Editing Thrust-specific fuel consumption (section)

==Significance of SFC==
SFC is dependent on engine design, but differences in the SFC between different engines using the same underlying technology tend to be quite small. Increasing [[overall pressure ratio]] on jet engines tends to decrease SFC.

In practical applications, other factors are usually highly significant in determining the fuel efficiency of a particular engine design in that particular application. For instance, in aircraft, turbine (jet and turboprop) engines are typically much smaller and lighter than equivalently powerful piston engine designs, both properties reducing the levels of [[Drag (physics)|drag]] on the plane and reducing the amount of power needed to move the aircraft. Therefore, turbines are more efficient for aircraft propulsion than might be indicated by a simplistic look at the table below.

SFC varies with throttle setting, altitude, climate. For jet engines, air flight speed is an important factor too. 
Air flight speed counteracts the jet's exhaust speed. (In an artificial and extreme case with the aircraft flying exactly at the exhaust speed, one can easily imagine why the jet's net thrust should be near zero.)  Moreover, since work is force (''i.e''., thrust) times distance, mechanical power is force times speed. Thus, although the nominal SFC is a useful measure of fuel efficiency, it should be divided by speed when comparing engines at different speeds.

For example, [[Concorde]] cruised at 1354&nbsp;mph, or 7.15 million feet per hour, with its engines giving an SFC of 1.195&nbsp;lb/(lbf·h) (see below); this means the engines transferred 5.98 million [[foot pound]]s per pound of fuel (17.9 MJ/kg), equivalent to an SFC of 0.50&nbsp;lb/(lbf·h) for a subsonic aircraft flying at 570&nbsp;mph, which would be better than even modern engines; the [[Rolls-Royce/Snecma Olympus 593|Olympus 593]] used in the Concorde was the world's most efficient jet engine.<ref>[https://www.pbs.org/wgbh/nova/transcripts/3203_concorde.html Supersonic Dream]</ref><ref>"[http://www.srmuniv.ac.in/downloads/turbofan-2012.pdf The turbofan engine] {{Webarchive|url=https://web.archive.org/web/20150418181832/http://www.srmuniv.ac.in/downloads/turbofan-2012.pdf |date=2015-04-18 }}", page 5. ''[[SRM Institute of Science and Technology]], Department of aerospace engineering''</ref> However, Concorde ultimately has a heavier airframe and, due to being supersonic, is less aerodynamically efficient, i.e., the [[lift to drag ratio]] is far lower. In general, the total fuel burn of a complete aircraft is of far more importance to the customer.