Thrust-specific fuel consumption

Template:Short description Thrust-specific fuel consumption (TSFC) is the fuel efficiency of an engine design with respect to thrust output. TSFC may also be thought of as fuel consumption (grams/second) per unit of thrust (newtons, or N), hence thrust-specific. This figure is inversely proportional to specific impulse, which is the amount of thrust produced per unit fuel consumed.

TSFC or SFC for thrust engines (e.g. turbojets, turbofans, ramjets, rockets, 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>Specific Fuel Consumption.</ref>

Specific fuel consumption of air-breathing jet engines at their maximum efficiency is more or less proportional to exhaust speed. The fuel consumption per mile or per kilometre is a more appropriate comparison for aircraft that travel at very different speeds.Template:Cn There also exists power-specific fuel consumption, which equals the thrust-specific fuel consumption divided by speed. It can have units of pounds per hour per horsepower.

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 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 mph, or 7.15 million feet per hour, with its engines giving an SFC of 1.195 lb/(lbf·h) (see below); this means the engines transferred 5.98 million foot pounds per pound of fuel (17.9 MJ/kg), equivalent to an SFC of 0.50 lb/(lbf·h) for a subsonic aircraft flying at 570 mph, which would be better than even modern engines; the Olympus 593 used in the Concorde was the world's most efficient jet engine.<ref>Supersonic Dream</ref><ref>"The turbofan engine Template:Webarchive", 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.

Units

	Specific impulse (by weight)	Specific impulse (by mass)	Effective exhaust velocity	Specific fuel consumption
SI	=X seconds	=9.8066 X N·s/kg	=9.8066 X m/s	=101,972 (1/X) g/(kN·s) / {g/(kN·s)=s/m}
Imperial units	=X seconds	=X lbf·s/lb	=32.16 X ft/s	=3,600 (1/X) lb/(lbf·h)

Typical values of SFC for thrust engines

Civil engines<ref>{{#invoke:citation/CS1|citation										CitationClass=web }}</ref>
Model	SL thrust	Template:Abbr	Template:Abbr	SL SFC	cruise SFC	Weight	Layout	cost ($M)	Introduction
GE GE90	Template:Cvt	8.4	39.3		Template:Cvt	Template:Cvt	1+3LP 10HP 2HP 6LP	11	1995
RR Trent	Template:Cvt	4.89-5.74	36.84-42.7		Template:Cvt	Template:Cvt	1LP 8IP 6HP 1HP 1IP 4/5LP	11-11.7	1995
PW4000	Template:Cvt	4.85-6.41	27.5-34.2	Template:Cvt		Template:Cvt	1+4-6LP 11HP 2HP 4-7LP	6.15-9.44	1986-1994
RB211	Template:Cvt	4.30	25.8-33		Template:Cvt	Template:Cvt	1LP 6/7IP 6HP 1HP 1IP 3LP	5.3-6.8	1984-1989
GE CF6	Template:Cvt	4.66-5.31	27.1-32.4	Template:Cvt	Template:Cvt	Template:Cvt	1+3/4LP 14HP 2HP 4/5LP	5.9-7	1981-1987
D-18	Template:Cvt	5.60	25.0		Template:Cvt	Template:Cvt	1LP 7IP 7HP 1HP 1IP 4LP		1982
PW2000	Template:Cvt	6	31.8	Template:Cvt	Template:Cvt	Template:Cvt	1+4LP 11HP 2HP 5LP	4	1983
PS-90	Template:Cvt	4.60	35.5		Template:Cvt	Template:Cvt	1+2LP 13HP 2 HP 4LP		1992
IAE V2500	Template:Cvt	4.60-5.40	24.9-33.40	Template:Cvt	Template:Cvt	Template:Cvt	1+4LP 10HP 2HP 5LP		1989-1994
CFM56	Template:Cvt	4.80-6.40	25.70-31.50	Template:Cvt	Template:Cvt	Template:Cvt	1+3/4LP 9HP 1HP 4/5LP	3.20-4.55	1986-1997
D-30	Template:Cvt	2.42			Template:Cvt	Template:Cvt	1+3LP 11HP 2HP 4LP		1982
JT8D	Template:Cvt	1.77	19.2	Template:Cvt	Template:Cvt	Template:Cvt	1+6LP 7HP 1HP 3LP	2.99	1986
BR700	Template:Cvt	4.00-4.70	25.7-32.1	Template:Cvt	Template:Cvt	Template:Cvt	1+1/2LP 10HP 2HP 2/3LP		1996
D-436	Template:Cvt	4.95	25.2		Template:Cvt	Template:Cvt	1+1L 6I 7HP 1HP 1IP 3LP		1996
RR Tay	Template:Cvt	3.04-3.07	15.8-16.6	Template:Cvt	Template:Cvt	Template:Cvt	1+3LP 12HP 2HP 3LP	2.6	1988-1992
RR Spey	Template:Cvt	0.64-0.71	15.5-18.4	Template:Cvt	Template:Cvt	Template:Cvt	4/5LP 12HP 2HP 2LP		1968-1969
GE CF34	Template:Cvt		21	Template:Cvt		Template:Cvt	1F 14HP 2HP 4LP		1996
AE3007	Template:Cvt		24.0	Template:Cvt		Template:Cvt
ALF502/LF507	Template:Cvt	5.60-5.70	12.2-13.8	Template:Cvt	Template:Cvt	Template:Cvt	1+2L 7+1HP 2HP 2LP	1.66	1982-1991
CFE738	Template:Cvt	5.30	23.0	Template:Cvt	Template:Cvt	Template:Cvt	1+5LP+1CF 2HP 3LP		1992
PW300	Template:Cvt	4.50	23.0	Template:Cvt	Template:Cvt	Template:Cvt	1+4LP+1HP 2HP 3LP		1990
JT15D	Template:Cvt	3.30	13.1	Template:Cvt	Template:Cvt	Template:Cvt	1+1LP+1CF 1HP 2LP		1983
WI FJ44-4A	Template:Cvt	3.28	12.80	Template:Cvt	Template:Cvt	Template:Cvt	1+1L 1C 1H 1HP 2LP		1992
WI FJ33-5A	Template:Cvt			Template:Cvt		Template:Cvt			2016

The following table gives the efficiency for several engines when running at 80% throttle, which is approximately what is used in cruising, giving a minimum SFC. The efficiency is the amount of power propelling the plane divided by the rate of energy consumption. Since the power equals thrust times speed, the efficiency is given by

<math>\eta=V/(SFC\times h)</math>

where V is speed and h is the energy content per unit mass of fuel (the higher heating value is used here, and at higher speeds the kinetic energy of the fuel or propellant becomes substantial and must be included).

citation

CitationClass=web }}</ref>
Turbofan	efficiency
GE90	36.1%
PW4000	34.8%
PW2037	35.1% (M.87 40K)
PW2037	33.5% (M.80 35K)
CFM56-2	30.5%
TFE731-2	23.4%

See also

• Template:Annotated link
• Template:Annotated link
• Template:Annotated link
• Template:Annotated link

External links

• GE CF6 website Template:Webarchive
• NASA Cruise SFC vs. Year
• SFC by Engine/Mfg Template:Webarchive