Thrust-specific fuel consumption: Difference between revisions

SFC is proportional to speed, not inversely proportional. (Error introduced in 2007!) Replaced Mach numbers with mph (since the conversion depends on temperature) and calculated energy imparted per weight of fuel for Concorde engine.
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(SFC is proportional to speed, not inversely proportional. (Error introduced in 2007!) Replaced Mach numbers with mph (since the conversion depends on temperature) and calculated energy imparted per weight of fuel for Concorde engine.)
TSFC may also be thought of as fuel consumption (grams/second) per unit of thrust (kilonewtons, or kN). It is thus thrust-specific, meaning that the fuel consumption is divided by the thrust.
 
TSFC or SFC for [[reaction engine|thrust engine]]s (e.g. [[turbojet]]s, [[turbofan]]s, [[ramjet]]s, [[rocket engine]]s, etc.) is the mass of [[fuel]] needed to provide the net thrust for a given period e.g. lb/(h·lbf) (pounds of fuel per hour-pound of thrust) or g/(s·kN) (grams of fuel per second-kilonewton). Mass of fuel is used rather than volume (gallons or litres) for the fuel measure since it is independent of temperature.<ref>[http://selair.selkirk.bc.ca/Training/Aerodynamics/range_prop.htm Specific Fuel Consumption<!-- Bot generated title -->] {{Webarchive|url=https://web.archive.org/web/20100809143435/http://selair.selkirk.bc.ca/training/Aerodynamics/range_prop.htm |date=2010-08-09 }}</ref>
 
Specific fuel consumption of air-breathing jet engines at their maximum efficiency variesis more or less inverselyproportional withto speed,. which means that theThe fuel consumption ''per mile'' or ''per kilometre'' can beis a more appropriate comparison for aircraft that travel at very different speeds.
 
This figure is inversely proportional to [[specific impulse]].
SFC varies with throttle setting,altitude and climate. For jet engines, flight speed also has a significant effect upon SFC; SFC is roughly proportional to air speed (actually exhaust velocity), but speed along the ground is also proportional to air speed. Since work done is force 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 to get a way to compare engines that fly at different speeds.
 
For example, [[Concorde]] cruised at [[Mach1354&nbsp;mph, number|Mach]]or 27.0515 million feet per hour, with its engines giving an SFC of 1.195&nbsp;lb/(lbf·h) (see below); this ismeans the engines transferred 5.98 million [[foot pound]]s per pound of fuel (17.9 MJ/kg), equivalent to an SFC of 0.5150&nbsp;lb/(lbf·h) for ana subsonic aircraft flying at Mach 0.85570&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]", page 5. ''[[SRM University]], 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.
 
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