How a turbofan jet engine works
The core airflow needs to be large enough to give sufficient core power to drive the fan. In the turbofan engine, the core engine is surrounded by a fan in the front and an additional turbine at the rear.
The case contains all the parts of the combustor, and inside it, the diffuser is the first part that does work. The diffuser slows down the air from the compressor, making it easier to ignite. The dome and swirler add turbulence to the air so it can more easily mix with fuel. From there, the liner is where the actual works happens. The liner has several inlets, allowing air to enter at multiple points how the combustion zone.
The last main part is the igniter, which is very similar to the spark plugs in your car or piston-engine engine. Once the igniter lights the fire, it is self-sustaining, and the igniter is turned off jet it's often used as a back-up in bad weather and icing conditions.
Once the air makes its way through the combustor, it flows through the turbine. The turbine is a series of airfoil shaped blades that are very similar to the blades in jet compressor. As the hot, high-speed air flows over the turbine blades, they extract energy from the air, spinning the turbine around in a circle, and turning the engine shaft that it's connected to.
This is the same shaft that the fan and compressor are connected to, so by spinning the turbine, the fan and compressor on the front of the engine continue sucking in more air that will soon be mixed with fuel and burned. The last step of the process happens in the nozzle.
The nozzle is essentially the works how of the engine, and it's where the high speed air shoots out the back. This is also the part where Sir Isaac Newton's third law comes into play: Put simply, by forcing air out the back of the engine at high speed, the airplane is pushed forward.
This makes them popular on planes used for short flights, where the time spent at low altitudes represents a greater percentage of the overall flight time.
The turbofan is something like a compromise between a pure turbojet and a turboprop. It works like the turbojet, except that the turbine shaft also drives an external fan, usually located at the front of the engine.
The fan has more blades than a propeller and spins much faster. It also features a shroud around its perimeter, which helps to capture and focus the air flowing through it. These features enable the fan to generate thrust at high altitudes, where a conventional propeller would be ineffective.
Some of the thrust still comes from the exhaust jet, but the addition of the fan makes the engine much more fuel efficient. It was followed by the aft-fan General Electric CF engine with a 2. There were at one time over CF aircraft in operation around the world, with an experience base of over 10 million service hours.
The core airflow needs to be large enough to give sufficient core power to drive the fan. To illustrate one aspect of how a turbofan differs from a turbojet, they may be compared, as in a re-engining assessment, at the same airflow to keep a common intake for example and the same net thrust i.
A bypass flow can be added only if the turbine inlet temperature is not too high to compensate for the smaller core flow.
How Does A Turbofan Engine Work?
The resulting turbofan, with reasonable efficiencies and duct loss for the added components, would probably operate at a higher nozzle pressure ratio than the turbojet, but with a lower exhaust temperature to retain net thrust. Since the temperature rise across the whole engine intake to nozzle would be lower, the dry power fuel flow would also be reduced, resulting in a better specific fuel consumption SFC.
Some low-bypass ratio military turbofans e. F have variable inlet guide vanes to direct air onto the first fan rotor stage. This improves the fan surge margin see compressor map. An afterburner is a combustor located downstream of the turbine blades and directly upstream of the nozzle, which burns fuel from afterburner-specific fuel injectors. The variable geometry nozzle must open to a larger throat area to accommodate the extra volume flow when the afterburner is lit.
Afterburning is often designed to give a significant thrust boost for take off, transonic acceleration and combat maneuvers, but is very fuel intensive. Consequently, afterburning can be used only for short portions of a mission. Unlike the main combustor, where the downstream turbine blades must not be damaged by high temperatures, an afterburner can operate at the ideal maximum stoichiometric temperature i. At a fixed total applied fuel: However, a high specific thrust turbofan will, by definition, have a higher nozzle pressure ratio, resulting in a higher afterburning net thrust and, therefore, a lower afterburning specific fuel consumption SFC.
However, high specific thrust engines have a high dry SFC.
The situation is reversed for a medium specific thrust afterburning turbofan: The former engine is suitable for a combat aircraft which must remain in afterburning combat for a fairly long period, but has to fight only fairly close to the airfield e.
However, the pilot can afford to stay in afterburning only for a short period, before aircraft fuel reserves become dangerously low. Low specific thrust is achieved by replacing the multi-stage fan with a single-stage unit. Unlike some military engines, modern civil turbofans do not have any stationary inlet guide vanes in front of the fan rotor.
The fan is scaled to achieve the desired net thrust. The core or gas generator of the engine must generate sufficient core power to at least drive the fan at its design flow and pressure ratio. Reducing the core mass flow tends to increase the load on the LP turbine, so this unit may require additional stages to reduce the average stage loading and to maintain LP turbine efficiency.
Reducing core flow also increases bypass ratio. Bypass ratios greater than 5: Further improvements in core thermal efficiency can be achieved by raising the overall pressure ratio of the core.
Improved blade aerodynamics reduces the number of extra compressor stages required. With multiple compressors i. For reasons of fuel economy, and also of reduced noise, almost all of today's jet airliners are powered by high-bypass turbofans.
Although modern combat aircraft tend to use low-bypass ratio turbofans, military transport aircraft e. The lower the specific thrust of a turbofan, the lower the mean jet outlet velocity, which in turn translates into a high thrust lapse rate i. See technical discussion below, item 2. Consequently, an engine sized to propel an aircraft at high subsonic flight speed e. Low specific thrust engines tend to have a high bypass ratio, but this is also a function of the temperature of the turbine system.
The turbofans on twin engined airliners are further more powerful to cope with losing one engine during take-off, which reduces the aircraft's net thrust by half. Modern twin engined airliners normally climb very steeply immediately after take-off. If one engine is lost, the climb-out is much shallower, but sufficient to clear obstacles in the flightpath.
How Gas Turbine Engines Work
The Soviet Union's engine technology was less advanced than the West's and its first wide-body aircraft, the Ilyushin Ilwas powered by low-bypass engines.
The Yakovlev Yaka medium-range, rear-engined aircraft seating up to passengers introduced in was the first Soviet aircraft to use high-bypass engines. Turbofan engines come in a variety of engine configurations.
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For a works engine cycle i. Off-design how and stability is, however, affected by engine configuration. As the design overall pressure ratio of an engine cycle increases, it becomes more difficult to operate at low rpm, without encountering an instability known as compressor surge.
This occurs when some of the compressor aerofoils stall like the wings of an aircraft causing a violent change in the direction of the airflow.
However, compressor stall can be avoided, at low rpm, by progressively:. Most modern American civil turbofans jet a relatively high-pressure-ratio high-pressure HP compressor, with many rows of variable stators to control surge margin at low rpm. As the HP compressor has a modest pressure ratio its speed can be reduced jet, without employing variable geometry.
However, because a shallow IP compressor working line is inevitable, the IPC has one stage of variable geometry on all variants except thewhich has none. Although far from common, the single-shaft turbofan is probably the simplest configuration, comprising a fan and high-pressure compressor driven by a single turbine unit, all on the same shaft.
Despite the simplicity of the turbomachinery configuration, the M53 requires a variable area mixer to facilitate part-throttle operation. Hot gas from the turbojet turbine exhaust expanded through the LP turbine, the fan blades being a radial extension of the turbine blades. One of the problems with the aft fan engine is hot gas leakage from the LP turbine to the fan. Many turbofans have the basic two-spool configuration where both the fan and LP turbine i. The BR is typical of this engine.
A turbofan engine is the most modern variation of the basic gas turbine engine. As with other gas turbines, there is how core enginewhose parts and operation are discussed on a separate works. In the turbofan engine, the core engine is surrounded by a fan in the front and an additional turbine at the rear.
The fan and fan turbine are composed of many blades, like the core compressor and core turbine, and are connected to an additional shaft. All of this additional turbomachinery is colored green on the schematic.Turbofan Engine: How It Works
As with the core compressor and turbine, some of the fan blades turn with the shaft and some blades remain stationary. The fan shaft passes through the core shaft for mechanical reasons. This type of arrangement is called a two spool engine one "spool" for the fan, one "spool" for the core.