Thrust-specific fuel consumption

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 (kilonewtons, or kN). It is thus thrust-specific, meaning that the fuel consumption is divided by the thrust.

TSFC or SFC for thrust engines (e.g. turbojets, turbofans, ramjets, rocket engines, 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.[1]

Specific fuel consumption of air-breathing jet engines at their maximum efficiency is more or less proportional to speed. The fuel consumption per mile or per kilometre is a more appropriate comparison for aircraft that travel at very different speeds. 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.

This figure is inversely proportional to specific impulse.

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 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 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.[2][3] 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

Specific fuel consumption (SFC), specific impulse, and effective exhaust velocity numbers for various rocket and jet engines.
Engine type Scenario Spec. fuel cons. Specific
impulse (s)
Effective exhaust
velocity
(m/s)
(lb/lbf·h) (g/kN·s)
NK-33 rocket engineVacuum 10.9 308 331[4] 3250
SSME rocket engineSpace shuttle vacuum 7.95 225 453[5] 4440
Ramjet Mach 1 4.5 130 800 7800
J-58 turbojetSR-71 at Mach 3.2 (Wet) 1.9[6] 54 1900 19000
Eurojet EJ200Reheat 1.66–1.73 47–49[7] 2080–2170 20400–21300
Rolls-Royce/Snecma Olympus 593 turbojetConcorde Mach 2 cruise (Dry) 1.195[8] 33.8 3010 29500
Eurojet EJ200Dry 0.74–0.81 21–23[7] 4400–4900 44000–48000
CF6-80C2B1F turbofanBoeing 747-400 cruise 0.605[8] 17.1 5950 58400
General Electric CF6 turbofanSea level 0.307[8] 8.7 11700 115000
Civil engines[9]
ModelSL thrustBPROPR SL SFCcruise SFCWeight Layoutcost ($M)Entry In Service
CFM5620,600–31,200 lbf
92–139 kN
4.80-6.4025.70-31.50 0.32–0.36 lb/(lbf⋅h)
9.1–10.2 g/(kN⋅s)
0.545–0.667 lb/(lbf⋅h)
15.4–18.9 g/(kN⋅s)
4,301–5,700 lb
1,951–2,585 kg
1+3/4LP 9HP
1HP 4/5LP
3.20-4.551986-1997
IAE V250022,000–33,000 lbf
98–147 kN
4.60-5.4024.9-33.40 0.34–0.37 lb/(lbf⋅h)
9.6–10.5 g/(kN⋅s)
0.574–0.581 lb/(lbf⋅h)
16.3–16.5 g/(kN⋅s)
5,210–5,252 lb
2,363–2,382 kg
1+4LP 10HP
2HP 5LP
1989-1994
GE CF349,220 lbf
41.0 kN
21 0.35 lb/(lbf⋅h)
9.9 g/(kN⋅s)
1,670 lb
760 kg
1F 14HP
2HP 4LP
1996
GE CF652,500–67,500 lbf
234–300 kN
4.66-5.3127.1-32.4 0.32–0.35 lb/(lbf⋅h)
9.1–9.9 g/(kN⋅s)
0.562–0.623 lb/(lbf⋅h)
15.9–17.6 g/(kN⋅s)
8,496–10,726 lb
3,854–4,865 kg
1+3/4LP 14HP
2HP 4/5LP
5.9-71981-1987
GE GE9090,000 lbf
400 kN
8.439.3 0.545 lb/(lbf⋅h)
15.4 g/(kN⋅s)
16,644 lb
7,550 kg
1+3LP 10HP
2HP 6LP
111995
JT8D21,700 lbf
97 kN
1.7719.2 0.519 lb/(lbf⋅h)
14.7 g/(kN⋅s)
0.737 lb/(lbf⋅h)
20.9 g/(kN⋅s)
4,515 lb
2,048 kg
1+6LP 7HP
1HP 3LP
2.991986
PW200038,250 lbf
170.1 kN
631.8 0.33 lb/(lbf⋅h)
9.3 g/(kN⋅s)
0.582 lb/(lbf⋅h)
16.5 g/(kN⋅s)
7,160 lb
3,250 kg
1+4LP 11HP
2HP 5LP
41983
PW400052,000–84,000 lbf
230–370 kN
4.85-6.4127.5-34.2 0.348–0.359 lb/(lbf⋅h)
9.9–10.2 g/(kN⋅s)
9,400–14,350 lb
4,260–6,510 kg
1+4-6LP 11HP
2HP 4-7LP
6.15-9.441986-1994
RR Spey9,900–11,400 lbf
44–51 kN
0.64-0.7115.5-18.4 0.56 lb/(lbf⋅h)
16 g/(kN⋅s)
0.800 lb/(lbf⋅h)
22.7 g/(kN⋅s)
2,287–2,483 lb
1,037–1,126 kg
4/5LP 12HP
2HP 2LP
1968-1969
RB21143,100–60,600 lbf
192–270 kN
4.3025.8-33 0.563–0.607 lb/(lbf⋅h)
15.9–17.2 g/(kN⋅s)
0.570–0.598 lb/(lbf⋅h)
16.1–16.9 g/(kN⋅s)
7,264–9,670 lb
3,295–4,386 kg
1LP 6/7IP 6HP
1HP 1IP 3LP
5.3-6.81984-1989
RR Trent71,100–91,300 lbf
316–406 kN
4.89-5.7436.84-42.7 0.557–0.565 lb/(lbf⋅h)
15.8–16.0 g/(kN⋅s)
10,550–13,133 lb
4,785–5,957 kg
1LP 8IP 6HP
1HP 1IP 4/5LP
11-11.71995
RR Tay13,850–15,400 lbf
61.6–68.5 kN
3.04-3.0715.8-16.6 0.43–0.45 lb/(lbf⋅h)
12–13 g/(kN⋅s)
0.690 lb/(lbf⋅h)
19.5 g/(kN⋅s)
2,951–3,380 lb
1,339–1,533 kg
1+3LP 12HP
2HP 3LP
2.61988-1992
ALF502/LF5076,970–7,000 lbf
31.0–31.1 kN
5.60-5.7012.2-13.8 0.406–0.408 lb/(lbf⋅h)
11.5–11.6 g/(kN⋅s)
0.414–0.720 lb/(lbf⋅h)
11.7–20.4 g/(kN⋅s)
1,336–1,385 lb
606–628 kg
1+2L 7+1HP
2HP 2LP
1.661982-1991
AE30077,150 lbf
31.8 kN
24.0 0.390 lb/(lbf⋅h)
11.0 g/(kN⋅s)
1,581 lb
717 kg
CFE7385,918 lbf
26.32 kN
5.3023.0 0.369 lb/(lbf⋅h)
10.5 g/(kN⋅s)
0.645 lb/(lbf⋅h)
18.3 g/(kN⋅s)
1,325 lb
601 kg
1+5LP+1CF
2HP 3LP
1992
JT15D3,045 lbf
13.54 kN
3.3013.1 0.560 lb/(lbf⋅h)
15.9 g/(kN⋅s)
0.541 lb/(lbf⋅h)
15.3 g/(kN⋅s)
632 lb
287 kg
1+1LP+1CF
1HP 2LP
1983
PW3005,266 lbf
23.42 kN
4.5023.0 0.391 lb/(lbf⋅h)
11.1 g/(kN⋅s)
0.675 lb/(lbf⋅h)
19.1 g/(kN⋅s)
993 lb
450 kg
1+4LP+1HP
2HP 3LP
1990
BR70014,845–19,883 lbf
66.03–88.44 kN
4.00-4.7025.7-32.1 0.370–0.390 lb/(lbf⋅h)
10.5–11.0 g/(kN⋅s)
0.620–0.640 lb/(lbf⋅h)
17.6–18.1 g/(kN⋅s)
3,520–4,545 lb
1,597–2,062 kg
1+1/2LP 10HP
2HP 2/3LP
1996
FJ441,900 lbf
8.5 kN
3.2812.8 0.456 lb/(lbf⋅h)
12.9 g/(kN⋅s)
0.750 lb/(lbf⋅h)
21.2 g/(kN⋅s)
445 lb
202 kg
1+1L 1C 1H
1HP 2LP
1992
D-3023,850 lbf
106.1 kN
2.42 0.700 lb/(lbf⋅h)
19.8 g/(kN⋅s)
5,110 lb
2,320 kg
1+3LP 11HP
2HP 4LP
1982
PS-9035,275 lbf
156.91 kN
4.6035.5 0.595 lb/(lbf⋅h)
16.9 g/(kN⋅s)
6,503 lb
2,950 kg
1+2LP 13HP
2 HP 4LP
1992
D-1851,660 lbf
229.8 kN
5.6025.0 0.570 lb/(lbf⋅h)
16.1 g/(kN⋅s)
9,039 lb
4,100 kg
1LP 7IP 7HP
1HP 1IP 4LP
1982
D-43616,865 lbf
75.02 kN
4.9525.2 0.610 lb/(lbf⋅h)
17.3 g/(kN⋅s)
3,197 lb
1,450 kg
1+1L 6I 7HP
1HP 1IP 3LP
1996

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 (probably the lower heating value is used here).

typical subsonic cruise, 80% throttle, min SFC[10]
Turbofanefficiency
GE9036.1%
PW400034.8%
PW203735.1% (M.87 40K)
PW203733.5% (M.80 35K)
CFM56-230.5%
TFE731-223.4%

See also

References

  1. Specific Fuel Consumption
  2. Supersonic Dream
  3. "The turbofan engine", page 5. SRM Institute of Science and Technology, Department of aerospace engineering
  4. "NK33". Encyclopedia Astronautica.
  5. "SSME". Encyclopedia Astronautica.
  6. Nathan Meier (21 Mar 2005). "Military Turbojet/Turbofan Specifications".
  7. "EJ200 turbofan engine" (PDF). MTU Aero Engines. April 2016.
  8. Ilan Kroo. "Data on Large Turbofan Engines". Aircraft Design: Synthesis and Analysis. Stanford University.
  9. Lloyd R. Jenkinson; et al. (30 Jul 1999). "Civil Jet Aircraft Design: Engine Data File". Elsevier/Butterworth-Heinemann.
  10. Ilan Kroo. "Specific Fuel Consumption and Overall Efficiency". Aircraft Design: Synthesis and Analysis. Stanford University. Archived from the original on November 24, 2016.
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