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Encyclopedia > Turbojet

Turbojets are the oldest kind of general purpose jet engines. Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently during the late 1930s, although credit for the first turbojet is given to Whittle. Image File history File links Question_book-3. ... For the transportation company in southern China, see TurboJET. Turbojets are the oldest kind of general purpose jet engines. ... A Pratt and Whitney turbofan engine for the F-15 Eagle is tested at Robins Air Force Base, Georgia, USA. The tunnel behind the engine muffles noise and allows exhaust to escape. ... Frank Whittle speaking to employees of the Flight Propulsion Research Laboratory (Now known as the NASA Glenn Research Center), USA, in 1946 Air Commodore Sir Frank Whittle, OM, KBE, FRS, Hon FRAeS (1 June 1907–9 August 1996) was an English Royal Air Force officer and is seen as the... Hans Joachim Pabst von Ohain (December 14, 1911 – March 13, 1998) was one of the inventors of jet propulsion. ...


Turbojets consist of an air inlet, an air compressor, a combustion chamber, a gas turbine (that drives the air compressor) and a nozzle. The air is compressed into the chamber, heated and expanded by the fuel combustion and then allowed to expand out through the turbine into the nozzle where it is accelerated to high speed to provide propulsion.


Turbojets are quite inefficient (if flown below about Mach 2) and very noisy. Most modern aircraft use turbofans instead for economic reasons. Schematic diagram of high-bypass turbofan engine CFM56-3 turbofan, lower half, side view. ...

Contents

History

Heinkel He 178, the world's first aircraft to fly purely on turbojet power
Heinkel He 178, the world's first aircraft to fly purely on turbojet power

On 27 August 1939 the Heinkel He 178 became the world's first aircraft to fly under turbojet power, thus becoming the first practical jet plane. The first two operational turbojet aircraft, the Messerschmitt Me 262 and the Gloster Meteor entered service towards the end of World War II in 1944. The Heinkel He 178 was the worlds first aircraft to fly under turbojet power, and the first practical jet plane. ... is the 239th day of the year (240th in leap years) in the Gregorian calendar. ... Year 1939 (MCMXXXIX) was a common year starting on Sunday (link will display the full calendar) of the Gregorian calendar. ... The Heinkel He 178 was the worlds first aircraft to fly under turbojet power, and the first practical jet plane. ... The Messerschmitt Me 262 Schwalbe (German: Swallow) was the worlds first operational turbojet fighter aircraft. ... The Gloster Meteor was the first British jet fighter and the Allies first operational jet. ... Combatants Allied powers: China France Great Britain Soviet Union United States and others Axis powers: Germany Italy Japan and others Commanders Chiang Kai-shek Charles de Gaulle Winston Churchill Joseph Stalin Franklin Roosevelt Adolf Hitler Benito Mussolini Hideki Tōjō Casualties Military dead: 17,000,000 Civilian dead: 33,000...


A turbojet engine is used primarily to propel aircraft. Air is drawn into the rotating compressor via the intake and is compressed to a higher pressure before entering the combustion chamber. Fuel is mixed with the compressed air and ignited by a 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 the turbine exit gas temperature and pressure, 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 momentum of the exhaust stream exceeds the momentum of the intake stream, the impulse is positive, thus, there is a net forward thrust upon the airframe. Flying machine redirects here. ... For other uses, see Fuel (disambiguation). ... A flame holder is a component of a jet engine designed to help maintain continual combustion. ... This article is about the chemical reaction combustion. ... A Siemens steam turbine with the case opened. ... Thrust is a reaction force described quantitatively by Newtons Second and Third Laws. ...


Early generation jet engines were pure turbojets with either an axial or centrifugal compressor. They were used because they were able to achieve very high altitudes and speeds, much higher than propeller engines, because of a better compression ratio. However they were not very fuel efficient. Modern jet engines are mainly turbofans, where a proportion of the air entering the intake bypasses the combustor; this proportion depends on the engine's bypass ratio. This makes turbofans much more efficient than turbojets. This article does not cite any references or sources. ... A centrifugal compressor, also called a radial blower, squirrel cage, or squirrel wheel compressor, consists of an axle to which is mounted a cylindrical assembly of compressor blades. ... For other uses, see Propeller (disambiguation). ... Schematic diagram of high-bypass turbofan engine CFM56-3 turbofan, lower half, side view. ... In aeronautical engineering, and jet engine design in particular, bypass ratio is a common measurement that compares the amount of air deliberately blown past the engine to that moving through the core. ...


Although ramjet engines are simpler in design as they have virtually no moving parts, they are incapable of operating at low flight speeds. For other uses, see Ramjet (disambiguation). ...


Air intake

An animation of an axial compressor. The darker colored blades are the stators.
An animation of an axial compressor. The darker colored blades are the stators.
Schematic diagram showing the operation of a centrifugal flow turbojet engine. The compressor is driven via the turbine stage and throws the air outwards, requiring it to be redirected parallel to the axis of thrust.
Schematic diagram showing the operation of a centrifugal flow turbojet engine. The compressor is driven via the turbine stage and throws the air outwards, requiring it to be redirected parallel to the axis of thrust.
Schematic diagram showing the operation of an axial flow turbojet engine. Here, the compressor is again driven by the turbine, but the air flow remains parallel to the axis of thrust.
Schematic diagram showing the operation of an axial flow turbojet engine. Here, the compressor is again driven by the turbine, but the air flow remains parallel to the axis of thrust.

Preceding the compressor is the air intake (or inlet). It is designed to be as efficient as possible at recovering the ram pressure of the air streamtube approaching the intake. The air leaving the intake then enters the compressor. The stators (stationary blades) guide the airflow of the compressed gases. Image File history File links Axial_compressor. ... Image File history File links Axial_compressor. ... Image File history File links Turbojet_operation-_centrifugal_flow. ... Image File history File links Turbojet_operation-_centrifugal_flow. ... Image File history File links Turbojet_operation-_axial_flow. ... Image File history File links Turbojet_operation-_axial_flow. ...


Compressor

The compressor is driven by the turbine. The compressor rotates at very high speed, adding energy to the airflow and at the same time squeezing (compressing) it into a smaller space. Compressing the air increases its pressure and temperature. This article is about pressure in the physical sciences. ... For other uses, see Temperature (disambiguation). ...


In most turbojet-powered aircraft, bleed air is extracted from the compressor section at various stages to perform a variety of jobs including air conditioning/pressurization, engine inlet anti-icing and turbine cooling. Bleeding air off decreases the overall efficiency of the engine, but the usefulness of the compressed air outweighs the loss in efficiency. Bleed air in jet turbines is compressed air taken from within the engine, after the compressor stage(s) and before the fuel is injected in the burners. ...


Several types of compressor are used in turbojets and gas turbines in general: axial, centrifugal, axial-centrifugal, double-centrifugal, etc. The worlds first commercial, oil-free gas turbine is manufactured by Capstone. ...


Early turbojet compressors had overall pressure ratios as low as 5:1 (as do a lot of simple auxiliary power units and small propulsion turbojets today). Aerodynamic improvements, plus splitting the compression system into two separate units and/or incorporating variable compressor geometry, enabled later turbojets to have overall pressure ratios of 15:1 or more. For comparison, modern civil turbofan engines have overall pressure ratios as high as 44:1 or more. The APU exhaust at the tail end of an Airbus A380 An auxiliary power unit (APU) is a device on a vehicle whose purpose is to provide energy for functions other than propulsion. ... Schematic diagram of high-bypass turbofan engine CFM56-3 turbofan, lower half, side view. ...


After leaving the compressor section, the compressed air enters the combustion chamber.


Combustion chamber

The burning process in the combustor is significantly different from that in a piston engine. In a piston engine the burning gases are confined to a small volume and, as the fuel burns, the pressure increases dramatically. In a turbojet the air and fuel mixture passes unconfined through the combustion chamber. As the mixture burns its temperature increases dramatically, but the pressure actually decreases a few percent. Components of a typical, four stroke cycle, DOHC piston engine. ... A combustion chamber is part of an engine in which fuel is burned. ...


The fuel-air mixture must be brought almost to a stop so that a stable flame can be maintained.
This occurs just after the start of the combustion chamber. The aft part of this flame front is allowed to progress rearward. This ensures that all of the fuel is burned, as the flame becomes hotter when it leans out, and because of the shape of the combustion chamber the flow is accelerated rearwards. Some pressure drop is required, as it is the reason why the expanding gases travel out the rear of the engine rather than out the front. Less than 25% of the air is involved in combustion, in some engines as little as 12%, the rest acting as a reservoir to absorb the heating effects of the burning fuel. AFT is a three-letter acronym that may refer to: American Farmland Trust Adiabatic flame temperature American Federation of Teachers Authenticated firewall traversal, in version 5 of SOCKS, an Internet protocol Americans For Fair Taxation Almost Free Text Ali Farka Toure (Freewood) Acres Fun Time Category: ...


Another difference between piston engines and jet engines is that the peak flame temperature in a piston engine is experienced only momentarily in a small portion of the full cycle. The combustor in a jet engine is exposed to the peak flame temperature continuously and operates at a pressure high enough that a stoichiometric fuel-air ratio would melt the can and everything downstream. Instead, jet engines run a very lean mixture, so lean that it would not normally support combustion. A central core of the flow (primary airflow) is mixed with enough fuel to burn readily. The cans are carefully shaped to maintain a layer of fresh unburned air between the metal surfaces and the central core. This unburned air (secondary airflow) mixes into the burned gases to bring the temperature down to something a turbine can tolerate. For other uses, see Temperature (disambiguation). ... In chemistry, stoichiometry is the study of the combination of elements in chemical reactions. ...


Turbine

Hot gases leaving the combustor are allowed to expand through the turbine. In the first stage the turbine is largely an impulse turbine (similar to a pelton wheel) and rotates because of the impact of the hot gas stream. Later stages are convergent ducts that accelerate the gas rearward and gain energy from that process. Pressure drops, and energy is transferred into the shaft. The turbine's rotational energy is used primarily to drive the compressor. Some shaft power is extracted to drive accessories, like fuel, oil, and hydraulic pumps. Because of its significantly higher entry temperature, the turbine pressure ratio is much lower than that of the compressor. In a turbojet almost two thirds of all the power generated by burning fuel is used by the compressor to compress the air for the engine. Pelton wheel from Walchensee, Germany hydro power station Figure from Peltons original patent (October 1880) Plan view of a Pelton turbine installation (courtesy Voith Siemens Hydro Power Generation). ... This gyroscope remains upright while spinning due to its angular momentum. ...


Nozzle

After the turbine, the gases are allowed to expand through the exhaust nozzle to atmospheric pressure, producing a high velocity jet in the exhaust plume. In a convergent nozzle, the ducting narrows progressively to a throat. The nozzle pressure ratio on a turbojet is usually high enough for the expanding gases to reach Mach 1.0 and choke the throat. Normally, the flow will go supersonic in the exhaust plume outside the engine.


If, however, a convergent-divergent "de Laval" nozzle is fitted, the divergent (increasing flow area) section allows the gases to reach supersonic velocity within the nozzle itself. This is slightly more efficient on thrust than using a convergent nozzle. There is, however, the added weight and complexity, since the con-di nozzle must be fully variable to cope basically with engine throttling. Diagram of a de Laval nozzle, showing approximate flow velocity increasing from green to red in the direction of flow A de Laval nozzle (or convergent-divergent nozzle, CD nozzle or con-di nozzle) is a tube that is pinched in the middle, making an hourglass-shape. ...


Net thrust

An equation for calculating the approximate net thrust of a turbojet is given by:

F_n = dot{m} (V_{jfe} - V_a)

where:

dot{m} is the intake mass flow rate
Vjfe is the fully-expanded jet velocity (in the exhaust plume)

dot{m} V_{jfe} represents the nozzle gross thrust


dot{m} V_a represents the ram drag of the intake.


Obviously, the jet velocity must exceed that of the flight velocity if there is to be a net forward thrust on the airframe.


Thrust to power ratio

A simple turbojet engine will produce thrust of approximately: 2.5 pounds force per horsepower (15 mN/W).


Afterburner

An afterburner or "reheat jetpipe" is a device added to the rear of the jet engine. It provides a means of spraying fuel directly into the hot exhaust, where it ignites and boosts available thrust significantly; a drawback is its very high fuel consumption rate. Afterburners are used mostly on military aircraft, but the two supersonic civilian transports, the Concorde and the TU-144, also utilized afterburners, as does Scaled Composites White Knight, a carrier aircraft for the experimental SpaceShipOne suborbital spacecraft. For other uses of afterburner, see Afterburner (disambiguation). ... For other uses, see Concorde (disambiguation). ... The Tupolev Tu-144 (NATO reporting name: Charger) was a supersonic airliner constructed under management of the Soviet Tupolev design bureau headed by Alexei Tupolev (1925-2001). ... SpaceShipOne and White Knight during a captive carry test flight The Scaled Composites Model 318 White Knight is a jet-powered carrier aircraft used to launch the SpaceShipOne experimental spacecraft. ... Scaled Composites SpaceShipOne SpaceShipOnes patch The Scaled Composites Model 316 SpaceShipOne is an experimental air-launched suborbital spaceplane that uses a hybrid rocket motor. ... A sub-orbital spaceflight (or sub-orbital flight) is a spaceflight that does not involve putting a vehicle into orbit. ...


Thrust reverser

A thrust reverser is, essentially, a pair of clamshell doors mounted at the rear of the engine which, when deployed, divert thrust normal to the jet engine flow to help slow an aircraft upon landing. They are often used in conjunction with spoilers. The accidental deployment of a thrust reverser during flight is a dangerous event that can lead to loss of control and destruction of the aircraft. Thrust reversers are more convenient than drogue parachutes. KLM Fokker 70 with reverse thrust applied. ... This KLM cityhopper Fokker 70 still has its spoilers deployed (the cream-coloured panels projecting above the top surface of the wing) after landing at Bristol International Airport, England. ... A B-52H Stratofortress from the 5 Bomb Wing deploying its drag chute for landing Apollo Command Module splashdown A drogue parachute is a type of parachute designed to be deployed from a rapidly moving object. ...


Cycle improvements

Thermodynamics of a Jet Engine is modelled approximately by a Brayton Cycle. The Brayton cycle is a constant-pressure cycle named after George Brayton (1830–1892), the American engineer who developed it. ...


Increasing the overall pressure ratio of the compression system raises the combustor entry temperature. Therefore, at a fixed fuel flow and airflow, there is an increase in turbine inlet temperature. Although the higher temperature rise across the compression system, implies a larger temperature drop over the turbine system, the nozzle temperature is unaffected, because the same amount of heat is being added to the system. There is, however, a rise in nozzle pressure, because overall pressure ratio increases faster than the turbine expansion ratio. Consequently, net thrust increases, while specific fuel consumption (fuel flow/net thrust) decreases.


Thus turbojets can be made more fuel efficient by raising overall pressure ratio and turbine inlet temperature in union. However, better turbine materials and/or improved vane/blade cooling are required to cope with increases in both turbine inlet temperature and compressor delivery temperature. Increasing the latter requires better compressor materials.


By Increasing the useful work to system , by minimizing the heat losses by conduction etc and minimizing the inlet temperature ratio up to a certain level will increase the themal efficiency of the turbo jet engine.


Early designs

Early German engines had serious problems controlling the turbine inlet temperature. A lack of suitable alloys due to war shortages meant the turbine rotor and stator blades would sometimes disintegrate on first operation and never lasted long. Their early engines averaged 10-25 hours of operation before failing—often with chunks of metal flying out the back of the engine when the turbine overheated. British engines tended to fare better, running for 150 hours between overhauls. A few of the original fighters still exist with their original engines, but many have been re-engined with more modern engines with greater fuel efficiency and a longer TBO (such as the reproduction Me-262 powered by General Electric J85s). One important measure of an aircraft engines overall economics is how often it has to be overhauled, the so-called time between overhaul, typically seen as TBO or TBOH. The time between overhauls is generally a function of the complexity of the engine. ... The Messerschmitt Me 262 Schwalbe (Swallow) was the first operational jet powered aircraft. ... The General Electric J85 was a small single-shaft turbojet engine, capable of generating up to 4000 lbf (18 kN) of dry thrust. ...


The United States had the best materials because of their reliance on turbo/supercharging in high altitude bombers of World War II. For a time some US jet engines included the ability to inject water into the engine to cool the compressed flow before combustion, usually during takeoff. The water would tend to prevent complete combustion and as a result the engine ran cooler again, but the planes would take off leaving a huge plume of smoke. Turbo redirects here. ... Combatants Allied powers: China France Great Britain Soviet Union United States and others Axis powers: Germany Italy Japan and others Commanders Chiang Kai-shek Charles de Gaulle Winston Churchill Joseph Stalin Franklin Roosevelt Adolf Hitler Benito Mussolini Hideki Tōjō Casualties Military dead: 17,000,000 Civilian dead: 33,000...


Today these problems are much better handled, but temperature still limits turbojet airspeeds in supersonic flight. At the very highest speeds, the compression of the intake air raises the temperatures throughout the engine to the point that the turbine blades would melt, forcing a reduction in fuel flow to lower temperatures, but giving a reduced thrust and thus limiting the top speed. Ramjets and scramjets do not have turbine blades, therefore they are able to fly faster. For other uses, see Ramjet (disambiguation). ... X-43A with scramjet attached to the underside A scramjet (supersonic combustion ramjet) is a variation of a ramjet with the key difference being that the flow in the combustor is supersonic. ...


At lower speeds, better materials have increased the critical temperature, and automatic fuel management controls have made it nearly impossible to overheat the engine.


Sources

  • Constructing A Turbocharger Turbojet Engine. Edwin H. Springer, Turbojet Technologies, 2001.

See also

Wikimedia Commons has media related to:
Turbojet engines
Image File history File links Commons-logo. ... The Brayton cycle is a constant-pressure cycle named after George Brayton (1830–1892), the American engineer who developed it. ... In a Variable Cycle Engine the engine cycle is deliberately modified from that which would occur under normal circumstances. ... Aviation encompasses all the activities relating to airborne devices created by human ingenuity, generally known as aircraft. ... This is a timeline of aviation history. ... This list of aircraft is sorted alphabetically, beginning with the name of the manufacturer (or, in certain cases, designer). ... This is a list of aircraft manufacturers (in alphabetic order). ... List of aircraft engines: // Two- and four-stroke rotary, radial, inline. ... This is a list of aircraft engine manufacturers both past and present. ... This is a list of airlines in operation (by continents and country). ... This is a list of air forces, sorted alphabetically by country, followed by a list of former countries air forces. ... This is an incomplete list of aircraft weapons, past and present. ... Below is a list of (links to pages on) missiles, sorted alphabetically by name. ... A Boeing 720 being flown under remote control as part of NASAs Controlled Impact Demonstration The following is a list of Unmanned aerial vehicles developed and operated by various countries around the world. ... This is a list of experimental aircraft. ... The SR-71 Blackbird is the current record holder. ... Flight distance records without refueling. ... It has been suggested that this article or section be merged with List of altitude records reached by different aircraft types. ... The flight endurance record is the amount of time spent in the air. ... Aircraft with a production run greater than 5,000 aircraft. ...

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