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The turbojet is an airbreathing jet engine, typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet, a compressor, a combustion chamber, and a turbine (that drives the compressor). The compressed air from the compressor is heated by burning fuel in the combustion chamber and then allowed to expand through the turbine. The turbine exhaust is then expanded in the propelling nozzle where it is accelerated to high speed to provide thrust. Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently into practical engines during the late 1930s. While the turbojet was the first form of gas turbine powerplant for aviation, it has largely been replaced in use by other developments of the original concept. In operation, turbojets typically generate thrust by accelerating a relatively small amount of air to very high supersonic speeds, whereas turbofans accelerate a larger amount of air to lower transonic speeds. Turbojets have been replaced in slower aircraft by turboprops because they have better specific fuel consumption. At medium to high speeds, where the propeller is no longer efficient, turboprops have been replaced by turbofans. At these transonic speeds, the turbofan is quieter and has better range-specific fuel consumption than the turbojet. Turbojets can be highly efficient for supersonic aircraft. Turbojets have poor efficiency at low vehicle speeds, which limits their usefulness in vehicles other than aircraft. Turbojet engines have been used in isolated cases to power vehicles other than aircraft, typically for attempts on land speed records. Where vehicles are "turbine-powered", this is more commonly by use of a turboshaft engine, a development of the gas turbine engine where an additional turbine is used to drive a rotating output shaft. These are common in helicopters and hovercraft. Turbojets were used on Concorde and the longer-range versions of the TU-144 which were required to spend a long period travelling supersonically. Turbojets are still common in medium range cruise missiles, due to their high exhaust speed, small frontal area, and relative simplicity. They are also still used on some supersonic fighters such as the MiG-25, but most spend little time travelling supersonically, and so employ turbofans and use afterburners to raise exhaust speed for supersonic sprints.

History

Early designs

Early German turbojets had severe limitations on the amount of running they could do due to the lack of suitable high temperature materials for the turbines. British engines such as the Rolls-Royce Welland used better materials giving improved durability. The Welland was type-certified for 80 hours initially, later extended to 150 hours between overhauls, as a result of an extended 500-hour run being achieved in tests. Despite their high maintenance, some of the early jet fighters are still operational with their original engines. General Electric in the United States was in a good position to enter the jet engine business due to its experience with the high-temperature materials used in their turbosuperchargers during World War II. Water injection was a common method used to increase thrust, usually during takeoff, in early turbojets that were thrust-limited by their allowable turbine entry temperature. The water increased thrust at the temperature limit, but prevented complete combustion, often leaving a very visible smoke trail. Allowable turbine entry temperatures have increased steadily over time both with the introduction of superior alloys and coatings, and with the introduction and progressive effectiveness of blade cooling designs. On early engines, the turbine temperature limit had to be monitored, and avoided, by the pilot, typically during starting and at maximum thrust settings. Automatic temperature limiting was introduced to reduce pilot workload and reduce the likelihood of turbine damage due to over-temperature.

Design

thumb|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

Air intake

An intake, or tube, is needed in front of the compressor to help direct the incoming air smoothly into the moving compressor blades. Older engines had stationary vanes in front of the moving blades. These vanes also helped to direct the air onto the blades. The air flowing into a turbojet engine is always subsonic, regardless of the speed of the aircraft itself. The intake has to supply air to the engine with an acceptably small variation in pressure (known as distortion) and having lost as little energy as possible on the way (known as pressure recovery). The ram pressure rise in the intake is the inlet's contribution to the propulsion system's overall pressure ratio and thermal efficiency. The intake gains prominence at high speeds when it generates more compression than the compressor stage. Well-known examples are the Concorde and Lockheed SR-71 Blackbird propulsion systems where the intake and engine contributions to the total compression were 63%/8% at Mach 2 and 54%/17% at Mach 3+. Intakes have ranged from "zero-length" on the Pratt & Whitney TF33 turbofan installation in the Lockheed C-141 Starlifter, to the twin, 65 feet-long, intakes on the North American XB-70 Valkyrie, each feeding three engines with an intake airflow of about 800 lb/sec.

Compressor

The compressor is driven by the turbine. It rotates at 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. The smaller the compressor, the faster it turns. At the large end of the range, the GE-90-115 fan rotates at about 2,500 RPM, while a small helicopter engine compressor rotates around 50,000 RPM. Turbojets supply bleed air from the compressor to the aircraft for the environmental control system, anti-icing, and fuel tank pressurization, for example. The engine itself needs air at various pressures and flow rates to keep it running. This air comes from the compressor, and without it, the turbines would overheat, the lubricating oil would leak from the bearing cavities, the rotor thrust bearings would skid or be overloaded, and ice would form on the nose cone. The air from the compressor, called secondary air, is used for turbine cooling, bearing cavity sealing, anti-icing, and ensuring that the rotor axial load on its thrust bearing will not wear it out prematurely. Supplying bleed air to the aircraft decreases the efficiency of the engine because it has been compressed, but then does not contribute to producing thrust. Bleed air for aircraft services is no longer needed on the turbofan-powered Boeing 787. Compressor types used in turbojets were typically axial or centrifugal. Early turbojet compressors had low pressure ratios up to about 5:1. Aerodynamic improvements including splitting the compressor into two separately rotating parts, incorporating variable blade angles for entry guide vanes and stators, and bleeding air from the compressor enabled later turbojets to have overall pressure ratios of 15:1 or more. For comparison, modern civil turbofan engines have overall pressure ratios of 44:1 or more. After leaving the compressor, the 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. In a turbojet, the air and fuel mixture burn in the combustor and pass through to the turbine in a continuous flowing process with no pressure build-up. Instead, a small pressure loss occurs in the combustor. The fuel-air mixture can only burn in slow-moving air, so an area of reverse flow is maintained by the fuel nozzles for the approximately stoichiometric burning in the primary zone. Further compressed air is introduced which completes the combustion process and reduces the temperature of the combustion products to a level which the turbine can accept. Less than 25% of the air is typically used for combustion, as an overall lean mixture is required to keep within the turbine temperature limits.

Turbine

Hot gases leaving the combustor expand through the turbine. Typical materials for turbines include inconel and Nimonic. The hottest turbine vanes and blades in an engine have internal cooling passages. Air from the compressor is passed through these to keep the metal temperature within limits. The remaining stages do not need cooling. 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. Energy is transferred into the shaft through momentum exchange in the opposite way to energy transfer in the compressor. The power developed by the turbine drives the compressor and accessories, like fuel, oil, and hydraulic pumps that are driven by the accessory gearbox.

Nozzle

After the turbine, the gases expand through the exhaust nozzle producing a high velocity jet. In a convergent nozzle, the ducting narrows progressively to a throat. The nozzle pressure ratio on a turbojet is high enough at higher thrust settings to cause the nozzle to choke. 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 divergent section. Additional thrust is generated by the higher resulting exhaust velocity.

Thrust augmentation

Thrust was most commonly increased in turbojets with water/methanol injection or afterburning. Some engines used both at the same time. Liquid injection was tested on the Power Jets W.1 in 1941 initially using ammonia before changing to water and then water-methanol. A system to trial the technique in the Gloster E.28/39 was devised but never fitted.

Afterburner

An afterburner or "reheat jetpipe" is a combustion chamber added to reheat the turbine exhaust gases. The fuel consumption is very high, typically four times that of the main engine. Afterburners are used almost exclusively on supersonic aircraft, most being military aircraft. Two supersonic airliners, Concorde and the Tu-144, also used afterburners as does Scaled Composites White Knight, a carrier aircraft for the experimental SpaceShipOne suborbital spacecraft. Reheat was flight-trialled in 1944 on the W.2/700 engines in a Gloster Meteor I.

Net thrust

The net thrust $F_N\;$ of a turbojet is given by: $F_N =\left( \dot_ + \dot_f\right) V_ - \dot_ V$ where: If the speed of the jet is equal to sonic velocity the nozzle is said to be "choked". If the nozzle is choked, the pressure at the nozzle exit plane is greater than atmospheric pressure, and extra terms must be added to the above equation to account for the pressure thrust.Cumpsty, Jet Propulsion, Section 6.3 The rate of flow of fuel entering the engine is very small compared with the rate of flow of air. If the contribution of fuel to the nozzle gross thrust is ignored, the net thrust is: $F_N = \dot_ \left(V_ - V\right)$ The speed of the jet $V_j\;$ must exceed the true airspeed of the aircraft $V\;$if there is to be a net forward thrust on the airframe. The speed $V_j\;$ can be calculated thermodynamically based on adiabatic expansion.

Cycle improvements

The operation of a turbojet is modelled approximately by the Brayton cycle. The efficiency of a gas turbine is increased by raising the overall pressure ratio, requiring higher-temperature compressor materials, and raising the turbine entry temperature, requiring better turbine materials and/or improved vane/blade cooling. It is also increased by reducing the losses as the flow progresses from the intake to the propelling nozzle. These losses are quantified by compressor and turbine efficiencies and ducting pressure losses. When used in a turbojet application, where the output from the gas turbine is used in a propelling nozzle, raising the turbine temperature increases the jet velocity. At normal subsonic speeds this reduces the propulsive efficiency, giving an overall loss, as reflected by the higher fuel consumption, or SFC. However, for supersonic aircraft this can be beneficial, and is part of the reason why the Concorde employed turbojets. Turbojet systems are complex systems therefore to secure optimal function of such system, there is a call for the newer models being developed to advance its control systems to implement the newest knowledge from the areas of automation, so increase its safety and effectiveness.SAMI 2010 • 8th IEEE International Symposium on Applied Machine Intelligence and Informatics • 28–30 January 2010 • Herl'any, Slovakia (Advanced methods of turbojet engines' control)(R. Andoga*,*** , L. Főző*,** , L. Madarász* and J. Judičák**** * Technical University of Košice, Department of Cybernetics and Artificial Intelligence, Košice, Slovakia ** Technical University of Košice, Department of Environmental Studies and Information Engineering, Košice,))

* Air-start system * Exoskeletal engine * Jet car * Turbine engine failure * Turbojet development at the RAE * Variable cycle engine

References

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