Editing Turbofan (section)

===Low-bypass turbofan===

[[File:Turbofan operation lbp.svg|thumb|Schematic diagram illustrating a 2-spool, low-bypass turbofan engine with a mixed exhaust, showing the low-pressure (green) and high-pressure (purple) spools. The fan (and booster stages) are driven by the low-pressure turbine, whereas the high-pressure compressor is powered by the high-pressure turbine.]]

A high-specific-thrust/low-bypass-ratio turbofan normally has a multi-stage fan behind inlet guide vanes, developing a relatively high pressure ratio and, thus, yielding a high (mixed or cold) exhaust velocity. The core airflow needs to be large enough to ensure there is sufficient [[core power]] to drive the fan. A smaller core flow/higher bypass ratio cycle can be achieved by raising the inlet temperature of the high-pressure (HP) turbine rotor.

To illustrate one aspect of how a turbofan differs from a turbojet, comparisons can be made at the same airflow (to keep a common intake for example) and the same net thrust (i.e. same specific thrust). A bypass flow can be added only if the turbine inlet temperature is not too high to compensate for the smaller core flow. Future improvements in turbine cooling/material technology can allow higher turbine inlet temperature, which is necessary because of increased cooling air temperature, resulting from an [[overall pressure ratio]] increase.

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 [[Thrust specific fuel consumption|specific fuel consumption]] (SFC).

Some low-bypass ratio military turbofans (e.g. [[General Electric F404|F404]], [[Pratt & Whitney JT8D|JT8D]]) have variable inlet guide vanes to direct air onto the first fan rotor stage. This improves the fan [[compressor stall|surge]] margin (see [[compressor map]]).

<gallery mode="packed" heights="120">
File:Pratt & Whitney JT8D-17A 1.JPG|The widely produced [[Pratt & Whitney JT8D]] used on many early narrowbody [[jetliners]]. The fan is located behind the inlet guide vanes.
File:Solowjow D-30 III.jpg|[[Soloviev D-30]] which powers the [[Ilyushin Il-76]] and [[Ilyushin Il-62|Il-62M]]; [[Mikoyan MiG-31]]; [[Xian H-6]]K and [[Xian Y-20|Y-20]]
File:AL-31FN.jpg|[[Saturn AL-31]] which powers the [[Chengdu J-10]] and [[Chengdu J-20|J-20]]; [[Shenyang J-11]], [[Shenyang J-15|J-15]] and [[Shenyang J-16|J-16]]; [[Sukhoi Su-30]] and [[Sukhoi Su-27|Su-27]]
File:Williams Research F107.jpg|[[Williams F107]] which powers the [[Raytheon]] [[Tomahawk (missile family)|BGM-109 Tomahawk]] cruise missile
File:AL-55 at the MAKS-2011 (01).jpg|[[NPO Saturn AL-55]] which powers certain [[HAL HJT-36 Sitara]]
File:Pratt & Whitney TF30 at Oakland Aviation Museum, June 2022.jpg|[[Pratt & Whitney TF-30]] which powers the [[Grumman F-14 Tomcat]]
File:Eurojet EJ200 for Eurofighter Typhoon PAS 2013 01 free.jpg|[[Eurojet EJ200]] which powers the [[Eurofighter Typhoon]]
File:XF3 KASM001.jpg|[[Ishikawajima-Harima F3]] which powers the [[Kawasaki T-4]]
File:GTX-35VS Kaveri.jpg|[[GTRE GTX-35VS Kaveri]] developed by [[Gas Turbine Research Establishment|GTRE]]
</gallery>