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INTRODUCTION

The jet engines are one of the most important inventions inthe 20th century use of which has become very widespread overtimeand which also have played an important role in history andstill plays an important role in our daily lives. In many warslike World War II or the Korean War, military airplanes poweredwith jet engines had a major role. Today in our daily lives theyhelp us travel through oceans and continents by powering thecivil airplanes.

These engines are called jet engines since they emit a jetused to propel the engine. Jet is actually a fluid that isprojected into the surrounding medium from a nozzle. A jet has aconsiderably higher momentum then surrounding fluid medium.

THE INVENTION OF THE JET PROPULSION ENGINE

The invention of the jet propulsion engine was achievedwith the contribution of multiple scientists/inventors. It wasSir Isaac Newton who first introduced the theory whichestablishes the basic principle of jet propulsion. Histheory, based on the well-known Newton’s Third Law of Motion,stated that an explosion, gaseous products of which arechannelled backward, could propel (push) a body forward with ahigh speed.

Wright Brothers (Wilbur and Orville Wright) were the firstpeople to introduce a machine which could benefit the use of jetpropeller. They built the first gas powered engine that waslight enough to make an aeroplane fly in 1903.

Before the World War II aircrafts were powered with piston-type engines that rotated a propeller, causing the movement ofthe air towards the rear of the aircraft to create thrust. Thefunction of jet engine is similar to its descendant propeller,although the propeller pushes a great volume of air with lowervelocity where jet engines push smaller volumes with greatervelocity.

It is not fair to give credit to a single person or asingle nation for the design of the jet engines since multiple

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nations worked on this project separately in the first half ofthe 20th century. When the success of the research process isconsidered, English aviation engineer Sir Frank Whittle (Born:June 1, 1907, Coventry, Warwickshire, England; Died: Aug. 8,1996, Columbia, Md., U.S.) and the German airplane designerDoctor Hans Von Ohain (Born: Dec. 14, 1911, Dessau, Germany;Died: March 13, 1998, Melbourne, Fla., U.S.) are regarded as thefathers of jet propulsion.

Whittle was a pilot at Royal Air Force, and he was 22 yearsold when he first thought of a turbo fan engine to power anairplane. His first attempt to get official support wasunsuccessful and he had to finance his research by his own andthis led him to get his first patent on turbojet propulsion in1930. After this achievement he got a private financial supportin 1935 and began to build his design. In April 1937 he was ableto test his first prototype in a laboratory. Although this testwasn’t intended to show its use on an airplane, the testsuccessfully demonstrated that turbojets would be of great usein that field. The modern turbojets used in many aircrafts arebased on this prototype of Frank Whittle.

He was associated to a firm called Power Jets Ltd. Thisfirm built an engine called W1 to fly a small aircraft. In1940 the first flight with a jet engine powered flight tookplace, the aircraft was called Pioneer.

Von Ohain was an airplane designer who also built anoperational jet engine. He had his doctorate in physics and wasa member of University of Gottingen when Ernst Heikel, a Germanaircraft builder, asked the university for assistance. He wasdoing some research for an engine without a propeller. He was 22years old when he got the idea of a jet engine. In 1934 he gothold of a patent similar to Whittle’s, but with differentinternal arrangement.

Hans Von Ohain joined Ernst Heinkel in 1936 and went onworking to develop his concept of jet propulsion. Heinkeldesigned a test aircraft to fly with their first jet engineHe178 (Fig 1) , its first flight took place on August 27, 1939successfully. During the World War II Germany builtapproximately 1300 airplanes that were fitted with turbojet

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engines. Germans were the first to produce a jet poweredaircraft for military purposes.

THE FUNCTIONING OF THE JET PROPULSION ENGINE

The Theory of Jet Propulsion

As stated before the basis of propulsion thrust lays on theNewton’s laws of motion. Newton’s 2nd law explains that force isequal to the product of mass and acceleration and the 3rd lawexplains that for every action there is an equal and oppositereaction (actio-reactio principle).

There are different types of jet engines but they allfunction on the same principles. The engine inlets air with helpof a fan in the front end. In a compressor that consists of fansattached to a shaft, the air is compressed to a smaller volume,thus the pressure of it is increased. Compressed air travelsinto the combustion chamber, it is mixed with fuel and ignitedwith electric sparks. The burnt gasses expand and have a greatamount of energy. These gasses are then expelled from the rearend of the engine through the nozzle to create thrust. They alsopass through a turbine attached to a shaft so that as gasses areexpelled fresh air can be inlet through the fan in the front endof the engine. (Fig 7)

The thrust is created according to the Newton’s 3rd law ofmotion. Exerted gasses apply a force in every directions of theengine. While vertical forces cancel out each other, horizontalforce towards the engine pushes the engine, thus the aircraftforward, and the gasses are pushed out of the rear end. Thisprinciple can be easily observed by releasing an inflatedballoon. As it is seen the jet propulsion actually works on avery simple principle.

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The Parts of a Jet Engine(Fig 2)

The basic jet engine consists of five major parts which are alsomentioned before.

The fan (Fig 2.1) is placed at the front end of the engine. Theblades of the fan is placed with such an angle that it sucks ahuge amount of air into the engine. These blades are mostly madeof titanium for an increased stability and to achieve a lowerweight. In a turbofan engine it splits air into “two pieces” andsends a part into the core of the engine where it is compressedand combusted. The other part bypasses the core and is releasedwith ignited gasses to increase thrust and decrease the noise ofthe engine.

The compressor (Fig 2.2) is the first part of the engine core.It exists of multiple fans attached to a shaft. The motion ofthe fans compresses the air by leading it to progressivelysmaller areas to increase its pressure. The compressed air thatcontains a high energy is then forced into the combustionchamber.

The combustion chamber (Fig 2.3) is where the air is mixed withfuel and ignited. There are multiple nozzles that spray fuel on the air and spark generators that ignite this mixture of fuel and air. Oxygen of the compressed air burns with fuel and creates expanding gasses with very high temperature up to 2700°C. Therefore the I nside of the combustion chamber is mostly made of ceramic materials to prevent the heat to damage the engine.

The turbine (Fig 2.4) is where the high-energy gasses reach after they leave the combustion chamber. The gasses bring the blades of the turbine into rotation. The turbine is attached to the same shaft as the fan and the compressor fans. The rotation of the turbine brings also those parts of the engine into motionto secure a constant air flow.

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The nozzle (Fig 2.5) works as the exhaust duct of the engine and is the place that the propulsion thrust is created. The hot gasses mix with the cold air to create an exhaust gas that creates the thrust when exerted from the rear end and this thrust moves the aircraft forward. The nozzle may be preceded with a mixer (Fig 2.6) , which combines the hot gasses and coldair to help decreasing the noise of the engine.

DIFFERENT TYPES OF JET ENGINES

There are different types of jet engines that differ in design but at the end do the same work, creating propulsion thrust; and stem on the same working principles.

Ramjets (Fig 3) are the simplest jet engines in design andthey don’t contain any moving parts. It consists of an air inletcalled the diffuser, a combustion chamber and a nozzle. The initial velocity of the engine “rams”, in other words forces airinto the engine. This air is then channelled to the combustor where it is ignited and the products of combustion are exerted from the nozzle. This simple design is able to fly an aircraft at supersonic speed. Supersonic speed is the speed from 1 to 5 mach (1 mach is the speed of the sound in air). Since an initialvelocity is required for the function of the ramjet it doesn’t develop any static thrust, therefore some form of an assisted take off is required. Furthermore the efficiency of the engine also depends on the forward speed. These facts result in a limited area of utilization. Ramjets are primarily used guided-missile systems and space vehicles.

Turbojets (Fig 4) are a more developed version of the ramjets. Additional to a ramjet it includes a compressor and itsdrive turbine. The compressor raises the air pressure and temperature to improve the efficiency of the engine. The air is not fully combusted inside combustions chamber and contains oxygen. Additional thrust can be gained by adding an after burner before exhaust gases reach the nozzle. The afterburner gets more efficient in low speeds, moreover it can be consideredwasteful in subsonic speeds therefore it is mostly used when a certain velocity is reached. Turbojets are most efficient over avelocity of 2.5 mach. Turbojets are very noisy and they’re

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replaced by turbofans. They are commonly used in cruise missiles.

Turbofans (Fig 5) are the most commonly used jet engines incommercial airplanes. They consist of two parts. An engine coreand the cold air duct. Additional to turbojets they include aturbine before the nozzle and a fan at the front end of theengine. The turbine is attached to this fan and the compressorfans with a shaft. A part of the energy of the exhaust gasses istransferred into rotational motion in the turbine and as aresult the fans of compressor and the additional fan in thefront end, thus a constant air flow in obtained. As the airenters the engine it is separated into two parts. The cold airflowing outside the core decreases the noise of the engine whichcontributes to its feasibility in commercial planes. Turbofansare most efficient at 500-1000km/h. When a continuousafterburner is installed turbofans can reach velocities up to 3mach and above.

Turboshaft and Turboprops (Fig 6) are used in low-speedaircrafts, or when instead of speed the propulsive efficiency isdesired. In turboshafts and turboprops the exhaust gasses playonly a minor role in propulsion. Most of the work is done by theturbines that convert the energy of gasses into rotationalmotion. These turbines are attached to a separate shaft todeliver the motion. This engine can power anything that makes arotational motion. If it powers the propeller of an aircraft itis called a turboprop, if it powers a rotor it is called aturboshaft. Turboprops are used for small aircrafts and for somelogistic purpose aircrafts. They usually provide subsonicvelocities.

JET FUEL

Many variables are involved in the determination of whichfuel to be used in jet engines. Military requirements aredifferent from the commercial requirements because of thedifferent operating conditions. Jet engines can actually operateusing multiple types of fuels but some are more suitable thanothers. Jet fuel is also used for cooling in engines when it isallowed to flow in suitable areas. Jet fuel is a mixture ofhydrocarbons. Most commercially used jet fuels are kerosene-type

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fuels Jet A and Jet A-1; also the naphtha-type fuel Jet B ispreferred for its enhanced cold-weather performance. Militaryfuels are encoded with letter JB and some of them are verysimilar to commercial fuels some of them are highly specializedfuels.

A very important point is that fuel should be kept freefrom water contamination. Since during the flight fuel tanksconfront low temperatures there exists the risk of waterprecipitation. When not prevented the water precipitation canblock fuel pipes. On the other hand removing all the water fromthe fuel is impractical; therefore fuel heaters are installed toprevent water from freezing.

ENVIROMENTAL EFFECTS OF JET ENGINES

The activities of jet engines have two major impacts onenvironment, first one is the contribution to climate change bytheir carbon emission, second one is the noise, and it’s furtherclaimed that they also contaminate air and water.

Since jet engines are internal combustion engines theycontribute to carbon emission by burning fossil-organic fuel andreleasing carbon dioxide and water vapour. Jet engines are morefuel-efficient than automobiles and therefore less polluting butthe air travel grew rapidly over the last two decades, to makeit more concrete for example the greenhouse gas emissions of airtravel has increased by 87% from 1990 to 2006. However they areresponsible for only 2% of the global carbon emission and 13% ofall the transport. To keep the carbon emission caused byaviation there taxes are applied.

The second effect is the noise that the jet enginesproduce. Since jet engines combust and release a huge mass ofair and release it they cause a lot of noise despite all thedevelopments in their design. Both physical and especially themental health of the people who live around airports and pathsof airplanes are affected by this noise. Studies since 1970sshow that aircraft noise is linked to stress, hypertension,

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sleep disturbances, decrease in work-performance and in academicperformance.

RELATION OF THE SUBJECT TO MECHANICAL ENGINEERING CURRICULUM

The design of the engine and the principles of jetpropulsion and their understanding requires an extensive formaleducation in engineering, especially in mechanical engineering.As we have seen in our courses designing a jet engine and makingit work is a subject of fluid mechanics which we will be taughtin the next terms. The design also requires knowledge inmaterial sciences and machine design, since determining theright material and using them to build the right shape has agreat effect on the efficiency and stability of the engine.These subjects are also available in the curriculum ofmechanical engineering.

This subject also offers a challenging research processsince the jet engines are not perfected yet. They can beconsidered very new since they’re approximately 75 years old andthere are still very important developments to be made. Althoughthey are more efficient than the conventional piston poweredmachines this efficiency can still be increased by a new design,new techniques of combustion or using a more proper fuel whichis to be invented. The noise of these machines is also a bigconcern when the environment and the flight comfort areconsidered. The revolutionary invention of turbofans had avaluable contribution to noise-decrease but the noise is stillan issue and development is still required.

ILLUSTRATIONS

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SOURCES

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Books

Charles Edward Chapel – Jet Aircraft Simplified (1954)

Jack V. Casamassa – Jet Aircraft Power Systems (1965)

Jack L. Kerrebrook – Aircraft Engines and Gas Turbines (1992)

William Thomson – Thrust for Flight ( 1969)

R. Douglas Archer, Maido Saarlas – An Introduction to Aerospace Propulsion (1996)

Internet

http://inventors.about.com/library/inventors/bljetengine.htm

http://inventors.about.com/library/inventors/blhowajetengineworks.htm

http://inventors.about.com/library/inventors/blhowajetengineparts.htm

http://inventors.about.com/od/jstartinventions/ss/jet_engine.htm

http://en.wikipedia.org/wiki/Jet_engine

http://en.wikipedia.org/wiki/Jet_fuel