# heat engine //موضوع كطريق استدلالي عن المكائن الحرارية



## حسن هادي (19 أغسطس 2007)

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 *
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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*


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## حسن هادي (19 أغسطس 2007)

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 (19 أغسطس 2007)

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 (19 أغسطس 2007)

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.

Rankine cycle (classical steam engine)
Regenerative cycle (steam engine more efficient than Rankine cycle)
Vapor to liquid cycle (Drinking bird)
Liquid to solid cycle (Frost heaving — water changing from ice to liquid and back again can lift rock up to 60 m.)
Solid to gas cycle (Dry ice cannon — Dry ice sublimes to gas.)


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## حسن هادي (19 أغسطس 2007)

Gas only cycles
In these cycles and engines the working fluid are always like gas:

Carnot cycle (Carnot heat engine)
Ericsson Cycle (Caloric Ship John Ericsson)
Stirling cycle (Stirling engine, thermoacoustic devices)
Internal combustion engine (ICE):
Otto cycle (eg. Gasoline/Petrol engine, high-speed diesel engine)
Diesel cycle (eg. low-speed diesel engine)
Atkinson Cycle (Atkinson Engine)
Brayton cycle or Joule cycle originally Ericsson Cycle (gas turbine)
Lenoir cycle (e.g., pulse jet engine)
Miller cycle


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## حسن هادي (19 أغسطس 2007)

*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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## حسن هادي (19 أغسطس 2007)

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 _d__W_ = − _P__d__V_ is the work extracted from the engine. (It is negative since work is _done by_ the engine.) _d__Q__h_ = _T__h__d__S__h_ is the heat energy taken from the high temperature system. (It is negative since heat is extracted from the source, hence ( − _d__Q__h_) is positive.) _d__Q__c_ = _T__c__d__S__c_ 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., _d__S__c_ = − _d__S__h_), keeping the overall change of entropy zero. Thus:



where _T__h_ is the absolute temperature of the hot source and _T__c_ that of the cold sink, usually measured in kelvin. Note that _d__S__c_ is positive while _d__S__h_ 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 Plants*_Power Plant__T__c_ (°C)_T__h_ (°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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## حسن هادي (19 أغسطس 2007)

_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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## حسن هادي (20 أغسطس 2007)

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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## حسن هادي (1 سبتمبر 2007)

دورة اوتو المثالية:6: 






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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## حسن هادي (1 سبتمبر 2007)

قانون الغاز المثالي:6: 




[FONT=Arial, Helvetica, sans-serif]Gases have various properties that we can observe with our senses, including the gas pressure *p*, temperature *T*, mass *m*, 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.
If we fix any two of the properties we can determine the nature of the relationship between the other two. You can explore the relationship between the variables at the animated gas lab. If the pressure and temperature are held constant, the volume of the gas depends directly on the mass, or amount of gas. This allows us to define a single additional property called the gas density *r*, which is the ratio of mass to volume. If the mass and temperature are held constant, the product of the pressure and volume are observed to be nearly constant for a real gas. The product of pressure and volume is exactly a constant for an *ideal gas*. This relationship between pressure and volume is called Boyle's Law in honor of Robert Boyle who first observed it in 1660. Finally, if the mass and pressure are held constant, the volume is directly proportional to the temperature for an ideal gas. This relationship is called Charles and Gay-Lussac's Law in honor of the two French scientists who discovered the relationship.
The gas laws of Boyle and Charles and Gay-Lussac can be combined into a single equation of state given in red at the center of the slide: 
p * V / T = n * Rbar 
where * denotes multiplication and / denotes division. To account for the effects of mass, we have defined the constant to contain two parts: a universal constant *Rbar* (on the figure, an R with a bar drawn over the top) and the mass of the gas expressed in moles *n*. Performing a little algebra, we obtain the more familiar form: 
*p * V = n * Rbar * T* 
A three dimensional graph of this equation is shown at the lower left. The intersection point of any two lines on the graph gives a unique state for the gas.
Engineers use a slightly different form of the equation of state that is specialized for a particular gas. If we divide both sides of the general equation by the mass of the gas, the volume becomes the specific volume, which is the inverse of the gas density. We also define a new gas constant *R*, which is equal to the universal gas constant divided by the mass per mole of the gas. The value of the new constant depends on the type of gas as opposed to the universal gas constant, which is the same for all gases. The value of the equation of state for air is given on the slide as .286 kilo Joule per kilogram per Kelvin. The equation of state can be written in terms of the specific volume or in terms of the air density as 
p * v = R * T 
p = r * R * T 
Notice that the equation of state given here applies only to an ideal gas, or a real gas that behaves like an ideal gas. There are in fact many different forms for the equation of state for different gases. Also be aware that the temperature given in the equation of state must be an absolute temperature that begins at absolute zero. In the metric system of units, we must specify the temperature in Kelvin (not degrees Celsius). In the Imperial system, absolute temperature is in Rankine (not degrees Fahrenheit).
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## حسن هادي (1 سبتمبر 2007)

[FONT=Arial, Helvetica, sans-serif]Thermodynamics is a branch of physics which deals with the energy and work of a system. Thermodynamics deals only with the large scale response of a system which we can observe and measure in experiments. Small scale gas interactions are described by the kinetic theory of gases. There are three principal laws of thermodynamics which are described on separate slides. Each law leads to the definition of thermodynamic properties which help us to understand and predict the operation of a physical system. We will present some simple examples of these laws and properties for a variety of physical systems, although we are most interested in the thermodynamics of propulsion systems and high speed flows. Fortunately, many of the classical examples of thermodynamics involve gas dynamics. 
In our observations of the work done on, or by a gas, we have found that the amount of *work depends not only on the initial and final states of the gas but also on the process*, or path which produces the final state. Similarly the *amount of heat transferred into, or from a gas also depends on the initial and final states and the process* which produces the final state. Many observations of real gases have shown that the difference of the heat flow into the gas and the work done by the gas depends only on the initial and final states of the gas and does *not* depend on the process or path which produces the final state. This suggests the existence of an additional variable, called the *internal energy* of the gas, which depends only on the state of the gas and not on any process. The internal energy is a state variable, just like the temperature or the pressure. The first law of thermodynamics defines the internal energy (E) as equal to the difference of the heat transfer (Q) *into* a system and the work (W) done *by* the system. 
E2 - E1 = Q - W 
We have emphasized the words "into" and "by" in the definition. Heat removed from a system would be assigned a negative sign in the equation. Similarly work done on the system is assigned a negative sign. 
The internal energy is just a form of energy like the potential energy of an object at some height above the earth, or the kinetic energy of an object in motion. In the same way that potential energy can be converted to kinetic energy while conserving the total energy of the system, the internal energy of a thermodynamic system can be converted to either kinetic or potential energy. Like potential energy, the internal energy can be stored in the system. _Notice, however, that heat and work can not be stored or conserved independently since they depend on the process._ The first law of thermodynamics allows for many possible states of a system to exist, but only certain states are found to exist in nature. The second law of thermodynamics helps to explain this observation. 
If a system is fully insulated from the outside environment, it is possible to have a change of state in which no heat is transferred into the system. Scientists refer to a process which does not involve heat transfer as an *adiabatic* process. The implementation of the first law of thermodynamics for gases introduces another useful state variable called the enthalpy which is described on a separate page. 
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## شكرى محمد نورى (1 سبتمبر 2007)

الأخ حسن هادي .

اجمل المنى .

عطاء دائم وموفق بأذنه تعالى .

[COLOR="Purple"[SIZE="5"]]نثمن جهود ومثابرتك على اطروحاتك المتجددة .[/SIZE][/COLOR]

مع التحية والسلام :31: .

البغدادي .:55:


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## حسن هادي (1 سبتمبر 2007)

نرجوا ان نكون قد وّفقنا في وضع الروابط المناسبة مع التوضيح الخاص بها لمنعفة الاخوة الاعضاء والله ولي التوفيق وندرج لكم في هذه المشاركة دورة كارنوت الحرارية وبامكانكم متابعة الدورات الاخرى في الروابط المدرجة في المشاركة المرقمة #5 مع التقدير *:6: 
********************************
*Carnot cycle*

*From Wikipedia, the free encyclopedia*


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The *Carnot cycle* is a particular thermodynamic cycle, modeled on the hypothetical Carnot heat engine, proposed by Nicolas Léonard Sadi Carnot in the 1820s and expanded upon by Benoit Paul Émile Clapeyron in the 1830s and 40s.
Every thermodynamic system exists in a particular state. A thermodynamic cycle occurs when a system is taken through a series of different states, and finally returned to its initial state. In the process of going through this cycle, the system may perform work on its surroundings, thereby acting as a heat engine.
A heat engine acts by transferring energy from a warm region to a cool region of space and, in the process, converting some of that energy to mechanical work. The cycle may also be reversed. The system may be worked upon by an external force, and in the process, it can transfer thermal energy from a cooler system to a warmer one, thereby acting as a refrigerator rather than a heat engine.
The Carnot cycle is a special type of thermodynamic cycle. It is special because it is the most efficient cycle possible for converting a given amount of thermal energy into work or, conversely, for using a given amount of work for refrigeration purposes.***************************************************
The Carnot cycle
The *Carnot cycle* when acting as a heat engine consists of the following steps:

*Reversible isothermal expansion of the gas at the "hot" temperature, TH (isothermal heat addition).* During this step (A to B on diagram) the expanding gas causes the piston to do work on the surroundings. The gas expansion is propelled by absorption of heat from the high temperature reservoir.
*Isentropic (Reversible adiabatic) expansion of the gas.* For this step (B to C on diagram) we assume the piston and cylinder are thermally insulated, so that no heat is gained or lost. The gas continues to expand, doing work on the surroundings. The gas expansion causes it to cool to the "cold" temperature, _T__C_.
*Reversible isothermal compression of the gas at the "cold" temperature, TC. (isothermal heat rejection)* (C to D on diagram) Now the surroundings do work on the gas, causing heat to flow out of the gas to the low temperature reservoir.
*Isentropic compression of the gas.* (D to A on diagram) Once again we assume the piston and cylinder are thermally insulated. During this step, the surroundings do work on the gas, compressing it and causing the temperature to rise to _T__H_. At this point the gas is in the same state as at the start of step 1.


 


Figure 1: *A Carnot cycle acting as a heat engine, illustrated on a temperature-entropy diagram. The cycle takes place between a hot reservoir at temperature TH and a cold reservoir at temperature TC. The vertical axis is temperature, the horizontal axis is entropy.*




*[edit] Properties and significance*


*[edit] The temperature-entropy diagram*



 


A generalized thermodynamic cycle taking place between a hot reservoir at temperature TH and a cold reservoir at temperature TC. By the second law of thermodynamics, the cycle cannot extend outside the temperature band from TC to TH. The area in red ΔQC is the amount of energy exchanged between the system and the cold reservoir. The area in white Δ W is the amount of work energy exchanged by the system with its surroundings. The amount of heat exchanged with the hot reservoir is the sum of the two. If the system is behaving as an engine, the process moves clockwise around the loop, and moves counter-clockwise if it is behaving as a refrigerator. The efficiency of the cycle is the ratio of the white area (work) divided by the sum of the white and red areas (total heat).


The behavior of a Carnot engine or refrigerator is best understood by using a temperature-entropy (TS) diagram, in which the thermodynamic state is specified by a point on a graph with entropy (S) as the horizontal axis and temperature (T) as the vertical axis. For a simple system with a fixed number of particles, any point on the graph will represent a particular state of the system. A thermodynamic process will consist of a curve connecting an initial state (A) and a final state (B). The area under the curve will be:



which is the amount of thermal energy transferred in the process. If the process moves to greater entropy, the area under the curve will be the amount of heat absorbed by the system in that process. If the process moves towards lesser entropy, it will be the amount of heat removed. For any cyclic process, there will be an upper portion of the cycle and a lower portion. For a clockwise cycle, the area under the upper portion will be the thermal energy absorbed during the cycle, while the area under the lower portion will be the thermal energy removed during the cycle. The area inside the cycle will then be the difference between the two, but since the internal energy of the system must have returned to its initial value, this difference must be the amount of work done by the system over the cycle. Mathematically, for a reversible process we may write the amount of work done over a cyclic process as:



Since _dU_ is an exact differential, its integral over any closed loop is zero and it follows that the area inside the loop on a T-S diagram is equal to the total work performed if the loop is traversed in a clockwise direction, and is equal to the total work done on the system as the loop is traversed in a counterclockwise direction.

*[edit] The Carnot cycle*



 


A Carnot cycle taking place between a hot reservoir at temperature _T_H and a cold reservoir at temperature _T_C.


Evaluation of the above integral is particularly simple for the Carnot cycle. The amount of energy transferred as work is



The total amount of thermal energy transferred between the hot reservoir and the system will be



and the total amount of thermal energy transferred between the system and the cold reservoir will be



. The efficiency η is defined to be:



where
Δ_W_ is the work done by the system (energy exiting the system as work), Δ_Q__H_ is the heat put into the system (heat energy entering the system), _T__C_ is the absolute temperature of the cold reservoir, and _T__H_ is the temperature of the hot reservoir. This efficiency makes sense for a heat engine, since it is the fraction of the heat energy extracted from the hot reservoir and converted to mechanical work. It also makes sense for a refrigeration cycle, since it is the ratio of energy input to the refrigerator divided by the amount of energy extracted from the hot reservoir.

*[edit] Carnot's theorem*

_Main article: Carnot's theorem (thermodynamics)_
It can be seen from the above diagram, that for any cycle operating between temperatures _T__H_ and _T__C_, none can exceed the efficiency of a Carnot cycle.


 


A real engine (left) compared to the Carnot cycle (right). The entropy of a real material changes with temperature. This change is indicated by the curve on a T-S diagram. For this figure, the curve indicates a vapor-liquid equilibrium (_See Rankine cycle_). Irreversible systems and losses of heat (for example, due to friction) prevent the ideal from taking place at every step.


*Carnot's theorem* is a formal statement of this fact: _No engine operating between two heat reservoirs can be more efficient than a Carnot engine operating between the same reservoirs._ Thus, Equation 3 gives the maximum efficiency possible for any engine using the corresponding temperatures. A corollary to Carnot's theorem states that: _All reversible engines operating between the same heat reservoirs are equally efficient._ Rearranging the right side of the equation gives what may be a more easily understood form of the equation. Namely that the theoretical maximum efficiency of a heat engine equals the difference in temperature between the hot and cold reservoir divided by the absolute temperature of the hot reservoir. To find the absolute temperature in kelvins, add 273 degrees to the Celsius temperature. Looking at this formula an interesting fact becomes apparent. Lowering the temperature of the cold reservoir will have more effect on the ceiling efficiency of a heat engine than raising the temperature of the hot reservoir by the same amount. In the real world, this may be difficult to achieve since the cold reservoir is often an existing ambient temperature.
In other words, maximum efficiency is achieved if and only if no new entropy is created in the cycle. Otherwise, since entropy is a state function, the required dumping of heat into the environment to dispose of excess entropy leads to a reduction in efficiency. So Equation 3 gives the efficiency of any reversible heat engine.

*[edit] Efficiency of real heat engines*

Carnot realized that in reality it is not possible to build a thermodynamically reversible engine, so real heat engines are less efficient than indicated by Equation 3. Nevertheless, Equation 3 is extremely useful for determining the maximum efficiency that could ever be expected for a given set of thermal reservoirs.
Although *Carnot's cycle* is an idealisation, the expression of Carnot efficiency is still useful. Consider the average temperatures,






at which heat is input and output, respectively. Replace _TH_ and _TC_ in Equation (3) by <_TH_> and <_TC_> respectively.
For the Carnot cycle, or its equivalent, <_TH_> is the highest temperature available and <_TC_> the lowest. For other less efficient cycles, <_TH_> will be lower than _TH_ , and <_TC_> will be higher than _TC_. This can help illustrate, for example, why a reheater or a regenerator can improve thermal efficiency.
_See also: Heat Engine (efficiency and other performance criteria)_ الروابط فعالة مع الود والاحترام


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## حسن هادي (1 سبتمبر 2007)

شكرى محمد نورى قال:


> الأخ حسن هادي .
> 
> اجمل المنى .
> 
> ...


 
حياك الله يا مشرفنا العزيز وتقبل تحياتنا 
تحياتي اخوكم حسن العراقي *


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## سنان عبد الغفار (6 سبتمبر 2007)

مشكور كثيرا على ابداعاتك ايها العضو الاكثر من المتميز جدا


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## حسن هادي (7 سبتمبر 2007)

سنان عبد الغفار قال:


> مشكور كثيرا على ابداعاتك ايها العضو الاكثر من المتميز جدا


اعتز بمداخلتك يا اخي الكريم سنان وحياك الله اخوكم حسن عراق


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## مهندس نورس (7 سبتمبر 2007)

عزيزي حسن هادي .

اطلعت على اغلب مواضيعك التي كتبتها ووجدتها هي بمثابة ملحمة هندسية رائعة وثرية ولم اجد لها

مثيل يذكر حقا انك انسان ومهندس منقطع النظير .

استمر على هذا النمط ويباركك الله على جهودك .


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## حسن هادي (8 سبتمبر 2007)

مهندس نورس قال:


> عزيزي حسن هادي .
> 
> اطلعت على اغلب مواضيعك التي كتبتها ووجدتها هي بمثابة ملحمة هندسية رائعة وثرية ولم اجد لها
> 
> ...


 
حياك الله اخي العزيز يا مهندس نورس وبارك الله فيك اخي الكريم وجعلنا الله من المؤهلين لنيل رضاه ورضاكم والله ولي التوفيق اخوكم حسن هادي


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