Amazing Analyses

FEA software is capable of more than ever, except engineering judgment.

January saw the death of British mathe-matician Olgierd Cecil Zienkiewicz, an early pioneer of the finite element method who pushed for its computerization and who recognized the potential for using the method to solve problems outside solid mechanics, purview of the famous Euler-Bernoulli beam equation.

With his passing, we pause to assess FEA’s past—looking at the pros and cons of the leap the method has made into the software realm—and to look at what the future holds for finite element analysis and its applications.

From its inception in the 1940s until about a decade and a half ago, finite element analysis had been performed exclusively by specialized analysts who held Ph.D.s in the subject and had devoted their careers to the discipline. But the FEA field has seen great change over the past 15 years, with a jump in the number of computer technologies available to an increasing number of engineers. And like all changes, it has brought risks and rewards.

In his lifetime, Zienkiewicz saw the finite element method move from a powerful technique originally developed to solve complex structural mechanical problems to its use today across nearly all engineering fields, including bioengineering—and its stretch into unrelated fields such as the simulation of weather patterns. Beginning in the 1960s, FEA researchers began to extend the method beyond linear structural analysis to nonlinear FEA and other engineering disciplines such as fluid dynamics, heat transfer, soil mechanics, wave propagation, and electromagnetics.

Since its inception in the 1940s, finite element analysis—originally developed for numerical solution of complex problems in structural mechanics—has become established in nearly all engineering fields, including bioengineering, where it plays a role in studying many parts of the body, such as nasal passages.

 

Zienkiewicz himself sought to move the finite element method from a research tool to a computer-based analysis method that could be called upon by designers and engineers. And in 1968 he founded the first journal dealing with computational mechanics, the International Journal for Numerical Methods in Engineering.

Today, high-end FEA software packages are available to solve complex problems across many disciplines. But they’re still not perfect; that is, the engineers and researchers who use these tools can spend a lot of time kitting them out and programming them for their own unique use.

Other FEA packages introduced within the past 15 years are now commonly integrated with computer-aided design applications, allowing product developers to analyze as they design. The integration is intended to speed up the design cycle because designers can analyze, then immediately update, their designs.

 

easy access
Without a doubt FEA’s move to the computer allowed it to become the widely used tool it is today, said Samer Adeeb, an assistant professor in the department of civil and environmental engineering at the University of Alberta in Edmonton. By helping move the tool to the computer, Zienkiewicz enabled its popularity and its growth, Adeeb added.

In the early 1970s, FEA was run only on mainframe computers owned mainly by companies in the aeronautics, defense, and nuclear industries. With the rapid decline in the cost of computers and the concomitant increase in computing power, today’s personal computers can now produce accurate FEA results. Adeeb pointed out that many engineering technology vendors are currently marketing simplified analysis programs, which walk users through a series of steps that allow them to define the analysis they want to run and then interpret the results.

“FEA has been around forever, but it’s grown to be very powerful because of the computational power that exists right now in computers,” Adeeb said.

“The problems we’re now solving with FEA couldn’t have been done ten years ago because you would have needed a mainframe,” he added. “Now a desktop has enough computation power that anyone can run FEA.”

FEA is now used by disparate industries for a range of
applications. The packaging industry calls upon it to
analyze designs for blow-molded products, including
bottles, top. Bioengineers call upon the software for
their own use, such as studying the growth of a human
bone, bottom.

 

That jump to the desktop makes for the packages that allow CAD users to run more up-front analysis during design, but it can also mislead untrained analysts, who may not fully understand the finite element method and won’t exactly know how to best input information or how to interpret results, Adeeb added.

“FEA is becoming so easy to run and is so highly integrated with all the CAD software now, but the output from the analysis is still the numbers a designer uses to determine if the part is safe or not safe,” he said.

“It’s sometimes too easy to toggle back and forth between CAD and FEA without really knowing what you’re doing,” he said. “Some people abuse FEA because it offers such a nice animation; so they try to get to the animation they want rather than to actually solve a problem that returns useful information.”

According to Adeeb, FEA software shouldn’t be relied upon as a black box that spits out numbers. Designers still need to know the proper inputs and to understand what those numbers mean. The old adage holds true, he said: garbage in, garbage out.

To get meaningful analysis results, designers need to know how to identify the problem that needs solving in the first place. That requires at least a basic understanding of the finite method, he added. Before beginning analysis, designers will need to simplify the problem at hand, to ask themselves whether the problem is linear or nonlinear, and to identify the forces that need to be analyzed.

As an instructor, Adeeb knows this line of questioning doesn’t come intuitively to users who have little or no understanding of the finite element method.

As a numerical technique, FEA allows engineers to find approximate solutions of partial differential and integral equations. FEA software simulates where structures bend or twist and indicates the distribution of stresses and displacements. The software uses a complex system of points to form a grid, or mesh, across a model. The engineer assigns nodes at a particular density throughout the material, often depending on the expected stress levels of a certain area. The mesh contains the material and structural properties that define how the part will react to certain load conditions.

“The first question I ask my students on exams is: what is FEA and why do you need it?” Adeeb said. “If people can’t offer the correct definition of what it is I don’t trust them using FEA. Within the definition itself lies the understanding of what they’re doing.”

Companies that hire designers to perform both CAD and FEA should offer new employees an introductory course to FEA, Adeeb said.

“Otherwise what they’re doing is just an animation, and that doesn’t differ from a computer scientist who is just drawing on the computer,” he said.

 

the simple stuff
Though desktop FEA has come a long way in the past few decades, everyday users—even those well trained in FEA—still face everyday problems when trying to analyze designs and behaviors as part of their jobs, said Rick James, vice president of consulting at the SimuTech Group of Rochester, N.Y.

One problem is that while many FEA applications are integrated with CAD systems today, many of these calculate what James called “the simple stuff”; that is, they perform relatively basic stress and fatigue analyses. Users will need to purchase a third-party analysis application to run advanced fatigue analysis or other types of analyses on top of the simple stuff, he said.

“Most FEA desktop software doesn’t do the niche stuff like crack-growth propagation, where you’re watching a crack form and move on the screen,” James said.

And advanced FEA add-ons require that users have much more information to hand.

According to James, “This advanced fatigue stuff asks for this load history and that load history and then this residual stress from welding.”

Thus, the everyday CAD users will likely need advanced training to best use advanced packages. Training costs and software costs can quickly add up. According to James, extra analysis software on top of the already existing FEA application can run companies anywhere from $30,000 to $60,000 depending on the package, the number of packages needed, and user needs.

But the companies that really need advanced FEA capabilities simply can’t get by with everyday FEA software alone, he said.

 

already here

But the good news is that for many uses the everyday FEA software of the type integrated with CAD systems has advanced enough to be of great help. In fact, according to Adeeb, the software is very user friendly and—when properly programmed by the user—adept at analyzing most FEA engineering problems.

“For certain applications, like analyzing engineering structures that behave according to theories developed one hundred years ago, FEA software doesn’t need advancing,” he said.

But the story differs when high-end FEA packages are used for applications not strictly related to engineering, such as biomedical problems, he said. The human body, after all, is nonlinear in behavior. So not only is defining the complex problems related to the body a challenge for a researcher, so is determining how to call upon FEA software to best solve them, Adeeb said.

For example, as part of his research, Adeeb needed to create and analyze a model of a human bone as it grew. For this, he used Abaqus, software marketed by Dassault Systèmes of Paris, a powerful, high-end package used to solve complex physical problems.

Software for complex simulations isn’t plug and play, Adeeb said. The software programs meant to model complex or unusual problems are built to allow their users to configure the software—to a certain degree—to their own unique needs.

“I can do the analysis, but it takes me a lot of trying to fool the software and coming up with workarounds; it’s not something I can directly model, and it takes me a long time to try to model something like that,” he said.

Researchers can be assured the vendors of these customizable, high-end systems like Abaqus and Ansys of Canonsburg, Pa., will be stepping up with software to suit specialized needs in the future. But for new and specialized applications like biomedical software, development takes time, James said.

“But FEA software has proven to be very lucrative so it’s worth it for these guys to work on development,” he added.

Still, whether researchers and engineers call upon FEA software to solve complex problems or to analyze fairly straightforward structures as they design, they’ll need to bring their own judgment to the problem at hand, James said.

“FEA is never going to be the ultimate decision-making tool, nor should it be,” he said. “It’s still used for mathematical equations and modeling physics, and you still have to use engineering judgment when calling out a result you don’t think is true, even if your software can do amazing feats.”

But the mix of engineering know-how, engineering judgment, and amazing feats of software make for analysis and design never dreamed possible before the age of FEA.

In 1998, upon acceptance of the Timoshenko Medal from ASME, Zienkiewicz speculated about the future of FEA by referring to Charles Duell, commissioner of the U.S. Office of Patents in 1899, who famously speculated that everything that could be invented had already been invented.

“I do not share this pessimistic view, and I think we shall see many exciting developments in the coming years,” Zienkiewicz said. “It is evident that both applied mechanicians and mathematicians will continue to contribute to the numerical analysis field.”

He said that more than a decade ago, and it still stands.

Renewable Sea Power

Waves, tides, and thermals—new research funding seeks to put them to work for us.

For the first time in more than a decade and a half, on September 18, 2008, the U.S. Department of Energy announced the funding of projects dealing with renewable marine energy technologies. The program will address one of the great sustaining resources of our planet to see if it can yield benefit to mankind in new ways.

The ocean has always been a provider. Its fish and plants nourish a large portion of the Earth’s population. It is a vast recycling engine that replenishes oxygen in our atmosphere. It is also an immense store of energy, much of it derived from the sun and most of it untapped.

The DOE’s program will explore technology that aims to harness some of the ocean’s energy and put it to work for us. While several awards specifically were made to private companies for technology development and market acceleration, DOE also created two National Marine Energy Centers, one in Hawaii operated by the School of Ocean and Earth Science and Technology at the University of Hawaii and the other in the Pacific Northwest under a partnership of Oregon State University and the University of Washington. They will study various technologies designed to generate power from forms of energy in the ocean.

The portion of the solar energy absorbed by the ocean is initially thermal in nature. We find that the surface waters heat during the day and cool during the night. The air adjacent to the water surface is also heated and cooled, and thermal currents occur in both the air and water. Hence, the absorbed radiant solar energy is partially transformed to thermal energy and, then, to hydraulic and pneumatic energies, giving rise to winds. Winds and the Earth’s rotation create waves on the water surface. These energy transformations give rise to the fields of ocean thermal energy conversion, ocean wave energy conversion, ocean current energy conversion, and offshore wind energy conversion.

The tide is also partly solar. Tides are predictable energy forms at any site in the world, since they are due to the gravitational attractions of the moon and the sun.

Offshore wind energy is an established technology. Tides are site-specific, and there are plants producing electricity at places like the Bay of Fundy in Canada and the Rance Estuary in France. The other forms of energy conversion are in development.

A conception of a field of water mills designed by Marine Current Turbines that turn the currents of tides into electricity.

 

The new Marine Energy Centers will both be active in the promotion of wave power conversion; Hawaii also will lead efforts to advance ocean thermal energy conversion, while the Northwest will be involved in tidal energy development.

Jefferson W. Tester, Croll Professor of Sustainable Energy Systems at Cornell University, and four researchers from Massachusetts Institute of Technology, Elisabeth M. Drake, Michael J. Driscoll, Michael W. Golay, and William A. Peters, jointly wrote a book, Sustainable Energy: Choosing Among Options, published in 2005 by MIT Press. In it they estimate the relative potentials of ocean energy resources. According to the authors, the wave action of oceans deliver 2,700 gigawatts of power, and the power available for use totals about 500 GW. Currents are estimated at a total of 5,000 GW, of which perhaps 50 GW is of practical use for energy conversion. Ocean thermal resources may be as high as 200,000 GW, although practical exploitation would be about 40 GW. Tides represent 2,500 GW and maybe 20 GW can be used for energy conversion. 

Ocean thermal energy conversion (often abbreviated as OTEC) uses the temperature differences between warm surface waters of the ocean and the cold deep water. The temperature of ocean water at a depth of 1,000 meters is only slightly above freezing. If surface waters are at least 18 oC warmer, the heat of the warm surface water can evaporate a fluid such as ammonia, which can drive a turbine. The fluid is then condensed by up-welled cold deep-ocean waters to begin the cycle again.

The ideal regions of the world for ocean thermal energy conversion are between 20o north latitude and 20o south, where the average surface temperature in tropical oceans can rise above 30 oC. Although the ideal latitude band is well away from much of the industrialized world, we can take advantage of OTEC by creating a product other than electricity. For example, if we build a floating aluminum plant at the OTEC site, and make the aluminum from imported bauxite, then we have produced an energy-intensive product, relieving a nation’s power grid from that task.

The state of Hawaii is in the ideal band for OTEC. In the late 1970s, a modest effort was started to build a land-based OTEC power plant. The electrical power output from the original power plant was of the order of tens of kilowatts. Over the years, this effort has expanded. The state of Hawaii recently agreed to have a 10 megawatt OTEC plant constructed.

 

riding the waves

Wave energy conversion devices exploit the rise and fall of waves, often to produce electricity. There are prototype systems deployed around the world, but no commercial-scale installations.

Michael Pleas and Douglas Hicks of the University of Delaware recognized in the 1970s that the production of potable water, rather than electricity, might be a more efficient use of converted wave energy. Efforts in this direction are now under way in the United States, Ireland, and elsewhere.

The average U.S. citizen, who requires electrical power of 1 kW, also requires approximately 60 gallons (about 227 liters) of water per day. For electricity production, consider a sinusoidal wave approaching a mid-Atlantic U.S. state. The wave might have a wave height of 1.5 meters and a period of 7.5 seconds. For this wave, the power per crest width is about 16.6 kW/m. Let us assume that this power is to be converted into electricity with a bus-bar efficiency of 25 percent. The electricity supplied to the grid, then, is 4.15kW per meter of converted wave crest. For a coastal town of 1,000 citizens, approximately 241m of the wave crest must be addressed.

The population of the same coastal town would require 227 kiloliters of potable water per day. In the same sea, a wave-powered desalination system operating at an average pressure of 60 atmospheres and pumping 379 kL of salt water per day to obtain the 227 kL of fresh water would require about 26.3 kW of power. For a 25 percent efficient system, only 6.34 meters of crest width would be required.

The WaveBob generates power by using the out-of-phase heaving motions of the float and a submerged inertial body.

 

A promising wave-powered electrical generating system is the Pelamis of Pelamis Wave Power in Scotland. It is an articulated-body system with an internal closed hydraulic system that is part of the power takeoff sub-system. It has four components, each 45 meters long. Three Pelamis units have been constructed for deployment 5 km from the coast of Portugal. Each unit is rated at 750 kW.

Other systems for capturing wave energy are buoy-like designs. One, PowerBuoy from Ocean Power Technology, has been deployed in Hawaiian waters. Another, WaveBob from WaveBob Ltd., has been deployed in Galway Bay off the coast of Ireland.

The use of the tides to produce electricity has been done on a commercial scale, but the energy resource is site-specific. Although there are numerous low-capacity tidal power plants along the coastal waters of the Chinese mainland, there are few high-capacity plants in existence in the rest of the world. The best locations for tidal power plants are the Bay of Fundy in Canada, the Severn Estuary in the United Kingdom, Port of Ganville and the Rance River at San Malo in France, Puerto Rio Gallegos in Argentina and, in Russia, the Bay of Mezen on the White Sea and Penzhinskaya Guba on the Sea of Okhotsk.

Tidal energy plants are costly. The turbines must be bi-directional, to take advantage of incoming and outgoing tides. They must also be of high capacity. Most tidal plants require construction of a barrage as well. But the amortized cost of electricity is relatively small because the static tidal power systems are robust and have a long operational life.

The French built a tidal power plant at St. Malo in the Rance estuary, where the mean tidal range is 8.55 meters. That power plant delivers an average of 240 MW of power (240,000 kW) at a cost of about 1.8 cents per kilowatt-hour, which is quite inexpensive.

More recently, attention has been focused on the dynamics of the tides in the form of tidal currents. To convert the hydrostatic tidal energy into electricity, tidal water mills are deployed. For example, in the East River at New York City, the Verdant Power Co. has installed submerged water mills. According to Verdant Power, six turbines in the East River will generate approximately 10 megawatts. A British company, Marine Current Turbines Ltd., has installed a 300 kW plant in the English Channel off the coast of Cornwall.

The tidal energy resource is both reliable and predictable. With the escalating costs of oil and natural gas, it will become a viable resource in the near future.

 

research partnerships

The Department of Energy’s new program in marine renewable energies is an attempt to tap into a vast resource.

The proposed level of funding for the National Marine Energy Centers in Hawaii and the Pacific Northwest currently stands at $1.25 million annually for five years, but overall cost-matching from non-federal money must be achieved. This requirement is meant, in particular, to foster active cooperation between academia and the private sector. 

The Pelamis wave-power electricity generator has a closed hydraulic system inside its articulated components.

 

Electrical utilities (e.g., HECO and MECO in Hawaii) and private companies (e.g., Ocean Power Technologies and Lockheed Martin) have made early commitments to participate in the centers, The scope of planned activities is, however, quite broad.

While conducting their own research, the universities will assist in the establishment of ocean field testing sites and help the DOE keep a recently created marine renewable energy data base up to date. Research areas themselves are expected to cover different aspects of marine renewable energy conversion. In Hawaii, for example, such issues as wave resource forecasting, novel wave power device testing, wave power focusing, OTEC environmental impact, OTEC heat exchanger testing, corrosion mitigation, and electrical grid stability have initially been considered.

The world’s appetite for energy can only continue to grow. As Tester and his co-authors point out, there are tremendous energy resources in the world’s oceans if we can develop the technology to harness them. We are hopeful that with research we will see one or more of the ocean energy options achieve its great promise.

کمپرسورهای تهویه مطبوع خودرو

  کمپرسورهای تهویه مطبوع خودرو

سه نوع عمومی از کمپرسورهای تهویه مطبوع اتومبیل وجود دارد

1-نوع رفت و برگشتی دو سیلندری Two cylinder reciprocating piston type
2-نوع چهار سیلندر دایره ای Four cylinder RADIAL type
3-نوع شش سیلندری محوریSix cylinder AXIAL type

ادامه نوشته

مزایای سیستم گرمایش از کف

در سيستم‌هاي گرمايشي متداول، تا 70% گرما نزديك سقف جمع مي‌شود و نزديك كف دماي پايين‌تري را داريم. اين شرايط محيطي با آسايش ما مطابقت ندارد. ما هنگامي آسوده‌تريم كه پاي ما گرم و سر ما خنك‌تر باشد. بنابراين گرما بايد در جايي توليد شود كه به آن بيشتر نياز است، يعني در كف.
سيستم گرمايش كفي انقلابي در نحوه‌ي گرم‌كردن ساختمان‌هاست. در اين سيستم، گردش آب گرم از درون شبكه‌اي از لوله‌هایی كه در زير كف نصب شده‌اند، حرارت را به آرامي توزيع مي‌كند.

 

ادامه نوشته

کمپرسورهای تبرید

انتخاب كمپرسور مناسب به شرايط و نوع بهره برداري بستگي دارد كه اهم آن به شرح ذیل مي‌باشد:

• فشار و دبي
• مورد نياز

   

   حساسيت به حضور روغن

   

     
• خواص فيزيكي و شيميايي 
• بهاي انرژي

   

   
• قابليت اعتماد

   

   
• هزينه‌هاي تعمير و نگهداري و قطعات يدكي قيمت اوليه

   

   
• حداكثر درجه حرارت قابل قبول

 

ادامه نوشته

خصوصیات نیروهای داخلی و تنش

تانسور تنش :

وقتی که یک نیروی خارجی بر یک جسم صلب وارد می شود اتمها یا ملکولهای آن را حرکت داده و باعث می شود از هم فاصله بگیرند .

می توان فرض کرد که اتمها به طور کامل از هم فاصله نمی گیرند ( اگر بار به اندازه کافی کوچک باشد ) زیرا آنها در مقابل نیروی خارجی وارد شده نیروی داخلی ایجاد می کنند که در برابر با وارد شده مقاومت می کند تا جسم به تعادل برسد . اگر نیروی داخلی نتواند در مقابل نیروی خارجی وارد شده مقاومت کند جسم ترک بر می دارد یا می شکند . با اینکه نیروی داخلی متناسب با اتمها یا ملکولهای درون جسم است ولی مطالعه تغییر شکل مکانیکی به وسیله علم اتم شناسی خیلی پیچیده است ( گر چه بعضی از افراد توانسته اند این کار را انجام دهند ) بنابراین ما خیلی از فرضهایی را که به کار می بریم قبول می کنیم و تحت این فرضها جزیئات اتمی جسم را نادیده گرفته و در عوض یک جسم صلب را درنظر گرفته به شرط اینکه آن جسم جامد باشد .

در اصل نظریه ما فقط برای مقیاس طول که خیلی بزرگتر از فاصله ی اتمی می باشد معتعبر باقی می ماند .

حالا یک جسم استوانه ای مانند را در نظر بگیرید که نیروی خارجی F  مانند شکل ( 1) بر آن وارد می شود

 

 

ادامه نوشته

مکانیک حرکت ماهی ها

ماهي چگونه حركت مي‌كند؟

بيشتر جانداران اين توانايي را دارند كه انواع مانورهاي حركتي را انجام دهند و همچنين مي‌توانند بخشي و يا همه بدن خود را در وضعيت خاص تعادلي نگهدارند. حركت و تعادل جانداران يك مسئله مهم مكانيكي است. جانداران دستگاه‌ها و ماشين‌هايي هستند كه به كمك نيروهايي به حركت درمي‌آيند و كار را انجام مي‌دهند و ساختمان‌هايي هستند كه تحت اثر نيروهاي وارده در حالت تعادل قرار مي‌گيرند. هر كدام از انواع جانوران داراي اعضاء و دستگاه‌هايي هستند كه حركت و تعادل آنها را امكان‌پذير مي‌سازد. براي نمونه جانوران زميني مانند انسان و چهارپايان به كمك سيستم استخوان‌بندي و ماهيچه‌هاتوانايي انجام حركات گوناگوني بر روي زمين را دارند. جانداران دريايي مانند ماهي‌ها و نهنگ‌ها داراي اعضايي مانند باله‌ها هستند و شكل عمومي آنها طوري است كه براي شرايط دريايي و حركت در آن مناسبت تمام دارد. در كل مي‌توان چنين گفت كه فرم ساختماني و عملكرد هريك از انواع جانوران با شرايط محيطي آن جانور هماهنگي دارد و به عبارت مهندسي، جانوران ماشين‌هايي هستند كه در طبيعت براي شرايط خاص محيطي خود و نيازهاي حياتي وابسته به آن طراحي و ساخته شده‌اند. همچنين رديابي انواع جانوران در طي دوران گذشته اين نتيجه را بدست داده است كه از ديدگاه تكاملي نيز تغييرات فرمي جانوران در طول زمان همواره در جهتي صورت گرفته است كه كارآيي مكانيكي آنها را به عنوان يك كاشين كامل افزايش بدهد.

 

ادامه نوشته

معرفی رشته بیومکانیک

  تقريبا در اوايل دهه 70 ميلادی، جامعه بين المللی واژه "بيو مکانيک" را برای دانش مطالعه سيستم های حياتی از ديد مکانيکی انتخاب نمود. بيو مکانيک از ابزار مکانيک برای مطالعات آناتوميکی و بررسی کارکرد اندام حياتی استفاده می کند. ااين علم طيف گسترده ای را از مطالعه تئوری تا کاربردهای عملی می پوشاند.
مطالعه کامل مکانيک شامل دو موضوع اساسی می باشد:
 استاتيک، که مطالعه اجسامی است که، در اثر نيرويی که بر آن ها ااعمال می شود، در حال سکوني يا وضعيت تعادل باقی می‌مانند و ديناميک، که مطالعه اجسام متحرک است.

ادامه نوشته

آسانسورهاي برقي

آسانسوربرقي با نيروي محركةكششي داراي اتاقكي است كه ازكابلهاي فولادي آويزان است و اين كابلها برروي قرقره محرك شيار دارحركت مي كنند.كابلهاي فولادي از يك طرف به بالاي اتاقك و از طرف ديگر به قاب وزنه تعادل متصل مي شوند.وزنه تعادل ازميزان بار روي موتور الكتريكي به اندازه اختلاف وزن موجود ميان اتاقك همراه با بار و وزنه تعادل يا اصطكاك كم مي كند.اين اختلاف وزن را ((بار غير متعادل))مي نامند.

وزنه تعادل معمولاً ۴۰ تا ۵۰ درصد وزن اتاقك به علاوه بار آن و اصطكاك وزن دارد. اصطكاك معمولاً ۲۰ درصد وزنه تعادل است. 

ادامه نوشته

مکانیک کلاسیک، مکانیک لاگرانژی

کانیک کلاسیک یکی از قدیمیترین و آشناترین شاخه‌های فیزیک است. این شاخه با اجسام در حال سکون و حرکت ، و شرایط سکون و حرکت آنها تحت تاثیر نیروهای داخلی و خارجی ، سرو‌ کار دارد. قوانین مکانیک به تمام گستره اجسام ، اعم از میکروسکوپی یا ماکروسکوپی، از قبیل الکترونها در اتمها و سیارات در فضا یا حتی به کهکشانها در بخش‌های دور دست جهان اعمال می‌شود.

سینماتیک حرکت:

سینماتیک به توصیف هندسی محض حرکت ( یا مسیرهای) اجسام ، بدون توجه به نیروهایی که این حرکت را ایجاد کرده‌اند ، می‌پردازد. در این بررسی عاملین حرکت (نیروهای وارد بر جسم) مد نظر نیست و با مفاهیم مکان ، سرعت ، شتاب ، زمان و روابط بین آنها سروکار دارد. در این علم ابتدا اجسام را بصورت ذره نقطه‌ای بررسی نموده و سپس با مطالعه حرکت جسم صلب حرکت واقعی اجسام دنبال می‌شود.

 

ادامه نوشته

مبانی گرما و انرژی در مولدهای بخار

مقدمه

امروزه، بیش از هر دوران در گذشته گرما ( انرژی ) کلید تمدن جدید است و منابع شناخته شده انرژی روز به روز تحلیل می روند بنابراین بدیهی است مهندسی که با تجهیزات استخراج گرما و تبدیل آن به انرژی سرو کار دارد باید درباره گرما اطلاعات کافی داشته باشد.

 

 

 

 

ادامه نوشته

backwards

Instead of carrying compressed oxygen or hydrogen, some advanced vehicles are generating critical gases directly from water.

by Robert Roy

It was a moment that bordered on catastrophe: A fire aboard the Mir space station in 1997 almost caused the loss of the orbiting platform and the deaths of its crew. With their route to an escape capsule blocked by the flames, the crew could only watch as the fire licked at the aluminum hull before dying out after 14 minutes. That fire was traced to an on-board oxygen generator. The generator relied upon an exothermic chemical reaction to create a breathable atmosphere for the crew of the space station. But as even a junior high school student knows, combining heat and oxygen can lead to fire. Oxygen for the crew of the International Space Station is supplied through an on-board electrolysis system. The successor to Mir is the International Space Station, and it too has an on-board oxygen generator, though of a radically different design. Installed in 2006 and successfully used since last June, the generator operates on a simple principle— electrolysis, the dissociation of oxygen and hydrogen from water through the application of electricity. What makes the generator safe? The device uses a proton-exchange membrane, similar to that used in a PEM fuel cell, to separate the generated hydrogen and oxygen. The fuel cell has been touted for decades as a device that can generate electricity from benign chemical reactions, such as reacting oxygen and methane to make water and carbon dioxide. Although fuel cells have successfully competed in many niche applications, they haven't exactly lived up to their potential—or the hype. Quietly, however, PEM electrolyzers have found their way into many high-profile vehicles over the past three decades. My colleagues at Hamilton Sundstrand and I believe that this technology soon may find its way into satellite thrusters or energy storage systems. An Old Idea Made New A PEM-based water electrolyzer works in much the same way as the archetypal junior high school chemistry experiment—the one with two wires attached to a lantern battery that are stuck into a beaker of dilute salt water— except that it uses a solid acid or membrane rather than a liquid solution. Water is oxidized at the oxygen electrode, or cell anode, to produce oxygen gas, releasing hydrogen ions, or protons, and electrons. The hydrogen ions migrate from the cell anode to the cell cathode, or hydrogen electrode, under the effect of the electric field imposed across the cell, while the electrons are transferred by a dc power source. The protons and electrons recombine at the cell cathode to produce hydrogen. Liquid water is also released at the cathode due to a process called electro-osmotic drag. Oxygen and hydrogen are generated in a stoichiometric ratio—two volume units of hydrogen for every one of oxygen—at a rate proportional to the applied cell current. The water to be electrolyzed can be either liquid or vapor. Indeed, in a liquid-phase setup, water is introduced on the cathode side and soaks through the membrane to the anode, where it is oxidized to make oxygen gas. As a result, the oxygen gas stream is produced without any additional liquid. Water can also be used to carry away any excess heat. The pressure of the produced gas can be ramped up as needed, as high as 3,000 pounds per square inch. For every factor of 10 increase in pressure, the equilibrium cell voltage need be increased by only 44 mV. And thanks to its solid membrane, a PEM electrolyzer is able to withstand large differentials in pressure. Because the membrane is non-porous, a PEM cell can safely separate product gases at superatmospheric pressures. Most of the design elements used to produce the water electrolysis cell stack were originally developed during the 1980s for submarine-based life support systems. The oxygen generating plant, for instance, was built for the U.S. Navy's Seawolf-class submarine, to produce oxygen and hydrogen for storage in high-pressure banks. At the time, however, the cell stack could operate only at relatively low pressure. The plastic frames that contained the fluids at their operating pressure within the cell cavities and the metal screens used to distribute water within the cell and provide support and electrical conductivity to the electrolysis cell membrane could withstand less than 200 psig. The risk for operating above that pressure was that hydrogen could leak into the submarine's atmosphere, or that because of a membrane breach, hydrogen and oxygen could mix. The design solution involved placing the cell stack within a pressurized vessel to balance the hoop stresses across the plastic frames, and providing pressure regulation between the oxygen and hydrogen fluid circuits to ensure that the proper cross-cell pressure differential is maintained during all aspects of system operation. Nitrogen provided by the shipboard's system served as a reference gas for the pressure control system. That limited the number of single barrier interfaces between oxygen and hydrogen to just the cell stack. It also served as the pressurized blanket for the cell stack housed in the pressure vessel. Also, because the Navy required the system to be as small as possible, the oxygen generating plant uses a liquid anode feed system so it can operate with a high current density. While the plant's electrolysis cell stack operates at up to 1,400 amps per square foot, there is a trade-off between volume and electrical efficiency. Typical efficiency for the cell stack is only about 65 percent at beginning of life; however, the submarine is not limited in either power or cooling resources. The oxygen generating plant is in service aboard the three Seawolf-class submarines—the USS Seawolf, the USS Connecticut, and the USS Jimmy Carter. However, the complex nature of the system and the need for high-pressure pumps and other components has resulted in high lifecycle costs for the plants. Additionally, nitrogen consumption is high, necessitating frequent recharges of the shipboard's nitrogen system. An alternate solution for providing respirable oxygen for submarine life support was needed. Making Oxygen in Mid-Air An advanced cell design was developed by Hamilton Sundstrand in the 1990s to permit high-pressure gas generation without the need for a pressure vessel or complex pressure control system. The new cell was fashioned from patterned foil layers that were bonded together using a fluoroelastomer coating. A sintered powder metal porous plate provided structural support to the electrolysis cell membrane while operating at high differential pressure, simultaneously allowing fluid transport both to and from the bulk water stream and the electrode surface. The first application for this new cell design was for an oxygen recharge system for commercial aircraft. Currently, aircraft are required to maintain the emergency oxygen system at 1,400 psia or higher prior to dispatch. The on-board oxygen generating system, or OBOGS, can recharge the high-pressure emergency oxygen system on commercial aircraft without the need for any oxygen servicing by ground personnel. The cell stack uses water from the galley to generate oxygen at up to 2,010 psia, while the hydrogen and water loop are maintained at essentially ambient pressure. The result of this advanced high differential pressure cell design is a greatly simplified fluid system, with only the oxygen fluid circuit maintained at elevated pressure. This has proved to be a robust design: Although the requirements called for 6,000 hours of operation, a single cell assembly operating at nominal recharge conditions operated in the laboratory for over 54,000 hours. The USS Jimmy Carter relies on an electrolyzer to generate oxygen. The high differential pressure cell design may also produce high-pressure hydrogen with oxygen at ambient pressure. A 65-cell stack currently in production for the U.S. Navy's latest fast-attack submarine class is capable of providing oxygen at ambient pressure and hydrogen at up to 800 psig. In this setup, the oxygen is vented directly to the shipboard ventilation system, while the hydrogen is at sufficient pressure to dump overboard. The Royal Navy has recently adopted this new cell technology for its new submarine class. Because of the limited availability of power aboard the International Space Station, energy efficiency is a high priority for the oxygen generating system that was launched in 2006 and first successfully operated in July 2007. The system employs a 28-cell liquid cathode feed PEM electrolyzer operating at a current density of only 200 amps per square foot to produce oxygen at ambient pressure with 80 percent efficiency. What's more, because the power from the photovoltaic cells rises and falls with the station's day-night cycle—53 minutes of daylight followed by 37 minutes of darkness—the cell stack can switch rapidly into a standby mode where its electrical draw drops by 97 percent. To ensure the safety of the space station crew, the oxygen generating system's electrolyzer as well as any hydrogen-containing components are enclosed in an evacuated dome. Any gas leaks are vented directly into space. Because it's critical to get as much use as possible from the water, the water-hydrogen mixture exiting the cell stack will be spun in a rotary device that separates the two components. The water will recirculate through the loop to a heat exchanger to reject waste heat from the process before returning to the cell stack. The hydrogen will be dumped overboard. (Eventually, the hydrogen will be sent to a Sabatier system, where it will combine with carbon dioxide to produce water and waste methane.) The water processing system is yet to be delivered, so for now the station will rely on a Russian-built system to replenish the atmosphere. In time, though, the generating system will be able to supply enough oxygen for up to seven astronauts. The Water Rocket The International Space Station is not the only orbital destination where this technology can be used. A high-pressure vapor feed electrolyzer was recently designed, built, and tested as an alternate propulsion system for advanced satellites. Developed as part of a DARPA program, the cell stack generated hydrogen and oxygen gases using similar design elements as the high differential pressure water electrolysis cell. The system, named the Water Rocket, would electrolyze water using power from the vehicle's solar array to charge storage tanks to 2,000 psia. When required, the high-pressure gases would be delivered to a hydrogen-oxygen thruster array to provide orbital reboost and maneuverability to satellites to improve their useful life, extend their capability, and enhance their overall versatility. The logistics of refueling the vehicle would also be simplified, requiring only the transfer of liquid water from one vehicle to the other rather than the more hazardous propellant fluids currently in use. Hamilton Sundstrand recently tested a five-cell Water Rocket stack. The breadboard demonstration generated oxygen and hydrogen at up to 2,000 psia. Another application for this technology would service individual spacesuits. The spacesuits onboard the International Space Station provide oxygen to the astronauts from oxygen bottles located within the suit's primary life support system. These bottles are charged from high-pressure oxygen storage tanks attached externally to the station's airlock. Those tanks are recharged using a mechanical compressor that takes boil-off from the Space Shuttle's cryogenic oxygen tank and compresses it to 2,700 psia into the tanks. Of course, such a system requires a visit from the shuttle, and since NASA is retiring the shuttle fleet in 2010, the agency is evaluating a high-pressure oxygen generating system that could recharge the tanks. Under contract to NASA, Hamilton Sundstrand has conducted system-level trade studies and is currently developing the cell stack to operate at a 3,000 pounds per square inch differential of oxygen over hydrogen. The oxygen would be dried prior to storage in the high-pressure tanks, while the hydrogen would be vented to space. Testing of a two-cell prototype stack assembly is scheduled for this spring. Oxygen compatibility testing of a mockup that simulates the water electrolysis cell stack is also planned early in the program to validate the system-level mitigation approach for managing catastrophic failures of the hardware. Extraterrestrial Applications Energy storage solutions using water electrolysis and fuel cell systems are being examined for applications ranging from backup power systems and lighter-than-air vehicles to extraterrestrial bases on the moon and Mars. The basic architecture of a regenerative fuel cell energy storage system includes a high-pressure water electrolysis system, a fuel cell, a fluid management and storage system, a thermal management system, and a power management system. Depending on whether the system has access to atmospheric oxygen, the oxygen from water electrolysis can either be stored as a pressurized gas or simply vented as a waste by-product. For extraterrestrial applications, the system would be used in tandem with a photovoltaic array. During the day, excess electricity would power the electrolysis system, which would store the produced hydrogen and oxygen. At night, the oxygen and hydrogen would be delivered to a fuel cell to provide power to the base; the product water from the fuel cell would be stored and subsequently consumed by the water electrolysis system, thereby closing the fluid cycle. The water electrolysis system and the fuel cell would be thermally linked so that neither would freeze when not in use. One day, electrolyzers may help fill space suits' air tanks during long missions. Recent studies have focused on oxygen and hydrogen storage pressures of between 1,000 and 2,000 psi, requiring the development of a high, balanced-pressure water electrolysis cell stack and balance of plant to safely manage these fluids. The choice of technology for the water electrolysis and fuel cell systems depends on a number of factors, including electrical efficiency, fluid purity, and ease of fluid management. Water electrolyzers may become more commonplace in the future as we move forward into a hydrogen-based economy. Fuel cell-powered vehicles hold the promise of reducing greenhouse gas emissions from the transportation sector, provided the hydrogen fuel is produced from a renewable energy source, such as a high-pressure water electrolyzer operating from wind, solar, or nuclear power. Backup power systems that currently depend upon lead-acid batteries may someday be supplanted by environmentally friendly regenerative fuel cell systems that are free of any lead content. Helium-buoyed airships may dot the skies, providing platforms for telecommunications payloads or surveillance equipment around the clock, thanks to an energy system that combines solar arrays and an onboard RFC power system. All that—based on a concept that can be demonstrated with a beaker of liquid, two wires, and a battery. Robert Roy is electrochemical programs engineering manager at Hamilton Sundstrand in Windsor Locks, Conn.

the many and the few

In a parade of machines, a young engineer saw the principle that everything flows one way, from high to low.

ME 101 is the  number of the thermodynamics course in mechanical engineering at Duke University. I cannot think of a better course for "ME101." It was put on the books by the old professors, the ones who had the vision to build Duke many decades ago. Today, I am sure, thermodynamics would get a less memorable course number.

Times change. Generations replace generations, but the principles remain. Contrivances, gadgets, and fads are the opposite. They parade in front of our eyes, but their impact on the "thin book" of fundamentals is nil.

I was reminded of this recently. I had just finished teaching thermodynamics for the semester when I visited two colleagues in Paris. With them I use constructal theory to predict the global weather. They work in a historic university called The National Conservatory of Arts and Professions (CNAM), a few blocks north of the Pompidou Center. It is historic for many ideas that persist today: evening classes for co-op students, continuing education, and the first museum of technology in the world.

A statue of French physicist Denis Papin (1647-1712), one of the inventors of the steam engine.

CNAM has a 300-year-old edifice with towers, an inner court, statues, clocks, and history that is chiseled in stone. In the front court, there is the statue of Denis Papin and the first piston-and-cylinder machine that expanded steam to produce work (1690). This was before the engine builders of Britain, and 80 years before James Watt.

Around the buildings, and around the ceilings of the oldest and most decorated classrooms, are the names of professors and students who breathed at CNAM. One name is Sadi Carnot. Without his work there would be no thermodynamics, engineering, or standard of living as we know them today.

Early in the 19th century, Sadi Carnot was a young engineer, not unlike my students today. He had graduated from the École Polytechnique. Tradition holds that he came to CNAM to contemplate, to think in quiet about the army of contrivances that was invading France: the steam engines. The industrial revolution was on the march. Britain had industrialized itself in the 1700s. A century later, it was the turn of the Continent to do the same.

Freedom Through Steam

Why were the steam engines invading? Because their effect on people's lives was good. It was dramatic. Engines were empowering people. They were liberating serfs, slaves, and animals. They were facilitating the movement of humanity all over the globe.

Sadi Carnot came to CNAM to study the machines that were on display. They were built by many hands-on engineers in Britain. These "many" fed the imagination of one individual. In turn, today Sadi Carnot's mental viewing feeds the minds of enormous numbers of builders of all sorts of machines throughout the world.

The principle that Sadi Carnot saw in that parade of machines is that everything flows one way, from high to low. Water flows through a pipe from high pressure to low pressure. Heat flows from high temperature to low temperature. This principle is known today as the second law of thermodynamics, irreversibility, dissipation, inefficiency, one way, water under the bridge, etc. Today, this is thermodynamics, the science of everything that kicks and moves.

The new principle that Carnot's visit at CNAM illustrates today is that one individual sustains the crowd, and vice versa. The big river sustains the many tiny streams of the river basin.

The reverse is equally true: The numerous sustain the singular. The river basin, like the tree of the lung, connects an entire area or volume to one point—flow resistances allocated to areas and volumes, all over the world, so the whole world flows best. This principle is the "constructal law"; to read more, see www.constructal.org.

Both principles—the second law and the constructal law—are in action. Their footprints persist, like the river beds and the beaten tracks. The river and the caravan that do not follow their beds and beaten tracks do not get far.

No flow system is an island. No river exists without its wet plain. No human settlement thrives without its farmland and open spaces. Everything that flowed to this day to "survive" is in an optimal balance with the flows that surround it and sustain it. The airflow to the alveolus is optimally matched to the blood flow through the vascularized tissue, and vice versa.

"Vascularized" is a good name for the energy systems that thermodynamics covers. The tissues of energy flows, like the fabric of society and all the tissues of biology, are optimized architectures. The climbing to this high level of performance is the transdisciplinary effort: the balance between seemingly unrelated flows, territories, and disciplines. This balancing act—the optimal distribution of imperfection—generates the very design of the process, power plant, city, geography, and economics.

All things that flow follow certain principles, whether it is the constructal design for maximum flow access from the center to the edge of a circle, or the delta of the River Lena in northern Siberia. The flow goes from high to low, with hierarchy and multiple scales.

The need for considering the whole—the macroscopic system—is great and universal. No matter how successful we are in discovering and understanding small-scale phenomena and processes, we are forced to face the challenge to assemble the invisible elements into palpable devices. The invisible grains must be kept alive with flows, which connect them and serve them. The challenge is to construct—that is, to connect and optimize while assembling.

This challenge is becoming increasingly difficult. While the smallest scales are becoming smaller, the number of components and the complexity of the useful device (always macroscopic) become greater.

A good example is the rush to nanotechnology. Technology means more than the new physical phenomena that may appear on the frontiers of progressively smaller scales. A technology is truly new when it is made useful in the form of macroscopic devices that improve our lives. Usefulness means that we must discover principles of constructing, connecting, and packing multiscale flow systems into macroscopic spaces.

And so I return to the start of this article. The new gadgets are like the engines in the invasion contemplated by Carnot. It's certain they will all be forgotten, unless there is a Sadi Carnot watching, to see a pattern and immortalize it with a short page in the thin book of principles. This happens only rarely, and when it does, it illustrates again the constructal principle of "one sustains the crowd."

Adrian Bejan is the J.A. Jones Distinguished Professor of
Mechanical Engineering at Duke University in Durham, N.C. His research covers thermodynamics, natural convection, heat and mass transfer, convection in porous media, and the constructal theory of organization in nature.

tracing the second law

Modern thermodynamics owes a debt to the contributions of a line of researchers extending back more than a century.

While man learned to control and use fire many thousands of years ago, only in the last 300 years has the nature of heat been given serious consideration. In this short time, it has been explained as phlogiston, a mysterious fluid created by fire, and as caloric, a material fluid flowing from hot to cold. The modern view, that heat is a convertible form of energy, is fewer than 200 years old.

In the 19th century, James Joule, an English physicist, experimented with converting mechanical energy into thermal effect. He discovered the equivalence of heat and work, and the First Law of Thermodynamics was firmly established. His tombstone is inscribed with a number, 722.55, which was his approximation to the now-accepted 778 foot-pounds per British thermal unit. In his honor, the units for energy in the S.I. system are designated in joules.

The Second Law developed in phases over some 125 years. It is one of the most abstract laws of physical science, and is the bane of students and others who try to understand its complexity. A professor once observed that all engineers could be divided into three groups: those who knew all about entropy, those who used it anyway, and those carefree souls into whose lives it never intrudes. Blissful ignorance, however, simply ignores the universal influence of the Second Law on every natural process. Even the simple act of taking a step would be impossible without the intrusion of an entropic effect.

SADI CARNOT, 1796-1832. The first phase in the evolution of the Second Law is due to Carnot.	JAMES JOULE, 1818-1889. He established the equivalence of heat and work.

The first phase in the evolution of the Second Law is older than Joule's work and is due to Sadi Carnot. A graduate of the famous École Polytechnique in Paris, he joined the French Army Corps of Engineers. In this capacity, he became fascinated with the development of the steam engine. He was also impressed that such useful machines could be developed in the total absence of guiding principles or analysis, but on empirical reasoning alone. His technical background gave him the incentive and preparation to try to remedy this condition, but he soon discovered that a major obstacle was the inability to give mathematical precision to the many irreversible events occurring in the operation. This led to his two most significant discoveries.

The first was the observation that all irreversible processes have associated with them an ideal process that is the limiting case when the irreversibility is progressively reduced to zero. This ideal case is amenable to mathematical analysis, using the appropriate available principles. The other discovery was the idea of combining a sequence of ideal processes so that the last one ended where the first one began, and the concept of a cycle was born. This was quite novel, since no steam engine operated on such a cycle.

By combining these two ideas, Carnot was able to analyze a number of cycles in which caloric flowed in and out of a cycle while producing useful work. His most famous cycle has the caloric added to the system at a constant high temperature and leaving the system at a constant lower temperature. For this cycle, he demonstrated that the efficiency depended only on the temperatures of the sources and sinks of the caloric, and was independent of the nature of the thermodynamic medium.

Carnot published his results in a book, Reflections on the Motive Power of Fire, in 1824. It was ignored for 25 years.

His work was done despite two major defects. One was that the prevailing temperature scale was based on the freezing and boiling points of water; the other was the prevailing concept of heat as a caloric fluid that had to be conserved during the cycle. His later writings indicate he was dissatisfied with the caloric theory and, had he not died in 1832, Carnot might have preempted Joule in rejecting it.

LORD KELVIN, 1824-1907. The second phase came in 1849, when he studied Carnot's work.	RUDOLF CLAUSIUS, 1822-1888. The third phase of the Second Law was carried out by Clausius.

The second phase in the evolution of the Second Law took place in 1849, when William Thomson (later Lord Kelvin) studied Carnot's work. Being familiar with the recent development of the first law, he discovered and corrected the error due to the caloric theory, which led him to his discovery of an absolute temperature scale. With this, he showed that the heat discharged to the low-temperature reservoir was only a fraction of the heat from the high-temperature source, leading to the relation,

1. Q_a/T_a = Q_r/T_r

where Q_a is the heat added at the temperature T_a, and Q_r is the heat removed at the temperature T_r , the difference between Q_a and Q_r is the work delivered by the ideal cycle.

The third phase of the evolution of the Second Law was carried out by a German professor of mathematical physics, Rudolf Clausius, who became aware of the work of Carnot, Joule, and Kelvin in 1850. He modified Kelvin's formula in two ways—first, by adopting the convention that heat added is considered to be positive, and heat removed negative; second, he recognized that heat might be added and removed at several points during the cycle. His revision took the form:

2. Q₁/T₁ + Q₂/T₂ + Q₃/T₃ + ... = 0

where Q may be positive and negative. He then recognized that a more general form of this relation for all ideal cycles could be stated in a simple calculus form as:

3. f dQ/T = 0

where T is the temperature at the surface across which the small quantities of heat flow.

During the next 15 years, Clausius undertook an investigation into the logic used by Carnot in his ideal cycle analysis to see if any general rule could be found for the form of the cyclic integral if an unspecified type and amount of irreversibility occurred during the cycle. His study was rewarded by the discovery of his famous inequality:

4. f dQ/T is less than or equal to 0

where the equality to zero holds for ideal cycles, and the negative inequality for all irreversible cycles. Clausius recognized that when the integral of a quantity around a complete cycle is zero, the integral of that quantity between different states is independent of the process and depends only on the change of a related property of the system. The simple steps required to demonstrate this are shown in all textbooks. When this is applied to the first law,

5. f (dQ - dW) = 0, then z²₁ dE = E₂ - E₁

where Q and W are the heat and work, and E is the internal energy property.

In order to apply this procedure to his inequality, Clausius devised a clever scheme to convert it into an equality. He postulated that when heat was added to the cycle, it was 'given a small amplification dQ', but when heat left the cycle, it was unchanged. He called dQ' the 'uncompensated heat,' and expressed equation 4 as:

6. f (dQ/T + dQ'/T) = 0, where Q' ³ 0.

There is no record of this idea ever having been taken seriously, but it did allow him to discover the property S, which he gave the name "entropy" in his 1865 paper. Willard Gibbs wrote a world-renowned book, On the Equilibrium of Heterogeneous Substances, in 1878, and completely ignored the inequality.

LARS ONSAGER, 1903-1976.
He carried out a fourth phase of the Second Law in 1931.

A fourth phase in the development of the Second Law was carried out by Lars Onsager in 1931. As an assistant professor of chemistry at Brown University, he became interested in applying the ideas of the earlier discoverers to chemically reacting systems. He devised an ingenious type of Carnot cycle in which a series of three chemical reactions were arranged in a cycle. He was able to analyze these reactions in sufficient detail to discover a chemical equivalent to Clausius's uncompensated heat. For reactions taking place irreversibly, he derived a quantity he called dissipation, designated f, which was produced by the irreversibility. For reversible reactions, this quantity is always zero. The positive-definite quality of this quantity enabled him to discover his famous reciprocal relations, Lij = Lji. He was awarded the Nobel Prize in 1975 for this work. His form of the Clausius inequality was:

7. f (dQ/T + df/T) = 0, with f ³ 0.

The fifth phase in the evolution of the Second Law was developed by Ilya Prigogine in 1945. As a doctoral student at the Free University of Brussels, he became interested in the new field of irreversible thermodynamics, where the properties of the system are allowed to vary from point to point as well as with time. To apply the laws to such systems, it is necessary to invoke the principle of local equilibrium, which states the specific dependent properties retain their dependence on the independent properties at a point as they do for a quasistatic system. Fortunately, many processes of practical importance conform to this condition, such as heat conduction, diffusion, and Newtonian viscous fluids. However, the flow through strong shock waves, violent chemical reactions, turbulence, and chaotic processes are "too" irreversible to be in this class.

Prigogine modified Clausius's equation 6 by splitting the change in the entropy into two parts, one of which replaced the uncompensated heat. This gave him a form of the Second Law that yielded clusters of terms for a general dissipation function, with each cluster containing pairs of terms for a particular irreversibility. He called these pairs products of generalized forces and fluxes, and since either can be positive or negative, the product is only positive. In order to meet this requirement, the forces are linearly related to the fluxes. As an example, for simple heat conduction,

f = -q dT/dx ³ 0,

setting q = - k dT/dx gives f = k (dT/dx) ² ³ 0 and the Second Law prescribes that heat can only flow down a temperature gradient. Similarly, more complex relations are found for natural processes. Prigogine was also awarded a Nobel Prize in 1977.

At this point, it is appropriate to show that the Clausius inequality can be transformed into an equality without resorting to strained justifications. All that is needed is the recognition that the magnitude of the negativeness of Clausius's inequality is monotonically related to the intensity of the irreversibility, and that the irreversibility may manifest itself at any point around the cycle. The first makes it possible to express equation 4 as:

8. f dQ/T = -s, with s ³ 0,

where s is the magnitude of the negative value of the integral for the entire cycle. Secondly, this total accumulates bit-by-bit throughout the cycle and is simply the sum of the bits, i.e.

9. s = f ds

Combining this with the above relation gives:

10. f (dQ/T + ds) = 0, with s ³ 0.

This is now in the proper form to be converted to a single process, as:

11. z²₁(dQ/T + ds) = z²₁dS = S₂ - S₁

With equation 11 as the basic form of the Second Law for closed systems, simple math requires each term be expressed in the same units, but there is no recognized name for them, like joules for the energy terms in the First Law.

Clarifying Terms

I propose that the units be called carnots, in honor of the originator. Second, that the unifying concept of the Second Law be called entropy, in honor of the contributions of Clausius, who coined the term. Third, that the property S be renamed the internal entropy by analogy with the internal energy of the First Law. The quantity Q/T is the entropy flow yoked to the heat flow. Last, the positive-definite quantity s be called the entropy creation, as a recognition of its source in irreversible effects. It is the one term that makes the Second Law unique in the physical world. If individual terms for the concepts could be adopted, much of the confusion of the Second Law would be mitigated.

The Second Law is a statement that the entropy content of a system may be increased or decreased by entropy exchanges with the environment, but may only be increased as irreversibilities cause entropy creation. It is not a conservation law, but simply a balancing of several entropic effects. Since real processes are irreversible, the 1865 statement by Clausius, "The entropy of the universe tends always toward a maximum," is certainly an ultimate truth.

Howard W. Butler is a former professor of mechanical engineering at Rensselaer Polytechnic Institute in Troy, N.Y., and the retired chair of the ME department at West Virginia University.

wankel engine

Wankel Engine
The Wankel radial engine is a fascinating beast that features a very clever rearrangment of the four elements of the Otto cycle.   It was developed by Felix Wankel in the 1950s.^1
In the Wankel a triangular rotor incorporating a central ring gear is driven around a fixed pinion within an oblong chamber.
The fuel/air mixture is drawn in the intake port during this phase of the rotation.
The mixture is compressed here.
The mixture burns here, driving the rotor around
And the exhaust is expelled here.
The rotory motion is transferred to the drive shaft via an eccentric wheel (illustrated in blue) that rides in a matching bearing in the rotor.   The drive shaft rotates once during every power stroke instead of twice as in the Otto cycle. The Wankel promised higher power output with fewer moving  parts than the Otto cycle engine, however technical difficulties have apparently interfered with widespread adoption.  In spite of valiant efforts by Mazda, the four stroke engine remains much more popular.

This is our aim

To keep pace with the rest of the world

موتور دورانی وانکل

سيستم انتقال قدرت

 

سيستم انتقال قدرت

جعبه دنده براي حفظ حركت نرم و روان تمام چرخ دنده ها و محورها نياز به روانكاري دارد اين كار با پر كردن محفظه جعبه دنده به طور جزئي با روغن چرخ دنده غليظ انجام مي شود
چنانچه با نشتي روغن مواجه شديد تا پيش از برطرف كردن آن كلاج را عوض نكنيد زماني كه محفظه هاي كلاج با روغن كثيف شده اند چاره اي جز تميز كردن آن نيست اما تنها راه برگرداندن عملكرد مناسب كلاج تعويض ديسك است

سيستم انتقال قدرت دو وظيفه را در اتومبيل به عهده دارد: انتقال قدرت از موتور به چرخ هاي محرك و تغيير مقدار گشتاور. در تشريح سيستم انتقال قدرت به كرات از دو عبارت توان و گشتاور استفاده مي شود كه توضيح كوتاهي درباره هركدام ضروري به نظر مي رسد. عبارت «توان» نرخ يا سرعت انجام كار است. «تورك» يا گشتاور به زبان ساده يعني گردش نيرو. با توجه به ارتباط بين دور موتور و توان، وجود جعبه دنده هاي چند نسبته ضروري است چرا كه موتور اتومبيل بيشينه توانش را در سرعت هاي معين تحويل مي دهد كه البته منظور از سرعت همان RPM يا دور در دقيقه است.
براي بهره گيري از همان دور موتورها در سرعت هاي مختلف حركت كه اينجا منظور از سرعت چيزي است كه در آمپر سرعت ديده مي شود، بايد نسبت چرخ دنده بين موتور و چرخ هاي محرك تغيير يابد. اتومبيل درست مثل يك دوچرخه بايد براي حركت در محدوده اي از سرعت ها، چرخ دنده ها را تعويض كند. اما برخلاف دوچرخه سيستم انتقال توان اتومبيل امكان عقب رفتن را نيز براي شما فراهم مي كند. در واقع دو مجموعه از چرخ دنده ها در سيستم انتقال توان وجود دارد: گيربكس يا جعبه دنده و دفرنسيال. وظيفه جعبه دنده تنظيم نسبت چرخ دنده است و ديفرانسيل نيز اجازه مي دهد تا چرخ ها در سرعت هاي گوناگون بچرخند. جعبه دنده هاي دستي معمولاً داراي چهار يا پنج سرعت هستند و اغلب از اوردرايو يا بيش ران (وسيله اي در جعبه دنده كه نسبت چرخ دنده را پايين مي آورد و مصرف سوخت را كاهش مي دهد) برخوردارند.
در واقع اور درايو به وضعيتي گفته مي شود كه در آن محور يا شفت ورودي مي تواند سريعتر از محور خروجي بچرخد كه در نتيجه ميزان مصرف سوخت در بزرگراه كاهش مي يابد. در برخي از جعبه دنده ها از كلاج الكتريكي و يك سوئيچ استفاده مي شود كه درگيري يا عدم درگيري اوردرايو را كنترل مي كند.
دستاورد جالبي كه در تعداد اندكي از اتومبيل ها ديده مي شود، جعبه دنده دستي بدون كلاج است. در اين نوع جعبه دنده يك دسته دنده و يك كلاج الكتريكي خودكار به كار مي رود. علاوه بر اين زماني كه راننده دنده ها را عوض مي كند، سنسورهاي سرعت و موقعيت، ميني كامپيوترها و تنظيم هاي گاز كاربراتور از افزايش بيش از حد دور موتور جلوگيري مي كند. در واقع مثل بسياري از نوآوري هاي دنياي اتومبيل اين هم يك ايده قديمي است كه امروز به بركت تحول كامپيوتر ميسر شده است.
جعبه دنده هاي خودكار براي تركيب سرعت و گشتاور عموماً از سه چرخ دنده رو به جلو استفاده مي كنند. در جعبه دنده هاي سه سرعته اولين چرخ دنده براي شروع حركت بيشترين گشتاور را در كمترين سرعت تحويل مي دهد. چرخ دنده دوم براي حالت هايي مثل افزايش سرعت و بالا رفتن از سربالايي ها مقدار گشتاور و سرعت متوسطي  را ارائه مي كند. سرانجام سومين چرخ دنده بيشترين سرعت با كمترين گشتاور را براي حركت در بزرگراه فراهم مي كند. يك چرخ دنده معكوس نيز حركت رو به عقب را ميسر مي سازد.
جعبه دنده دستگاهي براي تغيير توان و سرعت است كه در جايي بين موتور و چرخ هاي متحرك وسيله اي نصب مي شود. به عبارت ديگر اين دستگاه راهي براي تغيير نسبت بين دور موتور و دور چرخ ها فراهم مي كند. به گونه اي كه در موقعيت هاي خاص حركت بهترين حالت ممكن را داشته باشد. در برخي از انواع سيستم انتقال توان از وسيله اي موسوم به محور انتقال استفاده مي شود. اين وسيله به زبان ساده تركيبي از جعبه دنده و دفرنسيال است كه معمولاً مي توان آن را در اتومبيل هاي چرخ جلو متحرك يافت، اما در اتومبيل هاي موتور وسط يا عقب نيز ديده مي شود. البته در برخي از اتومبيل هاي كم نظير موتور در جلو قرار دارد و در عين حال براي بالانس بهتر وزن از يك محور انتقال در عقب استفاده مي شود.

گشتاور از توان به دست مي آيد. مقدار گشتاور قابل حصول از يك منبع توان، با فاصله اي از مركز دوران كه گشتاور در آن نقطه به كار مي رود متناسب است. بنابراين منطقي است كه اگر محوري (در اين بحث ميل لنگ) داشته باشيم كه با هر سرعت اعمال شده اي مي چرخد، مي توانيم چرخ دنده هايي با اندازه هاي گوناگون روي آن قرار دهيم و نتايج مختلفي به دست آوريم. چنانچه چرخ دنده  بزرگي روي محور نصب كنيم مي توانيم در لبه آن سرعت بيشتر و توان كمتري نسبت به يك چرخ دنده كوچكتر به دست آوريم. حال اگر محور دوم را موازي با محور محرك مان قرار دهيم و مطابق چرخ دنده هاي روي شفت متحرك، چرخ دنده هايي روي آن نصب كنيم، مي توانيم تقريباً هر تركيبي از توان و سرعت را كه در محدوده توانايي موتور باشد به دست آوريم. اين دقيقاً همان چيزي است كه جعبه دنده اتومبيل به كمك چرخ دنده ها و ديگر اجزا انجام مي دهد.
در يك نگاه كلي دو نوع جعبه دنده وجود دارد: دستي و خودكار. در حالت اول مجبور هستيد براي تعويض دنده ها معمولاً از يك دسته دنده واقع در كنسول و پدال كلاج استفاده كنيد. چنانچه جعبه دنده خودكار باشد خود مكانيسم بدون دخالت شما دنده ها را عوض مي كند. اين عمل از طريق يك سيستم كه توسط فشار روغن تغذيه مي كند، انجام مي شود. تعويض هر يك از دنده ها توسط يك سوپاپ تعويض كنترل مي شود.
در واقع تعويض دنده ها به سرعت، جاده و شرايط بار بستگي دارد. قسمت اساسي ديگر تمامي سيستم هاي انتقال قدرت يكي از انواع كلاج است. اين وسيله به موتور اجازه مي دهد تا هنگامي كه چرخ دنده ها و چرخ ها ثابت هستند به حركتش ادامه دهد. در اتومبيل هاي مجهز به جعبه دنده خودكار به جاي كلاج از مبدل گشتاور استفاده مي شود. از پشت موتور گرفته تا محل تماس لاستيك با جاده همگي عضو يكي از پيچيده ترين سيستم هاي اتومبيل تان به حساب مي آيند. به اعتقاد برخي نگاه كردن به يك جعبه دنده مغزشان را آزرده خاطر مي سازد.
همانطور كه گفته شد جعبه دنده دستي امكاني را فراهم مي كند تا نسبت بين سرعت موتور و سرعت چرخ ها تغيير كند. تغيير اين نسبت دنده ها باعث مي شود تا مقدار صحيح توان موتور در بيشتر سرعت هاي مختلف به دست آيد. جعبه دنده دستي براي به كارگيري و جابه جايي گشتاور موتور به محل ورودي جعبه دنده، نيازمند استفاده از كلاج است. كلاج باعث مي شود تا اين عمل به طور تدريجي اتفاق بيفتد و به همين علت اتومبيل مي تواند از يك توقف كامل شروع به حركت كند. در جعبه دنده هاي دستي مدرن هيچ كدام از چرخ دنده هاي رو به جلو از درگيري خارج نمي شوند. در واقع آنها از طريق استفاده از هماهنگي كننده ها به محورهايشان متصل مي شوند. حركت عكس نيز به كمك چرخ دنده  هرزگر معكوس كه به هنگام حركت رو به عقب اتومبيل درگير مي شود، به دست مي آيد.
برخي از جعبه دنده هاي دستي داراي اوردرايو هستند. اوردرايو بخشي مكانيكي است كه به پشت جعبه دنده پيچ مي شود عموماًَ اوردرايو را به اسم دنده پنجم مي شناسند.
زماني كه از آن استفاده مي كنيد سرعت يا همان دور موتور حدود يك سوم كاهش مي يابد در حاليكه آمپر سرعت اتومبيل تان همان سرعت حركت را نشان مي دهد. كمپاني كرايسلر در سال ۱۹۳۴ اولين جعبه دنده مجهز به اوردرايو را معرفي كرد.
بيشتر اتومبيل ها سه الي پنج دنده جلو و يك دنده عقب دارند، چنانچه در جعبه دنده اي يك چرخ دنده با ده دندانه چرخ دندانه ديگري با بيست دندانه را بگرداند گفته مي شود كه حركت داراي نسبت دو به يك است. در واقع نسبت حركت دوچرخ دندانه برابر است با نسبت تعداد دندانه هاي چرخ دنده دوم به اول. اولين دنده توان موتور را از طريق يك جفت مجموعه چرخ دنده  كاهنده كه به هنگام آغاز حركت توان را افزايش و سرعت را كاهش مي دهد، به چرخ هاي محرك مي رساند. در اين حالت موتور بسيار سريعتر از محور خروجي مي چرخد، معمولاً با نسبت چهار به يك. سرعت هاي متوسط با تغيير نسبت دنده تا نزديكي هاي يك به يك و سرانجام سرعت نمايي معمولاً با اتصال مستقيم محورهاي ورودي و خروجي با نسبت حركت دقيقاً يك به يك به دست مي آيد. به كارگيري يك مجموعه متحرك از چرخ دنده ها با ابعاد متفاوت، دستيابي به چندين مقدار از گشتاور خروجي را ممكن مي سازد. چرخ دنده محرك دفرنسيال اتومبيل كه توسط شفت متحرك به حركت درمي آيد چرخ دنده حلقوي (چرخ دنده اي شبيه حلقه در دفرنسيال اتومبيل كه پينيون يا همان چرخ دنده كوچك متصل به ميل گاردان آن را مي چرخاند و نيروزا از طريق دفرنسيال به اكسل مي دهد) را مي چرخاند. در واقع اين دوچرخ دنده مثل يك جعبه دنده تك سرعته عمل مي كنند و باعث كاهش بيشتر دور موتور و افزايش گشتاور با يك نسبت ثابت مي شوند. چرخ دنده ها دقيقاً مشابه اهرم ها كار مي كنند. چرخ دنده كوچكتر درحالي همتاي بزرگترش را مي چرخاند كه ميزان گشتاور افزايش و سرعت كاهش يافته است.

موتور شش زمانه

موتورهای شش زمانه

لفظ موتورهای شش زمانه به موتورهايی اطلاق می شود که ريشه ترکيبی از دو موتور، دو و چهار زمانه دارد. از محاسن اين موتور می توان به توان و گشتاور بالای آن (حدوداً 23% افزايش در مقايسه با موتورهای بنزينی چهار زمانه) اشاره نمود همچنين مصرف سوخت و آلايندگی کمتر بدليل به سوزی سوخت در اين موتور.
اين موتور دارای يک پيستون با قطر کمتر در بالای پيستون اصلی می باشد که حرکت خود را با استفاده از يک رابط (زنجير يا تسمه) از ميل لنگ می گيرد و در حين حرکت خود در راستای حرکت پيستون اصلی علاوه بر تغيير حجم موتور در کورس های مختلف (مکش، تراکم، انبساط و تخليه) کار سوپاپهای هوا و دود را نيز انجام می دهد.
اين موتور برپايه يک موتور Ducati نمونه سازی و تست شده است. از آنجاييکه اين موتور تنها نمونه سازی شده و در مرحله patent می باشد و هنوز تجاری نشده است، لذا اطلاعات زيادی در مورد نحوه عملکرد آن در دست نمی باشد.

اميدوارم که تصاوير زير در درک بهتر مطلب شما را ياری کند.

شبيه سازی يک موتور شش زمانه


نمای برش خورده موتور شش زمانه؛ دقت کنيد که هر سيلندر دارای دو شمع می باشد.


موتور شش زمانه نمونه سازی شده بر پايه موتور Ducati V-Twin

 

 

This is our aim

To keep pace with the rest of the world

فن آوری موتورهای آينده در خودروهاي سواری

مقدمه:

 

آلايندگي و مصرف سوخت خودروها بدليل محدوديتهاي زيست محيطي، ظرف 10 سال آينده بايستي به ميزان قابل توجهي بهبود يابد. فن آوريهاي جديد در زمينه موتورهاي بنزيني، نظیر كوچك سازي موتورها به لحاظ اندازه (Downsizing) با استفاده از تقويت بالای آنها (High Boosted)، موتورهاي با تزريق مستقيم (GDI) و سيستم سوپاپهاي كاملاً متغير (Fully Variable Valve Train) هم اکنون در حال توسعه می باشند. در مورد موتورهاي ديزل نيز بخشهايی که انتظار می رورد توسعه يابند، شامل انژكتورهاي پيزو الكتريك، فيلترهاي ذرات معلق و سيستم كاتاليستهاي DeNOx می باشند.

در اين يادداشت ابتدا به بررسی الزامات استانداردهای آلايندگی پرداخته و پس از آن تمهيداتی که جهت دستيابی به اين استانداردها در موتورهای بنزينی و ديزل بکار گرفته شده است را معرفی می نماييم. در بخش اول موتورهای بنزينی را مورد بحث و بررسی قرار خواهيم داد.

 

الزامات استانداردهای آلايندگی در آينده:

قوانين اروپايي روي آلاينده هاي خطرناك اگزوز كه در سال 2000 نسبتاً سختگيرانه به اجرا در آمد بار ديگر در سال 2005 سختگيرانه تر خواهد شد. محدوديتهاي استاندارد آلايندگي EURO IV براي آلاينده هاي HC و NOX و ذرات معلق حدود 50% سطح كنوني اين گازهاي مضر مي‌باشد (استاندارد آلایندگی اروپا در سال 2004 مطابق با استاندارد EURO III است). مرحله بعد در استانداردهاي اروپايي كهEURO V  ناميده ميشود احتمالاً با تمركز روي ذرات معلق، به بهينه سازي بيشتري نياز دارد.

از سوی دیگر در استاندارد آمريكايي TIER 2 كاهش مرحله به مرحله NMOG (گازهاي اورگانيك غير متان) و کاهش متوسط NOX ناشی از ناوگان اتوبوسرانی از سال 2004 تا 2007 مد نظر است . از سال 2003 به بعد در كاليفرنيا ميبايستي حداقل 10% فروش هر سازنده اتومبيل، خودروهایي با آلايندگي صفر يا معادل آن باشد. نگراني در مورد اثر گازهای گلخانه ای، خودروسازان اروپايي را وادار كرده است كه تا سال 2008 خودروهايی توليد نمايند كه متوسط CO2 منتشره از آنها زير 140 gr/Km باشد. يعني كاهش مصرف سوخت بايستي به ميزان بيش از 25% در مقايسه با سطح تعيين شده در سال 1995 باشد. همچنین كاهش بيشتر به سطح 120 gr/Km تا سال 2012 نيز در سال 2003 تحت بحث و بررسي قرار گرفت.

از طرفی همزمان با طرح مباحث آلایندگی، مشتريان نيازمند ايمني و آسايش بيشتري نسبت به سابق خواهند بود كه این مساله تنها با افزایش وزن خودرو ميسر خواهد شد و واضح است که اين موضوع با مصرف كمتر انرژي منافات دارد. همچنين ضمن حفظ حداقل عملكرد خودرو، در عین حال نباید هزينه مالكيت خودرو افزايش يابد.

 

 

فن آوري آينده در موتورهاي بنزيني:

هدف اصلي در توسعه موتورهاي اشتعال جرقه اي، بهبود مصرف سوخت و در نتـيجه كاهش انتشار گاز CO2 ميباشد. از ديدگاه ترموديناميكي، دستيابي به راندمان بيشتر، با عملكرد موتور در بارهاي زياد و كاهش در افت تبادل گاز و حرارت در بارهاي جزيي ممكن ميباشد. راه حلهاي فني براي اين منظور عبارتند از :كوچك سازي سايز موتورها و استفاده از سوپر شارژ، فن آوري سوپاپهاي كاملاً متغير و پاشش مستقيم.

 

كوچك سازي(DOWNSIZING )

يك استراتژي براي بهبود قابل توجه در مصرف سوخت، كاهش حجم جابجايي موتور با حفظ شكل منحني گشتاور ميباشد.با افزايش فشار تغذيه تا 2.5 بار و كاهش نسبت تراكم در بارهاي زياد مي توان به اين هدف دست يافت. در شكل شماره يك، منحني هاي گشتاور و مصرف سوخت دو موتور يكي موتور 3 ليتري با تنفس طبيعي و ديگري موتور 1.5 ليتري با سوپر شارژ بالا، با يكديگر مقايسه شده است. همانگونه كه شكل نشان مي دهد، مصرف سوخت ويژه در بارهاي جزيي در حدود 2.5% بهبود يافته است. فن آوري جديد مورد نياز براي اين منظور در سمت راست شكل نشان داده شده است.

سوپر شارژهاي مكانيكي با راندمان بالا دستيابي به گشتاورهاي لحظه اي و بالا را فراهم مي نمايد.  استفاده از سوپرشارژ منجر به پديده ناك یا ضربه در بارهاي زياد مي گردد. براي احتراز از اين موضوع يك سيستم نسبت تراكم متغير ابداع شده است (پايين سمت راست شكل) تا با كاهش نسبت تراكم،  دستيابي به فرايند احتراق بدون ناك را در بارهاي زياد ممكن سازد؛ در حاليكه قادر است در بارهاي جزيي، تراكم بهينه را حفظ نمايد.

 

 

سيستم سوپاپ بندي كاملاً متغير:

با سيستم سوپاپ بندي كاملاً متغير مي توان روشهاي مديريت سيلندر و سوپاپها را معرفي نمود. همانطور كه در شكل دو نشان داده شده است، در حال حاضر سوپاپهایي ساخته شده اند كه قادرند با استفاده از نيروي الكترومغناطيسي و يك بازو مابين فنرهاي مكانيكي، هرگونه پروفيل باز و بسته شدني را براي سوپاپها ايجاد نمايند. با كنترل جريان الكتريكي، بازو ميتواند در موقعيت انتهايي خود نگه داشته شود بنابراين سوپاپ مطابق با نياز ميتواند باز يا بسته نگه داشته شود. از آنجائيكه زمانبندي سوپاپها ميتواند بصورت آزادانه تنظيم شود، جرم هواي ورودي و گازهاي باقيمانده را ميتوان با سوپاپها تعيين نمود.

بدين وسيله ميتوان از افت دريچه گاز اجتناب كرد و ميزان تشكيل NOX را در بارهاي جزئي كاهش داد.  از انجاييكه در اين روش زمان بندي هر سوپاپ براي هر سيلندر را مي توان بصورت جداگانه تنظيم نمود، بنابراين فعال يا غير فعال كردن هر سيلندر با اين روش ميسر ميگردد(Cylinder Cut Off) . مكانهايي كه در آنها سيلندرها غير فعال ميگردد يا سوپاپهاي آنها باز ميگردد در منحني عملكردي موتور در شكل دو نشان داده شده است. اندازه گیری مصرف سوخت نمونه هاي ساخته شده بر اساس این تکنیک، كاهش مصرف سوخت تا 15% و در صورت بكارگيري فرايند غير فعال سازي سيلندرها تا 20% را نشان مي دهد.

 

پاشش مستقيم:

ابداع سيستمهاي جديد تزريق با فشار بالا و پيشرفت در سيستم كاتاليستهاي DeNOx منجر به اولين توليد انبوه موتورهاي پاشش مستقيم بنزيني با شارژطبقه اي (stratified charge direct injection gasoline engine) شده كه کاهش مصرف سوخت بين 10% تا 15% را به ارمغان آورده است. همانطور كه شكل سه نشان ميدهد،براي دستيابي به بهترين مصرف سوخت، اين موتورها در بارهای جزئي و مخلوط هوا و سوخت بسيار رقيق با نسبتي تا 3 كار ميكنند.

در بارهاي زياد يا بار كامل به منظور تامين ماكزيمم قدرت خروجي، مخلوط هوا و سوخت بصورت همگن وارد محفظه احتراق می گردد. جهت پايداري فرايند احتراق و اجتناب از تشكيل SOOT (دوده) در بار های جزئي، حالت مخلوط هوا و سوخت با حركت كنترل شده هواي ورودي تامين می گردد. با طراحي نشان داده شده در سمت راست بالاي شكل سه، مخلوط با حركت پيچشي رو به جلو (Forward Air Tumble) در  فاصله هوايی شمع پايدار ميگردد. شايان ذکر است که پايداری فرآيند احتراق در موتورهای GDI، بدليل نسبت هوا به سوخت بالا(رقيق سوز بودن)، از چالشهای اساسی اين نوع موتورها می باشد. در اين حالت از يك انژكتور نوع چرخشي(Swirl-Type) استفاده مي شود. نسل بعدي سيستمهاي پاشش مستقيم که در شكل پائين سمت راست نمايش داده شده است، عملکردی  شبيه به فرآيند احتراق درموتورهاي ديزل خواهند داشت؛ یعنی پاشش توام با فرايند احتراق.

از آنجائيكه كاتاليستهاي سه راهه تنها در شرايط استوكيومتريك عمل مي كنند، يك سيستم  کاتاليستی DeNOx بايستي به مجموعه افزوده گردد تا كاهش آلاينده ها را در فرآيند شارژ طبقه ای، مطمئن سازد. به همين دليل سوخت مورد استفاده نيز بايد عاري از گوگرد باشد.    

 

 

 

the little engine

the little engine

Can a gas turbine with tolerances smaller than a wavelength of light make power more portable than ever?

 

batteries are too big. And they don't last long enough—just ask any soldier, laptop user, or TV cameraman. But Alan Epstein, a professor of aeronautics and astronautics at the Massachusetts Institute of Technology, hopes to change all that with a gas turbine engine made of silicon. It's no larger than a quarter and can be stamped out a hundred at a time.

Epstein and his colleagues have been working on the little engine for more than a decade now, and they may currently be just months away from an actual working model. It's hard to tell exactly, because, unlike the fixes that might be needed to nudge a full-size turbine to readiness, every change Epstein's team makes means starting over and building the engine again.

"That's the big difference between something built in silicon, and something built conventionally," Epstein said. "If it's conventional, and you decide something's too big, you take it apart, take it down to the machine shop, then reassemble it. With our engine, once you've built it, it's one solid piece of silicon, and to make a change you have to start from scratch."

Microengines for microprocessors: These tiny silicon gas-turbine engines may soon power laptops or cell phones. And, they'll do so efficiently.

However long and difficult the design cycle, a big surprise for Epstein was discovering how similar the overall concepts of a microengine were to a turbine of any size. "We thought we'd have problems that were very different from a large engine, but in retrospect, we haven't. Our solutions are designed differently, but the challenges are the same: bearings and rotor dynamics," Epstein said.

Contrary to previous analysis, the fluid mechanics at the size Epstein hoped to build his engine turned out to be the same as those of larger engines. As long as the passages made for gas flow are larger than a micrometer in diameter, molecular kinetics are not an issue. The size of the tubes is not so small that at the molecular level the behavior of the fluid against the passage walls changes.

That said, the size of the engine does alter the design, of course—mostly thanks to the limited way tiny things are built in silicon. Whereas a larger engine might first be designed for efficiency—with the question of how to actually manufacture it put off till later—the unique problems of manufacturing in the minuscule dictate the design from the get-go.

Fine Etchings

To make whole sheets of the little turbines all at once, they are built with nine etched and bonded silicon wafers (earlier versions used only six). The virgin silicon is first coated with a photoresist, then a design pattern is applied on top. Next, the wafer is developed and baked. The silicon that remains exposed is then etched, either chemically or with a plasma. To protect the resulting vertical walls from being worn away, they are dusted with a Teflon-like polymer. (The area covered by the pattern is actually etched as well, but as the rest of the silicon is removed somewhere between 50 and 100 times faster than the pattern, the desired depth is achieved.) By repeating this process, a single wafer can have several layers. Smooth slopes may someday be achieved with a gray- scale pattern being developed at MIT's partner, the University of Maryland.

The rotor and its airfoils are carved out of a single wafer. Additional plumbing and bearings are etched onto the wafers that are to sandwich the rotor. All the layers must then be bonded together. Silicon bonds well to silicon, it turns out, and the bonded areas are just as strong as the material itself—but only if the surfaces are kept perfectly clean. A dust particle no bigger than a millionth of a meter in diameter can keep an area the size of the engine itself from bonding.

Although it would be feasible to place a separate rotor into the middle of a silicon engine, the cost and time required for such a procedure would be prohibitive—making the engines impossible to produce cheaply by the hundreds out of a single silicon sheet. Instead, the rotor is made entirely out of one of the wafer layers, but it cannot be completely freed during etching or this most crucial element may fall out during the rest of the manufacturing process. To keep from losing it, Epstein's team keeps it attached until the very end, either with a glue that can later be dissolved or with thin silicon tabs that are easily broken.

This cross-section is of an earlier concept using only five layers. The center wafer contains the etched rotor (disc and airfoils) and the rest of the sandwich consists primarily of bearings and plumbing.

Combustion occurs just outside the rotor, at the same wafer level, spinning it by pushing on its airfoils from the outside. At more than a million rpm the heat produced by the spinning rotor threatens to actually weaken the silicon, so cooling becomes a major issue. To pump out the heat more quickly, the shaft that would normally be in the center of the rotor is removed. A side benefit of the high rpms is that to human ears the turbine is silent. Electricity will be produced with either a tiny magnetic generator, or an electrostatic induction machine.

"To date, these have been driven by micro air turbines for test purposes, rather than the micro gas turbine, which has yet to produce positive mechanical power," Epstein said. With the air turbine the magnetic generator has been shown to produce 10 watts of power.

Although the turbine size is not small enough to change the behavior of fluids, it is small enough to make any fine tuning of the plumbing difficult. On large turbines, for instance, changes in fluid density are handled with tapered passages. Such tubing is currently impossible on an engine of this size, although changing the rotor's airfoil thickness can help the problem somewhat. There's also no way to make tubes with gentle curves—passages are necessarily either etched straight down through a wafer, or across it—so the plumbing has to change directions at right angles. Both limitations reduce the overall efficiency.

From top to bottom, left: (1) A magnetic generator, 4 mm in diameter; it's almost 60 percent efficient. (2) A 6 mm diameter turbine nested neatly within the cumbustor. (3) The airfoils on this silicon wafer have thick trailing edges to make up for an inability to taper fluid paths. (4) A diamond saw will separate turbines along the lines. Empty white circles test how the material responds to processing, and squares within the squares contain MIT's logo.

Small losses like these can add up and it's been shown that in general the smaller the engine, the lower the efficiency. (Epstein, however, points out that this may have more to do with the funding available for small engines than it does with fluid flow.)

According to Epstein, the fuel source could be packaged with the engine or come as a cartridge like a cigarette lighter. "Do you refuel it like you refuel a lighter?" he asked. "Do you sell cartridges? What you rapidly realize is that it's all fuel. How you choose to market and package it is a market question."

In order to have any longevity with the high rotor speed of an engine of this size, the bearings must be low friction.

Epstein's group considered magnetic bearings early on, but found that in addition to the manufacturing difficulties, the magnetic materials had too low a Curie point and would not stay magnetic at the temperatures at which the engine would operate. Instead, they chose pressurized gas bearings, which conveniently can hold more weight relative to their size as they get smaller. Thrust bearings with spiral grooves and holes in their centers are self-pumping and keep the rotor free and in the right position.

Bearing the Loads

"You can indeed make million-rpm air bearing systems out of silicon and have them run reliably," Epstein said. But the bearings on such an engine have to be able to withstand not only the forces going on within, but also the sudden acceleration that might occur when, say, a cell phone is yanked off a table—or dropped on a sidewalk.

The concern is even greater for Epstein's lab since his prototypes take months to put together and are assembled one at a time. "These things are fragile, and if someone drops the wafers—it's happened at Intel. People drop things that are worth tens of millions of dollars."

As for the primary materials in use, with the etching process, the options were few. "Our choices were silicon, silicon, silicon, so we chose silicon," said Epstein. However restricted they were in their materials selection, silicon turns out not to be too bad: It can go to higher temperatures than the materials used in larger engines, and is stronger, too. Silicon nitride and silicon carbide would work well in larger turbines if it weren't for the fact that they are difficult to manufacture in large sizes without introducing flaws.

While there is clearly plenty of room for improving efficiency, the microengine may very well end up as the only real way to power, say, a laptop, an iPod, or a soldier's thermal weapon sight, to say nothing of a palm-size plane. In terms of power per pound, the little engine will easily beat batteries with an output of somewhere between 50 and 100 watts and a 100:1 thrust ratio. Overall it will perform as well as the gas turbines made in the 1940s.

So what, then, is the holdup? "We're at the stage where we chose to demonstrate each part separately. All of them work as individual devices," Epstein said. "It's getting them all to work on the same day and at the same place that's the challenge."

مقدمه اي بر سيستمهاي هيدروليک و نيوماتيک

امروزه در بسياري از فرآيندهاي صنعتي ، انتقال قدرت آن هم به صورت کم هزينه و با دقت زياد مورد نظر است در همين راستا بکارگيري سيال تحت فشار در انتقال و کنترل قدرت در تمام شاخه هاي صنعت رو به گسترش است. استفاده از قدرت سيال  به دو شاخه مهم هيدروليک و نيوماتيک ( که جديدتر است ) تقسيم ميشود .

از نيوماتيک در مواردي که نيروهاي نسبتا پايين (حدود يک تن) و سرعت هاي حرکتي بالا مورد نياز باشد (مانند سيستمهايي که در قسمتهاي محرک رباتها بکار مي روند) استفاده ميکنند در صورتيکه کاربردهاي سيستمهاي هيدروليک عمدتا در مواردي است که قدرتهاي بالا و سرعت هاي کنترل شده دقيق مورد نظر باشد(مانند جک هاي هيدروليک ، ترمز و فرمان هيدروليک و...).

حال اين سوال پيش ميايد که مزاياي يک سيستم هيدروليک يا نيوماتيک نسبت به ساير سيستمهاي مکانيکي يا الکتريکي چيست؟در جواب مي توان به موارد زير اشاره کرد:

 

1.طراحي ساده      2.قابليت افزايش نيرو        3. سادگي و دقت کنترل

4. انعطاف پذيري      5. راندمان بالا                6.اطمينان

در سيستم هاي هيدروليک و نيوماتيک نسبت به ساير سيستمهاي مکانيکي قطعات محرک کمتري وجود دارد و ميتوان در هر نقطه به حرکتهاي خطي يا دوراني با قدرت بالا و کنترل مناسب دست يافت ، چون انتقال قدرت توسط جريان سيال پر فشار در خطوط انتقال (لوله ها و شيلنگ ها) صورت ميگيرد ولي در سيستمهاي مکانيکي ديگر براي انتقال قدرت از اجزايي مانند بادامک ، چرخ دنده ، گاردان ، اهرم ، کلاچ و... استفاده ميکنند.

در اين سيستمها ميتوان با اعمال نيروي کم به نيروي بالا و دقيق دست يافت همچنين ميتوان نيرو هاي بزرگ خروجي را با اعمال نيروي کمي (مانند بازو بسته کردن شيرها و ...) کنترل نمود.

استفاده از شيلنگ هاي انعطاف پذير ،  سيستم هاي هيدروليک و نيوماتيک را به سيستمهاي انعطاف پذيري تبديل ميکند که در آنها از محدوديتهاي مکاني که براي نصب سيستمهاي ديگر به چشم مي خورد خبري نيست.  سيستم هاي هيدروليک و نيوماتيک به خاطر اصطکاک کم و هزينه پايين از راندمان بالايي برخوردار هستند همچنين با استفاده از شيرهاي اطمينان و سوئيچهاي فشاري و حرارتي ميتوان سيستمي مقاوم در برابر بارهاي ناگهاني ، حرارت يا فشار بيش از حد ساخت که نشان از اطمينان بالاي اين سيستمها دارد.

 اکنون که به مزاياي سيستم هاي هيدروليک و نيوماتيک پي برديم به توضيح ساده اي در مورد طرز کار اين سيستمها خواهيم پرداخت.

 

براي انتقال قدرت به يک سيال تحت فشار (تراکم پذير يا  تراکم ناپذير) احتياج داريم که توسط  پمپ هاي هيدروليک ميتوان نيروي مکانيکي را تبديل به قدرت سيال تحت فشار نمود. مرحله بعد انتقال نيرو به نقطه دلخواه است که اين وظيفه را لوله ها، شيلنگ ها و بست ها به عهده ميگيرند .

بعد از کنترل فشار و تعيين جهت جريان توسط شيرها سيال تحت فشار به سمت عملگرها (سيلندرها يا موتور هاي هيدروليک ) هدايت ميشوند تا قدرت سيال به نيروي مکانيکي مورد نياز(به صورت خطي يا دوراني ) تبديل شود.

اساس کار تمام سيستم هاي هيدروليکي و نيوماتيکي بر قانون پاسکال استوار است.

 

قانون پاسکال:

1.    فشار سرتاسر سيال در حال سکون يکسان است .(با صرف نظر از وزن سيال)

2.    در هر لحظه فشار استاتيکي در تمام جهات يکسان است.

3.    فشار سيال در تماس با سطوح بصورت عمودي وارد ميگردد.

همانطور که در شکل 1 مي بينيد يک نيروي ورودي  نيوتني ميتواند نيروي مورد نياز چهار سيلندر ديگر را تامين کند.

 

 شکل (1)

 

يا در شکل 2  داريم :

 

 شکل (2)        

کار سيستمهاي نيوماتيک مشابه سيستم هاي هيدروليک است فقط در آن به جاي سيال تراکم ناپذير مانند روغن از سيال تراکم پذير مانند هوا استفاده مي کنند . در سيستمهاي نيوماتيک براي دست يافتن به يک سيال پرفشار ، هوا را توسط يک کمپرسور فشرده کرده تا به فشار دلخواه برسد سپس آنرا در يک مخزن ذخيره مي کنند، البته دماي هوا پس از فشرده شدن بشدت بالا ميرود که مي تواند به قطعات سيستم آسيب برساند لذا هواي فشرده قبل از هدايت به خطوط انتقال قدرت بايد  خنک شود. به دليل وجود بخار آب در هواي فشرده و پديده ميعان در فرايند خنک سازي بايد از يک واحد بهينه سازي براي خشک کردن هواي پر فشار استفاده کرد.

اکنون بعد از آشنايي مختصر با طرز کار سيستمهاي هيدروليکي و نيوماتيکي به معرفي اجزاي يک سيستم هيدروليکي و نيوماتيکي مي پردازيم.

 

اجزاي تشکيل دهنده سيستم هاي هيدروليکي:

1- مخزن : جهت نگهداري سيال

2- پمپ :   جهت به جريان انداختن سيال در سيستم که توسط الکترو موتور يا 3- موتور هاي احتراق داخلي به کار انداخته مي شوند.

4- شيرها : براي کنترل فشار ، جريان و جهت حرکت سيال

5- عملگرها : جهت تبديل انرژي سيال تحت فشار به نيروي مکانيکي مولد کار(سيلندرهاي هيدروليک براي ايجاد حرکت خطي و موتور هاي هيدروليک براي ايجاد حرکت دوراني).

شکل 3 يک سيستم هيدروليکي را نشان ميدهد.

 

شکل(3)

 

اجزاي تشکيل دهنده سيستم هاي نيوماتيکي:

1- کمپرسور

2- خنک کننده و خشک کننده هواي تحت فشار

3- مخزن ذخيره هواي تحت فشار

4- شيرهاي کنترل

5- عملگرها

شکل 4 يک سيستم نيوماتيکي را نشان ميدهد.

 

 شکل (4)

 

   يک مقايسه کلي بين سيستمهاي هيدروليک و نيوماتيک:

 

1- در سيستمهاي نيوماتيک از سيال تراکم پذير مثل هوا و در سيستمهاي هيدروليک از سيال تراکم ناپذير مثل روغن استفاده مي کنند.

2- در سيستمهاي هيدروليک روغن علاوه بر انتقال قدرت وظيفه روغن کاري قطعات داخلي سيستم را نيز بر عهده دارد ولي در نيوماتيک علاوه بر روغن کاري قطعات، بايد رطوبت موجود در هوا را نيز از  بين برد ولي در هر دو  سيستم سيال بايد عاري از هر گونه گرد و غبار و نا خالصي باشد

3- فشار در سيستمهاي هيدروليکي بمراتب بيشتر از فشار در سيستمهاي نيوماتيکي مي باشد ، حتي در مواقع خاص به 1000 مگا پاسکال هم ميرسد ، در نتيجه قطعات سيستمهاي هيدروليکي بايد از مقاومت بيشتري برخوردار باشند.

4- در سرعت هاي پايين دقت محرک هاي نيوماتيکي  بسيار نامطلوب است در صورتي که دقت محرک هاي هيدروليکي در هر سرعتي رضايت بخش است .

5- در سيستمهاي نيوماتيکي با سيال هوا نياز به لوله هاي بازگشتي و مخزن نگهداري هوا نمي باشد.

6- سيستمهاي نيوماتيک از بازده کمتري نسبت به سيستمهاي هيدروليکي برخوردارند

 

گاردريل هاي خود بازساز

مواد سفارشي، امروزه براي امنيت در تصادفات به عنوان عامل جذب كننده‌ ي ضربه در مكان هاي مستعد تصادف در بزرگراه ها در نظر گرفته مي شود. ايالات متحده ي، به زوي خداحافظي تلخي را با شبكه هاي شني كناره ي بزرگراه ها (محافظ تصادف) خواهد داشت. با گسترش موانع خود باز ساز در اطراف بزرگراه ها مي توان امنيت اين مكان ها را بالا برد. اين مواد...

بعد از چند دقيقه به حالت اوليه ي خود باز مي گردند. از اين مواد مي توان در مكان هاي مستعد تصادف استفاده كرد، كه به سرعت بعد از اولين تصادف خود را براي تصادف بعدي بازسازي مي كنند.
ماده ي مورد استفاده نوعي از پوليورتان (polyurethane) مي باشد كه طي يك فرآيند تخصصي، از ميزان ترد بودن است كاسته مي شود. اين ماده بايد از خاصيت پس زدگي و انعكاس (Rebound) زيادي برخوردار نباشد، چرا كه اين باعث مي شود خودروي تصادف كرده به مسير باز گردد كه اين خود بسيار خطر ناك مي باشد. اين مواد مي توانند به روش هاي مختلف ساخته شوند و براي استفاده هاي مختلف تغيير كنند. در بسياري از مواد هر چه سرعت ضربه زيادتر باشد، رفتار ترديت بيشتري از خود نشان مي دهند، اما اين ماده تحت شكست ترد قرار نمي گيرد و مقدار انرژي جذب شده در شكل فوق الاستيك پوليورتان يكي از كارآيي هاي يكتاي اين ماده است.
يكي از كاربردهاي اين ماده در مسابقات ماشين راني (Nascar) مي باشد، جايي كه امنيت بسيار زيادي براي ديوارهاي جدا كننده ي پيست لازم است. يك نمونه ي اوليه توانسته به صورت رودرو و زاويه 20درجه، ضربه اي با سرعت 161متربرثانيه را دريافت كند و به حالت اوليه ي خود برگردد، همچنين مي توان از اين ماده در تجهيزات نظامي نيز استفاده كرد.
بسياري از پوليورتان ها و پلاستيك ها مي توانند بعد از اولين ضربه تا 95 درصد به حالت اوليه باز گردند،‌90درصد براي دومين بار و 85درصد براي سومين بار و سرانجام به حدي مي رسند كه ديگر به حالت اوليه باز نمي گردند كه اين رفتار مواد در محدوده ي پلاستيك مي‌باشد. همچنين حدي وجود دارد كه در آن با كشيده شدن ماده مي برد. اما پوليورتان هاي جديد، كه توسط ضربه ي ماشين هاي Nascar و ماشين هاي با وزن 4/3 تن به تعداد 12 بار تست شدند مي توانند باز هم به حالت اوليه ي خود باز گردند. زمان كمي براي بازگشتن به حالت اوليه نياز است. براي بازسازي تا 98درصد، 15دقيقه زمان نيز است و براي بازسازي به حالت اصلي 30 دقيقه زمان نياز است. يكي از مزيت هاي عملي اين مواد در مكان هايي همچون كناره ي پل ها و موانع ساروجي – شني براي جدا كردن خطوط جاده ها مشخص مي گردد، زيرا افراد به دلايلي تمايل دارند كه هنگام تصادف به طرف اين موانع بروند. فناوري استفاده شده ي امروزي، از شبكه هاي شني يا آبي يا نرده هاي فولادي مي باشد كه يكبار مصرف هستند و مي بايستي بعد از هر تصادف جايگزين شوند. در ايالت Ohio، حدوداً 2هفته زمان براي تعمير موانع لازم است و اگر روزانه 100000 خودرو عبور كند احتمال اينكه تصادفي دوباره رخ دهد و جان كارگران رابه خطر بياندازد. وجود دارد.
هم اكنون 2شركت به ارزيابي و سنجش اين ماده مي پردازند كه تحت نظر Adminuistration the federal highwoy safety مي باشند. پيش بيني مي شود كه محاسبات و ارزيابي ها تا طي چند ماه اخير پايان يابد و اين محصول در سال 2007 به بازار عرضه گردد.

منبع: www.technologyreview.com

ECU

 مخفف Electronic Control Unit يا واحد کنترل الکترونيک مي باشد و نقش هدايت و کنترل يک خودروي انژکتوري را بر عهده دارد. همانطور که مي دانيد خودروهاي انژکتوري بدليل عملکرد بهتر و توانايي پاس کردن استانداردهاي آلودگي، بطور کامل در تمام دنيا جايگزين خودروهاي کاربراتوري شده اند و مغز اين سيستم ECU مي باشد. ECU با توجه به سنسورهايي که به موتور متصل است وضعيت و شرايط خودرو را تحليل کرده و پاسخهاي لازم را به خروجيها که عبارتند از: انژکتورها، جرقه زنها و ... اعمال مي کند. سنسورهاي کيت هاي انژکتوري مختلف هستند که هر چه تعداد آنها بيشتر باشد ECU بهتر مي تواند شرايط موتور را درک کند. سنسورهاي مهم خودروهاي انژکتوري عبارتند از: سنسور دور يا RPM، سنسور فشار داخل مانيفولد يا MAP، سنسور دريچه گاز يا TPS، سنسور دماي آب يا CTS، سنسور دماي هوا ATS، سنسور اکسيژن يا لاندا، سنسور ضربه و ...

سازندگان معروف ECU چه شرکتهايي هستند؟

1) شرکت Bosch آلمان: اين شرکت بهترين و معروفترين سازنده ECU و کيت انژکتوري در دنيا مي باشد و در اغلب خودروهاي پيشرفته جهان نشاني از آن را مي توان يافت. چند مدل از زانتيا موجود در ايران داراي کيت انژکتوري Bosch مي باشد.

۲) شرکت Delco آمريکا: اين شرکت يکي از قديمي ترين شرکتهاي سازنده ECU مي باشد و ECU آن در اغلب خودروهاي آمريکايي بخصوص خودروهاي شرکت GM يا جنرال موتورز بکار رفته است مانند کاديلاک، پونتياک و... همچنين در خودروهاي دوو کره مانند دوو ESPERO.

۳) شرکت Ford آمريکا: اين شرکت سازنده خودرو، سازنده ECU البته براي خودروهاي فورد مي باشد و اولين بار ايده کنترل تطبيقي يا خود-يادگير در خودروهاي اين شرکت عملا پياده سازي شد.

۴) شرکت Siemens آلمان: فعاليت اين شرکت گرچه به اندازه رقيب آلماني آن يعني Bosch نيست اما ECU هاي خوبي مي سازد. ECU پرايد انژکتوري موجود در ايران طراحي اين شرکت است.

۵) شرکت Magneti Marelli ايتاليا: اين شرکت در اروپا محبوبيت زيادي داشته و بر روي اغلب خودروهاي اروپايي کيت آن نصب است. به عنوان مثال خودروهاي فيات مدل PUNTO و فولکس واگن مدل GOLF IV، مزدا ۳۲۳.

۶) شرکت Sagem فرانسه: بر روي اغلب ماشينهاي فرانسوي ECU اين شرکت نصب است. بنابراين پژو ۲۰۶، مدلهايي از زانتيا؛ همچنين خودروهاي ايراني مانند سمند و پيکان انژکتوري.

۷) شرکت Nippon Denso ژاپن: اين شرکت توسط شرکت تويوتا تاسيس شده و بخش عمده سهام آن را دارا مي باشد البته ۶ درصد سهام آن متعلق به شرکت Bosch است. ECU اغلب خودروهاي تويوتا (مانند تويوتا لندکروز ) و برخي خودروهاي ژاپني مانند نيسان، هوندا، سوزوکي و ... متعلق به اين شرکت مي باشد.


شرکتهاي ديگري هم هستند مانند HITACHI، MATSUHITA، LOTUS و ...







UNICHIP يا فن آوري تنظيم ECU

امروزه موتورهاي انژكتوري نقشي بسيار اساسي در موفقيت صنايع خودروسازي ايفاء مي‌نمايند و كيفيت و قابليتهاي آن، درصد كارايي خودرو را نشان مي‌دهد. همانطور كه مي‌دانيم كنترل كننده موتورهاي انژكتوري، بردي الكترونيكي به نام ECU مي‌باشد و در واقع كارايي اين بخش تعيين كننده كيفيت يك موتور و در ابعادي ديگر كيفيت خودرو خواهد بود؛ بدين معني كه هرچقدر ECU يك موتور بهتر طراحي شده باشد، آن موتور كيفيت بهتري خواهد داشت.

ECU بر اساس سنسورهايي كه بدان متصل است شرايط كار موتور را درك كرده و فرامين مناسب را به انژكتورها و شمعها صادر مي‌كند. از آنجا كه ديناميك خودرو بسيار پيچيده و غير خطي مي‌باشد، طراحان ECU براي سهولت كار، جداولي را به نام map داخل حافظه ECU مي‌ريزند كه در آن مقدار پاشش سوخت و زاويه آوانس در هر دور و بار موتور مشخص شده است. هر چه دقت اين جداول بيشتر باشد، دقت عملكرد ECU بيشتر خواهد بود.

نكته‌اي كه بايد توجه كرد اينست كه مقادير اين جدولها وابستگي مستقيمي به پارامترهاي جغرافيايي موتور، نظير فشار و دماي هوا دارد. شركتهاي خودروسازي، ECU را براي يك آب و هواي خاص طراحي نمي‌كنند بلكه مقادير map را بگونه‌اي تنظيم مي‌كنند كه براي انواع شرايط جغرافيايي جوابي بهينه و معقول بدهد. بنابراين map، در اين حالت براي تمام خودروهاي از يك مدل بهينه است نه هر خودروي خاص؛ زيرا هيچ دو خودرويي، حتي از يك مدل كاملاً مانند يكديگر نيستند.

اگر سيستمي بتواند اين نقيصه را از ECUها برطرف كند، آنگاه مي‌توان به طور اختصاصي map هر خودرو را كاليبره كرده و توان آن را افزايش داد.

امروزه تيونينگ ECU خودروها، بحث جا افتاده اي است و شرکتهاي بسياري در اين زمينه فعاليت مي کنند بطور کلي دو روش براي تيونينگ خودروهاي انژکتوري وجود دارد. روش اول خواندن ديتاهاي (map) ECU و دادن ديتاهاي جديد که شرکتهاي بسياري در اين زمينه فعالند از جمله: Eurochip، Chip Tuning، Tech TV، Autospeed و ...يکي از اشکالات اين روش اينست که بشدت وابسته به ساختار ECU است و با پيچيده شدن سخت افزار ECU امکان خواندن و تغيير ديتاهاي آن مشکل و گاهي غيرممکن مي شود مگر آنکه شرکت سازنده ECU خود نحوه دسترسي به اطلاعات را در اختيار شرکتهاي تيونينگ بگذارد. روش دوم اضافه کردن يک سخت افزار جانبي به ECU جهت تغيير پارامترهاي ECU است. اين روش گرچه گرانتر تمام مي شود اما وابسته به نوع ECU نيست. يکي از شرکتهايي که در اين زمينه فعال است

، شرکت Dastek است. شرکتي که در آفريقاي جنوبي قرار دارد و با پرسنلي در حدود 30 نفر توانسته موفقِِِِت چشمگيري داشته باشد.جالب است بدانيد که اين شرکت بظاهر کوچک توانسته است محصول خود را به کشورهاي مختلف دنيا صادر کند و بيش از 300 نمايندگي فروش در سرتاسر دنيا دارد كه فقط 100 تا از آنها در ايالات متحده آمريكا هستند. نام اين محصول UNICHIP است.

اصول عملکرد UNICHIP بدين صورت كه سنسورهاي اصلي در يك موتور انژكتوري (MAP, RPM) را خوانده و سپس با توجه به نقطه كار موتور، مقاديري مجازي از اين دو سنسور را به ECU اعمال مي‌كند؛ بگونه‌اي كه رفتار ECU نسبت به حالت قبل بهبود پيدا مي‌كند.

آمارها نشان مي‌دهد كه موفقيت UNICHIP در اين زمينه بسيار بالا بوده است:از هر 400 خودرو، فقط يك خودرو ممكن است با UNICHIP بهينه نگردد، 80% خودروهايي كه در آفريقاي جنوبي استفاده مي‌شوند، UNICHIP را در خودروهاي خود نصب كرده‌اند، UNICHIP بر روي بيش از 320 مدل موتور از خودروسازان بزرگ دنيا پياده شده است.

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خودروی هیبریدی چگونه کار می کند؟

مقدمه

امروزه با توجه به آلودگي‌هاي ناشي از خودروها و محدوديت‌هاي سوخت فسيلي، كارخانه‌هاي خودروسازي گام مهمي در مقابله با اين امر برداشته‌اند كه از جمله آنها می توان به خودروهاي هيبریدي (Hybrid Vehicle)، تکنولوژی پيل سوختي (Fuel Cell)، موتورهای با پاشش مستقيم‌ بنزيني (GDI)، موتورهاي HCCI و خودروهاي دو گانه سوز (Bifuel) اشاره کرد.



بازدة بالا، آلايندگي كم، مسافت قابل پيمايش بالا، ايمني مطلوب و قيمت قابل رقابت با خودروهاي متداول از جمله ويژگيهاي حائز اهميت براي خودروهاي هيبريدي است. بسياري از خودروسازان بزرگ مبادرت به توليد اين خودروها در سطحی گسترده نموده‌اند. در اين قسمت به شماي كلي از نحوة عملکرد، حالتهاي كاركردي، مزايا، معايب و تقسيم‌بندي سيستم‌هاي مختلف خودروي هيبريدي خواهيم پرداخت.



تاريخچة خودروي هيبريدي

يك مهندس آمريكائي به نام H.Piper در 23 نوامبر 1905 يك ماشين هيبريدي ساخت كه قادر بود در طي 10 ثانيه تا 25 مايل شتاب بگيرد. موتور اين خودرو تركيبی از موتور بنزيني و موتور الكتريكي بود كه امروزه به عنوان موتور هيبريدي شناخته مي‌شود. Piper در سه سال و نيم بعد، اختراع خود را ثبت نمود؛ اما پيشرفت سريع موتورهای احتراق داخلی با قدرت و گشتاور بالا در آن دوره، همچنين قابليت استارت بدون هندل آنها و از همه مهمتر پايين بودن قيمت سوختهای فسيلی و مطرح نبودن آلودگی محيط زيست، سبب عدم توجه به اين نوع خودروها شد. در پي بحرانهاي نفتي سالهاي 1970 دوباره اين خودروها مورد توجه قرار گرفتند ولي تا سال 1990 که كار اصولي با مشاركت PNGV (Partnership for a New Generation Vehicle) در آمريكا آغاز گرديد، این خودروها به طور جدی پيگيری نشدند.



امروزه خودروهاي هيبريدي مورد توجه كمپانيهاي بزرگ جهان قرار گرفته اند كه از آن جمله مي‌توان به شركتهايي مانند: تويوتا، هندا، ميتسوبيشي، فورد، فيات، جنرال موتورز، دايملر كرايسلر، نيسان و پژو و ... اشاره نمود. توفيق اين محصولات به حدي چشمگير بوده كه از دسامبر سال 1997 تا ابتداي سال 2000 بيش از چهل هزار محصول پريوس كمپاني تويوتا به فروش رسيده است.



ويژگيها

خودروهاي هيبريدي، نوع تعميم يافته خودروهاي برقي خالص مي‌باشند كه معايب خودروهاي برقي خالص تا حدود زيادي در آنها برطرف گرديده است و مي توان گفت معايب خودروهاي احتراق داخلي نيز تا حدودي در آنها برطرف شده است. از مزاياي مهم اين خودروها نسبت به خودروهاي احتراق داخلي، كاركرد در دور و بار ثابت بوده و به اصطلاح در نقطة بهينة خود كار مي‌كنند كه اين امر باعث بالا رفتن بازده موتور و كاهش آلودگي و پايين آمدن مصرف سوخت مي‌گردد و ديگر اينكه به هنگام ترمزگيري و يا شتاب منفي، انرژي به صورت الكتريكي در باطری ها ذخيره مي‌شود و همين امر باعث كاركرد كمتر موتور احتراقي خواهد شد و در نتيجه منجر به كاهش آلودگي و پايين آمدن مصرف سوخت مي‌گردد. به عنوان مثال تويوتا پريوس (Toyota Prius) با موتور ۴ سيلندر ۱۵۰۰ سی سی مصرف سوختی معادل ۲/۴ ليتر در ۱۰۰ کيلومتر دارد. مزيت ديگر اين خودروها نسبت به خودروي برقي خالص، قابليت پيمودن مسيرهاي طولاني در هر بار شارژ كردن باطری مي‌باشد.

سيستمهاي ذخيره سازي انرژي :

خودروهای هيبريدی از ساختارهاي مختلفي برخوردارند. اما الزاما" يك خودروي هيبريدي از يك سيستم ذخير ساز انرژي، يك واحد توليد قدرت و يك سيستم انتقال قدرت تشكيل شده است. انتخابهاي اوليه براي سيستم ذخيره ساز انرژي باطريها، خازنها و فلايويل‌ها هستند. اگر چه باطريها عمده‌ترين انتخاب در اين زمينه مي‌باشند اما تحقيق بر روي زمينه‌هاي ديگر ذخيره‌سازي انرژي آغاز شده است. باطری ها، بدلیل ارزان و تجاري بودن و نداشتن قسمتهاي متحرک اولين وسيله ذخيره انرژي و همانطور که گفته شد متداولترين است اما بزرگترین عیبشان عمر كوتاه آنها می باشد. البته باطريها با تكنولوژي جديد بسيار گران مي‌باشند و امروزه تعداد زيادي از باطريهاي جديد در حال توسعه هستند.

انواع خودروهاي هيبريدي :

با توجه به ساختار كنترلي و طريقه اتصال اجزاء به يكديگر، خودروهاي هيبريدي به سه نوع سري، موازي و سری-موازی تقسيم‌بندي مي‌شوند.

سيستم هيبريدي سري :

در اين دسته از خودروها موتور احتراق داخلي يك ژنراتور را مي‌چرخاند و اين ژنراتور، هم باطري را شارژ می كند و هم يك موتور الكتريكي را به حركت درمي‌آورد و بدین صورت انتقال قدرت صورت می گيرد. در اين ساختار موتور احتراقي مستقيم به سيستم انتقال قدرت وصل نمي‌شود.

اين سيستم به خاطر اين سري ناميده مي‌شود كه قدرت، به صورت سري به چرخ‌ها منتقل مي‌گردد و از آن براي رانش موتورهاي با قدرت كم و با رنج كاركرد بهينه استفاده می شود.





سيستم هيبريدي موازي :

در اين نوع سيستم، موتور احتراقي و موتور الكتريكي به صورت موازي چرخها را به حركت درمي‌آورند. در اين سيستم موتور الكتريكي توسط باطري و موتور احتراقي توسط منبع سوخت فسيلي مستقيما" تغذيه مي‌گردند. در اين حالت ژنراتور حذف شده و باطري با تغيير حالت موتور الكتريكي به ژنراتور شارژ مي‌گردد. از آنجائيكه این سيستم فقط يك موتور دارد موتور الكتريكي نمي‌تواند همزمان هم باطري را شارژ كند و هم باعث رانش چرخها گردد. يك تصوير ساده از اين سيستم در ذيل نشان داده شده است.





سيستم هيبريدي سري ـ موازي:

اين طرح بگونه ای است كه مي‌توان از آن در شرايط مختلف به صورت هيبريد سري يا موازي استفاده نمود. در اين سيستم با بهره‌گيري از فن‌آوري پيشرفته امكان استفاده از سيستم احتراقي و سيستم الكتريكي بطور جداگانه و همزمان وجود دارد. به اين ترتيب در مواقع شهري كاملا" الكتريكي و بدون آلودگي و در سرعتهاي بالا و در محدودة برون شهري مي‌تواند بطور مستقل احتراقي و يا تركيبي از دو سيستم باشد. در مواقعي چون شتابگيري سريع، هر دو سيستم با هم عمل مي‌كنند. چنين ايده‌اي فقط بكمك يك فن‌آوري مدرن در يك خودرو سواري قابل اجراست. معمولا" چنين سيستمهايي از نوع تركيبي هستند و با بهره‌گيري از يك استراتژي كنترلي مناسب عملا" همراه با فراهم آوردن عملكرد مناسب، سطح شارژ باطريها نيز در حد خوبی نگهداري مي‌شود بدين ترتيب اين خودرو مي‌تواند چه در شهر و چه در جاده به يك خودروي متداول تبديل گردد. در اين سيستم دو موتور الكتريكي وجود دارد كه بسته به شرايط مي‌تواند تركيبي از آنها به كار آيند و قابليت تبديل به ژنراتور را نيز دارند.

اين سيستم در خودرو Prius و Estima شرکت تويوتا استفاده شده است.




مقايسه چند نوع سيستم هيبريدي:

در شكل‌ (1) مقايسه‌اي كلي از سه نوع سيستم هيبريدي صورت گرفته است كه در شكل (2) مي‌توان مزايا و معايب سيستمها را در كنار هم مشاهده نمود.




با توجه به جدول فوق می توان خصوصیات زير را برای خودروهای هيبريد سری- موازی برشمرد:

۱- كاهش اتلاف انرژي: سيستم بطور اتوماتيك در حالت idle (درجا) خاموش مي‌شود و بدين ترتيب از به هدر رفتن انرژي جلوگيری می شود.

۲- ذخيره‌سازي و برگرداندن انرژي: انرژيي كه در هنگام شتاب منفي و ترمزگيري هدر مي‌رود را به انرژي الكتريكي تبديل نموده و از اتلاف آنها جلوگيري مي‌كند.

۳- كمك به كاركرد موتور احتراقي: موتور الكتريكي در زمان شتابگيري به كاركرد موتور احتراقي كمك مي‌كند.

۴- كاركرد با بازدهي بالا: اين سيستم با يک استراتژی کنترلی مناسب، بازدهی کلی خودرو را در تمام شرايط کاری در حالت بيشينه نگه می دارد؛ بدين صورت که موتور الکتريکی مانند يک جبران ساز در شرايطی که قدرت موتور احتراق داخلی کافی نيست وارد عمل می گردد و در مواقعی که قدرت موتور احتراق داخلی بيشتر از نياز خودرو است، انرژی مازاد در باطريها ذخيره می گردد.





حالتهاي عملكردي موتور هيبريدي سری-موازی:

حالتهاي عملكردي يك موتور هيبريدي سری-موازی را مي‌‌توان به شش قسمت تقسيم نمود:



حالت روشن شدن و دورهاي پايين و متوسط:



در اين حالت موتور احتراقي كه بازدهي مناسبی ندارد كاملا" خاموش است و فقط موتور الكتريكي توان مورد نياز خودرو را تأمين مي‌نمايد (A)






حركت در حالتهاي معمولي:

قدرت ناشي از موتور احتراقي توسط تقسيم كنندة قدرت (Power split device) به دو بخش تقسيم مي‌گردد قسمتي از قدرت آن به ژنراتور مي‌رود كه منجر به حركت در آوردن موتور الكتريكي مي‌گردد (B) و مابقي چرخها را مستقيما" به حركت درمي‌آورد (C) قدرت موتورا حتراقي در اين مرحله در حداكثر بازدهي است.




شتابگيري سريع:

در حاليكه قدرت يكنواختي از مسيرهاي B و C به چرخها منتقل می گردد توان اضافي توسط باطري نيز جهت افزايش توان موتور الکتريکی (A) تامين می گردد.



شتاب كندشونده و يا ترمزگيري:

موتور با قدرت بالا به ژنراتور با قدرت بالايي تبديل مي‌گردد كه توسط چرخها به حركت درمي‌آيد. در اين حالت انرژي جنبشي به انرژي مكانيكي تبديل شده و در باطري ذخيره مي‌گردد. (D)







شارژ شدن باطري:

براي اينكه باطريها هميشه در حد قابل قبولي انرژي داشته باشند. در حالت ضروري كه احتياج باشد توسط موتور احتراقي شارژ مي‌گردد.(E)



حالت استراحت:

موتور به حالت اتوماتيك خاموش مي‌گردد.

the road not yet taken

the road not yet taken

To end our dependence on rapidly dwindling oil supplies, switching to hybrid vehicles and ethanol fuel from corn simply isn't enough.

in his State of the Union speech this past January, President George W. Bush declared, "It's in our vital interest to diversify America's energy supply." He then went on to outline what he called "ambitious goals": reducing gasoline consumption by 20 percent in 10 years, boosting use of gasoline alternatives to 35 billion gallons over the same period, and incrementally increasing automobile fuel efficiency standards.

One word that was barely uttered in the speech was hydrogen. It was just a few years ago that hydrogen promised to deliver the clean, fuel-efficient transportation system President Bush has called for. But as knowledgeable scientists and engineers have pointed out, hydrogen is not a fuel source, but merely an energy carrier that must be manufactured. And a "cradle-to-grave"—or, more accurately, a "well-to-wheel"—analysis clearly demonstrates one thing: There is no currently available pathway to produce hydrogen, store it, transport it as an energy carrier, and use it to generate heat or electricity as efficiently as using the heat or electric power from the primary energy source (fossil or nuclear fuels, or sunlight) directly.

Although the current electric grid will have to be strengthened as more electric power is needed, the cost of expanding the existing grid would be much less than building a new hydrogen distribution and storage system from scratch. The cost of building a hydrogen distribution system has been estimated by various sources as costing from 500 billion to one trillion dollars.

These facts are especially relevant to building a secure transportation system for the United States. The U.S. transportation system depends almost entirely on oil. Imports have risen steadily since 1973 as demand increased, and domestic supplies reached a peak and began to decrease. Today, more than 60 percent of the oil consumed in the U.S. is imported, and the dependence on foreign oil, much of it from countries hostile to the United States, is bound to increase. Moreover, oil demand in developing countries, especially China and India, is increasing rapidly just as worldwide production is beginning to approach its peak. Once the world oil peak is reached and oil production begins to drop, the cost of fuel will increase steeply. Unless demand can be curtailed and alternative fuels can be supplied from domestic sources soon, an unprecedented social and economic crisis is likely to ensue.

Replacing the petroleum-based transportation system is of the utmost importance. We have available today options that will enable the development of a transportation system that is more efficient, more secure, and has less negative impact on the environment than the one the United States has currently. Access to petroleum is a problem today. Giving it up will actually be a blessing.

Some energy economists have claimed that there will be no oil supply crisis: Thanks to the so-called magic of the market, as oil becomes more expensive, producers will have incentive to provide more of it. Unfortunately, geology is not subject to the market. Even if the amount of ultimately recoverable oil reserves were to increase from the Energy Information Agency's mean estimate of three trillion barrels to its maximum estimate of four trillion barrels, that only pushes back the peak of production by 11 years. No matter how many places we open up to exploration, production will peak in this generation.

An electric car (top) or plug-in hybrid (above) gets power by plugging into the electrical grid (below).

Unconventional supplies of oil also won't provide relief any time soon. The vast oil-shale deposits in Colorado, Utah, and Wyoming have long been hailed as a future energy source. However, more than half a century of research has not found an economical and environmentally benign way to use oil shale. Therefore, we cannot bank on this resource to help us now. We must instead start to supplement oil as the primary transportation fuel because an orderly transition to develop petroleum substitutes will take time and careful planning.

For instance, turnover in the national automobile fleet is achingly slow. The number of cars that are retired each year, according to statistics compiled by the U.S. Government, can be approximated as a steady 5 percent over 20 years until none remain on the road. The number of new cars added each year can be approximated as 7 percent of the existing fleet. With these rates of turnover and assuming that suddenly all new cars were high-efficiency vehicles, after 10 years 41 percent of all vehicles on the road still would be from the initial low-efficiency fleet, and only after 20 years would essentially all vehicles be high-efficiency. Any realistic scenario would require much longer to convert the fleet to high-efficiency vehicles. To make a large difference in fleet fuel efficiency, then, changes need to be initiated immediately and must be substantial.

Hybrid electric vehicles like the ones on the road today are twice as fuel efficient as the current average vehicle. But the near-term reduction in fuel consumption of hybrid vehicles has been overstated. Even if starting tomorrow half of all new cars and trucks sold in the U.S. were hybrid electric—an absurd proposition—the annual fuel savings in 10 years' time would be less than 15 percent. Even after 20 years, the cumulative savings would be less than one-sixth of what would be otherwise consumed.

Therefore, we need to introduce technologies that use even less petroleum. One technology that can achieve this is the plug-in hybrid electric vehicle. A plug-in hybrid can run moderate distances drawing only on its stored electricity, like a pure electric vehicle, then switch on the engine to extend its range when the battery is drawn down.

The diesel engine is inherently 25 to 30 percent more efficient than the spark-ignition (Otto cycle) engine. Diesel engines are now much cleaner and quieter than they were in the past. In Europe, roughly half of all new vehicles sold are diesel-powered. Furthermore, diesel fuel is more readily produced from coal and biomass than is gasoline. The gasoline engine in hybrid and plug-in hybrid vehicles could just as well be diesel engines to further improve efficiency.

The ultimate gasoline savings that a plug-in hybrid can provide depends on the size of its on-board battery pack, and the driving profile of the vehicle. They can be designed with different all-electric ranges. A PHEV60, a plug-in hybrid electric vehicle that could travel 60 miles on batteries alone, would see a greater number of miles traveled per year in all-electric mode than a PHEV20, with an all-electric range of 20 miles.

According to a study by the Electric Power Research Institute in Palo Alto, Calif., about one-third of the annual mileage for a typical PHEV20 would be electric-powered (EPRI, Technical Report 1009299, May 2004). Given the excellent efficiency of all-electric drivetrains (more than 80 percent, according to recent EPRI data), plug-in hybrids can reach parity with conventional vehicles in terms of life-cycle costs if the price per kilowatt-hour of battery storage were to come down to $316 per kilowatt-hour for a PHEV20 with gasoline at $1.75 a gallon. We have calculated that parity can be reached at a battery cost of about $1,600 per kWh if gasoline costs $2.50 a gallon.

The assumptions made in the EPRI report are very conservative because plug-in hybrid electric vehicles and battery technology are developing rapidly. There are companies, such as Hybrid Prius Inc. and CalCars, that claim PHEV30s can achieve 100 mpg.

One way to obtain liquid fuel from coal is the Fischer-Tropsch process, in which a synthetic gas made from coal is catalyzed. The liquid is similar to petroleum-derived diesel.

Also important is that plug-in hybrid vehicle technology provides utilities with a new and sustainable market for off-peak electric power. According to EPRI, consumer demand for electric power peaks during the day, while more than 40 percent of U.S. generating capacity sits idle or operates at reduced loads overnight. Vehicles could be recharged during these off-peak hours by installing software in the cars that would initiate battery charging only when excess power is available. This arrangement would also even out electricity consumption. Moreover, since no new production facilities or infrastructure would be required, the cost of recharging plug-in hybrids during off-peak hours would be only the extra fuel and operation and maintenance, much less than average utility rates.

High-efficiency vehicles won't solve the gasoline consumption problem alone. Even if every new car from this point forward were a diesel plug-in hybrid or battery electric vehicle, it would take until 2025 at the earliest to achieve a cumulative reduction in gasoline consumption equal to half what an all-gasoline fleet would use. Demand-side solutions are critical, but what is needed is a new way to obtain automotive fuel.

Fortunately, there are options beyond petroleum. Coal, natural gas, and biomass can be transformed chemically into liquid fuels. In the United States, the conversion of coal to liquid fuel has received a great deal of attention of late. The governor of Montana, Brian Schweitzer, as well as senators from Pennsylvania and other political leaders have been promoting the idea as a means of achieving energy independence.

To make coal into a vehicle fuel, it must first be converted to a synthesis gas of hydrogen and carbon monoxide. The sulfur contained in the coal is converted to hydrogen sulfide gas and captured; metals are removed as slag. The gaseous product may then be reacted to one of several chemical products that can be used as vehicle fuel. A big advantage of most liquid fuels is that they can use existing distribution infrastructure with little change, although high concentrations of ethanol require different storage materials.

What's more, the gasification process lends itself to the capture and sequestration of carbon dioxide, with an overall efficiency penalty of about 2 percent. Between the capture of CO₂ at the point of manufacture and the greater efficiency of the plug-in hybrid vehicles, such an integrated system could greatly reduce the nation's greenhouse gas emissions.

The most commonly cited method for turning synthetic gas into liquid fuel is the Fischer-Tropsch process, which was invented by German scientists early in the last century and is used today in South Africa by Sasol to make diesel fuel. The Fischer-Tropsch reaction results in a liquid fuel consisting of approximately 75 percent synthetic diesel and 25 percent naphtha that is used to make synthetic gasoline.

Today, Sasol Ltd., the world's largest maker of motor fuel from coal, produces 160,000 barrels per day in Secunda, South Africa. The 50-year-old plant provides 28 percent of South Africa's supplies of such fuels as diesel, gasoline, and kerosene. Several large liquid-fuel projects are in progress in the Middle East, starting with natural gas that is otherwise flared.

Ramping up production of synthetic fuel won't happen overnight. The estimated time of construction for a plant is four to five years, and the capital investment is large. For example, the capital cost of a coal-gasification Fischer-Tropsch synthesis plant with a capacity to produce 20,000 barrels of liquid fuel per day is estimated to be on the order of $1.2 billion. At present, the U.S. uses something on the order of 20 million barrels of liquid fuel each day.

Opponents of coal gasification claim that there will be excessive greenhouse gas pollution from the process. However, in the future, vehicle fuel-cycle emissions of carbon dioxide can be reduced below those of gasoline-only powered vehicles, by the use of plug-in hybrid electric vehicles and by sequestration of the carbon dioxide from the fuel production process. And coal is far from the only feedstock available for the process. Natural gas can be reacted with steam to make synthetic gas that can be processed in the same way as coal. Indeed, the gas-to-liquid technology is so well developed that four major projects, totaling more than 360,000 barrels a day in production, have been announced in the past two years, including a 32,000 barrel-a-day joint project between Sasol and Qatar Petroleum and a 34,000 barrel-a-day ChevronTexaco facility under construction in Nigeria.

Biomass can be gasified either alone or in combination with coal and converted to liquid fuels by the same process as gasified coal. It can also be pyrolyzed and then processed into vehicle fuels.

Before choosing the direction of synthetic fuels, however, it is important to look at the efficiency of the process. Energy is lost in the conversion of coal, natural gas, or biomass into a vehicle fuel, and the energy efficiency of these conversion processes is important in determining the overall efficiency from well (or mine or farm) to wheel of these alternative pathways.

Ethanol from corn is a rapidly growing vehicle fuel, due largely to a federal subsidy of approximately 50 cents per gallon to the producer. This makes ethanol about the same price per gallon as gasoline, though it is still higher per mile driven. Although there has been controversy about the energy efficiency of ethanol from corn, it has been amply demonstrated that ethanol as currently produced from corn contains 1.25 to 1.3 times more energy than the source energy (not including the solar input to the crop) required to produce it (Farrell, et al., Science, Vol. 311, 506-508, 27 January, 2006, and rael.berkeley.edu/ EBAMM/Farrell).

All fossil fuel-based energy sources produce less energy than is input; gasoline contains only about 0.9 times the energy of the petroleum used to produce it, making it one of the most efficiently produced of fossil fuels. Other issues associated with corn-based ethanol are that corn is part of the food-supply chain, and its use results in a rather small reduction in CO₂ emissions. Ethanol from sugar cane, or from nonfood crops such as switch grass, has a much higher energy output per fossil input than corn, and causes a much larger reduction in CO₂ emissions.

One thing that the experience of the past half-century should teach us is not to rely too heavily on one source of energy for our transportation system. Instead of replacing a petroleum-fueled, internal combustion-powered system with one based entirely on hydrogen or biomass or fuel cells, we should identify the best two or three or four combinations of fuel and vehicle. And we should begin to switch to these new technologies immediately. As we have shown, even a radical change will take time to have a noticeable effect.

Fortunately, there are already several vehicle and fuel technologies available that can help us. Plug-in hybrid electric vehicles, for one, combine the best of both electric vehicles and hybrid technologies. Like electric vehicles, plug-in hybrids can be fueled with electricity generated from domestic sources and produce fewer CO₂ emissions than conventional spark-ignition vehicles do, because of their improved mileage. Like any hybrid, the plug-in variety can run on liquid fuel for acceptable driving range. Because of the reduced fuel
consumption, it may be possible to provide the fuel entirely from domestic sources in the future.

But to ensure that the transition to plug-in hybrids or to some other technology happens rapidly, policy changes must be made. We need a stiff tax on carbon fuels to encourage efficiency, and we need CAFE standards tough enough to prod manufacturers into selling diesel, hybrid, and plug-in hybrid vehicles. Federal programs could spur the development of vehicles with a greater reliance on electric drive and the commercialization of coal- and biomass-based diesel fuel.

There is also a need for immediate research into a number of other related technologies that will be needed in the coming decades. Most importantly, we need to develop processes to produce ethanol from cellulosic material at a reasonable cost and investigate photochemical and high-temperature solar thermal reactions that can produce fuels, including hydrogen. We need to improve the performance of electrical storage in batteries or ultracapacitors. And we must develop technologies that can capture and store carbon dioxide. It's vital that these technologies are available within a generation, when they will be needed to augment or replace parts of the new transportation system we've outlined.

President Bush's ultimate goal of increased energy security is laudable, but the proposals that he calls ambitious don't go far enough. The suggestions we have presented have a better chance to provide us not only with increased energy security, but also, eventually, with energy independence, and they may help reduce the long-term threat to the nation from climate change. We believe this new system will work—and will do so in a way that should not be disruptive. Indeed, doing nothing—allowing the nation's fuel supply and vehicle fleet to remain unchanged right up to the moment when petroleum production begins to decline—would be a catastrophe, one that could be avoided if we were to take action now.

This article is based largely on a paper by West and Kreith in the Journal of Energy Resource Technology (Vol. 128, Sept. 2006, pages 236-243).

Frank Kreith is professor emeritus of mechanical engineering and Ron West is professor emeritus of chemical engineering, both at the University of Colorado in Boulder. The authors have been investigating questions regarding future energy supply for the past six years.

Fahrenheit 3,600

Fahrenheit 3,600

Everywhere you look, the gas turbine industry is running hot.

One of the basic rules of gas turbines is that the hotter the gas that enters the work-producing

turbine from the combustor, the greater the thermal efficiency and output. Still, there are limits.Turbine inlet temperatures in the gas path of modern high-performance jet engines usually don't exceed 3,000°F, while non-aviation gas turbines operate at 2,700°F or lower.

But 3,600°F? That temperature exceeds the melting point of iron and the boiling point of molten silver. And yet the turbine airfoils in the new F135 jet engine that powers the Joint Strike Fighter Lightning II are capable of operating at these extreme temperatures. The F135 gas turbine is the first production jet engine in this new 3,600°F class, designed to withstand these highest, record-breaking turbine inlet temperatures.

There have been, in fact, quite a few accomplishments in the gas turbine industry over the last year. GE put into operation a simple-cycle 100 MW turbine that runs at 46 percent efficiency. Pratt & Whitney ramped up production of engines for a new class of aircraft, the very light jet. And construction of the first pebble bed nuclear reactor, set to be built in South Africa, was placed on the schedule.

But 3,600°F? That's hot. The JSF engine represents a bold—and necessary—step forward. This 40,000-pound thrust engine will power all three variants of the JSF: an Air Force fighter that takes off conventionally, a carrier-based Navy jet, and a short takeoff/vertical landing aircraft for the Marines. The STOVL version is the first aircraft to be able to do the "Hat Trick"—take off in a short distance, go into supersonic flight, then hover and land vertically. These varied missions require a very high thrust-to-weight ratio, and thus high turbine inlet temperatures.

The powerful engine for the new Joint Strike Fighter on
its test platform in Florida (above) will develop 40,000 lbs. of thrust. The jet engine will come in Navy, Marine, and Air Force versions (below).

Last December, at Pratt & Whitney's Middletown, Conn., plant, Ed Crow, retired senior vice president and head of engineering at Pratt, took a few of us from the University of Connecticut Mechanical Engineering Department to view a F135 engine disassembled after 600 to 800 hours of operation. The blades and vanes of the high turbine, clad with ceramic thermal barrier coatings, are made of single crystal superalloys, which soften and melt at temperatures between 2,200 and 2,600°F. (Single crystal alloys were the subject of an article, "Crown Jewels," in ME magazine in February 2006.) Turbine airfoils closest to the combustor operate in a gas stream that can exceed their superalloy melting point by 1,000°F.

So how do turbine airfoils survive running conditions in this 3,600°F class engine? The vanes and blades are cooled to maintain acceptable service temperatures, some eight-tenths to nine-tenths of their melting temperature. Each high-temperature turbine airfoil is formed from an elaborate investment casting to accommodate the intricate internal passages and surface hole patterns necessary to channel and direct cooling air (bled from the compressor) within and over external surfaces of the airfoil structure. An error in airfoil cooling hole location or in cooling air pressure ratios could cause airfoil gas path inhalation rather than film cooling exhalation, which at the JSF's high turbine gas path temperatures would induce airfoil expiration. The JSF turbine film cooling design is based on some 30 years of gas turbine industry film cooling research and development, and unequivocally pushes forward the state-of-the-art of turbine performance and durability.

The JSF engine is just one product in the $3.7 billion military gas turbine market, which includes jet engine production for the world's fighter aircraft—such as the F15, F16, F22, F35, and Typhoon—military cargo, transport, refueling, and special-purpose aircraft. And that's just a fraction of the total worldwide gas turbine market.

A Steep Climb

David Franus of Forecast International in Newton, Conn., has again this year provided me with values of gas turbine manufacturing production, based on FI's proprietary databases and computer models. FI's values of production of gas turbines are unique in that they are for both aviation and non-aviation, the two disparate parts of the industry, usually reported on separately in trade journals. Worldwide gas turbine production for 2006 amounted to $27.6 billion, up significantly from $22 billion in 2005, but still below the 10-year average of $28.5 billion.

The aviation portion, all for manned aircraft jet and turboprop engines, amounted to $18.5 billion, two-thirds of the 2005 total value of gas turbine production.

The value of gas turbine production for commercial aviation is three to four times that of military, $14.8 billion in 2006. There is a prediction of $16.9 billion in 2010 (a 14 percent increase). This upward trend reflects the growth of the airline industry, evidenced by increased passenger loads (especially for Asian travel) since 9/11 and SARS, and an increase in the number of new airlines. Sales of existing models of Boeing and Airbus aircraft, using a variety of General Electric, Pratt & Whitney, Rolls-Royce, and Snecma engines, are strong, and both airframe companies are developing new models. Boeing has the new subjumbo 787, designed to serve what the company sees as the future demand of air travel, as well as a "new" superjumbo 747-8 family. (The 747 is an incredibly long-lived product line. I remember working on the first JT9D 747 jet engines, back in the 1960s at Pratt & Whitney Aircraft.). Airbus is developing its trouble-plagued superjumbo A380.

The air cargo market is strong and orders for new jet engine-powered freighters are high. Jet engine demand is also strong in regional airline and business aircraft markets.

The Siemens SGT-8000H gas turbine, shown here in a cutaway diagram, is the world's largest, rated at 340 MW with an efficiency of 60 percent.

A booming area for new jet engines is the very light jet, or "air taxi" market (the subject of the article "Very Light and Fast" in January). A VLJ twin-engine aircraft with a pilot and from five to eight passengers, could provide point-to-point, on-demand air taxi service to some of the 5,000 local airports in North America. For flights shorter than 500 miles, VLJ aircraft use could enable air travelers to circumvent the bottleneck created by airport security and could eliminate layovers caused by the existing hub-and-spoke airline system.

Eclipse Aviation, Honda Aircraft Co., Cessna Aircraft Co., Citation, Embraer, and Adam Aircraft have entered the VLJ market, and Eclipse reports orders for 2,500 of its jets. Pratt & Whitney Canada is in full production for several thousand of the VLJ engines, in the 1,000- to 3,000-pound thrust range. Other VLJ engine OEMs are Williams International and Honda/GE.

In contrast with the steadily climbing aircraft market, the value of production for non-aviation gas turbines shows a boom-and-bust quality, rising to a peak of nearly $26 billion in 2001 before dropping back to around $8 billion a few years later. That behavior is caused by the rapid growth in—and sometimes speculative nature of—the electric power market, during this recent era of piecemeal utility deregulation.

Non-aviation gas turbines consist of electrical power generation, mechanical drive (mostly used to drive natural gas pipeline compressors), and marine (Navy, cruise ships, and ferry propulsion). The largest segment of that market by far is electrical power generation, in simple cycle (gas turbine only), combined cycle (gas turbine with its exhaust producing steam for steam turbine generation), and cogeneration (gas turbine, with its exhaust producing steam for heat, as described, for instance, in "Campus Heat and Power," Dec. 2006).

Forecast International predicts significant growth in coming years in demand for gas turbine electrical power generation, rising from $8.6 billion in 2006 to a projected $13.5 billion in 2008, a 60 percent increase. Based on a small sample of OEMs that I interviewed at the big Power-Gen conference and exhibit in Orlando last December, I agree with FI's predictions. In particular, two U.S. OEMs said that the cogeneration market for gas turbines was much stronger in Europe than in the U.S. In the words of one OEM exhibitor, "The sales are strong in those countries that signed the [Kyoto] treaty." Such an observation would seem to be at odds with assertions made by U.S. officials that signing the Kyoto treaty on greenhouse gas emissions would put the U.S. at an economic disadvantage.

Cleaning Coal

In many countries, such as the United States, South Africa, and China, coal is the major energy source, and it is used to produce electricity in steam Rankine cycle plants. At the Sino-American Technology and Engineering Conference I attended in Beijing last October, Xu Kuangdi, the president of the Chinese Academy of Engineering, remarked that of every three power plants currently being built in the world, two were in China, where the major fuel is coal.

Companies and government have been launching projects to design and develop integrated gasification combined-
cycle power plants. These IGCC plants convert coal into syngas, a low calorific value gas composed of carbon monoxide and hydrogen; the syngas is then used as fuel for a gas turbine, whose exhaust provides heat to generate steam to run a steam turbine. Using the same fuel twice, in essence, a combined-cycle power plant can have thermal efficiencies as high as 60 percent. There are now only two IGCC plants in operation in the United States, compared with 1,100 pulverized coal steam power plants, all with thermal efficiencies much, much lower than 60 percent. If IGCCs prove to have reasonable capital costs per kilowatt, the market for gas turbines could be very promising.

The first standardized commercial IGCC plants are being built by GE Energy and Bechtel, for American Electric Power, the U.S.'s largest electrical generator. Also, the U.S. Department of Energy has initiated FutureGen, a program to build the first integrated sequestration and hydrogen production plant. This is to be a zero-emissions fossil fuel plant using, of course, gas turbines.

The very largest electric power gas turbines are identified as H class, a designation that has lightheartedly been interpreted as an abbreviation for "humongous" (see "A Year of Turbulence," ME magazine's Power & Energy, June 2004). A General Electric GE Energy 9H gas turbine weighs in at 405 tons (367,900 kg), and the first one went into natural gas fuel operation at Baglan Bay, Wales, in 2003. In combined-cycle operation this unit can input 520 MW into the U.K.'s electric power grid, at a plant thermal efficiency of just under 60 percent.

Siemens' first H class gas turbine combined-cycle plant is now under construction in Irsching, Germany. It's also slated to have a thermal efficiency over 60 percent, and a plant output of 530 MW. The Siemens SGT-8000H gas turbine itself is rated at 340 MW, making it the world's largest.

The two companies differ in their design philosophy on turbine cooling systems. GE Energy H units are steam cooled—closely tying together the steam (Rankine) and gas turbine (Brayton) cycles—while the Siemens H gas turbine will be cooled by air bled from the compressor.

Wild animals graze along the road to the Koeberg nuclear power station (below). The site, home to a 1,800 MW conventional nuclear power station, will see construction of the world's first pebble bed nuclear reactor in 2008. The new reactor will have an output of 165 MW.

While H machines are designed mainly for base load electric power markets, General Electric's new LMS100 gas turbine is aimed at the mid-merit and daily cycling segments—the difficult-to-predict, must-be-ready-to-start electric peak power providers. The LMS100 is rated at 100 MW and, at 46 percent, has the highest efficiency of any simple cycle gas turbine. It is the first modern production electric power gas turbine that has an intercooler. This is a water-cooled, shell-and-tube heat exchanger through which gas path flow between the high and low compressor is cooled, making for less compressor work. The resulting heated intercooler water can then be used for some other purposes, but more importantly, the net gas turbine output is increased and colder turbine cooling air is made available, boosting thermal efficiency.

The LMS100 is an aeroderivative, based on GE's CF6-80C1 jet engine, but perhaps should be called a hybrid aeroderivative since the machine's low compressor is derived from GE's heavy-frame MS6001FA gas turbine. The first production unit of this innovative, intercooled gas turbine went into operation at Groton, S.D., last year.

This past February, while in Cape Town, South Africa, I visited what will be the site of the world's first nuclear-powered gas turbine electric power plant. The consortium Pebble Bed Modular Reactor (Pty) Ltd. will begin construction by May 2008, and Westinghouse of the U.S., Mitsubishi Heavy Industries of Japan, Nukem of Germany, and South Africa's utility, Eskom, are all participating.

This first PBMR unit will have an output of 165 MW provided by a closed-cycle gas turbine designed and developed by Mitsubishi and operating with helium gas. The helium is heated in a nuclear graphite-modulated, high-temperature reactor, approximately 88 feet high and 20 feet in diameter. The reactor is filled with 450,000 fuel "pebbles," managed in such a way that the reactor need not be shut down for refueling. Each 6 cm diameter graphite pebble (about the size of a tennis ball) is heated by nuclear reactions going on in some 15,000 kernels of uranium dioxide, each about 0.5 mm diameter, dispersed in the pebble, and individually encased in protective layers of carbon and silicon carbide.

The helium enters the pebble bed at 500°C and 9 Mpa, and is heated to about 900°C before it enters the turbine, then on to a recuperator, compressor, intercooler, recuperator, and then back into the pebble bed reactor, thus producing a nuclear-heated, Brayton thermodynamic closed cycle. In a closed-cycle operation, electric load variation is accomplished by varying the amount of helium in the system (A book on the subject, Closed-Cycle Gas Turbines, by Hans Frutschi is available from ASME Press). The PBMR is designed to have a relatively high thermal efficiency: 41 percent, compared to 33 percent for a conventional light water reactor using a Rankine cycle.

One selling point of the design is that any loss of coolant will shut down the nuclear reactions. This first PBMR unit, in fact, will be built right next to Eskom's Koeberg 1,800 MW Rankine cycle nuclear power plant. That facility is located on 7,500 acres of the Koeberg Nature Reserve, on the Atlantic coast less than 20 miles north of Cape Town. It's a very picturesque location for a generating station of any sort, and probably the only nuclear power plant in the world patrolled by wild springboks and zebras.

Lee S. Langston is professor emeritus of mechanical engineering at the University of Connecticut in Storrs. A frequent contributor to Mechanical Engineering, he is the former editor of ASME's Journal of Engineering for Gas Turbines and Power.

پیل سوختی

پيل سوختي

 

اولين پيل سوختي توسط سر ويليام گرو در سال 1839 توسعه يافت، ولي تا مدتها از آن استفاده عملي نشد، تا اينكه در سال 1960، ناسا اين فن‌‌آوري را براي توليد الكتريسيته بر روي فضاپيماهاي آپولو و جميني نصب كرد. پيلهاي سوختي در مقايسه با سيستمهاي توليد توان متداول، مزاياي بسياري دارند؛ بازده الكتريكي نسبتاً بالاتر، ميزان انتشار آلاينده‌هاي كم، انعطاف پذيري در مورد محل نصب، قابليت اطمينان بالا، ملزومات و هزينه تعمير و نگهداري كم، كارآيي part-load فوق العاده، تعديل پذير بودن و قابليت بكارگيري سوختهاي مختلف. به خاطر بازده و مزاياي زيست محيطي‌شان، فن‌‌آوري پيل سوختي يك راه حل بسيار جذاب و مفيد  در قرن بيست و يكم براي رفع مشكلات انرژي خواهد بود.

 

فرايند الكتروشيميايي در پيل سوختي

 

پيل سوختي (FC) فن‌‌آوري است كه بدون داشتن اجزاي متحرك، انرژي شيميايي را به انرژي الكتريكي تبديل مي‌كند. پيل سوختي نيز مانند باتريها، بر اساس اصول الكترو شيميايي كار مي‌كند. يك پيل سوختي شامل دو الكترود (آند و كاتد) است كه به وسيله يك ماده الكتروليت از هم جدا شده‌اند. بخشي از يك پيل سوختي كه شامل الكترودها و مواد الكتروليت است، «استك» ناميده مي‌شود و بطور كلي بخش هزينه‌بَر آن محسوب مي‌شود. با وجود اينكه پيلهاي سوختي از نظر مواد الكتروليتي متفاوتند، ولي همه آنها با اصول پايه يكساني كار مي‌كنند.

شكل1 فرايند الكتروشيميايي در پيل سوختي

پيلهاي سوختي را از نظر نوع الكتروليت بكار رفته در آن تقسيم بندي مي‌كنند. در حال حاضر پنج نوع پيل سوختي وجود دارد: پيلهاي سوختي آلكالين (AFC)، پيلهاي سوختي اسيد فسفريك (PAFC)، پيلهاي سوختي پليمري (PEFC) (در بعضي مراجع از آن به پيلهاي سوختي غشاء تبادل پروتون (PEMFC) ياد شده است)، پيلهاي سوختي كربنات مذاب (MCFC) و پيلهاي سوختي اكسيد جامد (SOFC).

سوخت اين پيلها هيدروژن است. هيدروژن را هم مي‌توان به صورت مستقيم بكاربرد، و هم مي‌توان آن را از منابعي كه از لحاظ هيدروژن غني هستند، استخراج كرده و سپس به مصرف رساند. اين منابع عموماً هيدروكربنهايي نظير بنزين، پروپان، متانول، گاز طبيعي و غيره هستند. براي استخراج هيدروژن از اين منابع به يك دستگاه جانبي بنام مبدل (reformer) كه پيش از ورود سوخت به پيل قرار مي‌گيرد، نياز است.

روش كار به اين صورت است كه، سوخت هيدروژن به آند پيل سوختي تغذيه شده و اكسيژن (يا هوا) نيز از طريق كاتد وارد پيل سوختي مي‌شوند. به كمك يك كاتاليست اتمهاي هيدروژن به يونهاي مثبت و منفي تبديل مي‌شوند. يونهاي مثبت كه حاوي پروتونها (H+) هستند از ميان الكتروليت عبور كرده و به سمت كاتد مي‌روند، ولي يونهاي منفي (الكترونها) نمي‌توانند از الكتروليت عبور كنند. به همين خاطر الكترونها از يك مدار خارجي عبور داده مي‌شوند. در ضمن عبور الكترونها از اين مدار خارجي، الكتريسيته توليد ميشود و سرانجام الكترونها به كاتد مي‌رسند. در كاتد يونهاي مثبت و منفي در مجاورت يك اكسيد كننده (اكسيژن يا هوا) با هم دوباره تركيب شده و با اكسيژن موجود تشكيل آب و گرما مي‌دهند.

 

كاربرد پيل سوختي در توليد همزمان

 

استفاده از محتواي انرژي گرمايي حاصله از واكنش الكتروشيميايي پيل سوختي، باعث مي‌شود كه بازده اينگونه سيستمها به مقدار قابل ملاحظه‌اي بهبود يابد. عموماً گرما به شكل آب داغ يا بخار فشار پايين (كمتر از 30psig ) بازيافت مي‌شود، ولي كيفيت گرماي حاصله به نوع پيل سوختي و دماي كاري آنها بستگي دارد (جدول1). در بين انواع پيل سوختي پايين­ترين كيفيت دمايي را پيلهاي نوع PEFC، با دماي كاري بين 65 تا 85 درجه سانتيگراد دارند. همانطور كه مشاهده مي‌شود پيلهاي سوختي نوع SOFC و MCFC با گستره دمايي بين 650 تا 1000 درجه سانتيگراد، در كاربردهاي CHP كه نياز به توليد بخار فشار بالا و متوسط است، بسيار مناسب مي‌باشند. بطور كلي گرماي بازيافت شده از سيستمهاي CHP پيل سوختي، براي گرمايش فضاي ساختمانها، گرمايش آب آشاميدني و فرآيندهايي كه نياز به دماهاي پايين و متوسط دارند، مناسب مي‌باشد.

جدول1 دماي كاري و بازده انواع پيل سوختي

نوع پيل سوختي	بازده الكتريكي	بازده به صورت CHP	دماي كاري (C°)
آلكالين (AFC)	32%-40%	NA	90-260
اسيد فسفريك (PAFC)	35%-45%	75%	190-210
پليمري (PEFC)	25%-35%	72%	65-85
كربنات مذاب (MCFC)	40%-50%	70%	650-700
اكسيد جامد (SOFC)	45%-55%	70%	750-1000

مطالب قدیمی‌تر

با سلام,
من راشد جعفری دانشجوی رشته ی مهندسی مکانیک در دانشگاه ازاد اسلامی واحد اهواز هستم. این وبلاگ را برای علاقهمندان به این رشته درست کرده ام. امیدوارم که رضایت شما را جلب کند. نظرات خود را به ادرس [e-mail] ایمیل کنید.همچنین اگر علاقهمند هستید میتوانید عضو خبر نامه وبلاگ شوید.برای این کار اول گزینه ی close all of the menus  را کلیک کرده و بعد بر روی
open all of the menus (بالا سمت چپ) کلیک کنید.در آخرین منوی سمت چپ اطلاعات مورد نیاز را وارد کنید.
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