What is the heat engine heater for?
A heat engine is a device that converts the internal energy of fuel into mechanical energy. The main parts of a heat engine: the heater, the working body, and the cooler. To produce useful work, you must make the work of gas compression less than the work of expansion. This requires that each volume at compression corresponds to a lower pressure than at expansion. Therefore, the gas has to be cooled before it is compressed.In order for an engine to do work, a pressure difference is required on both sides of the engine piston or turbine blades. In all heat engines, this pressure difference is achieved by increasing the temperature of the working body (gas) by hundreds or thousands of degrees compared to the ambient temperature. This increase in temperature occurs when fuel is burned. One of the main parts of an engine. A vessel filled with gas with a moving piston. The working body of all heat engines is the gas that performs the work of expansion. Denote the initial temperature of the working body (gas) by T1. This temperature in steam turbines or machines is obtained by steam in the steam boiler. In internal combustion engines and gas turbines, the temperature increase is due to the combustion of fuel inside the engine itself. Temperature T1 is the temperature of the heater.’
Let’s consider the example of ideal thermal machine.
A heat machine consists of three parts: a heat sink, a working body and a heat recipient. The heat source has a temperature T1 and gives some heat Q1 to the working body. Working body (gas, steam, hot liquid) performs work. over, not all the heat Q1 turns into work, but only some part of it
Another part of the heat Q2 is transferred to a body with a lower temperature (T2), the heat recipient. Thus the essence of the work of a heat machine consists not only in getting heat Q1 from a heat source and doing the work A, but also in transferring some quantity of heat Q2 to a heat recipient whose temperature is lower than the temperature of the heat source (T1 T2). The perpetual motion machine of the second kind consists of the first two parts, that is, the heat Q1 completely turns into work A, and it is impossible. Where there is no temperature difference (T1 = T2), it is impossible to convert heat into work.
To get a mathematical expression for the second principle of thermodynamics, consider the action of an ideal thermal machine. An ideal machine is one which works without friction or heat loss. In it the working body is an ideal gas. The operation of the machine is based on the principle of a reversible thermodynamic cycle called the Carnot cycle.
The Carnot cycle consists of four consecutive processes: isothermal expansion, adiabatic expansion, isothermal compression, adiabatic compression of gas. All processes are reversible, with the result that the gas returns to its original position.
As a result of mathematical transformations we get
where h is the coefficient of performance (efficiency) of the heat machine.
The rotor fitted on the shaft is rigidly connected to a gear wheel which meshes with a stationary gear. The toothed rotor seems to run round the pinion. Its faces slide over the epitrochoidal surface of the cylinder and cut off the alternating volumes of the chambers in the cylinder.
This design allows a 4-stroke cycle without a special gas distribution mechanism. Sealing of the chambers is achieved by radial and axial sealing plates that are pressed against the cylinder by centrifugal forces, gas pressure and band springs.
Mixing, ignition, lubrication, cooling and starting are fundamentally the same as for an ordinary reciprocating internal combustion engine.
Engines with triangular rotors, with the ratio of the gear wheel to gear wheel radius: r: R = 2: 3, which are installed in automobiles, boats, etc., are of practical application.п. The mass and dimensions of a Wankel engine are two to three times less than those of a conventional combustion engine in terms of power.
Air first enters the cylinder, is compressed and heated to a high temperature. Self-igniting and quick-burning fuel is injected into the heated air with the help of a nozzle and the motor begins to run. These engines need special diesel fuel. We all know from physics class that thermal energy can be converted into mechanical energy. This is what happens when the fuel burns in the engine cylinder. The heat is converted into mechanical work and begins to move the piston, which moves reciprocatingly in the cylinder. The crankshaft, which is connected to the piston by a connecting rod, rotates.
During operation, the piston moves in and out of the crankshaft. When the two parts come together, then the combustible mixture enters the cylinder. When the cylinder moves in the opposite direction, it increases the pressure in the cylinder. the compressed combustible mixture is ready for combustion. once the spark ignites, the mixture ignites easily and gases are released, propelling the engine. The cylinder is connected to a pipe which drains the exhaust gases from the engine.
One movement of the piston to or from the crankshaft is called a stroke. If the shaft makes two revolutions around its axis in four strokes of the piston, then the so-called work cycle is complete. The engine, the working cycle of which is made for two revolutions of the crankshaft, is called a quadruple. There are also quadruple engines. They complete the work cycle in two strokes of the piston and one revolution of the crankshaft. Such engines are hardly ever used in automobiles, but they are widely used in motorcycles.
The greater the pressure on the piston during combustion of the combustible mixture, the greater the power of the engine. It is therefore advantageous to increase the compression ratio in an engine. That way, they get more work out of the same amount of fuel. Many motorists try to tune the engine themselves so that it consumes less fuel, but does not lose power. But do not get carried away with it, because when the compression ratio is greatly increased, the combustible mixture burns too quickly (this process is called detonation), which causes unstable engine operation. During this process, a knocking sound can be heard in the running engine, the output is significantly reduced, and there is black smoke coming out of the muffler.
What is a heater for a heat engine?
But such a one-time act of converting heat into work is of no interest for technology. Current combustion engines (steam engines, internal combustion engines, etc.) are not designed to work in cycles. д.) work cyclically. The process of heat transfer and the conversion of the amount of heat received into work is periodically repeated. For this, the working body must perform a circular process or thermodynamic cycle in which the initial state is periodically restored. Circular processes are depicted in the diagram (p, V) of a gaseous working body by means of closed curves During expansion gas performs positive work A1, equal to the area under the curve abc, during compression gas performs negative work A2, equal in modulo to the area under the curve cda. The total work per cycle A = A1 A2 in diagram (p, V) is equal to the area of the cycle. Work A is positive if the cycle is bypassed clockwise, and A is negative if the cycle is bypassed in the opposite direction.
The circular process in the diagram (p, V). abc. expansion curve, cda. contraction curve. The work A in the circular process is equal to the area of the figure abcd.
A common property of all circular processes is that they cannot be carried out by bringing the working body into thermal contact with only one thermal reservoir. You need at least two. A heat reservoir with a higher temperature is called a heater, and one with a lower temperature is called a cooler. Performing the circular process, the working body receives from the heater a certain amount of heat Q1 0 and gives away to the refrigerator the amount of heat Q2 0. The total amount of heat Q received by the working body per cycle is
When the cycle is bypassed, the working body returns to the initial state, hence the change of its internal energy equals zero (ΔU = 0). According to the first law of thermodynamics, ΔU = Q. A = 0. Hence: A = Q = Q1. |Q2. The work A done by the working body during the cycle is equal to the heat quantity Q. The ratio of the work A to the quantity of heat Q1, received by the working body during the cycle from the heater, is called the efficiency η of the heat machine: А Q1. |Q2| η = η. =.- Q Q1 Efficiency factor indicates what part of thermal energy received by the working body from the “hot” heat reservoir, turned into useful work. The rest (1. η) was “uselessly” transferred to the refrigerator. The efficiency of a heat machine is always less than unity (η 1). The energy diagram of the heat machine is shown in Fig.2.
Fig. 2 Energy diagram of a heat machine: 1. heater; 2. refrigerator; 3. working body in a circular process. Q1 0, A 0, Q2 0; T1 T2.
Various circular processes are used in the engines used in engineering. In Fig. the cycles used in a gasoline carburetor engine and in a diesel engine are shown. In both cases the working body is a mixture of gasoline or diesel fuel vapor with air. The cycle of a carbureted internal combustion engine consists of two isochores (1-2, 3-4) and two adiabats (2-3, 4-1). An internal combustion diesel 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 of a carburetor engine is about 30%, of a diesel engine. about 40%.
Figure 3 Cycles of carbureted internal combustion engine (1) and diesel engine (2).
In 1824 French engineer S. Carnot considered a circular process consisting of two isotherms and two adiabats. This circular process played an important role in the development of the doctrine of thermal processes. It is called the Carnot cycle
The Carnot cycle is performed by the gas in the cylinder under the piston. At isothermal section (1-2) gas is brought into thermal contact with hot thermal reservoir (heater) having temperature T1. Gas isothermally expands making work A12, at that some heat Q1 = A12 is supplied to the gas. Further on adiabatic section (2-3) gas is placed in adiabatic shell and continues expanding in absence of heat exchange. At this point, the gas performs work A23 0. The temperature of the gas during adiabatic expansion drops to T2. In the next isothermal section (3-4) gas is brought into thermal contact with cold thermal reservoir (cooler) at temperature T2 T1. The process of isothermal compression takes place. The gas works A34 0 and gives off heat Q2 0 equal to the work A34. Internal energy of gas does not change. Finally, in the last part of adiabatic contraction, the gas is again put into an adiabatic shell. During compression gas temperature increases up to the value T1, gas performs the work A41 0. The total work A performed by the gas per cycle is equal to the sum of the work at the individual sites: A = A12 A23 A34 A41. In the diagram (p, V) this work is equal to the area of the cycle. The work done by the gas in the two adiabatic sections of the Carnot cycle is equal in modulo and opposite in sign A23 =.A41. By definition, the efficiency η of the Carnot cycle is C. Carnot expressed the efficiency of the cycle through the temperatures of the heater T1 and cooler T2:
The Carnot cycle is remarkable in that on all its parts there is no contact of bodies with different temperatures. Any state of the working body (gas) on the cycle is quasi-equilibrium, t. е. infinitely close to a state of thermal equilibrium with the surrounding bodies (heat reservoirs or thermostats). The Carnot cycle excludes heat exchange at a finite temperature difference between the working body and the environment (thermostats), when heat can be transferred without committing work. Therefore, the Carnot cycle is the most efficient circular process of all possible at given heater and cooler temperatures: η Carnot = ηmax. Any section of the Carnot cycle and the cycle as a whole can be traversed in either direction. The clockwise bypass of the cycle corresponds to a heat engine, when the heat received by the working body is partially transformed into useful work. The counterclockwise bypass corresponds to a refrigerator, where some amount of heat is withdrawn from the cold reservoir and transferred to the hot reservoir by performing external work. This is why an ideal device which works according to the Carnot cycle is called a reversible heat engine.
In real refrigerating machines different cyclic processes are used. All refrigeration cycles on the diagram (p, V) circle counterclockwise. Energy diagram of the refrigerating machine is presented in Fig. 5.
Energy diagram of a refrigerating machine. Q1 0, A 0, Q2 0, T1 T2.
A device operating according to the refrigeration cycle can have a dual purpose. If a useful effect is to extract some heat |Q2| from the cooled bodies (e.g. from the food in the refrigerator chamber), such a device is an ordinary refrigerator. Efficiency of operation of the refrigerator can be characterized by the ratio t. е. operating efficiency of the refrigerator is the amount of heat taken away from the cooled bodies per 1 joule of work expended.
What is the heater of a heat engine for?
A heat engine is a device that converts the internal energy of fuel into mechanical energy. The main parts of a heat engine are the heater, the working body, and the refrigerator. In order to obtain useful work, it is necessary to make the work of gas compression less than the work of expansion. This requires that each volume at compression corresponds to a smaller pressure than that at expansion. Therefore, the gas must be cooled before compression.In order for an engine to work, a pressure difference is required on both sides of the engine piston or turbine blades. In all thermal engines, this pressure difference is achieved by raising the temperature of the working body (gas) by hundreds or thousands of degrees compared to the ambient temperature. This temperature increase occurs when the fuel burns.One of the main parts of an engine. a vessel filled with gas with a moving piston. The working medium in all heat engines is gas, which performs work during expansion. Let us denote by T1 the initial temperature of the working body (gas). This temperature in steam turbines or machines is obtained by steam in the steam boiler. In internal combustion engines and gas turbines, the temperature increase is due to the combustion of fuel inside the engine itself. The temperature T1 is the temperature of the heater.’
Let’s take the example of an ideal heat machine.
Any heat machine consists of three parts: a heat sink, a working body and a heat receiver. The heat sink has a temperature T1 and gives some heat Q1 to the working body. Working body (gas, steam, hot liquid) performs the work. Not all of the heat Q1 is converted into work, but only some of it
Another part of the heat Q2 is transferred to a body with a lower temperature (T2), the heat acceptor. Thus, the essence of the heat machine is not only to receive heat Q1 from the heat sink and do the work A, but also to transfer a certain amount of heat Q2 to the receiver, whose temperature is lower than the temperature of the heat sink (T1 T2). A perpetual motion machine of the second kind consists of the first two parts, that is, the heat Q1 is completely converted into work A, and this is impossible. Where there is no temperature difference (T1 = T2), it is impossible to turn heat into work.
To get a mathematical expression of the second principle of thermodynamics, consider the action of an ideal heat machine. An ideal machine is one that works without friction or heat loss. In this cycle the working body is an ideal gas. The operation of the machine is based on the principle of a reversible thermodynamic cycle, called the Carnot cycle.
The Carnot cycle consists of four consecutive processes: isothermal expansion, adiabatic expansion, isothermal compression, adiabatic compression of gas. All processes are reversible, as the result of which the gas returns to its initial position
As a result of mathematical transformations we get
where h is the coefficient of performance (efficiency) of the heat machine.
The rotor mounted on the shaft is rigidly connected to a cogwheel that engages with a stationary gear. The toothed rotor seems to run round the pinion. Its facets slide on the epitrochoidal surface of the cylinder and cut off the variable volumes of the chambers in the cylinder.
This design enables the 4-stroke cycle without a dedicated timing mechanism. Sealing of the chambers is achieved by radial and axial sealing plates that are pressed against the cylinder by centrifugal forces, gas pressure and band springs.
Mixture, ignition, lubrication, cooling, starting up are in principle the same as for an ordinary reciprocating internal combustion engine.
Motors with triangular rotors, with the ratio of the gear wheel to gear wheel radius: r: R = 2: 3, which are installed in automobiles, boats, etc., are of practical application.п. The mass and dimensions of a Wankel engine are two to three times smaller than those of a conventional combustion engine.
The air first enters the cylinder, is compressed and heated to a high temperature. A jet injector fills the hot air with fuel that can ignite quickly and combustibly and gets the engine running. Such engines require special diesel fuel. We all know from physics lessons that thermal energy can be converted into mechanical. This is what happens when the fuel burns in the engine cylinder. Heat, when converted into mechanical work, begins to move the piston, which moves reciprocatingly in the cylinder. The crankshaft, which is connected to the piston by a connecting rod, rotates.
During operation, the piston moves in and out of the crankshaft. When the two parts come together, the combustible mixture enters the cylinder. When the cylinder goes in reverse, it gets pressurized. The compressed combustible mixture at this point is ready for combustion, it is barely worth a spark, the mixture is easily ignited and emits the gases that are needed to set the motor in motion. The cylinder is connected to a pipe that discharges the exhaust gases from the engine.
One movement of the piston to or from the crankshaft is called a stroke. If the shaft completes two revolutions about its axis in four strokes, the so-called work cycle is complete. An engine that performs a work cycle of two crankshaft revolutions is called a four-stroke engine. There are also twin engines. They work over two strokes of the piston and one revolution of the crankshaft. Such engines are not used in automobiles, but are widely used in motorcycles.
The greater the pressure on the piston during combustion of the combustible mixture, the greater the power of the engine. It is therefore advantageous to increase the compression ratio in the engine. In this case more useful work is obtained from the same portion of fuel. Many motorists try to tune the engine themselves so that it consumes less fuel, but does not lose power. But don’t be too keen on it, because when the compression ratio is increased, the combustible mixture burns too quickly (this process is called detonation), which causes unstable engine operation. At the same time in the working engine can be heard a knocking, the power is significantly reduced, and black smoke comes out of the muffler.
What is a heat engine heater for?
Thermal machines in thermodynamics are periodic heat engines and refrigeration machines (thermocompressors). A variety of refrigeration machines are heat pumps.
Devices, which perform mechanical work due to the internal energy of fuel, are called thermal machines (thermal engines). For a heat machine to function, the following components are necessary: 1) a heat source with a higher temperature t1, 2) a heat source with a lower temperature t2, 3) a working body. In other words: All heat machines (heat engines) consist of a heater, a refrigerator and a working body.
Heat engine. Experience in physics
Gas or steam are used as the working body, because they are well compressible, and depending on the type of engine can be fuel (gasoline, kerosene), water vapor, etc. The heater transfers to the working body a certain amount of heat (Q1), and its internal energy increases, at the expense of this internal energy mechanical work is done (A), then the working body gives a certain amount of heat to the refrigerator (Q2) and cools down at this to the initial temperature. The described scheme represents a cycle of engine operation and is general, in real engines the role of the heater and refrigerator can be performed by different devices. The refrigerant can be the environment.
Since in an engine, part of the energy of the working body is transferred to the cooler, it is clear that not all of the energy it receives from the heater is used to perform work. Accordingly, the efficiency of an engine is equal to the ratio of the work done (A) to the amount of heat received by it from the heater (Q1):
Internal combustion engine (ICE)
There are two types of internal combustion engines (ICE): carbureted and diesel. In a carburetor engine, the fuel mixture (fuel-air mixture) is prepared outside the engine in a special device and from that mixture enters the engine. In a diesel engine, the fuel mixture is prepared inside the engine.
Internal combustion engine consists of a cylinder in which a piston moves, the cylinder has two valves, through one of which the combustible mixture enters the cylinder, and through the other exhaust gases are let out of the cylinder. The piston is connected by a crank mechanism to the crankshaft, which rotates as the piston moves forward. Cylinder is closed with a cap.
The internal combustion engine cycle includes four cycles: intake, compression, stroke, exhaust. During intake, the piston moves down and the pressure in the cylinder is reduced and the mixture (carburetor) or air (diesel) enters the cylinder through the valve. The valve is closed at this time. At the end of the mixture intake, the valve closes.
During the second stroke, the piston moves up, the valves are closed, and the mixture or air is compressed. At the same time the gas temperature rises: the combustible mixture in the carburetor engine heats to 300-350 ° C, and the air in the diesel engine. up to 500-600 ° C. At the end of the compression stroke in a carburettor engine, a spark ignites the combustible mixture. In a diesel engine, fuel is injected into the cylinder and the resulting mixture ignites itself.
During combustion, the gas expands and pushes the piston and the crankshaft connected to it, generating mechanical work. This causes the gas to cool.
When the piston reaches its lowest point, the pressure in the piston decreases. When the piston moves upward, the valve opens and the exhaust gas is released. At the end of this cycle the valve closes.
A steam turbine is a disc attached to the shaft on which the blades are attached. Steam is delivered to the blades. The vapor heated to 600 °C flows into the nozzle and expands in it. When the steam expands, its internal energy is converted into the kinetic energy of the directional motion of the steam jet. The steam jet streams from the nozzle onto the turbine blades and transfers some of its kinetic energy into the blades, causing the turbine to rotate. Turbines usually have several disks, each of which transfers some of the energy of the steam. The rotation of the disk is transmitted to the shaft with which the electric current generator is connected.
Specific heat of combustion
Burning different fuels of the same mass gives out different amounts of heat. For example, it is well known that natural gas is a more energy efficient fuel than firewood. This means that in order to obtain the same amount of heat, the mass of firewood, which must be burned, must be significantly greater than the mass of natural gas. Consequently, different types of fuel from the energy point of view are characterized by the value called the calorific value of fuel.
The calorific value of fuel is a physical quantity showing how much heat is released during total combustion of 1 kg of fuel.
The specific heat of combustion is marked with q, the unit of which is 1 J/kg.
The specific heat is determined experimentally. Hydrogen has the greatest specific heat of combustion, and gunpowder has the lowest.
Specific heat of combustion of oil is 4,410 7 J/kg. This means that the complete combustion of 1 kg of oil releases a quantity of heat of 4,410 7 J. In general, if the mass of fuel is equal to m, the quantity of heat Q evolved at its complete combustion is equal to the product of the specific heat of combustion q by its mass:
Q = qm.
Lesson outline for physics in 8th grade. ICE. Specific heat of combustion.
What function does each element perform?
From the heater, the working body gas, or steam, receives a reserve of thermal energy Then, the resulting energy is divided into two, usually unequal parts. Work is done by one part.
and the remainder is transferred to a refrigerant (for example, to the atmosphere) and dissipated by the surrounding medium.
Role of the refrigerator in a heat engine
When making work, the working body, the expanding gas, is being cooled. The temperature \(T_\) to which the gas has cooled is called the refrigerant temperature.
As gas, expanding, is cooling, and at cooling the energy should be somewhere to go, so no thermal machine without a fridge will be able to work. In order to function, a heat machine must give up some of its heat energy to a refrigerator.
Normally the temperature \(T_\) is slightly higher than the ambient temperature. But in the case of steam engines equipped with a condenser, which is a special device for condensing and cooling the exhaust steam, the temperature \(T_\) can be somewhat lower than the ambient temperature
Note: The steam condenser is only used in steam engine designs.
The working body of a heat engine
In order to do useful work, it is necessary to create motion under the action of a force. Such motion in a heat engine is accomplished by the expansion of a portion of a gas called the working body. In all heat engines, the working body receives heat from the heater, then expands, performing work. As it expands, it cools and gives off heat to the Condenser.
For all applied thermal engines the cooling agent is the environment. Heaters, on the other hand, depend on the type of engine. For a steam engine The heater is the furnace of the steam boiler. For internal combustion engines, the heater is the working medium itself. a combustible gas mixture.
Types of jet engines
All the different types of jet engines consist of the following main parts: 1) a tank with fuel; 2) a combustion chamber; 3) the devices which control the fuel supply to the combustion chamber and the combustion products exhaust. Depending on the type of fuel used, jet engines are divided into two large groups: solid fuel engines, liquid fuel engines.
The simplest example of a solid fuel engine is a gunpowder rocket. In a rocket, the combustion of gunpowder produces gases which are ejected from the body of the rocket, creating jet propulsion.
Liquid propellants (petroleum products, alcohol, etc) burn in liquid propellant engines. д.). Liquid jet engines were used at the end of World War II for long-range projectile airplanes. Velocity of rocket planes was up to 5400 km/h with flight range of 290-300 km and trajectory altitude of 100 km.
This type of engine also included the rocket engine for interplanetary communication, invented by C. Э. Tsiolkovsky.
Energy Conversion in Thermal Engines
The steam engine heralded the beginning of the scientific and technological revolution, but the steam engine itself was imperfect at first. They developed great power, but consumed too much fuel.
If we compare the performance of the first engines with the horsepower of a horse, we find that the horse uses “fuel” oats and hay much more efficiently. Scientists have noted that the body “burns” food: after all, humans and animals inhale oxygen and exhale carbon dioxide and water vapor; so does the furnace with burning firewood.
This is when calorie counting was first learned. The energy of food was estimated by the amount of heat that would be released by burning it. On the “calorie” scale, oats, coal, and gasoline could be compared. And on this scale, the first steam engines were extremely inefficient: only 1\% 2\% of the calories burned were converted into useful work.
Attempts were made to improve the machines, sometimes they produced a better effect, sometimes a worse one; a theoretical basis was needed in order to achieve the best version.
The founders of thermodynamics first of all solved the question: can all the heat, transferred by a steam engine, be converted into work?? In mechanics, the conversion of potential energy into kinetic energy can occur with very little loss. Mostly friction interferes, but in many problems friction can be neglected. Let’s imagine that we also reduce to zero friction of the piston on the cylinder, unproductive loss of thermal energy. Can we imagine an ideal cyclic motor in which all heat is converted into work??
According to the first principle of thermodynamics, heat is consumed by work and an increase in internal energy:
Let DU = 0. The heat caused the steam to expand, the steam set the piston in motion and the piston did the work. the temperature of the steam and its internal energy has not changed, let’s ignore the losses and assume that all heat is converted into mechanical work: Q = A
But we are looking at a cyclic motor. The piston has moved, having done work; it must now be returned to its original state.
If you move the piston, compressing the steam, you have to do as much work as A. But this means that there is no gain, and the efficiency is zero, even if there is no loss!
To reduce the work of moving the piston back, let the internal energy change. If the steam is cooled, its pressure will decrease, and the work of moving the piston will be less than that done in the working cycle.
This difference of work is the efficiency of engine.
In the diagram p(v) the forward and reverse stroke of the piston is shown by the lines abc and cda which form a closed figure. The area of the closed figure abcd corresponds to the useful work. The area V1abcV2 is the forward work and the area V2cdaV1 corresponds to the reverse work.
Thus, a heat engine needs not only a heater, but also a cooler; most often the role of the cooler is played by the environment, to which the residual heat is transferred
In the ideal case, the work accomplished during the cycle corresponds to the difference between the heat which the heated working body had and the heat which was left by the working body after cooling down:
The efficiency of an ideal motor is equal to the ratio of the work to the heat received from the heater:
This formula shows the efficiency limit that cannot be exceeded by a heat engine at certain heater and cooler parameters. The actual efficiency of an engine depends on its design, and is always less than the ideal value.
So, the efficiency of an engine is always less than unity, because part of the heat energy must be given to the cooler. This is a reflection of the second principle of thermodynamics
One of the formulations of the second principle of thermodynamics:
It is impossible to have a circular process, the only result of which would be to produce work by cooling the heat reservoir. (This process is called the Thomson process).
The physical basis of a heat engine
The performance of mechanical work in modern machines and mechanisms is mainly due to the internal energy of substances.
Thermal motor. A device that converts the internal energy of fuel into mechanical energy
It is impossible to imagine modern civilization without heat engines.
The mechanical work in the engine is done by the expansion of the working substance that moves the piston in the cylinder. For the cyclic, continuous operation of an engine, the piston must return to its original position, t.е. compression of working substance. Easily compressible substance is the substance in gaseous state, so as the working substance in the heat engine is used gas or steam.
The work of a heat engine consists of the intermittent processes of gas expansion and compression. Gas compression cannot be spontaneous, it occurs only under the action of an external force, for example, at the expense of energy stored by the engine flywheel during gas expansion.
The total mechanical work A is the sum of the gas expansion work Arash and the gas compression work Acj, performed by the gas pressure forces during its compression. Since at compression DV0, Aszh =. |Acj |0, therefore
In order to obtain positive total mechanical work (A0), the work of gas compression must be less than the work of gas expansion.
The change in volume DV of gas during expansion and compression must be the same due to the cyclic operation of the engine.
Consequently, the pressure of the gas during compression must be less than its pressure during expansion. For the same volume of gas, the smaller the lower its temperature, so the gas must be cooled before compression, t.е. is in contact with a cooling machine. by a body having a lower temperature. To produce mechanical work in a heat engine in a cyclic process, gas expansion must occur at a higher temperature than compression.
Prerequisite for the cyclic production of mechanical work in a heat engine. presence of a heater and a refrigerator.
Efficiency of a heat engine
Purpose of a heat engine.- produce mechanical work. But only part of the heat received by the engine is expended to.
The impossibility of the complete conversion of the internal energy of a gas into the work of a heat engine is due to the irreversibility of processes in nature. If the heat could spontaneously return from the refrigerator to the heater, the internal energy could be completely transformed into useful work by any heat engine.
According to the law of conservation of energy, the work done by the engine is equal:
where. the amount of heat received from the refrigerator, and. amount of heat given to the refrigerator.
The coefficient of efficiency of a heat engine is the ratio of work. made by the motor, to the amount of heat received from the heater:
Since all engines have some amount of heat transferred to the refrigerator, 1.
The efficiency of a heat engine is proportional to the temperature difference between the heater and the refrigerator. At
the motor can not work. Maximum efficiency of a heat engine
The laws of thermodynamics allow us to calculate the maximum possible efficiency of a heat engine operating with a heater having a temperature T 1. and the refrigerator with temperature T 2. This was first done by the French engineer and scientist Sadi Carnot. Carnot invented an ideal heat machine with an ideal gas as the working medium. The amount of heat received by the working body from the heater during isothermal expansion is equal to
Similarly, in isothermal compression, the working body gave to the refrigerator
Hence the efficiency factor of the Carnot heat machine is
The last expression shows that the efficiency of the Carnot heat machine depends only on the temperatures of the heater and the refrigerator. It also follows that the efficiency can be 100% only if the refrigerant temperature is absolute zero, which is unattainable. It can be shown that the efficiency of any heat machine operating on a cycle other than the Carnot cycle will be less than the efficiency of a Carnot heat machine operating at the same heater and cooler temperatures.
Transfer of heat from the heater to the working body and from the working body to the cooler takes place in the Carnot cycle in the absence of a temperature difference. Благодаря этому цикл Карно обратим (передача тепла при наличии конечной разности температур всегда необратима согласно постулату Томсона). Но при отсутствии разности температур тепло передается бесконечно медленно. Поэтому мощность тепловой машины Карно равна нулю.