An engine that converts the internal energy of the fuel that is burned into mechanical work.

Any heat engine consists of three main parts: heater, working body(gas, liquid, etc.) and refrigerator. The operation of the engine is based on a cyclic process (this is a process in which the system returns to its original state).

Direct Cycle Heat Engine

A common feature of all cyclic (or circular) processes is that they cannot be carried out by bringing the working fluid into thermal contact with only one heat reservoir. They need at least two. A heat reservoir with a higher temperature is called a heater, and a heat reservoir with a lower temperature is called a refrigerator. Making a circular process, the working fluid receives a certain amount of heat Q 1 from the heater (expansion occurs) and gives the refrigerator the amount of heat Q 2 when it returns to its original state and contracts. The total amount of heat Q=Q 1 -Q 2 received by the working fluid per cycle is equal to the work performed by the working fluid in one cycle.

Reverse Chiller Cycle

In the reverse cycle, expansion occurs at a lower pressure, and compression occurs at a higher pressure. Therefore, the work of compression is greater than the work of expansion; the work is performed not by the working body, but by external forces. This work turns into warmth. Thus, in the refrigeration machine, the working fluid takes a certain amount of heat Q 1 from the refrigerator and transfers a larger amount of heat Q 2 to the heater.

Efficiency

Direct Loop:

Chiller Efficiency Index:

Carnot cycle

In heat engines, they strive to achieve the most complete conversion of thermal energy into mechanical energy. Maximum efficiency.

The figure shows the cycles used in a gasoline carburetor engine and in a diesel engine. In both cases, the working fluid is a mixture of gasoline or diesel fuel vapors with air. The cycle of a carburetor internal combustion engine consists of two isochores (1–2, 3–4) and two adiabats (2–3, 4–1). A diesel internal combustion engine operates on a cycle consisting of two adiabats (1–2, 3–4), one isobar (2–3), and one isochore (4–1). The real efficiency for a carburetor engine is about 30%, for a diesel engine - about 40%.

The French physicist S. Carnot developed the work of an ideal heat engine. The working part of a Carnot engine can be thought of as a piston in a cylinder filled with gas. Since the Carnot engine - machine is purely theoretical, that is, ideal, the friction forces between the piston and the cylinder and the heat losses are assumed to be zero. Mechanical work is maximum if the working fluid performs a cycle consisting of two isotherms and two adiabats. This cycle is called Carnot cycle.

section 1-2: the gas receives an amount of heat Q 1 from the heater and expands isothermally at a temperature T 1
section 2-3: the gas expands adiabatically, the temperature decreases to the refrigerator temperature T 2
section 3-4: the gas is exothermically compressed, while it gives the refrigerator the amount of heat Q 2
section 4-1: the gas is compressed adiabatically until its temperature rises to T 1 .
The work performed by the working body is the area of ​​the resulting figure 1234.

Such an engine functions as follows:

1. First, the cylinder comes into contact with a hot reservoir, and the ideal gas expands at a constant temperature. During this phase, the gas receives some heat from the hot reservoir.
2. The cylinder is then surrounded by perfect thermal insulation, whereby the amount of heat available to the gas is conserved and the gas continues to expand until its temperature drops to that of the cold thermal reservoir.
3. In the third phase, the thermal insulation is removed, and the gas in the cylinder, being in contact with the cold reservoir, is compressed, while giving off part of the heat to the cold reservoir.
4. When the compression reaches a certain point, the cylinder is again surrounded by thermal insulation, and the gas is compressed by raising the piston until its temperature equals that of the hot reservoir. After that, the thermal insulation is removed and the cycle repeats again from the first phase.

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.

heat engine efficiency. 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.

The efficiency of a 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

The maximum value of the efficiency of heat 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 a 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 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: At these temperatures, the maximum efficiency value is:

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

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

Thermal engines and nature conservation. The widespread use of heat engines in order to obtain energy convenient for use to the greatest extent, compared 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 reaches 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 far from exhausts the negative consequences of the use of heat engines. Furnaces of thermal power plants, internal combustion engines of 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. Special danger in this respect represent motor vehicles, the number of which is growing alarmingly, 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; achieve more complete combustion of fuel in automobile 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?

A thermal engine is an engine that performs work at the expense of a source of thermal energy.

Thermal energy ( Q heater) from the source is transferred to the engine, while part of the received energy the engine spends on doing work W, unspent energy ( Q refrigerator) is sent to a refrigerator, the role of which can be performed, for example, by ambient air. The heat engine can only work if the temperature of the refrigerator is less than the temperature of the heater.

The coefficient of performance (COP) of a heat engine can be calculated by the formula: Efficiency = W/Q ng.

Efficiency = 1 (100%) if all thermal energy is converted into work. Efficiency=0 (0%) if no thermal energy is converted into work.

The efficiency of a real heat engine lies in the range from 0 to 1, the higher the efficiency, the more efficient the engine.

Q x / Q ng \u003d T x / T ng Efficiency \u003d 1- (Q x / Q ng) Efficiency \u003d 1- (T x / T ng)

Considering the third law of thermodynamics, which states that the temperature of absolute zero (T=0K) cannot be reached, we can say that it is impossible to develop a heat engine with efficiency=1, since T x >0 is always.

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

Efficiency factor (COP) is a measure of the efficiency of a system in terms of energy conversion or transfer, which is determined by the ratio of the energy usefully used to the total energy received by the system.

efficiency- the value is dimensionless, it is usually expressed as a percentage:

The coefficient of performance (COP) of a heat engine is determined by the formula: , where A = Q1Q2. The efficiency of a heat engine is always less than 1.

Carnot cycle- This is a reversible circular gas process, which consists of two consecutive isothermal and two adiabatic processes performed with a working fluid.

The circular cycle, which includes two isotherms and two adiabats, corresponds to the maximum efficiency.

The French engineer Sadi Carnot in 1824 derived a formula for the maximum efficiency of an ideal heat engine, where the working fluid is an ideal gas, the cycle of which consisted of two isotherms and two adiabats, that is, the Carnot cycle. The Carnot cycle is the real working cycle of a heat engine that performs work due to the heat supplied to the working fluid in an isothermal process.

The formula for the efficiency of the Carnot cycle, i.e., the maximum efficiency of a heat engine, is: , where T1 is the absolute temperature of the heater, T2 is the absolute temperature of the refrigerator.

Heat engines- These are structures in which thermal energy is converted into mechanical energy.

Heat engines are diverse both in design and purpose. These include steam engines, steam turbines, internal combustion engines, jet engines.

However, despite the diversity, there are common features in the principle of operation of various heat engines. The main components of each heat engine:

• heater;
• working body;
• refrigerator.

The heater releases thermal energy, while heating the working fluid, which is located in the working chamber of the engine. The working fluid can be steam or gas.

Having accepted the amount of heat, the gas expands, because. its pressure is greater than the external pressure, and moves the piston, producing positive work. At the same time, its pressure drops, and its volume increases.

If we compress the gas, passing through the same states, but in the opposite direction, then we will perform the same absolute value, but negative work. As a result, all the work for the cycle will be equal to zero.

In order for the work of a heat engine to be nonzero, the work of compressing the gas must be less than the work of expansion.

In order for the work of compression to become less than the work of expansion, it is necessary that the compression process take place at a lower temperature, for this the working fluid must be cooled, therefore, a refrigerator is included in the design of the heat engine. The working fluid gives off the amount of heat to the refrigerator when in contact with it.