of heat given to the refrigerator.
The coefficient of performance (COP) of a heat engine is the ratio of work 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 maximum value of the efficiency of heat engines.
The laws of thermodynamics allow us to calculate the maximum possible thermal efficiency an engine operating with a heater having a temperature of T 1 and a refrigerator with a temperature of T 2 , and also to determine ways to increase it.
For the first time, the maximum possible efficiency of a heat engine was calculated 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 in a cycle consisting of two isotherms and two adiabats, and these processes are considered reversible (Fig. 13.14). First, a vessel with gas is brought into contact with a heater, the gas expands isothermally, doing positive work, at a temperature T 1 , while it receives an amount of heat Q 1 .
Then the vessel is thermally insulated, the gas continues to expand already adiabatically, while its temperature decreases to the temperature of the refrigerator T 2 . After that, the gas is brought into contact with the refrigerator, under isothermal compression, it gives off the amount of heat Q 2 to the refrigerator, compressing to a volume V 4< V 1 . Затем сосуд снова теплоизолируют, газ сжимается адиабатно до объёма V 1 и возвращается в первоначальное состояние. Для КПД этой машины было получено следующее выражение:
As follows from formula (13.17), the efficiency of the Carnot machine is directly proportional to the difference in the absolute temperatures of the heater and refrigerator.
The main meaning of this formula is that it indicates the way to increase the efficiency, for this it is necessary to increase the temperature of the heater or lower the temperature of the refrigerator.
Any real heat engine operating with a heater having a temperature T 1 and a refrigerator with a temperature T 2 cannot have an efficiency exceeding the efficiency of an ideal heat engine: 
The processes that make up the cycle of a real heat engine are not reversible.
Formula (13.17) gives a theoretical limit for the maximum value of the efficiency of heat engines. It shows that a heat engine is more efficient, the greater the temperature difference between the heater and the refrigerator.
Only at the temperature of the refrigerator, equal to absolute zero, η = 1. In addition, it has been proved that the efficiency calculated by formula (13.17) does not depend on the working substance.
But the temperature of the refrigerator, the role of which is usually played by the atmosphere, practically cannot be lower than the ambient temperature. You can increase the temperature of the heater. However, any material (solid body) 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.
For a steam turbine, the initial and final steam temperatures are approximately as follows: T 1 - 800 K and T 2 - 300 K. At these temperatures, the maximum efficiency is 62% (note that efficiency is usually measured as a percentage). The actual value of the efficiency due to various kinds of energy losses is approximately 40%. Diesel engines have the maximum efficiency - about 44%.
Environmental protection.
It is difficult to imagine the modern world without heat engines. They provide us with a comfortable life. Heat engines drive vehicles. About 80% of electricity, despite the presence of nuclear power plants, is generated using heat engines.
However, during the operation of heat engines, inevitable environmental pollution occurs. This is a contradiction: on the one hand, every year humanity needs more and more energy, the main part of which is obtained by burning fuel, on the other hand, combustion processes are inevitably accompanied by environmental pollution.
When fuel is burned, the oxygen content in the atmosphere decreases. In addition, the combustion products themselves form chemical compounds that are harmful to living organisms. Pollution occurs not only on the ground, but also in the air, since any aircraft flight is accompanied by emissions of harmful impurities into the atmosphere.
One of the consequences of the operation of the engines is the formation of carbon dioxide, which absorbs infrared radiation from the Earth's surface, which leads to an increase in the temperature of the atmosphere. This is the so-called greenhouse effect. Measurements show that the temperature of the atmosphere rises by 0.05 °C per year. Such a continuous increase in temperature can cause the ice to melt, which in turn will lead to a change in the water level in the oceans, i.e., to the flooding of the continents.
We note one more negative point when using heat engines. So, sometimes water from rivers and lakes is used to cool engines. The heated water is then returned back. The increase in temperature in water bodies disrupts the natural balance, this phenomenon is called thermal pollution.
To protect the environment, various cleaning filters are widely used to prevent the emission of harmful substances into the atmosphere, and engine designs are being improved. There is a continuous improvement of fuel, which gives less harmful substances during combustion, as well as the technology of its combustion. Alternative energy sources using wind, solar radiation, and core energy are being actively developed. Electric vehicles and vehicles powered by solar energy are already being produced.
The work done by the engine is:
This process was first considered by the French engineer and scientist N. L. S. Carnot in 1824 in the book Reflections on the driving force of fire and on machines capable of developing this force.
The purpose of Carnot's research was to find out the reasons for the imperfection of heat engines of that time (they had an efficiency of ≤ 5%) and to find ways to improve them.
The Carnot cycle is the most efficient of all. Its efficiency is maximum.
The figure shows the thermodynamic processes of the cycle. In the process of isothermal expansion (1-2) at a temperature T 1 , the work is done by changing the internal energy of the heater, i.e., by supplying the amount of heat to the gas Q:
A 12 = Q 1 ,
Cooling of the gas before compression (3-4) occurs during adiabatic expansion (2-3). Change in internal energy ΔU 23 in an adiabatic process ( Q=0) is completely converted into mechanical work:
A 23 = -ΔU 23 ,
The temperature of the gas as a result of adiabatic expansion (2-3) decreases to the temperature of the refrigerator T 2 < T 1 . In the process (3-4), the gas is isothermally compressed, transferring the amount of heat to the refrigerator Q2:
A 34 = Q 2,
The cycle is completed by the process of adiabatic compression (4-1), in which the gas is heated to a temperature T 1.
The maximum value of the efficiency of heat engines operating on ideal gas, according to the Carnot cycle:
.
The essence of the formula is expressed in the proven WITH. Carnot's theorem that the efficiency of any heat engine cannot exceed the efficiency of the Carnot cycle carried out at the same temperature of the heater and refrigerator.
Class: 10
Lesson type: Lesson learning new material.
The purpose of the lesson: Explain the principle of operation of a heat engine.
Lesson objectives:
Educational: to introduce students to the types of heat engines, to develop the ability to determine the efficiency of heat engines, to reveal the role and importance of TD in modern civilization; generalize and expand students' knowledge of environmental issues.
Developing: develop attention and speech, improve presentation skills.
Educational: to instill in students a sense of responsibility to future generations, in connection with which, consider the impact of heat engines on the environment.
Equipment: computers for students, teacher's computer, multimedia projector, tests (in Excel), Physics 7-11 Library of electronic visual aids. "Cyril and Methodius".
During the classes
1. Organizing moment
2. Organization of students' attention
The topic of our lesson is "Heat engines". (Slide 1)
Today we will recall the types of heat engines, consider the conditions for their effective operation, and talk about the problems associated with their mass application. (Slide 2)
3. Actualization of basic knowledge
Before moving on to learning new material, I suggest checking how you are ready for this.
Front poll:
- State the first law of thermodynamics. (The change in the internal energy of the system during its transition from one state to another is equal to the sum of the work of external forces and the amount of heat transferred to the system. U \u003d A + Q)
– Can a gas heat up or cool down without heat exchange with the environment? How does this happen? (For adiabatic processes.)(Slide 3)
– Write the first law of thermodynamics in the following cases: a) heat transfer between bodies in a calorimeter; b) heating water on an alcohol lamp; c) body heating upon impact. ( a) A=0,Q=0, U=0; b) A=0, U=Q; c) Q=0, U=A)
- The figure shows a cycle performed by an ideal gas of a certain mass. Draw this cycle on the p(T) and T(p) graphs. In what parts of the cycle does the gas release heat and in which parts does it absorb?
(In sections 3-4 and 2-3, the gas releases some heat, and in sections 1-2 and 4-1, heat is absorbed by the gas.) (Slide 4)
4. Learning new material
All physical phenomena and laws find application in everyday human life. The reserves of internal energy in the oceans and the earth's crust can be considered practically unlimited. But having these reserves is not enough. It is necessary at the expense of energy to be able to set in motion devices capable of doing work. (Slide 5)
What is the source of energy? (various fuels, wind, solar, tidal power)
There are various types of machines that realize in their work the transformation of one type of energy into another.
A heat engine is a device that converts the internal energy of a fuel into mechanical energy. (Slide 6)
Consider the device and principle of operation of a heat engine. The heat engine works cyclically.
Any heat engine consists of a heater, a working fluid and a refrigerator. (Slide 7)
Closed loop efficiency (Slide 8)
Q 1 - the amount of heat received from heating Q 1 >Q 2
Q 2 - the amount of heat given to the refrigerator Q 2 A / = Q 1 - |Q 2 | is the work done by the engine per cycle?< 1. Cycle C. Carnot (Slide 9) T 1 - heating temperature. T 2 - refrigerator temperature. Heat engines are predominantly used in all major types of modern transport. On rail transport until the middle of the 20th century. the main engine was a steam engine. Now diesel locomotives and electric locomotives are mainly used. In water transport, steam engines were also used at first, now both internal combustion engines and powerful turbines for large ships are used. Of greatest importance is the use of heat engines (mainly powerful steam turbines) in thermal power plants, where they drive the rotors of electric current generators. About 80% of all electricity in our country is generated at thermal power plants. Thermal engines (steam turbines) are also installed at nuclear power plants. Gas turbines are widely used in rockets, in rail and road transport. On automobiles, piston internal combustion engines with an external formation of a combustible mixture (carburetor engines) and engines with the formation of a combustible mixture directly inside the cylinders (diesels) are used. In aviation, piston engines are installed on light aircraft, and turboprop and jet engines, which also belong to heat engines, are installed on huge liners. Jet engines are also used in space rockets. (Slide 10) (Showing video clips of the operation of a turbojet engine.) Let us consider in more detail the operation of an internal combustion engine. Viewing a video clip. (Slide 11) The operation of a four-stroke internal combustion engine. Heat engines and environmental protection (Slide 13) The steady growth of energy capacities - the ever-increasing spread of tamed fire - leads to the fact that the amount of heat released becomes comparable to other components of the heat balance in the atmosphere. This cannot but lead to an increase in the average temperature on Earth. Rising temperatures could pose a threat of melting glaciers and catastrophic sea level rise. But this does not exhaust the negative consequences of the use of heat engines. The emission of microscopic particles into the atmosphere is growing - soot, ash, crushed fuel, which leads to an increase in the "greenhouse effect" due to an increase in the concentration of carbon dioxide over a long period of time. This leads to an increase in the temperature of the atmosphere. Toxic combustion products emitted into the atmosphere, products of incomplete combustion of fossil fuels, have a harmful effect on flora and fauna. Cars are a particular danger in this regard, the number of which is growing alarmingly, and the purification of exhaust gases is difficult. All this poses a number of serious problems for society. (Slide 14) It is necessary to improve the efficiency of structures that prevent the emission of harmful substances into the atmosphere; achieve more complete combustion of fuel in automobile engines, as well as increase the efficiency of energy use, save it in production and at home. Alternative engines: Ways to solve environmental problems: Use of alternative fuel. Use of alternative engines. Improvement of the environment. Education of ecological culture. (Slide 16) All of you will have to pass the unified state exam in just a year. I suggest you solve several problems from part A of the physics demo for 2009. You will find the task on the desktops of your computers. More than 240 years have passed since the first steam engine was built. During this time, heat engines have greatly changed the content of human life. It was the use of these machines that allowed mankind to step into space, to reveal the secrets of the deep sea. Gives grades for class work. Before leaving the class, please complete the table. I worked in class active / passive With my work in the classroom, I happy / not happy The lesson seemed to me short / long for the lesson i not tired / tired 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. 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. 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. 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. 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 again thermally insulated. In this case, the gas is adiabatically compressed to its original volume and state. 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. 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. 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.
1 stroke: inlet.
2 beat: compression.
3 stroke: working stroke.
4 beat: release.
Device: cylinder, piston, crankshaft, 2 valves (inlet and outlet), candle.
Dead spots - the extreme position of the piston.
Let's compare the performance characteristics of heat engines.5. Fixing the material
6. Summing up the lesson
7. Homework:
§ 82 (Myakishev G.Ya.), exercise. 15 (11, 12) (Slide 17)
8. Reflection
Heat engine device
Operating principle
Heat engine operation
Carnot engine
Varieties
Other types of heat engines
How can you increase efficiency
