Turbofan engine
A turbofan engine , also bypass engine , dual flow jet engine factory , twin-flow turbine jet engine (CTL) or fan engine - English. Turbofan - called, is a jet engine in which an outer air flow envelops the inner "core flow", which takes part in the actual thermodynamic cycle (compressing, heating, expanding and supplying energy). In modern engines, the bypass flow usually supplies the majority of the thrust , often over 80%, depending on the bypass ratio . The core engine is therefore sometimes seen primarily as a drive for fans and thus bypass flow. That is why it is sometimes referred to as the “hot gas generator” for the fan-driven turbine. The bypass flow causes a reduction in the jet speed (ejected air / exhaust gas jet) with the consequence of lower fuel consumption and lower sound emissions compared to a single - flow jet engine with the same thrust.
Almost all jet-powered civil aircraft are nowadays equipped with turbofan engines because of the increased economic efficiency and the noise-reducing effect of the bypass flow. In combat aircraft, however , the bypass flow component in favor of a maximum top speed is low to very low, since this advantage is lost at high speeds (greater than Mach 0.7-0.9).
functionality
Air is sucked in at the inlet of the turbofan engine. The first, large airfoil wheel ( fan , see below) accelerates this air in the outside area ( bypass flow). The bypass flow is bypassed by the rest of the engine and ejected from the rear - its increased speed when exiting provides a large part of the thrust. In the inner area ( core flow ) of the first large airfoil wheel, the air is more compressed and slowed down slightly (relative to the engine). The core air flow is directed into the "actual engine", a gas turbine , where the thermodynamic cycle generates the drive energy. For this purpose, the fan is first followed by an (axial) compressor , which further compresses the core flow. The combustion chamber follows after the compressor . Here, fuel is injected into the compressed air and burned, which supplies the energy for the drive: the temperature is increased significantly, the theoretical increases in volume and pressure lead to a strong acceleration of the core flow (so strong that the pressure over the combustion chamber even decreases slightly; the combustion chamber does not become significantly larger towards the outlet, the volume directly increases at most insignificantly). The core flow is passed through the turbine after the combustion chamber . This converts part of the energy supplied in the combustion chamber into mechanical power, so that the compressor and fan are driven (by means of a forward shaft ). The remaining energy (pressure, flow velocity) in the core flow is converted into thrust by the thrust nozzle located at the end of the engine . In today's civil turbofan engines, the thrust from the core engine is low compared to the thrust from the turbofan generated by the fan.
Since, from the point of view of the fan, the core engine primarily serves to generate hot high-pressure (exhaust) gas for its turbine stages, the core engine is sometimes also referred to as a “(hot) gas generator”.
fan
The fan is sometimes also called a blower or blower in German , rarely a fan (see also: ducted propeller ). Mostly it is arranged in front of the compressor, in rare cases, for example in the General Electric CJ805-23 and the General Electric CF700 , the fan is behind the core engine . This arrangement is called the aft fan (see below for how it works). The fan of a normal turbofan engine has the task of sucking in large air masses and accelerating them; if it is in front of the compressor, the area that leads into the core engine can already be designed more for compression than for acceleration.
In most engines, the fan is considered to be part of the (low-pressure) compressor (its “first stage”), especially if it already causes significant compression for the “actual” compressor. After the fan, the airflow splits into one
- inner air flow (primary flow), which gets into the core engine (a gas turbine ) (with the "actual" compressor, combustion chamber and turbine), and a
- external air flow (bypass or secondary flow) that is routed outside the core engine.
The fan acts like a jacketed propeller and generates around 80% of the propulsion in modern commercial engines. In general, with a turbofan engine, more energy is withdrawn from the primary flow by the turbine than with a single-flow jet engine of the same size as the core engine, since the fan must be driven with this energy.
Characteristic of a turbofan engine, the bypass ratio . It is the ratio of the amount of air in the bypass flow to the amount of air that flows through the gas turbine. Modern bypass engines in civil aircraft have a bypass ratio of 4 (80% bypass, 20% core flow) to 9 (90% bypass, 10% core flow). Secondary and primary currents together create the total thrust. The Kuznetsov NK-93 shows an extreme design , in which a propeller turbine acts on two counter-rotating, encapsulated propellers. A bypass ratio of 16.6 is achieved here.
The compressor is often also called a compressor .
Wave structure
Due to the mostly large fan diameter (for example 2.95 m in the Rolls-Royce Trent 900 ), the fan can no longer be coupled to the compressor and turbine via a single shaft: The compressor and turbine usually work at high speeds at which the blade tips of the Fans would reach too high a speed. Therefore, either a transmission gear ( geared turbofan, see below) or two to three shafts are used in order to be able to operate the core engine and fan at different speeds . In the case of a multi-shaft engine without a gear, there is no mechanical coupling between the shafts; each shaft is only driven by “its” turbine stages.
A multi-shaft system is implemented by a coaxial shaft arrangement : an outer hollow primary shaft (high pressure shaft) through which the thinner, but longer secondary (low pressure) shaft runs inside.
In addition to a different speed, an opposite direction of rotation (of the coaxial shafts or the geared turbofan) is also possible. In the case of several shafts, the different directions of rotation are achieved by opposing angles of incidence of the blades of the compressor and turbine stages mounted on one shaft; in the case of the geared turbofan, the different directions of rotation are achieved by the gearbox.
A three-shaft system works in some turbofan engines, for example in the Rolls-Royce-Trent series. This enables a further subdivision of the compressor and turbine stages with the advantage of being able to operate the respective components with speeds better adapted to their optimal operating point ; Furthermore, the acceleration behavior is improved by the lower masses of the individual shaft systems.
Effects, advantages and disadvantages
With turbofan engines at speeds between 600 and 850 km / h, a high air flow rate can be achieved with lower fuel consumption, which lowers costs. The air in the secondary flow forms a buffer layer between the hot exhaust gases and the ambient air, which reduces the noise emissions of the exhaust gas jet.
Today, dual-flow engines are used almost exclusively, as they offer greater efficiency and greater safety than single-flow engines. The bypass flow ratio is different depending on the intended use. For high speeds up to the supersonic range , such as in the EJ200 for the Eurofighter Typhoon , the thrust is in the foreground, which is why the bypass ratio is low. With civil and military passenger and transport machines, low consumption, wear and noise values are in the foreground, which is why the bypass flow ratio is very high here.
Geared turbofan
As geared (engl. Geared Turbofan ) are referred to by two or three waves turbofan engines which a reduction gear (about 3: 1: 1 to 4) between the low pressure shaft and fan (wave). Since the speed of the fan can be reduced and that of the low-pressure turbine / compressor increased, both components can work in their respective optimal speed range. This significantly reduces consumption and noise levels.
The additional mass of the gearbox is offset by the lower mass of the high-speed low-pressure turbine, so that a geared turbofan does not have to be heavier. Such an engine has existed in the thrust class for business aircraft since 1972 with the Honeywell TFE731 . Another geared engine is the Lycoming ALF 502, which has been driving the four- engine BAe 146 since 1981 . Its further development, the Avro RJ, was powered by the more powerful ALF 507 engine version.
The first attempt to use this technology in larger engines was made in 1986 by International Aero Engines (IAE) under the name SuperFan - an engine for the A340, which, however, was not fully developed due to technical risks. Pratt & Whitney , a partner of IAE, continued to pursue the concept and presented demonstration engines (the Advanced Ducted Prop and Advanced Technology Fan Integrator ) with 236 kN and 56 kN thrust respectively in 1992 and 2001 .
Since this technology enables a reduction in fuel consumption of well over 10% compared to the current engines due to the easier to achieve increase in the bypass flow ratio , Pratt & Whitney and MTU Aero Engines developed another demonstration engine based on the Pratt & Whitney PW6000 , which was launched in 2008 by P&W was tested on a company's own Boeing 747SP and in flight together with Airbus on an A340-600.
The success of this demonstration engine led to the development of the Pratt & Whitney PW1000G , which is marketed under the name PurePower . This has been selected for the three newly developed aircraft Airbus A220 (first flight September 16, 2013), Mitsubishi SpaceJet (first flight November 11, 2015) and Irkut MS-21 (first flight May 28, 2017). In December 2010, Airbus announced that it would be able to deliver its bestseller, the A320 family of aircraft, as the A320neo (New Engine Option) from 2016. Customers of this type can choose between the PW1000G and the CFM-LEAP-X engine (without gearbox).
Aft fan
An aft-fan engine differs from normal turbofan engines in that the fan is not in front of the rest of the engine, but behind it. So far, this configuration - as far as is known - has only been used by General Electric with the General Electric CJ805-23B and C engine types and the General Electric CF700 in order to derive turbofans from single-shaft turbojet engines. GE switched a freewheel turbine behind the respective engine . However, the exhaust gases from the engine flow around only the inner part of the blades as a turbine, causing it to rotate. The outer part of the blades, on the other hand, rotates in the jacket as a fan and accelerates the jacket flow there.
Open rotor
If the fan is not encased, it is called an "open rotor" design. It represents an intermediate form between turbofan and turboprop design. Both “pusher” configurations (similar to the aft fan design) and “pullers” in which the open fan is positioned in front of the core engine are examined .
history
Pratt & Whitney JT9D turbofan engine on a Boeing 747
Two General Electric TF-39 turbofan engines on the wing of a Lockheed C-5
The first functional turbofan engine was the Daimler-Benz DB 670 (also 109-007), the first test bench run of which took place on April 1, 1943. At Escher-Wyss in Switzerland, a single-shaft engine with bypass flow was being developed until development was discontinued in 1947, while Sulzer engineers had proposed a twin-shaft turbofan engine for the N-20 aircraft as early as 1946 , which, however, was also not implemented has been.
The Rolls-Royce Conway (first flight in 1954 and originally designed for the Handley Page Victor ) was ready in 1959, as was the Pratt & Whitney JT3D (actually for the Boeing B-52 H). Both were modifications of turbojet engines and had a low bypass ratio. They were originally military developments. The civil registration of the JT3D followed a few months later than the Conway.
The Soviet Solovyov D-20 followed in 1960 and was designed for civil aviation from the outset. The Tupolev Tu-124 was the first short-haul service with jet engines.
The development of the turbofan engines with a high bypass ratio in use today goes back to a tender by the USAF for a turbo engine for the Lockheed C-5 Galaxy military transporter , since turbojet or turbofan engines with a low bypass ratio would have consumed too much fuel for this long-haul transport aircraft, which weighs more than 350 t and were too weak. General Electric won the competition with the General Electric TF39 . The Pratt & Whitney JT9D is based on a competition design for this competition and was used in the first versions of the Boeing 747 .
Manufacturer
Western manufacturers of turbofan jet engines are
and cooperations of the same
Awiadwigatel manufactures bypass motors in Russia .
Related topics
- The Blade Off Test determines the effects of a lost turbine blade at maximum speed.
- The abbreviation CROR stands for English counter rotating open rotor , a turbo-jet engine with counter-rotating , open fan.
literature
- Andreas Linke-Diesinger: Turbofan engine systems. Functions of the engine systems of commercial aircraft. Springer Vieweg, Berlin / Heidelberg 2014, ISBN 978-3-662-44569-3 .
Web links
- Video explaining how a turbofan works (accessed December 30, 2014)
- State-of-the-Art Subsonic Engine SFC , NASA diagram of specific fuel consumption (SFC) of different jet engine designs in the subsonic range, English language
Individual evidence
- ^ Klaus Hünecke: Jet Engines. P. 9, Figures 1-7 , Motorbooks International, Osceola WI (USA) 1997, ISBN 0-7603-0459-9 .
- ^ Klaus Hünecke: Aero engines. Your technology and function. Motorbuchverlag, Stuttgart 1987/1998, ISBN 3-87943-407-7 .
- ↑ HS.146 Progress Report (1974). (PDF) In: flightglobal.com. Flightglobal, April 1974, accessed July 28, 2010 (Archived scan of a page from the printed 1974 edition).
- ↑ FliegerRevue October 2008, pp. 32–33, Rotating with a gearbox - engines of the future
- ↑ Ghim-Lay Yeo: Sources: Airbus prepares to release A320neo details. In: flightglobal.com. Flightglobal, January 14, 2011. Accessed June 11, 2016.
- ↑ See X-ray crack of the General Electric CF 700. ( Memento of the original from March 3, 2016 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. In: aircraftenginedesign.com. Retrieved June 11, 2016.
- ^ Georges Bridel: Swiss jet aircraft and jet engines. Schweizerische Bauzeitung, Volume 95, Issue 32, Page 542, August 11, 1977
- ^ Georges Bridel: Swiss jet aircraft and jet engines. Schweizerische Bauzeitung, Volume 95, Issue 10, Page 140, March 10, 1977
