An engine in which the internal energy of the fuel that burns is converted into mechanical work.
Any heat engine consists of three main parts: heater, working fluid(gas, liquid, etc.) and refrigerator. The operation of the engine is based on a cyclic process (this is the process as a result of which the system returns to its original state).
Direct cycle heat engine
A common property 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. You need at least two of them. A heat reservoir with a higher temperature is called a heater, and a heat reservoir with a lower temperature is called a refrigerator. Carrying out a circular process, the working fluid receives a certain amount of heat Q 1 from the heater (expansion occurs) and transfers an amount of heat Q 2 to the refrigerator 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.
Refrigerator reverse cycle
In the reverse cycle, expansion occurs at lower pressure and compression occurs at higher pressure. Therefore, the work of compression is greater than the work of expansion; the work is performed not by the working fluid, but by external forces. This work turns into heat. Thus, in a 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 cycle:
Refrigerator efficiency indicator:
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 a diesel engine. In both cases, the working fluid is a mixture of gasoline or diesel fuel vapor 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 actual efficiency of a carburetor engine is about 30%, and that of a diesel engine is about 40%.
The French physicist S. Carnot developed the operation of an ideal heat engine. The working part of a Carnot engine can be imagined as a piston in a gas-filled cylinder. Since the Carnot engine is the car is purely theoretical, that is, ideal, friction forces between the piston and cylinder and heat losses are considered equal to 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 temperature T 1
section 2-3: the gas expands adiabatically, the temperature drops to refrigerator temperature T 2
section 3-4: the gas is exothermically compressed, while it transfers the amount of heat Q 2 to the refrigerator
Section 4-1: The gas is compressed adiabatically until its temperature rises to T1.
The work performed by the working fluid 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 a certain amount of heat from the hot reservoir.
2. The cylinder is then surrounded by perfect thermal insulation, due to which the amount of heat available in the gas is conserved and the gas continues to expand until its temperature drops to the temperature 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, giving off some 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 is equal to the temperature of the hot reservoir. After this, the thermal insulation is removed and the cycle is repeated 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 goal 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. During isothermal expansion (1-2) at temperature T 1 , work is done due to a change in the internal energy of the heater, i.e. due to the supply of heat to the gas Q:
A 12 = Q 1 ,
Gas cooling before compression (3-4) occurs during adiabatic expansion (2-3). Change in internal energy ΔU 23 during an adiabatic process ( Q = 0) is completely converted into mechanical work:
A 23 = -ΔU 23 ,
The gas temperature as a result of adiabatic expansion (2-3) drops to the temperature of the refrigerator T 2 < T 1 . In process (3-4), the gas is isothermally compressed, transferring the amount of heat to the refrigerator Q 2:
A 34 = Q 2,
The cycle ends with the process of adiabatic compression (4-1), in which the gas is heated to a temperature T 1.
Maximum efficiency value of ideal gas heat engines 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 a Carnot cycle carried out at the same temperature of the heater and refrigerator.
Efficiency factor (efficiency) is a characteristic of the system's performance in relation to the conversion or transfer of energy, which is determined by the ratio of the useful energy used to the total energy received by the system.
Efficiency- a dimensionless quantity, usually expressed as a percentage: 
The coefficient of performance (efficiency) 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 is a reversible circular gas process, which consists of sequentially standing two isothermal and two adiabatic processes performed with the working fluid. 
A circular cycle, which includes two isotherms and two adiabats, corresponds to maximum efficiency.
The French engineer Sadi Carnot in 1824 derived the 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, i.e. 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, has the form: 
, 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, and jet engines.
However, despite the diversity, in principle the operation of various heat engines has common features. The main components of every heat engine are:
- heater;
- working fluid;
- fridge.
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 external pressure, and moves the piston, producing positive work. At the same time, its pressure drops and its volume increases.
If we compress a gas, going through the same states, but in the opposite direction, then we will do the same absolute value, but negative work. As a result, all work per cycle will be zero.
In order for the work of a heat engine to be different from zero, the work of gas compression 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, which is why a refrigerator is included in the design of the heat engine. The working fluid transfers heat to the refrigerator upon contact with it.
The topic of the current lesson will be the consideration of processes occurring in very concrete, and not abstract, as in previous lessons, devices - heat engines. We will define such machines, describe their main components and operating principle. Also during this lesson, we will consider the issue of finding efficiency - the efficiency factor of heat engines, both real and maximum possible.
Topic: Fundamentals of thermodynamics
Lesson: How a Heat Engine Operates
The topic of the last lesson was the first law of thermodynamics, which specified the relationship between a certain amount of heat that was transferred to a portion of a gas and the work done by this gas during expansion. And now the time has come to say that this formula is of interest not only for some theoretical calculations, but also in quite practical application, because the work of gas is nothing more than useful work, which we extract when using heat engines.
Definition. Heat engine- a device in which the internal energy of the fuel is converted into mechanical work (Fig. 1).
Rice. 1. Various examples of heat engines (), ()
As you can see from the figure, heat engines are any device that operates on the above principle, and they range from incredibly simple to very complex in design.
Without exception, all heat engines are functionally divided into three components (see Fig. 2):
- Heater
- Working fluid
- Fridge
Rice. 2. Functional diagram of a heat engine ()
A heater is the process of combustion of fuel, which during combustion transfers a large amount of heat to the gas, heating it to high temperatures. The hot gas, which is the working fluid, expands due to an increase in temperature and, consequently, pressure, doing work. Of course, since there is always heat transfer with the engine body, surrounding air, etc., the work will not be numerically equal to the heat transferred - part of the energy goes to the refrigerator, which, as a rule, is the environment.
The easiest way to imagine the process occurring is in a simple cylinder under a moving piston (for example, the cylinder of an internal combustion engine). Naturally, for the engine to work and make sense, the process must occur cyclically, and not one-time. That is, after each expansion, the gas must return to its original position (Fig. 3).
Rice. 3. Example of cyclic operation of a heat engine ()
In order for the gas to return to its initial position, some work must be done on it (the work of external forces). And since the work of the gas is equal to the work on the gas with the opposite sign, in order for the gas to perform a total positive work over the entire cycle (otherwise there would be no point in the engine), it is necessary that the work of external forces be less than the work of the gas. That is, the graph of the cyclic process in P-V coordinates should have the form: a closed loop with a clockwise traversal. Under this condition, the work done by the gas (in the section of the graph where the volume increases) is greater than the work done on the gas (in the section where the volume decreases) (Fig. 4).
Rice. 4. An example of a graph of a process occurring in a heat engine
Since we are talking about a certain mechanism, it is imperative to say what its efficiency is.
Definition. Efficiency (Coefficient of Performance) of a Heat Engine- the ratio of useful work performed by the working fluid to the amount of heat transferred to the body from the heater.
If we take into account the conservation of energy: the energy leaving the heater does not disappear anywhere - part of it is removed in the form of work, the rest goes to the refrigerator:
We get:
This is an expression for efficiency in parts; if you need to get the efficiency value in percent, you must multiply the resulting number by 100. Efficiency in the SI measurement system is a dimensionless quantity and, as can be seen from the formula, cannot be more than one (or 100).
It should also be said that this expression is called real efficiency or efficiency of a real heat engine (heat engine). If we assume that we somehow manage to completely get rid of the shortcomings of the engine design, then we will get an ideal engine, and its efficiency will be calculated using the formula for the efficiency of an ideal heat engine. This formula was obtained by the French engineer Sadi Carnot (Fig. 5):
The operation of many types of machines is characterized by such an important indicator as the efficiency of the heat engine. Every year engineers strive to create more advanced technology, 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 performs work using internal energy. Any heat engine that turns into a mechanical one uses the thermal expansion of substances as the temperature increases. In solid-state engines, it is possible not only to change the volume of a substance, but also the shape of the body. The action 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 operating principle of heat engines (heat engine efficiency) depends on their type. Often the refrigerator is a steam condenser, and the heater is any type of fuel that burns in the firebox. The efficiency of an ideal heat engine is found by the following formula:
Efficiency = (Theating - Cooling) / Theating x 100%.
In this case, the efficiency of a real engine can never exceed the value obtained according to this formula. Also, this figure will never exceed the above-mentioned value. To increase efficiency, most often the heater temperature is increased and the refrigerator temperature is decreased. Both of these processes will be limited by the actual operating conditions of the equipment.
When a heat engine operates, work is done, as the gas begins to lose energy and cools to a certain temperature. The latter is usually several degrees higher than the surrounding atmosphere. This is the temperature of the refrigerator. This special device is designed for cooling and subsequent condensation of exhaust steam. Where condensers are present, the temperature of the refrigerator is sometimes lower than the ambient temperature.
In a heat engine, when a body heats up and expands, it is not able to give up 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 heat is inevitably lost. During fuel combustion, the working fluid receives a certain amount of heat Q 1 from the heater. At the same time, it still performs 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 energy expended, A is the 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 in the form of the following formula: η = (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 using 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 observed in the Carnot device. This is due to the fact that in this system it depends only on the absolute temperature of the heater (Tn) and cooler (Tx). 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. This indicator was first calculated by the French scientist and engineer Sadi Carnot. He invented a heat engine that operated on an ideal gas. It operates in a cycle of 2 isotherms and 2 adiabats. The principle of its operation is quite simple: a heater is connected to a 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. Afterwards the vessel is thermally insulated. Despite this, the gas continues to expand, but adiabatically (without heat exchange with the environment). At this time, its temperature drops to that of a refrigerator. At this moment, the gas comes into contact with the refrigerator, as a result of which it gives off 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. 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 burning fuel is converted into mechanical energy. Such devices can be gas and liquid. There are 2-stroke and 4-stroke engines. They can have a continuous duty cycle. According to the method of preparing the fuel mixture, such engines are carburetor (with external mixture formation) and diesel (with internal). Based on the type of energy converter, they are divided into piston, jet, turbine, and combined. The efficiency of such machines does not exceed 0.5. A Stirling engine is a device in which the working fluid is located in a confined space. It is a type 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 changes 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 at thermal power plants. Turbine (rotary) internal combustion engines are used at thermal power plants in peak mode. Not as widespread as others. A turbine engine generates some of its thrust through its propeller. It gets the rest from exhaust gases. Its design is a rotary engine on the shaft of which a propeller is mounted. Rocket, turbojet and those that receive thrust due to the return of exhaust gases. Solid state engines use solid matter as fuel. During operation, it is not its volume that changes, but its shape. When operating 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. She studies the mutual transformations of different types of energy. It has been established that all available mechanical, etc., cannot be used. At the same time, their conversion into thermal 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 a body heats up, the faster its constituent molecules will move. The movement of particles will become even more erratic. Along with this, everyone knows that order can easily be turned into chaos, which is very difficult to order.Heat engine operation
Carnot engine
Varieties
Other types of heat engines
How can you increase efficiency
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