The principle of action of thermal motors. The efficiency of the efficiency (efficiency) of thermal engines is a hypermarket of knowledge. The principles of the operation of thermal motors of the heat engine efficiency is equal to what

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Tasks in physics on the efficiency of the thermal engine

Task for calculating the efficiency of the heat engine №1

Condition

Water weighing 175 g is heated on the alcohol. While the water was heated from T1 \u003d 15 to T2 \u003d 75 degrees Celsius, the mass of the alcohol decreased from 163 to 157 g. Calculate the efficiency of the installation.

Decision

The efficiency of the useful operation can be calculated as the ratio of useful work and the total amount of heat generated by the alcohol:

Useful work in this case is the equivalent of the amount of heat that went exclusively for heating. It can be calculated according to the well-known formula:

The total amount of heat is calculated, knowing the mass of burnt alcohol and its specific heat of combustion.

We substitute the values \u200b\u200band calculate:

Answer: 27%

Task for calculating the efficiency of the heat engine №2

Condition

The old engine made a job of 220.8 MJ, while spending 16 kilograms of gasoline. Calculate the engine efficiency.

Decision

We find the total amount of heat that produced the engine:

Or, multiplying by 100, we obtain the value of the efficiency in percent:

Answer: 30%.

Task for calculating the efficiency of the heat engine №3

Condition

The heat machine works on the carno cycle, with 80% of the heat obtained from the heater, is transmitted to the refrigerator. For one cycle, the working body receives 6.3 J heat from the heater. Find the work and efficiency of the cycle.

Decision

Efficiency of the perfect heat machine:

By condition:

Calculate first work and then efficiency:

Answer: twenty%; 1.26 J.

Task for calculating the efficiency of the heat engine №4

Condition

The diagram depicts a diesel engine cycle consisting of adiabat 1-2 and 3-4, isobara 2-3 and isoohra 4-1. Gas temperatures at points 1, 2, 3, 4 are t1, T2, T3, T4, respectively. Find the CCD cycle.

Decision

Let us analyze the cycle, and the efficiency will be calculated through the subordinate and allotted amount of heat. On adiabats, heat is not supplied and not allocated. On Isobara 2 - 3 heat is supplied, the volume increases and, accordingly, the temperature grows. On isoore, 4 - 1 heat is given, and the pressure and temperature fall.

Similarly:

We get the result:

Answer: See above.

The challenge for calculating the efficiency of the heat engine No. 5

Condition

The thermal car operating on the carboy cycle makes it possible for one cycle A \u003d 2.94 kJ and gives the cooler for one cycle the amount of heat Q2 \u003d 13.4 kJ. Find the CCD cycle.

Decision

We write a formula for efficiency:

Answer: 18%

Questions on the topic of thermal engines

Question 1. What is a heat engine?

Answer. The heat engine is a machine that makes work due to the energy coming to it in the process of heat transfer. The main parts of the thermal engine: heater, refrigerator and working body.

Question 2. Give examples of thermal motors.

Answer. The first thermal engines that have received widespread were steam machines. Examples of a modern heat engine can serve:

• rocket engine;
• aircraft engine;
• gas turbine.

Question 3. Can the engine efficiency be equal to one?

Answer. Not. The efficiency is always less than one (or less than 100%). The existence of an engine with an efficiency is equal to one contrary to the first beginning of thermodynamics.

The efficiency of real engines rarely exceeds 30%.

Question 4. What is kpd?

Answer. Efficiency (efficiency) - the ratio of the work that the engine performs to the amount of heat obtained from the heater.

Question 5. What is the specific heat combustion of fuel?

Answer. Specific heat combustion q. - The physical value that shows how much heat is released when combustion of fuel weighing 1 kg. When solving the Tasks of the efficiency, it is possible to determine the power of the engine n and the amount of fuel incinerated per unit of time.

Tasks and questions about the carno cycle

By affecting the theme of heat engines, it is impossible to leave the carno cycle aside - perhaps the most famous cycle of the heat machine in physics. We give additionally several tasks and questions to the carno cycle with the solution.

The cycle (or process) of carno is the perfect circular cycle consisting of two adiabat and two isotherms. Named so in honor of the French engineer Sadi Carno, who described this cycle in his scientific work "On the driving force of fire and about machines that can develop this power" (1894).

Task on the carno cycle №1

Condition

The ideal thermal car operating on the carboy cycle makes it possible for one cycle a \u003d 73.5 kJ. Heater temperature T1 \u003d 100 ° С, temperature of the refrigerator T2 \u003d 0 ° C. Find the efficiency of the cycle, the amount of heat obtained by the machine for one cycle from the heater, and the amount of heat released for one cycle of the refrigerator.

Decision

Calculate the CPD of the cycle:

On the other hand, to find the amount of heat obtained by the machine, we use the ratio:

The amount of heat, given to the refrigerator, will be equal to the difference in total heat and useful work:

Answer: 0.36; 204.1 kJ; 130.6 kJ.

The task of the carno cycle №2

Condition

The ideal thermal car operating in the carno cycle makes it possible for one cycle a \u003d 2.94 kJ and gives the amount of heat quality Q2 \u003d 13.4 kJ for one cycle. Find the CCD cycle.

Decision

Formula for efficiency cycle carno:

Here a - perfect work, and Q1 is the amount of heat that needed to make it. The amount of heat that the perfect car gives the refrigerator, equal to the difference of these two values. Knowing it, we find:

Answer: 17%.

Task on carno cycle №3

Condition

Picture carno cycle on the diagram and describe it

Decision

The carno cycle on the PV diagram is as follows:

• 1-2. Isothermal extension, the working body receives from the heater the amount of heat Q1;
• 2-3. Adiabatic expansion, heat is not supplied;
• 3-4. Isothermal compression, during which the heat is transferred to the refrigerator;
• 4-1. Adiabatic compression.

Answer: see above.

Question on the carno cycle №1

Word the first carno theorem

Answer. The first carno theorem states: the efficiency of the heat machine operating on the carno cycle depends only on the temperatures of the heater and the refrigerator, but does not depend on the machine device, nor from the type or properties of its working fluid.

Question on the carno cycle №2

Can the efficiency in a carno cycle to be 100%?

Answer. Not. The efficiency of the carno cycle will be 100% only if the temperature of the refrigerator is equal to the absolute zero, and this is not possible.

If you have any questions about the theme of thermal engines and the carno cycle, you can safely ask them in the comments. And if you need help in solving tasks or other examples and tasks, contact

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?

Thermal is called the engine performing work due to the source of thermal energy.

Thermal energy ( Q Heater) From the source is transmitted to the engine, while the engine of the engine received is spent on performance W., unspent energy ( Q Refrigerator) Departs to the refrigerator, the role of which can perform, for example, the ambient air. The thermal motor can only work if the temperature of the refrigerator is less than the temperature of the heater.

The efficiency (efficiency) of the thermal engine can be calculated by the formula: Efficiency \u003d w / q ng.

Efficiency \u003d 1 (100%) in the event that all heat energy turns into operation. KPD \u003d 0 (0%) if no heat energy turns into operation.

The efficiency of the real heat engine lies between 0 to 1, the higher the efficiency, the more efficient the engine.

Q x / q ng \u003d t x / t ng kpd \u003d 1- (q x / q ng) kpd \u003d 1- (t x / t ng)

Given the third principle of thermodynamics, which it states that the temperature of the absolute zero (T \u003d 0К) cannot be achieved, it can be said that it is impossible to develop a heat engine with efficiency \u003d 1, since always t x\u003e 0.

The efficiency of the heat engine will be the greater the higher the temperature of the heater, and below the temperature of the refrigerator.

Physics, grade 10

Lesson 25. Thermal motors. Efficiency of thermal engines

The list of questions considered in the lesson:

1) the concept of the thermal engine;

2) the device and the principle of operation of the thermal engine;

3) the efficiency of the thermal engine;

4) Carno cycle.

Glossary on the topic

Thermal motor -a device in which the internal fuel energy turns into mechanical.

Efficiency (the efficiency) is the ratio of useful work performed by this engine, to the amount of heat obtained from the heater.

Internal combustion engine - The engine in which the fuel combines directly in the engine operating chamber (inside).

Jet engine - Engine that creates the power of thrust through the transformation of the internal energy of fuel into the kinetic energy of the reactive jet of the working fluid.

Carno cycle - This is the perfect circular process consisting of two adiabatic and two isothermal processes.

Heater - The device from which the working fluid gets energy, part of which goes to work.

Refrigerator - body absorbing part of the energy of the working fluid (environment or special cooling devices and condensation of the spent steam, i.e. condensers).

Working body - The body that expanding, makes a job (it is gas or steam)

The main and additional literature on the subject of the lesson:

1. Myakyshev G.Ya., Bukhovtsev B.B., Sotsky N.N. Physics.10 class. Tutorial for general educational organizations M.: Education, 2017. - P. 269 - 273.

2. Rymkevich A.P. Collection of tasks in physics. 10-11 class. -M.: Drop, 2014. - P. 87 - 88.

Open electronic resources on the subject of lesson

Theoretical material for self-study

Fairy tales and myths of different nations testify that people always dreamed of quickly moved from one place to another or quickly perform one work. To achieve this goal, needed devices that could work or move in space. Watching the world around the world, the inventors came to the conclusion that to facilitate labor and rapid movement, it is necessary to use the energy of other bodies, for example, water, wind, etc. Is it possible to use the internal energy of the powder or other type of fuel for its purposes? If we take a test tube, hill the water, close it with a plug and be heated. When heated, the water will boil, and the water pairs poured the plug. Couple expanding makes work. In this example, we see that the internal energy of fuel has become the mechanical energy of a moving tube. When replacing the cork with the piston capable of moving inside the tube, and the tube itself is a cylinder, then we will get the simplest thermal engine.

Thermal motor -the thermal engine is called a device in which the internal energy of fuel turns into mechanical.

Recall the structure of the simplest internal combustion engine. The internal combustion engine consists of a cylinder, inside which the piston moves. The piston with a connecting rod is connected to the crankshaft. At the top of each cylinder there are two valves. One of the valves is called intake, and the other is graduation. To ensure the smoothness of the stroke of the piston on the crankshaft, a heavy flywheel has been strengthened.

The operating cycle of the engine consists of four clocks: inlet, compression, work move, release.

During the first time the intake valve opens, and the exhaust valve remains closed. Moving down piston sucks into the cylinder combustible mixture.

In the second tact, both valves are closed. Moving up the piston compresses the fuel mixture, which is heated when compressed.

In the third tact, when the piston turns out to be in the upper position, the mixture is mounted on electrical spark candles. The inflammable mixture forms hot gases, the pressure of which is 3 -6 MPa, and the temperature reaches 1600-2200 degrees. Pressure force pushes the piston down, the movement of which is transmitted to the crankshaft with the flywheel. Having received a strong push, the flywheel will continue to rotate on the inertia, ensuring the movement of the piston and during subsequent tacks. During this tact, both valves remain closed.

In the fourth tact, an exhaust valve opens and the exhaust gases by the moving piston are pushed through the silencer (not shown in the figure) into the atmosphere.

Any thermal engine includes three main elements: heater, working fluid, refrigerator.

To determine the efficiency of the thermal engine, the concept of the efficiency is introduced.

The ratio of useful acts is called the ratio of useful work performed by this engine, to the amount of heat obtained from the heater.

Q 1 - the amount of heat obtained from heating

Q 2 - the amount of heat, given to the refrigerator

- Work performed by the engine per cycle.

This efficiency is real, i.e. Just this formula and are used to characterize real thermal motors.

Knowing the power of n and the time of operation T engine work performed by the cycle can be found by the formula

Transfer of unused energy to the refrigerator.

In the XIX century, as a result of work on heat engineering, the French engineer Sadi Karo proposed another method for determining the efficiency (through thermodynamic temperature).

The main value of this formula is that any real heat machine operating with a heater having a temperature of T 1 and a refrigerator with a temperature T 2 cannot have an efficiency exceeding the efficiency of the perfect heat machine. Sadi Carno, finding out with what a closed process the heat engine will have the maximum efficiency, proposed to use a cycle consisting of 2 adiabatte and two isothermal processes

Carno cycle is the most effective cycle having a maximum efficiency.

There is no thermal engine, which has an efficiency \u003d 100% or 1.

The formula 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 \u003d 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.

Increasing the efficiency of thermal engines and approaching it to the maximum possible - the most important technical task.

Heat engines - steam turbines, also set on all nuclear power plants to produce high temperature steam. In all major types of modern transport, thermal motors are mainly used: on automotive - piston internal combustion engines; on water - internal combustion engines and steam turbines; on railway - diesel locomotives with diesel installations; In aviation - piston, turboctive and jet engines.

Compare operational characteristics of thermal motors.

Steam engine - 8%.

Steam turbine - 40%.

Gas turbine - 25-30%.

Internal combustion engine - 18-24%.

Diesel engine - 40-4%.

Jet engine - 25%.

The widespread use of thermal motors does not pass without a trace for the environment: gradually decreases the amount of oxygen and increases the amount of carbon dioxide in the atmosphere, the air is contaminated with chemical compounds harmful to human health. There is a threat to climate change. Therefore, finding ways to reduce environmental pollution is one of the most pressing scientific and technical problems.

Examples and analysis of task solutions

1 . What kind of average power develops the engine of the car, if at a speed of 180 km / h gasoline consumption is 15 liters per 100 km of way, and the engine efficiency is 25%?