Turbojets are the oldest kind of general purpose jet engines. specific --->A jet engine is a Reaction engine that discharges a fast moving jet of Fluid to 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. Air Commodore Sir Frank Whittle, OM, KBE, FRS, Hon FRAeS (1 June 1907 &ndash 9 August 1996 was an English Royal The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom, the UK or Britain,is a Sovereign state located Hans Joachim Pabst von Ohain ( December 14 1911 March 13 1998) was one of the inventors of jet propulsion. Germany, officially the Federal Republic of Germany ( ˈbʊndəsʁepuˌbliːk ˈdɔʏtʃlant is a Country in Central Europe.
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. A turbofan is a type of Jet engine, similar to a Turbojet. It essentially consists of a Ducted fan with a smaller diameter turbojet engine Turbojets are still very common in medium range cruise missiles, due to their high speed, low frontal area and relative simplicity. A cruise missile is a guided Missile that carries an explosive payload and uses a lifting wing and a propulsion system usually a Jet engine, to allow
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History
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 Heinkel He 178 was the world's first Aircraft to fly under Turbojet power and the first practical jet plane the pioneering example of this type Events 479 BC - Greco-Persian Wars: Persian forces led by Mardonius are routed by Pausanias, the Spartan 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 world's first Aircraft to fly under Turbojet power and the first practical jet plane the pioneering example of this type 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. WikipediaWikiProject Aircraft. Please see WikipediaWikiProject Aircraft/page content for recommended layout WikipediaWikiProject Aircraft. Please see WikipediaWikiProject Aircraft/page content for recommended layout World War II, or the Second World War, (often abbreviated WWII) was a global military conflict which involved a majority of the world's nations, including
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. Fuel is any material that is burned or altered in order to obtain energy A flame holder is a component of a Jet engine designed to help maintain continual Combustion. This combustion process significantly raises the temperature of the gas. Combustion or burning is a complex sequence of Exothermic chemical reactions between a Fuel and an Oxidant accompanied by the production of Hot combustion products leaving the combustor expand through the turbine where power is extracted to drive the compressor. A turbine is a rotary Engine that extracts Energy from a Fluid flow 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. Thrust is a reaction force described quantitatively by Newton 's Second and Third Laws.
Early generation jet engines were pure turbojets with either an axial or centrifugal compressor. Axial compressors are rotating aerofoil based compressors in which the working fluid principally flows parallel to the axis of rotation Centrifugal compressor, (sometimes referred to as radial compressors) are a special class of radial-flow work-absorbing Turbomachinery that includes pumps 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. A propeller is essentially a type of fan which transmits power by converting Rotational motion into Thrust for propulsion of a vehicle such as an 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. A turbofan is a type of Jet engine, similar to a Turbojet. It essentially consists of a Ducted fan with a smaller diameter turbojet engine Description Jet engines are generally able to create considerably more energy than they can use in moving air through the engine core This makes turbofans much more efficient than turbojets.
Although ramjet engines are simpler in design as they have virtually no moving parts, they are incapable of operating at low flight speeds. A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of Jet engine that contains no major Moving parts.
Air intake
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.
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. In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός Compressing the air increases its pressure and temperature. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature
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. Bleed air in gas turbine engines is compressed air taken from within the engine after the compressor stage(s and before the fuel is injected in the burners Bleeding air off decreases the overall efficiency of the engine, but the usefulness of the compressed air outweighs the loss in efficiency.
Several types of compressor are used in turbojets and gas turbines in general: axial, centrifugal, axial-centrifugal, double-centrifugal, etc. A gas turbine, also called a combustion turbine, is a rotary Engine that extracts energy from a flow of Combustion gas
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). An auxiliary power unit ( APU) is a device on a vehicle whose purpose is to provide energy for functions other than propulsion 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. A turbofan is a type of Jet engine, similar to a Turbojet. It essentially consists of a Ducted fan with a smaller diameter turbojet engine
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. A reciprocating engine, also often known as a piston engine, is a Heat engine that uses one or more reciprocating Pistons to convert 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. A combustion chamber is the part of an Engine in which Fuel is burned As the mixture burns its temperature increases dramatically, but the pressure actually decreases a few percent.
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. For the acronym see AFT. Aft, in naval Terminology, is an Adjective or Adverb meaning 'towards 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.
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. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Stoichiometry (sometimes called reaction stoichiometry to distinguish it from composition stoichiometry is the Calculation of Quantitative (measurable 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.
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. The Pelton wheel is among the most efficient types of Water turbines. 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. In Physics, the angular momentum of a particle about an origin is a vector quantity equal to the mass of the particle multiplied by the Cross product of the position 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.
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. 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 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.
Net thrust
An equation for calculating the approximate net thrust of a turbojet is given by:
where:
is the intake mass flow rate- Vjfe is the fully-expanded jet velocity (in the exhaust plume)
represents the nozzle gross thrust
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. The AfterBurner is a lighting solution for the Game Boy Advance system that was created by Triton-Labs 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. WikipediaWikiProject Aircraft. Please see WikipediaWikiProject Aircraft/page content for recommended layout WikipediaWikiProject Aircraft. Please see WikipediaWikiProject Aircraft/page content for recommended layout The Scaled Composites Model 318 White Knight is a jet-powered carrier Aircraft used to launch the SpaceShipOne experimental Spacecraft. SpaceShipOne is a Spaceplane that completed the first privately funded Human spaceflight on June 21, 2004. A sub-orbital spaceflight (or sub-orbital flight is a Spaceflight in which the Spacecraft reaches space, but its Trajectory intersects
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. Thrust reversal, also called reverse thrust, is the temporary diversion of an Aircraft engine 's exhaust or changing of propeller pitch so that the thrust produced They are often used in conjunction with spoilers. In Aeronautics a spoiler (sometimes called a lift dumper) is a device intended to reduce lift in an aircraft 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. A drogue parachute is a 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 that describes the workings of the Gas turbine engine basis of the Jet engine and others
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 engine 's overall economics is how often it has to be overhauled the so-called time between overhaul, typically seen as TBO or WikipediaWikiProject Aircraft. Please see WikipediaWikiProject Aircraft/page content for recommended layout J85 redirects here for the British Halcyon class minesweeper see HMS Seagull (J85, for the Abscess of lung and mediastinum ICD-10 code see ICD-10 Chapter X Diseases of the
The United States had the best materials because of their reliance on turbo/supercharging in high altitude bombers of World War II. The United States of America —commonly referred to as the A turbocharger, or turbo, is an air Compressor used for forced-induction of an Internal combustion engine. World War II, or the Second World War, (often abbreviated WWII) was a global military conflict which involved a majority of the world's nations, including 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.
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. A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of Jet engine that contains no major Moving parts. A scramjet ( s upersonic c ombustion ramjet) is a variation of a Ramjet with the distinction being that some or all of the combustion process
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
The ' Brayton cycle' is a constant-pressure cycle that describes the workings of the Gas turbine engine basis of the Jet engine and others In a Variable Cycle Engine (VCE the engine cycle is deliberately modified from that which would occur under normal circumstancesDictionary
turbojet
-noun
- A jet engine that develops thrust using a turbine compressor which is propelled by high speed exhaust gases expelled as a jet.
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