محرك الطائرة

[خالد الحموري]

محرك الطائـــــــرة

المحرك من الأجزاء الرئيسية في الطائرة وهو لتأمين قوة دفع للطائرة (لسحب
الهواء ودفعه للخلف بقوة لتتقدم الطائرة للأمام ) وهي على نوعين، فإما أن يكون المحرك:

1-محرك مكبسي (Piston Engine) : (محرك احتراق داخلي كالموجود في السيارات)
يقوم بإدارة المروحة (Propeller) في مقدمة الطائرة أو عدة مراوح على الأجنحة(وهي
كالمراوح المنزلية تدفع الهواء إلى الأمام , لكن في الطائرة فهي تسحب الهواء
وتدفعه إلى الخلف بقوة لتتقدم الطائرة للأمام ) .


2-المحرك التوربيني ( Turbine-engine) و هو على شكلين، فإما أن تستخدم
طاقة الدوران في إدارة مراوح الطائرة مثل المحركات المكبسية ، و إما أن يتم استخدام
قوة نفث كمية من الهواء الحار للخلف لدفع الطائرة (هنا لا حاجة إلى وجود المراوح).

كل أنواع المحركات التوربينية أو النفاثة تعمل بنفس المبدأ إذ يمتص المحرك النفاث الهواء
من المقدمة بواسطة المروحة و يضغطه عن طريق سحبه في سلسلة من المراوح ذات الشفرات الصغيرة والمتصلة بعمود إدارة shaft و من ثم يخلط بالوقود , و يشعل مزيج الهواء والوقود بواسطة شرارة كهربائية و ينفجر المزيج بقوة وتتمدد الغازات المحترقة و تتجه نحو التوربين
وهو عدة مراوح تدور وبدورانها تحرك المراوح التي في المقدمة عن طريق العمود المربوطة به , والغازات تتجه بقوة بعدئذ إلى المؤخرة عبر فوهات العادم، هذه القوة المتجهة للخلف تدفع المحرك النفاث و الطائرة للأمام .
الصورة أسفل توضح كيفية تدفق الهواء من خلال المحرك فبعض الهواء يدخل قلب المحرك
وبعضه يتدفق حوله لعملية خفض صوت المحرك ومن ثم يخلط مع الهواء الحار لزيادة قوة الدفع.

بناء على ما سبق يمكن تقسيم المحرك التقليدي إلى: ( المروحة Fan) - (الضاغطCompressor )- (غرفة الإحتراقCombustor) - )عنفة أو توربينTurbine ) – (مخرج أو عادم Exhaust nozzle ) ووظائفها كالتالي :

- مدخل الهواء أو المروحة : لسحب الهواء و إدخاله للمحرك وزيادة سرعته وتوجيهه للضاغط .
- الضاغط : وهو عبارة عن مراوح عدة ذات شفرات صغيرة تكون متسلسة خلف بعضها وهي
لضغط الهواء عن طريق عصره في مناطق صغيرة وبعد إرتفاع ضغط الهواء يدخل على غرفة الإحتراق.
- غرفة الإحتراق : عند دخول الهواء لها يتعرض لرش من الوقود عن طريق أنابيب صغيرة ومن ثم يتعرض للشرر من عدة قوابس تكون موزعة بشكل دائري و بدرجة حرارة تصل أحياناً إلى 2700 درجة يتمدد الهواء بهذه الحرارة العالية ويندفع للتوربين.
- التوربين : بدورانه تدور الضواغط و المروحة فهو موصول بها عن طريق عمود الإدارة ليساعد في إدارتها و له عدة خدمات ومن خدماته أنه يمد نظام التكييف بالهواء المضغوط وكذلك يدير تروس إضافية ملتصقة بالمحرك من الخارج وتخدم هذه التروس الإضافيه مولدات الكهرباء بالطائرة ومضخات عدة.
- العادم : وهو المكان الذي تخرج منه قوة الدفع Thrust ومنه يتم إخراج الهواء الساخن والمندفع للخلف ومزجه بالهواء البارد القادم من حول المحرك .
أنواع محركات التوربين :

1- المحرك النفاث التوربيني ( Turbojet ):

محرك مثالي للمحركات التوربينية حيث المروحة و الضواغط و غرفة الاحتراق و التوربين و فوهة العادم، كل الهواء المسحوب إلى داخل الضواغط من المروحة يمر عبر نواة المحرك ثم يحرق ثم يتم إفلاته، وهنا ينشأ الدفع المقدم من قبل المحرك عن قوة سرعة إفلات غازات العادم من المؤخرة.
ولزيادة قوة الدفع لبعض المحركات النفاثة لدى الطائرات المقاتلة يوجد هناك قسم ما بعد الإحراق
( Afterburner) ويوضع قبل العادم وهو عبارة عن أنابيب صغيرة موزعة بشكل منتظم لنشر رذاذ الوقود على الهواء المحترق والقادم من المحرك مما يزيد من حرارة الهواء وتمدده , وبزيادة هذه الحرارة تزيد قوة الدفع بحوالي 40% أثناء الإقلاع و تزيد أكثر أثناء الطيران بسرعات عالية والصورة التالية لمحرك نفاث مع Afterburner.

2-المحرك التوربيني ذو المروحة (Turbofan ):

وهو المحرك الشائع لدى أغلب الطائرات المدنية في يومنا هذا , حيث تمت إضافة مروحة كبيرة في مقدمة قسم الضواغط ، تسحب هذه المروحة كميات هائلة من الهواء إلى داخل غلاف المحركات إلا أن كمية صغيرة نسبياً منه فقط تذهب عبر النواة للقيام بعملية الاحتراق وأما الباقي فيندفع خارج غلاف النواة وضمن غلاف المحرك( وهذا ما يجعله مختلف عن المحرك النفاث) ليساعد في خفض صوت المحرك و يختلط مع الهواء الحار في العادم مما يزيد قوة الدفع ويقلل إستهلاك الوقود.
وتكون محركات Turbojet ,Turbofan فعالة للسرعات فوق 800 كم/س .

3- المحرك المروحي التوربيني ( Turboprop):

وهو محرك نفاث يدير عمود موصل بمروحة كمروحة المحرك المكبسي , و كثير من الطائرات الصغيرة الاستثمارية تستخدم المحرك المروحي التوربيني، وهذه المحركات فعالة عند الارتفاعات المنخفضة و السرعات المتوسطة حوالي 640 كم/س (400 ميل بالساعة)، الفرق بين Turbofan و Turboprop : أن Turbofan في مروحته Fan ليست لتوليد الدفع و إنما لسحب الهواء و الدفع ناتج عن نفث الغازات، أما المروحة الدافعة Propeller فوظيفتها إنتاج الدفع فيما يكون لنفث الغازات من المحرك دفعاً صغيراً يصل إلى 15% من دفع المحرك بشكل عام.
والمحركات الجديدة من هذا النوع زودت بمراوح قصيرة الطول لكن كثيرة العدد وعدل في حوافها لأكثر فعالية في السرعات العالية .

4- محرك عمود الإدارة التوربيني ( Turboshaft) :

محرك شبيه بالمحرك المروحي التوربيني لكنه لا يدير المروحة بل لإدارة مراوح الهيلوكوبتر , وهو يستخدم بأكثر طائرات الهيلوكوبتر الموجودة حالياُ , و المحرك مصمم بحيث أن سرعة المراوح مستقلة عن سرعة المحرك مما يتيح لسرعة المراوح أن تكون ثابتة حتى لو تغيرت سرعات المحرك ليتكيف مع الطاقة المنتجة , وبما أن أغلب الطائرات المستخدمة لهذا المحرك تكون على إرتفاعات منخفظة فإن الغبار والأتربة قد تسبب عائقاً له لذا فقد أضيف له عند مدخل الهواء عازل ومصفي من الأتربة .

5- المحرك النفاث التضاغطي(Ramjet ) :

وفكرة هذا المحرك بسيطة وهي الإستغناء عن الضواغط والتوربين , و السماح للمحرك بنفسه بالتعامل مع الهواء بضغطه وتسخينه ودفعه إلى الخلف .
وهذا النوع من المحركات لا يعمل إلا أن يكون متحركاً بسرعة 485كم/س تقريباً ( للسماح بالهواء للدخول بسرعة وضغطه ) , وهو جداً فعال في السرعات العالية تقريباً 3 ماخ ( 3600 كم/س ) ويستخدم غالباً في الصواريخ طويلة المدى والمركبات الفضائية .

6- المحرك الصاروخي ( Rocket engine) :

يعمل محرك الصاروخ بنفس المبدأ، عدا أنه في مجال عديم الهواء في الفضاء يجب على الصاروخ أن يحمل على ظهره هواءه الخاص بشكل وقود صلب أو سائل قابل للتأكسد من أجل القيام بعملية الانفجار.
​

 

[ابن المغرب البار]

بارك الله فيك .. موضوع جميل ومفيــد​

 

[BERKUT_SU]

سيتم تثبيت الموضوع حتى يستفيد منه الاخوه اكثر

وهذه معلومات ولكن بالانجليزيه.......

Military jet engines

A jet engine is an engine that discharges a fast moving jet of fluid to generate thrust in accordance with Newton's third law of motion. This broad definition of jet engines includes turbojets, turbofans, rockets and ramjets and water jets, but in common usage, the term generally refers to a gas turbine used to produce a jet of high speed exhaust gases for special propulsive purposes.
History

In the 1930s, the piston engine in its many different forms (rotary and static radial, aircooled and liquid-cooled inline) was the only type of powerplant available to aircraft designers. However, engineers were beginning to realize conceptually that the piston engine was self-limiting in terms of the maximum performance which could be attained; the limit was essentially one of propeller efficiency. This seemed to peak as blade tips approached the speed of sound. If engine, and thus aircraft, performance were ever to increase beyond such a barrier, a way would have to be found to radically improve the design of the piston engine, or a wholly new type of powerplant would have to be developed. This was the motivation behind the development of the gas turbine engine, commonly called a "jet" engine, which would become almost as revolutionary to aviation as the Wright brothers' first flight.

The key to a practical jet engine was the gas turbine, used to extract energy to drive the compressor from the engine itself. In 1929, Aircraft apprentice Frank Whittle formally submitted his ideas for a turbo-jet to his superiors. On 16 January 1930 in England, Whittle submitted his first patent (granted in 1932). The patent showed a two-stage axial compressor feeding a single-sided centrifugal compressor. Whittle would later concentrate on the simpler centrifugal compressor only, for a variety of practical reasons. In 1935 Hans von Ohain started work on a similar design in Germany, seemingly unaware of Whittle's work. Whittle had his first engine running in April 1937. It was liquid-fuelled, and included a self-contained fuel pump. Von Ohain's engine, as well as being 5 months behind Whittle's, relied on gas supplied under external pressure, so was not self-contained. Whittle unfortunately failed to secure proper backing for his project, and so fell behind Von Ohain in the race to get a jet engine into the air.

One problem with both of these early designs, which are called centrifugal-flow engines, was that the compressor worked by "throwing" (accelerating) air outward from the central intake to the outer periphery of the engine, where the air was then compressed by a divergent duct setup, converting its velocity into pressure. An advantage of this design was that it was already well understood, having been implemented in centrifugal superchargers. However, given the early technological limitations on the shaft speed of the engine, the compressor needed to have a very large diameter to produce the power required. A further disadvantage was that the air flow had to be "bent" to flow rearwards through the combustion section and to the turbine and tailpipe.

Austrian Anselm Franz of Junkers' engine division (Junkers Motoren or Jumo) addressed these problems with the introduction of the axial-flow compressor. Essentially, this is a turbine in reverse. Air coming in the front of the engine is blown towards the rear of the engine by a fan stage (convergent ducts), where it is crushed against a set of non-rotating blades called stators (divergent ducts). The process is nowhere near as powerful as the centrifugal compressor, so a number of these pairs of fans and stators are placed in series to get the needed compression. Even with all the added complexity, the resulting engine is much smaller in diameter. Jumo was assigned the next engine number, 4, and the result was the Jumo 004 engine. After many lesser technical difficulties were solved, mass production of this engine started in 1944 as a powerplant for the world's first jet-fighter aircraft, the Messerschmitt Me 262. After the end of the war the German Me 262 aircraft were extensively studied by the victorious allies and contributed to work on early Soviet and US jet fighters.

Centrifugal-flow engines have improved since their introduction. With improvements in bearing technology, the shaft speed of the engine was increased, greatly reducing the diameter of the centrifugal compressor. The short engine length remains an advantage of this design. Also, its engine components are robust; axial-flow compressors are more liable to foreign object damage.

British engines also were licensed widely in the US. Their most famous design, the Nene would also power the USSR's jet aircraft after a technology exchange. American designs would not come fully into their own until the 1960s.


TurbojetGeneric term for simple turbine engineSimplicity of designBasic design, misses many improvements in efficiency and power


TurbofanFirst stage compressor greatly enlarged to provide bypass airflow around engine coreQuieter due to greater mass flow and lower total exhaust speed, more efficient for a useful range of subsonic airspeeds for same reason, cooler exhaust temperatureGreater complexity (additional ducting, usually multiple shafts), large diameter engine, need to contain heavy blades. More subject to FOD and ice damage. Top speed is limited due to the potential for shockwaves to damage engine



RamjetIntake air is compressed entirely by speed of oncoming air and duct shape (divergent)Very few moving parts, Mach 0.8 to Mach 5+, efficient at high speed (> Mach 2.0 or so), lightest of all airbreathing jets (thrust/weight ratio up to 30 at optimum speed)Must have a high initial speed to function, inefficient at slow speeds due to poor compression ratio, difficult to arrange shaft power for accessories, usually limited to a small range of speeds, intake flow must be slowed to subsonic speeds, noisy, fairly difficult to test, finicky to kept lit.

ScramjetSimilar to a ramjet without a diffuser; airflow through the entire engine remains supersonicFew mechanical parts, can operate at very high Mach numbers (Mach 8 to 15) with good efficiencies.
Still in development stages, must have a very high initial speed to function (Mach >6), cooling difficulties, very poor thrust/weight ratio (~2), extreme aerodynamic complexity, airframe difficulties, testing difficulties/expense

 

[BERKUT_SU]

PulsejetAir is compressed and combusted intermittently instead of continuously. Some designs use valves.Very simple design, commonly used on model aircraftNoisy, inefficient (low compression ratio), works poorly on a large scale, valves on valved designs wear out quickly

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Pulse detonation engineSimilar to a pulsejet, but combustion occurs as a detonation instead of a deflagration, may or may not need valvesMaximum theoretical engine efficiencyExtremely noisy, parts subject to extreme mechanical fatigue, hard to start detonation, not practical for current use

RocketCarries all propellants onboard, emits jet for propulsionVery few moving parts, Mach 0 to Mach 25+, efficient at very high speed (> Mach 10.0 or so), thrust/weight ratio over 100, no complex air inlet, high compression ratio, very high speed (hypersonic) exhaust, good cost/thrust ratio, fairly easy to test, works in a vacuum-indeed works best exoatmospheric which is kinder on vehicle structure at high speed.Needs lots of propellant- very low specific impulse typically 100-450 seconds. Extreme thermal stresses of combustion chamber can make reuse harder. Typically requires carrying oxidiser onboard which increases risks. Extraordinarily noisy.

 

[BERKUT_SU]

Turbojet engines

A turbojet engine is a type of internal combustion engine often used to propel aircraft. Air is drawn into the rotating compressor via the intake and is compressed, through successive stages, to a higher pressure before entering the combustion chamber. Fuel is mixed with the compressed air and ignited by flame in the eddy of a flame holder. This combustion process significantly raises the temperature of the gas. Hot combustion products leaving the combustor expand through the turbine, where power is extracted to drive the compressor. Although this expansion process reduces both the gas temperature and pressure at exit from the turbine, both parameters are usually still well above ambient conditions. The gas stream exiting the turbine expands to ambient pressure via the propelling nozzle, producing a high velocity jet in the exhaust plume. If the jet velocity exceeds the aircraft flight velocity, there is a net forward thrust upon the airframe.
Under normal circumstances, the pumping action of the compressor prevents any backflow, thus facilitating the continuous-flow process of the engine. Indeed, the entire process is similar to a four-stroke cycle, but with induction, compression, ignition, expansion and exhaust taking place simultaneously, but in different sections of the engine. The efficiency of a jet engine is strongly dependent upon the overall pressure ratio (combustor entry pressure/intake delivery pressure) and the turbine inlet temperature of the cycle.
It is also perhaps instructive to compare turbojet engines with propeller engines. Turbojet engines take a relatively small mass of air and accelerate it by a large amount, whereas a propeller takes a large mass of air and accelerates it by a small amount. The high-speed exhaust of a jet engine makes it efficient at high speeds (especially supersonic speeds) and high altitudes. On slower aircraft and those required to fly short stages, a gas turbine-powered propeller engine, commonly known as a turboprop, is more common and much more efficient. Very small aircraft generally use conventional piston engines to drive a propeller but small turboprops are getting smaller as engineering technology improves.
The turbojet described above is a single-spool design, in which a single shaft connects the turbine to the compressor. Higher overall pressure ratio designs often have two concentric shafts, to improve compressor stability during engine throttle movements. The outer high pressure (HP) shaft connects the HP compressor to the HP turbine. This HP Spool, with the combustor, forms the core or gas generator of the engine. The inner shaft connects the low pressure (LP) compressor to the LP Turbine to create the LP Spool. Both spools are free to operate at their optimum shaft speed.
Turbofan engines

Most modern jet engines are actually turbofans, where the low pressure compressor acts as a fan, supplying supercharged air to not only the engine core, but to a bypass duct. The bypass airflow either passes to a separate 'cold nozzle' or mixes with low pressure turbine exhaust gases, before expanding through a 'mixed flow nozzle'.
Forty years ago there was little difference between civil and military jet engines, apart from the use of afterburning in some (supersonic) applications.
Civil turbofans today have a low specific thrust (net thrust divided by airflow) to keep jet noise to a minimum and to improve fuel efficiency. Consequently the bypass ratio (bypass flow divided by core flow) is relatively high (ratios from 4:1 up to 8:1 are common). Only a single fan stage is required, because a low specific thrust implies a low fan pressure ratio.
Today's military turbofans, however, have a relatively high specific thrust, to maximize the thrust for a given frontal area, jet noise being of little consequence. Multi-stage fans are normally required to achieve the relatively high fan pressure ratio needed for a high specific thrust. Although high turbine inlet temperatures are frequently employed, the bypass ratio tends to be low (usually significantly less than 2.0).

 

[BERKUT_SU]

Manufactures

The main manufactures of military jet engines today are :

• Pratt & Whitney (US; F-16, F-22)
• General Electric (US; B-1, B-2)
• Rolls-Royce (UK; Harrier)
• Tumansky (Soviet Union; Mig-25, Mig-29)
• Lyulka/Saturn (Soviet Union; SU-27, SU-37)
• Klimov (Soviet Union; Mig-17)
• Turbo-Union (UK, Germany, Italy; Tornado)
• EuroJet (UK, Germany, Italy, Spain; Eurofighter Typhoon)
• SNECMA (France; Mirage-2000, Rafale)

 

[BERKUT_SU]

Military aircraft jet-engines in more detail

History

"After World War Two, piston engines continued to power civil airliners for many years, but in the field of military aircraft they were rapidly displaced by the gas turbine. Fighters and bombers switched to the turbojet, transports and maritime-patrol aircraft used turboprops, and helicopters benefited greatly from changing to turboshaft engines. The change meant more power for less weight, far greater reliability, no cooling problems and safer kerosene-type fuels.
With extraordinary reluctance, designers eventually recognized that the turbofan, offering a wide choice of bypass ratio (BPR - the mass flow of air in the bypass duct divided by that through the core), could with advantage replace the turbojet. In supersonic aircraft the need to minimize frontal area means that BPR is seldom as high as 1, and even then the installation must be done with great care. When the J79 turbojet of 79.63 kN thrust installed in the McDonnell Douglas F-4 Phantom was replaced in the British versions by the Rolls-Royce Spey turbofan of 91.25 kN the change made the aircraft slower in level flight, while giving improvements in take-off and climb performance!
Today the turbojet is almost extinct, except for some countries like China, where different criteria apply. Elsewhere, the trend has been towards achieving greater power with engines that are not only lighter but also smaller and dramatically simpler. For example, the Spey Mk 202, the engine of the RAF Phantoms, had a total of 17 stages of blading in the compressors (5+12 flow pressure+high pressure) and four stages of blading in the turbines (2+2). The next-generation RB. 199, engine of the Tornado, has 12 stages of compression (3+3+6) and again four stages of expansion through the turbines (1+1+2), whereas today's Eurojet EJ200, engine of the Eurofighter, has only eight compressor stages (3+5) and two turbine stages (1+ 1).
In general, the more stages of blading an axial-flow compressor has, the greater the overall pressure ratio (OPR) and the better the fuel economy (and thus, for a given aircraft tankage, the greater the range and endurance). One might therefore think that the simpler compressors have been achieved at the expense of more rapid fuel burn, but in fact the reverse is true. The OPR of the Phantom's Spey was 20, the figure for the Tornado engine is 23, and for the Eurofighter it has gone up to 26. Indeed, the next-generation fighter engine could have an OPR of 35, with only six or seven stages of blading.
Benefits of Simplicity

Simpler engines mean greater reliability, better resistance to battle damage, easier maintenance, and several other advantages including lower cost, though cost is not as dominant as it is in the civil sector. In the immediate postwar era, up to 1970, it was normal practice not to introduce an engine to the airlines until hundreds or even thousands had gained experience in fighters and bombers. The two families then diverged. Airliners needed engines offering the lowest possible fuel consumption and lowest possible noise at airports, and these (surprisingly slowly) eventually led to today's engines with a BPR of from 5 to 9, with enormous fans. Combat aircraft need slim engines, as already noted, so military experience is seldom much help to civil engines (though the best-selling CFM56 has the core of a long-established military engine, the F101 used in the Rockwell B-1B Lancer).
Today, the military trend towards greater simplicity is being echoed by civil engines. Nearly 30 years ago, special turbojets and turbofans were being produced purely to lift VTOL (vertical take-off and landing) aircraft. They were used only at take-off and landing, so were made as simple as possible. Like other engines, they sometimes had two spools (low-pressure and high-pressure compressors, each driven by its own turbine), and the aerodynamicists found that by making the spools rotate in opposite daemons, it was possible to do away with at least some of the stator (fixed) blades ahead of the turbine rotors.

 

[الضاري]

موضوع و لا أروع , مجهود ممتاز , أهنئك عليه .

 

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رد: محرك الطائرة

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