Turbofan

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CFM56-3 turbofan, lower half, side view.

Boeing 747 jet engine up close

The turbofan is a type of airplane engine. Notably, all of the jet-engines that power current commercial jets are turbofans. Turbofans are also used in military jets and (rarely) in other vehicles such as jet-powered cars. The turbofan has evolved from the axial-flow turbojet engine, essentially by increasing the relative size of the Low Pressure (LP) Compressor to the point where some (or in some cases, most) of the air exiting the unit actually bypasses the core (or gas generator). This bypass air either expands through a separate propelling nozzle, or is mixed with the hot gases leaving the Low Pressure (LP) Turbine, before expanding through a Mixed Stream Propelling Nozzle.

If the turboprop is better at moderate flight speeds and the turbojet is better at very high speeds, it might be imagined that at some speed range in the middle a mixture of the two is best. Such an engine is the turbofan (originally termed bypass turbojet by the inventors at Rolls Royce). Another sometimes used is ducted fan, though that term is also is used for propellers and fans used in vertical flight applications.

The difference between a turbofan and a propeller besides direct thrust, is that the duct slows the air before it arrives at the fan. As both propeller and fan blades must operate subsonically to be efficient, ducted fans allow efficient operation at higher vehicle speeds.

Depending on specific thrust (i.e. net thrust/intake airflow), ducted fans operate best from about 400 to 2000 km/h (250 to 1300 mph), which is why turbofans are the most common type of engine for aviation use today in airliners and supersonic military fighter jets. Turboprop engines are also gas turbine engines that deliver all of their power to a shaft to drive a propeller. Turboprops remain popular on very small or slow aircraft such as small commuter airliners and combat transports such as the C-130 Hercules and P-3 Orion.

In a turbofan, the LP Compressor is often called a fan. Civil turbofans usually have a single fan stage, whereas most military turbofans have multi-stage fans.

Bypass ratio (the ratio of bypassed air mass to combustor air mass) is a parameter often used for classifying turbofans, although specific thrust is a better parameter.

The noise of any type of jet engine is strongly related to the velocity of the exhaust gases. High bypass ratio (i.e. low specific thrust) turbofans are relatively quiet compared to turbojets and low bypass ratio (i.e. high specific thrust) turbofans. A low specific thrust engine has a low jet velocity almost by definition, as the following approximate equation for net thrust implies:

[F_n = \dot m \cdot (V_ - V_a)]

where:

[\dot m = \,]intake mass flow

[V_ =\,] fully expanded jet velocity (in the exhaust plume)

[V_a =\,] aircraft flight velocity

Rearranging the above equation, specific thrust is given by:

[\frac = (V_ - V_a)]

So for zero flight velocity, specific thrust is directly proportional to jet velocity.

Jet aircraft are often considered loud, but a conventional piston engine or a turboprop engine delivering the same power would be much louder. (NASA has a [web page] with details on jet noise.)

Low-bypass turbofans

Early turbojet engines were very fuel-inefficient, as their overall pressure ratio and turbine inlet temperature were severely limited by the technology available at the time. Improved materials, and the introduction of twin compressors such as in the Pratt & Whitney JT3C engine, increased the overall pressure ratio and thus the thermodynamic efficiency of engines, but led to a poor propulsive efficiency, as pure turbojets have a low mass flow, high velocity exhaust.

The original low-bypass turbofan engines were designed to improve propulsive efficiency by reducing the exhaust speed to a value closer to aircraft speeds. The Rolls-Royce Conway, the first turbofan, had a bypass ratio of 0.3, similar to the modern General Electric F404 fighter engine. Civil turbofan engines of the 1960s, such as the Pratt & Whitney JT8D and the Rolls-Royce Spey had bypass ratios closer to 1.

The unusual General Electric [CF700] turbofan engine was developed as an aft-fan engine with a 2.0 bypass ratio. This was derived from the T-38 Talon and the Learjet General Electric [J85/CJ610]turbojet (2,850 lbf or 12,650 N) to power the larger Rockwell Sabreliner 75/80 model aircraft, as well as the Dassault Falcon 20 with about a 50% increase in thrust (4,200 lbf or 18,700 N). The CF700 was the first small turbofan in the world to be certificated by the Federal Aviation Administration (FAA). There are now over 400 CF700 aircraft in operation around the world, with an experience base of over 10 million service hours. The CF700 turbofan engine was also used to train Moon-bound astronauts in Project Apollo as the powerplant for the Lunar Landing Research Vehicle.

Afterburning turbofans

Since the 1970s, most jet fighter engines have been low-bypass turbofans with a mixed exhaust and afterburners – the first afterburning turbofan was the Pratt & Whitney TF30. A few low-bypass ratio military turbofans (e.g. F404) have Variable Inlet Guide Vanes, with piano-style hinges, to direct air onto the first rotor stage. This improves the fan surge margin (see compressor map) in the mid-flow range. The swing wing F-111 achieved a very high range / payload capability by pioneering the use of this engine, and it was also the heart of the famous F-14 Tomcat air superiority fighter which used the same engines in a smaller, more agile airframe to achieve efficient cruise and Mach 2 speed.

Imagine a retrofit situation where a new low bypass ratio, mixed exhaust, turbofan is replacing an old turbojet, in a particular military application. Say the new engine is to have the same airflow and net thrust (i.e. same specific thrust) as the one it is replacing. A bypass flow can only be introduced if the turbine inlet temperature is allowed to increase, to compensate for a correspondingly smaller core flow. Improvements in turbine cooling/material technology would facilitate the use of a higher turbine inlet temperature, despite increases in cooling air temperature, resulting from a probable increase in overall pressure ratio.

Efficiently done, the resulting turbofan 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).

Modern low-bypass military turbofans include the Pratt & Whitney F119, the Eurojet EJ200 and the General Electric F110, all of which feature a mixed exhaust, afterburner and variable area propelling nozzle. Non-afterburning engines include the Rolls-Royce/Turbomeca Adour (afterburning in the SEPECAT Jaguar) and the unmixed, vectored thrust, Rolls-Royce Pegasus.

High-bypass turbofan engines

Schematic diagram illustrating a 2-spool, high-bypass turbofan engine with an unmixed exhaust. The low-pressure spool is coloured green and the high-pressure one purple. The fan is driven by the low-pressure spool.

The introduction of variable compressor stators enabled high pressure ratio compressors to work surge-free at all throttle settings. This innovation made its debut in the General Electric J79, a single-shaft turbojet for supersonic military aircraft. When variable stators were combined with multiple compressors, dramatic increases in overall pressure ratio became possible. Higher turbine inlet temperatures (through improvements in turbine cooling/material technology) enabled relatively small mass flow gas generators to be employed. Coupling this with significant increases in fan mass flow, made the high-bypass turbofan engine feasible. Bypass ratios of 5 or more are now common.The turbofan is mounted in a large diameter bypass duct that surrounds the turbojet. It works like a huge propeller, blowing cold air around the outside of the turbojet to help cool it. The air emerging form the bypass duct then mixes with the hot exhaust from the turbojet, making the engine much quieter. The air travelling through the bypass duct does more than just cool the turbojet, it also produces most of the power. as much as 80% of the engines thrust comes directly from the huge fan at the front.

The first high-bypass turbofan engine was the General Electric TF39, built to power the Lockheed C-5 Galaxy military transport aircraft. The civil General Electric CF6 engine used a related design. Other high-bypass turbofans are the Pratt & Whitney JT9D, the three-shaft Rolls-Royce RB211 and the CFM International CFM56. More recent large high-bypass turbofans include the Pratt & Whitney PW4000, the three-shaft Rolls-Royce Trent, the General Electric GE90, and the General Electric GEnx.

The tremendously higher thrust provided by high-bypass turbofan engines also made civil wide-body aircraft practical and economical. In addition to the vastly increased thrust, these engines are also generally quieter. This is not so much due to the higher bypass ratio, but as to the use of low pressure ratio, single stage, fans, which significantly reduce specific thrust and, thereby, jet velocity. The combination of a higher overall pressure ratio and turbine inlet temperature improves thermal efficiency. This, together with a lower specific thrust (better propulsive efficiency), leads to a lower specific fuel consumption.

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 military aircraft tend to use low bypass ratio turbofans, military transport aircraft (e.g. C17 ) mainly use high bypass ratio turbofans (or turboprops) for fuel efficiency.

The Soviet Union's engine technology was less advanced than the West's and its first wide-body aircraft, the Ilyushin Il-86, was powered by low-bypass engines. The Yakovlev Yak-42, a medium-range, rear-engined aircraft seating up to 120 passengers was the first Soviet aircraft to use high-bypass engines.

Cycle improvements

Consider a mixed turbofan with a fixed bypass ratio and airflow. Increasing the overall pressure ratio of the compression system raises the combustor entry temperature. Therefore, at a fixed fuel flow 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 mixed 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, causing an increase in the hot mixer entry pressure. Consequently, net thrust increases, whilst specific fuel consumption (fuel flow/net thrust) decreases. A similar trend occurs with unmixed turbofans.

So turbofans can be made more fuel efficient by raising overall pressure ratio and turbine inlet temperature in unison. 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 may require better compressor materials.

Thrust growth

Thrust growth is obtained by increasing core power (the residual power available in the expansion process, after the power requirements of the core compression have been met). There are two basic routes available:

a) hot route: increase HP turbine rotor inlet temperature

b) cold route: increase core flow

The hot route may require changes in turbine blade/vane materials and/or better cooling. The cold route can be obtained by one of the following:

adding T-stage/s to the LP/IP compression
adding a zero-stage to the HP compression
improving the compression process, without adding stages (e.g. higher fan hub pressure ratio)
increasing core size (expensive, because a new turbine system is also required).

Changes must also be made to the fan to absorb the extra core power. On a civil engine, jet noise considerations mean that any significant increase in Take-off thrust must be accompanied by a corresponding increase in fan mass flow (to maintain a T/O specific thrust of about 30lbf/lb/s), usually by increasing fan diameter. On military engines, the fan pressure ratio would probably be increased to improve specific thrust, jet noise not normally being an important factor.1

Technical Discussion

1) Specific Thrust (net thrust/intake airflow) is an important parameter for turbofans and jet engines in general. Imagine a fan (driven by an appropriately sized electric motor) operating within a pipe, which is connected to a propelling nozzle. Fairly obviously, the higher the Fan Pressure Ratio (discharge pressure/inlet pressure), the higher the jet velocity and the corresponding specific thrust. Now imagine we replace this set-up with an equivalent turbofan - same airflow and same fan pressure ratio. Obviously, the core of the turbofan must produce sufficient power to drive the fan via the Low Pressure (LP) Turbine. If we choose a low (HP) Turbine Inlet Temperature for the gas generator, the core airflow needs to be relatively high to compensate. The corresponding bypass ratio is therefore relatively low. If we raise the Turbine Inlet Temperature, the core airflow can be smaller, thus increasing bypass ratio. Raising turbine inlet temperature tends to increase thermal efficiency and, therefore, improve fuel efficiency.

2) Naturally, as altitude increases there is a decrease in air density and, therefore, the net thrust of an engine. There is also a flight speed effect, termed Thrust Lapse Rate. Consider the approximate equation for net thrust again:

[F_n = m \cdot (V_ - V_a)]

With a high specific thrust (e.g. fighter) engine, the jet velocity is relatively high, so intuitively one can see that increases in flight velocity have less of an impact upon net thrust than a medium specific thrust (e.g. trainer) engine, where the jet velocity is lower. The impact of thrust lapse rate upon a low specfic thrust (e.g. civil) engine is even more severe. At high flight speeds, high specific thrust engines can pick-up net thrust through the ram rise in the intake, but this effect tends to diminish at supersonic speeds because of shock wave losses.

3) Thrust growth on civil turbofans is usually obtained by increasing fan airflow, thus preventing the jet noise becoming too high. However, the larger fan airflow requires more power from the core. This can be achieved by raising the Overall Pressure Ratio (combustor inlet pressure/intake delivery pressure) to induce more airflow into the core and by increasing turbine inlet temperature. Together, these parameters tend to increase core thermal efficiency and improve fuel efficiency.

4) Some high bypass ratio civil turbofans use an extremely low area ratio (less than 1.01), convergent-divergent, nozzle on the bypass (or mixed exhaust) stream, to control the fan working line. The nozzle acts as if it has variable geometry. At low flight speeds the nozzle is unchoked (less than a Mach Number of unity), so the exhaust gas speeds up as it approaches the throat and then slows down slightly as it reaches the divergent section. Consequently, the nozzle exit area controls the fan match and, being larger than the throat, pulls the fan working line slightly away from surge. At higher flight speeds, the ram rise in the intake increases nozzle pressure ratio to the point where the throat becomes choked (M=1.0). Under these circumstances, the throat area dictates the fan match and, being smaller than the exit, pushes the fan working line slightly towards surge. This is not a problem, since fan surge margin is much better at high flight speeds.

5) The off-design behaviour of turbofans is illustrated under compressor map and turbine map.

Recent developments in blade technology

The turbine blades in a turbofan engine are subject to high heat and stress, and require special fabrication. New material construction methods and material science have allowed blades, which were originally polycrystalline (regular metal), to be made from lined up metallic crystals and more recently mono-crystalline (i.e. single crystal) blades, which can operate at higher temperatures with less distortion.

Although turbine blade (and vane) materials have improved over the years, much of the increase in (HP) turbine inlet temperatures is due to improvements in blade/vane cooling technology. Relatively cool air is bled from the compression system, bypassing the combustion process, and enters the hollow blade or vane. After picking up heat from the blade/vane, the cooling air is dumped into the main gas stream. If the local gas temperatures are low enough, downstream blades/vanes are uncooled and solid.

Strictly speaking, the HP Turbine Rotor Inlet Temperature (after the temperature drop across the HPT stator) is more important than the (HP) turbine inlet temperature. Although some modern military and civil engines have peak RIT's of the order of 3300R (2840F), such temperatures are only experienced for a short time (during Take-off) on civil engines.

Turbofan engine manufacturers

The turbofan engine market is dominated by General Electric, Rolls-Royce plc and Pratt & Whitney, in order of market share.

General Electric

GE Aircraft Engines, part of the General Electric Conglomerate, currently has the largest share of the turbofan engine market. Some of their engine models include the CF6 (available on the Boeing 767, 747, Airbus A330 and more), GE90 (only the Boeing 777) and GENx (developed for the Airbus A350 & Boeing 787 currently in development) engines.Through joint ventures CFM International and Engine Alliance, they have created the very successful CFM56 series and the new GP7200.

Rolls-Royce

Rolls-Royce plc is the second largest manufacturer of turbofans and is most noted for their RB211 and Trent series, as well as their joint venture engines for the Airbus A320 and Boeing MD-90 families (IAE V2500), the Panavia Tornado (Turbo-Union RB199) and the Boeing 717 (BR700). As owners of the Allison Engine Company, their engines power the C-130 Hercules and several Embraer regional jets.

Pratt & Whitney

Pratt & Whitney is behind GE and Rolls-Royce, the JT9D has the proud distinction of being chosen by Boeing to power the original 747 "Jumbo jet". They current supply the PW4000 to some Airbus A330 and Boeing 777. Their engines can also be found on 747 and other old jets.

Extreme bypass jet engines

In the 1970s Rolls-Royce/SNECMA tested a M45SD-02 turbofan fitted with variable pitch fan blades to improve handling at ultra low fan pressure ratios and to provide thrust reverse down to zero aircraft speed. The engine was aimed at ultra quiet STOL aircraft operating from city center airports.

In a bid for increased efficiency with speed, a development of the turbofan and turboprop , known as a propfan engine, was created that had an unducted fan. The fan blades are situated outside of the duct, so that it appears like a turboprop with wide scimitar-like blades. Both General Electric and Pratt & Whitney/Allison demonstrated propfan engines in the 1980s. Excessive cabin noise and relatively cheap jet fuel prevented the engines being put into service.

Other meanings

The Unicode standard includes a turbofan character, #274B, in the dingbats range. Its official name is "HEAVY EIGHT TEARDROP-SPOKED PROPELLER ASTERISK = turbofan". In appropriately-configured browsers, it should appear in quotes here: "❋";

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