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.Specific Fuel Consumption.

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. Power-specific fuel consumption, the thrust-specific fuel consumption divided by speed, 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, for a subsonic aircraft flying at 570 mph, to an SFC of 0.50 lb/(lbf·h), better than subsonic engines of the time. The Olympus 593 used in the Concorde was the world's most efficient jet engine.Supersonic Dream"The turbofan engine ", page 5. SRM Institute of Science and Technology, Department of aerospace engineering 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)Effectiveexhaust velocitySpecific fuel consumptionSI=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 Model SL thrust SL SFC cruise SFC Weight Layout cost ($M) Introduction GE GE90 8.4 39.3 1+3LP 10HP2HP 6LP 11 1995 RR Trent 4.89-5.74 36.84-42.7 1LP 8IP 6HP1HP 1IP 4/5LP 11-11.7 1995 PW4000 4.85-6.41 27.5-34.2 1+4-6LP 11HP2HP 4-7LP 6.15-9.44 1986-1994 RB211 4.30 25.8-33 1LP 6/7IP 6HP1HP 1IP 3LP 5.3-6.8 1984-1989 GE CF6 4.66-5.31 27.1-32.4 1+3/4LP 14HP2HP 4/5LP 5.9-7 1981-1987 D-18 5.60 25.0 1LP 7IP 7HP1HP 1IP 4LP 1982 PW2000 6 31.8 1+4LP 11HP2HP 5LP 4 1983 PS-90 4.60 35.5 1+2LP 13HP2 HP 4LP 1992 IAE V2500 4.60-5.40 24.9-33.40 1+4LP 10HP2HP 5LP 1989-1994 CFM56 4.80-6.40 25.70-31.50 1+3/4LP 9HP1HP 4/5LP 3.20-4.55 1986-1997 D-30 2.42 1+3LP 11HP2HP 4LP 1982 JT8D 1.77 19.2 1+6LP 7HP1HP 3LP 2.99 1986 BR700 4.00-4.70 25.7-32.1 1+1/2LP 10HP2HP 2/3LP 1996 D-436 4.95 25.2 1+1L 6I 7HP1HP 1IP 3LP 1996 RR Tay 3.04-3.07 15.8-16.6 1+3LP 12HP2HP 3LP 2.6 1988-1992 RR Spey 0.64-0.71 15.5-18.4 4/5LP 12HP2HP 2LP 1968-1969 GE CF34 21 1F 14HP2HP 4LP 1996 AE3007 24.0 ALF502/LF507 5.60-5.70 12.2-13.8 1+2L 7+1HP2HP 2LP 1.66 1982-1991 CFE738 5.30 23.0 1+5LP+1CF2HP 3LP 1992 PW300 4.50 23.0 1+4LP+1HP2HP 3LP 1990 JT15D 3.30 13.1 1+1LP+1CF1HP 2LP 1983 WI FJ44-4A 3.28 12.80 1+1L 1C 1H1HP 2LP 1992WI FJ33-5A2016

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

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).

+ typical subsonic cruise, 80% throttle, min SFC 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

Notes

References

External links GE CF6 website NASA Cruise SFC vs. Year SFC by Engine/Mfg