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heat engine //موضوع كطريق استدلالي عن المكائن الحرارية

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    الصورة الرمزية حسن هادي
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    heat engine //موضوع كطريق استدلالي عن المكائن الحرارية

    heat engine //موضوع كطريق استدلالي عن المكائن الحرارية وعن كيفية عملها وطرق حساب الكفاءة الحراة والتطرق الى الخسائر التي تحصل في الطاقة من خلال باقة مشاركات نرجو ان تنال استحسانكم *
    ************************************************** ********************************

    A heat engine is a physical or theoretical device that converts thermal energy to mechanical output. The mechanical output is called work, and the thermal energy input is called heat. Heat engines typically run on a specific thermodynamic cycle. Heat engines are often named after the thermodynamic cycle they are modeled by. They often pick up alternate names, such as gasoline/petrol, turbine, or steam engines. Heat engines can generate heat inside the engine itself or it can absorb heat from an external source. Heat engines can be open to the atmospheric air or sealed and closed off to the outside (Open or closed cycle).
    In engineering and thermodynamics, a heat engine performs the conversion of heat energy to mechanical work by exploiting the temperature gradient between a hot "source" and a cold "sink". Heat is transferred from the source, through the "working body" of the engine, to the sink, and in this process some of the heat is converted into work by exploiting the properties of a working substance (usually a gas or liquid).



    Figure 1: Heat engine diagram

  2. [2]
    حسن هادي
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    عضو متميز
    الصورة الرمزية حسن هادي


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    Overview
    Heat engines are often confused with the cycles they attempt to mimic. Typically when describing the physical device the term 'engine' is used. When describing the model the term 'cycle' is used.
    In thermodynamics, heat engines are often modeled using a standard engineering model such as the Otto cycle (4-stroke/2-stroke). Actual data from an operating engine, one is called an indicator diagram, is used to refine the model. All modern implementations of heat engines do not exactly match the thermodynamic cycle they are modeled by. One could say that the thermodynamic cycle is an ideal case of the mechanical engine. One could equally say that the model doesn't quite perfectly match the mechanical engine. However, understanding is gained from the simplified models, and ideal cases they may represent.
    In general terms, the larger the difference in temperature between the hot source and the cold sink, the larger is the potential thermal efficiency of the cycle. On Earth, the cold side of any heat engine is limited to close to the ambient temperature of the environment, or not much lower than 300 kelvins, so most efforts to improve the thermodynamic efficiencies of various heat engines focus on increasing the temperature of the source, within material limits.
    The efficiency of various heat engines proposed or used today ranges from 3 percent [1](97 percent waste heat) for the OTEC ocean power proposal through 25 percent for most automotive engines, to 45 percent for a supercritical coal plant, to about 60 percent for a steam-cooled combined cycle gas turbine. All of these processes gain their efficiency (or lack thereof) due to the temperature drop across them.
    OTEC uses the temperature difference of ocean water on the surface and ocean water from the depths, a small difference of perhaps 25 degrees Celsius, and so the efficiency must be low. The combined cycle gas turbines use natural-gas fired burners to heat air to near 1530 degrees Celsius, a difference of a large 1500 degrees Celsius, and so the efficiency can be large when the steam-cooling cycle is added in. [

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    حسن هادي
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    عضو متميز
    الصورة الرمزية حسن هادي


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    examples//
    ملاحظة الروابط تعمل
    Examples of everyday heat engines include: the steam engine, the diesel engine, and the gasoline (petrol) engine in an automobile. A common toy that is also a heat engine is a drinking bird. All of these familiar heat engines are powered by the expansion of heated gases. The general surroundings are the heat sink, providing relatively cool gases which, when heated, expand rapidly to drive the mechanical motion of the engine.

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    حسن هادي
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    عضو متميز
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    Phase change cycles
    In these cycles and engines, the working fluids are gases and liquids. The engine converts the working fluid from a gas to a liquid.

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    Energy efficiency


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    For energy efficiency in relation to energy economics, see Efficient energy use In physics and engineering, including mechanical and electrical engineering, energy efficiency is a dimensionless number, with a value between 0 and 1 or, when multiplied by 100, is given as a percentage. The energy efficiency of a process, denoted by eta, is defined as
    where output is the amount of mechanical work (in watts) or energy released by the process (in joules), and input is the quantity of work or energy used as input to run the process.
    Due to the principle of conservation of energy, energy efficiency within a closed system can never exceed 100%.

    [edit] Energy efficiency and global warming

    Making homes, vehicles, and businesses more energy efficient is seen as a largely untapped solution to addressing global warming and energy security. Many of these ideas have been discussed for years, since the 1973 oil crisis brought energy issues to the forefront. In the late 1970s, physicist Amory Lovins popularized the notion of a "soft path" on energy, with a strong focus on energy efficiency. Among other things, Lovins popularized the notion of negawatts -- the idea of meeting energy needs by increasing efficiency instead of increasing energy production.
    Energy efficiency has proved to be a cost-effective strategy for building economies without necessarily growing energy consumption, as environmental business strategist Joel Makower has noted. For example, the state of California began implementing energy-efficiency measures in the mid-1970s, including building code and appliance standards with strict efficiency requirements. As a result, the state's energy consumption has remained flat over 30 years while national U.S. consumption doubled. As part of its strategy, California implemented a three-step plan for new energy resources that puts energy efficiency first, renewable electricity supplies second, and new fossil-fired power plants last.
    Still, efficiency often has taken a secondary position to new power generation as a solution to global warming in creating national energy policy. Some companies also have been reluctant to engage in efficiency measures, despite the often favorable returns on investments that can result. Lovins' Rocky Mountain Institute points out that in industrial settings, "there are abundant opportunities to save 70% to 90% of the energy and cost for lighting, fan, and pump systems; 50% for electric motors; and 60% in areas such as heating, cooling, office equipment, and appliances." In general, up to 75% of the electricity used in the U.S. today could be saved with efficiency measures that cost less than the electricity itself.
    Other studies have emphasized this. A report published in 2006 by the McKinsey Global Institute, asserted that "there are sufficient economically viable opportunities for energy-productivity improvements that could keep global energy-demand growth at less than 1 percent per annum" -- less than half of the 2.2 percent average growth anticipated through 2020 in a business-as-usual scenario. Energy productivity -- which measures the output and quality of goods and services per unit of energy input -- can come from either reducing the amount of energy required to produce something, or from increasing the quantity or quality of goods and services from the same amount of energy.

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  7. [7]
    حسن هادي
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    عضو متميز
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    Efficiency
    The efficiency of a heat engine relates how much useful power is output for a given amount of heat energy input.
    From the laws of thermodynamics:
    where dW = − PdV is the work extracted from the engine. (It is negative since work is done by the engine.) dQh = ThdSh is the heat energy taken from the high temperature system. (It is negative since heat is extracted from the source, hence ( − dQh) is positive.) dQc = TcdSc is the heat energy delivered to the cold temperature system. (It is positive since heat is added to the sink.) In other words, a heat engine absorbs heat energy from the high temperature heat source, converting part of it to useful work and delivering the rest to the cold temperature heat sink.
    In general, the efficiency of a given heat transfer process (whether it be a refrigerator, a heat pump or an engine) is defined informally by the ratio of "what you get" to "what you put in."
    In the case of an engine, one desires to extract work and puts in a heat transfer.
    The theoretical maximum efficiency of any heat engine depends only on the temperatures it operates between. This efficiency is usually derived using an ideal imaginary heat engine such as the Carnot heat engine, although other engines using different cycles can also attain maximum efficiency. Mathematically, this is due to the fact that in reversible processes, the change in entropy of the cold reservoir is the negative of that of the hot reservoir (i.e., dSc = − dSh), keeping the overall change of entropy zero. Thus:
    where Th is the absolute temperature of the hot source and Tc that of the cold sink, usually measured in kelvin. Note that dSc is positive while dSh is negative; in any reversible work-extracting process, entropy is overall not increased, but rather is moved from a hot (high-entropy) system to a cold (low-entropy one), decreasing the entropy of the heat source and increasing that of the heat sink.
    The reasoning behind this being the maximal efficiency goes as follows. It is first assumed that if a more efficient heat engine than a Carnot engine is possible, then it could be driven in reverse as a heat pump. Mathematical analysis can be used to show that this assumed combination would result in a net decrease in entropy. Since, by the second law of thermodynamics, this is forbidden, the Carnot efficiency is a theoretical upper bound on the efficiency of any process.
    Empirically, no engine has ever been shown to run at a greater efficiency than a Carnot cycle heat engine.

    Figure 2: Carnot cycle efficiency



    Figure 3: Carnot cycle efficiency


    Here are two plots, Figure 2 and Figure 3, for the Carnot cycle efficiency. One plot indicates how the cycle efficiency changes with an increase in the heat addition temperature for a constant compressor inlet temperature, while the other indicates how the cycle efficiency changes with an increase in the heat rejection temperature for a constant turbine inlet temperature.

    [edit] Other criteria of heat engine performance

    One problem with the ideal Carnot efficiency as a criterion of heat engine performance is the fact that by its nature, any maximally-efficient Carnot cycle must operate at an infinitesimal temperature gradient. This is due to the fact that any transfer of heat between two bodies at differing temperatures is irreversible, and therefore the Carnot efficiency expression only applies in the infinitesimal limit. The major problem with that is that the object of most heat engines is to output some sort of power, and infinitesimal power is usually not what is being sought.
    A different measure of heat engine efficiency is given by the endoreversible process, which is identical to the Carnot cycle except in that the two processes of heat transfer are not reversible. As derived in Callen (1985), the efficiency for such a process is given by:
    This model does a better job of predicting how well real-world heat engines can do, as can be seen in the following table (Callen):
    Efficiencies of Power PlantsPower PlantTc (°C)Th (°C)η (Carnot)η (Endoreversible)η (Observed)West Thurrock (UK) coal-fired power plant255650.640.400.36CANDU (Canada) nuclear power plant253000.480.280.30Larderello (Italy) geothermal power plant802500.330.1780.16
    As shown, the endoreversible efficiency much more closely models the observed data.

    [edit] Heat engine enhancements

    Engineers have studied the various heat engine cycles extensively in an effort to improve the amount of usable work they could extract from a given power source. The Carnot Cycle limit cannot be reached with any gas-based cycle, but engineers have worked out at least two ways to possibly go around that limit, and one way to get better efficiency without bending any rules.
    1) Increase the temperature difference in the heat engine. The simplest way to do this is to increase the hot side temperature, and is the approach used in modern combined-cycle gas turbines. Unfortunately, NOx production and material limits (melting the turbine blades) place a hard limit to how hot you can make a workable heat engine. Modern gas turbines are about as hot as they can become and still maintain acceptable NOx pollution levels. Another way of increasing efficiency is to lower the output temperature. Once new method of doing so is to use mixed chemical working fluids, and then exploit the changing behavior of the mixtures. One of the most famous is the so-called Kalina Cycle, which uses a 70/30 mix of ammonia and water as its working fluid. This mixture allows the cycle to generate useful power at considerably lower temperatures than most other processes.
    2) Exploit the physical properties of the working fluid. The most common such exploit is the use of water above the so-called critical point, or so-called supercritical steam. The behavior of fluids above their critical point changes radically, and with materials such as water and carbon dioxide it is possible to exploit those changes in behavior to extract greater thermodynamic efficiency from the heat engine, even if it is using a fairly conventional Brayton or Rankine cycle. A newer and very promising material for such applications is CO2. SO2 and xenon have also been considered for such applications, although SO2 is a little toxic for most.
    3) Exploit the chemical properties of the working fluid. A fairly new and novel exploit is to use exotic working fluids with advantageous chemical properties. One such is nitrogen dioxide (NO2), a toxic component of smog, which has a natural dimer as di-nitrogen tetraoxide (N2O4). At low temperature, the N2O4 is compressed and then heated. The increasing temperature causes each N2O4 to break apart into two NO2 molecules. This lowers the molecular weight of the working fluid, which drastically increases the efficiency of the cycle. Once the NO2 has expanded through the turbine, it is cooled by the heat sink, which causes it to recombine into N2O4. This is then fed back to the compressor for another cycle. Such species as aluminum bromide (Al2Br6), NOCl, and Ga2I6 have all been investigated for such uses. To date, their drawbacks have not warranted their use, despite the efficiency gains that can be realized. [3]

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  8. [8]
    حسن هادي
    حسن هادي غير متواجد حالياً
    عضو متميز
    الصورة الرمزية حسن هادي


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    نرجوا ان يكون الموضوع مفيدا والله ولي التوفيق



    For the forty years following the first flight of the Wright brothers, airplanes used internal combustion engines to turn propellers to generate thrust. Today, most general aviation or private airplanes are still powered by propellers and internal combustion engines, much like your automobile engine. On this page we will discuss the fundamentals of the internal combustion engine using the Wright brothers' 1903 engine, shown in the figure, as an example.
    The brothers' design is very simple by today's standards, so it is a good engine for students to study to learn the fundamentals of engine operation. This type of internal combustion engine is called a four-stroke engine because there are four movements (strokes) of the piston before the entire engine firing sequence is repeated. In the figure, we have colored the fuel/air intake system red, the electrical system green, and the exhaust system blue. We also represent the fuel/air mixture and the exhaust gases by small colored balls to show how these gases move through the engine. Since we will be referring to the movement of various engine parts, here is a figure showing the names of the parts:




    Mechanical Operation
    At the end of the power stroke the exhaust has been expanded into the cylinder to a moderate pressure and temperature by the motion of the piston to the left. From our considerations of the engine cycle, we designate this condition as Stage 5 of the Otto cycle. The intake valve and exhaust valve are closed and the electrical contact is open. The exhaust has done work on the piston but there is some residual heat in the exhaust gas. As the piston comes to a halt near the crankshaft, the residual heat is quickly transferred to the water in the water jacket surrounding the cylinder. In theory, the transfer proceeds so quickly that we can consider the piston to be motionless and the volume of the combustion chamber and cylinder to be a constant. The end of the heat rejection process is designated Stage 6 of the engine cycle and is the beginning of the exhaust stroke.

    Thermodynamics
    Because the intake and exhaust valves are closed, the heat transfer from the exhaust gas takes place in a nearly constant volume vessel. The heat transfer decreases the temperature of the exhaust gas. From considerations of the first law of thermodynamics, the temperature decrease is given by:
    T6 = T5 - Q /cv
    where Q is the heat rejected, T is the temperature, and cv is the specific heat at constant volume, From the equation of state, we know that:
    p6 = p5 * (T6 /T5)
    where p is the pressure. The numbers indicate the two stages of the cycle. Since Q is a positive number, T6 is less than T5 and p6 is less than p5. Temperature and pressure in the cylinder both decrease during the cooling process. The final value of the pressure is atmospheric pressure and this determines the amount of heat that is rejected. In theory, the heat transfer takes place instantaneously when the piston is motionless. In reality, the heat is transferred throughout the exhaust stroke. The effect is the same, but reality is so much harder to analyze that we make the assumption of instantaneous heat release to obtain an initial estimate of the heat transferred.


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  9. [9]
    حسن هادي
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    عضو متميز
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    تحياتي لكل الاعضاء

    Steam engine in action (animation). Note that movement of the connecting linkage from the centrifugal governor operating the steam throttle is shown for illustrative purpose only, in practice this link only operates when the engine speeds up or slows down.




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  10. [10]
    حسن هادي
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    عضو متميز
    الصورة الرمزية حسن هادي


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    دورة اوتو المثالية


    To move an airplane through the air, thrust is generated by some kind of propulsion system. Beginning with the Wright brothers' first flight, many airplanes have used internal combustion engines to turn propellers to generate thrust. Today, most general aviation or private airplanes are powered by internal combustion (IC) engines, much like the engine in your family automobile. When discussing engines, we must consider both the mechanical operation of the machine and the thermodynamic processes that enable the machine to produce useful work. On this page we consider the thermodynamics of a four-stroke IC engine.
    To understand how a propulsion system works, we must study the basic thermodynamics of gases. Gases have various properties that we can observe with our senses, including the gas pressure p, temperature T, mass, and volume V that contains the gas. Careful, scientific observation has determined that these variables are related to one another, and the values of these properties determine the state of the gas. A thermodynamic process, such as heating or compressing the gas, changes the values of the state variables in a manner which is described by the laws of thermodynamics. The work done by a gas and the heat transferred to a gas depend on the beginning and ending states of the gas and on the process used to change the state. It is possible to perform a series of processes, in which the state is changed during each process, but the gas eventually returns to its original state. Such a series of processes is called a cycle and forms the basis for understanding engine operation.
    On this page we discuss the Otto Thermodynamic Cycle which is used in all internal combustion engines. The figure shows a p-V diagram of the Otto cycle. Using the engine stage numbering system, we begin at the lower left with Stage 1 being the beginning of the intake stroke of the engine. The pressure is near atmospheric pressure and the gas volume is at a minimum. Between Stage 1 and Stage 2 the piston is pulled out of the cylinder with the intake valve open. The pressure remains constant, and the gas volume increases as fuel/air mixture is drawn into the cylinder through the intake valve. Stage 2 begins the compression stroke of the engine with the closing of the intake valve. Between Stage 2 and Stage 3, the piston moves back into the cylinder, the gas volume decreases, and the pressure increases because work is done on the gas by the piston. Stage 3 is the beginning of the combustion of the fuel/air mixture. The combustion occurs very quickly and the volume remains constant. Heat is released during combustion which increases both the temperature and the pressure, according to the equation of state. Stage 4 begins the power stroke of the engine. Between Stage 4 and Stage 5, the piston is driven towards the crankshaft, the volume in increased, and the pressure falls as work is done by the gas on the piston. At Stage 5 the exhaust valve is opened and the residual heat in the gas is exchanged with the surroundings. The volume remains constant and the pressure adjusts back to atmospheric conditions. Stage 6 begins the exhaust stroke of the engine during which the piston moves back into the cylinder, the volume decreases and the pressure remains constant. At the end of the exhaust stroke, conditions have returned to Stage 1 and the process repeats itself.
    During the cycle, work is done on the gas by the piston between stages 2 and 3. Work is done by the gas on the piston between stages 4 and 5. The difference between the work done by the gas and the work done on the gas is the area enclosed by the cycle curve and is the work produced by the cycle. The work times the rate of the cycle (cycles per second) is equal to the power produced by the engine.
    The area enclosed by the cycle on a p-V diagram is proportional to the work produced by the cycle. On this page we have shown an ideal Otto cycle in which there is no heat entering (or leaving) the gas during the compression and power strokes, no friction losses, and instantaneous burning occurring at constant volume. In reality, the ideal cycle does not occur and there are many losses associated with each process. These losses are normally accounted for by efficiency factors which multiply and modify the ideal result. For a real cycle, the shape of the p-V diagram is similar to the ideal, but the area (work) is always less than the ideal value.

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