Physics:Thrust-specific fuel consumption

Short description: Fuel efficiency of an engine design with respect to thrust output

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 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. 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. 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
Model	SL thrust	BPR	OPR	SL SFC	cruise SFC	Weight	Layout	cost ($M)	Introduction
GE GE90	90,000 lbf 400 kN	8.4	39.3		0.545 lb/(lbf⋅h) 15.4 g/(kN⋅s)	16,644 lb 7,550 kg	1+3LP 10HP 2HP 6LP	11	1995
RR Trent	71,100–91,300 lbf 316–406 kN	4.89-5.74	36.84-42.7		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	11-11.7	1995
PW4000	52,000–84,000 lbf 230–370 kN	4.85-6.41	27.5-34.2	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	6.15-9.44	1986-1994
RB211	43,100–60,600 lbf 192–270 kN	4.30	25.8-33		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	5.3-6.8	1984-1989
GE CF6	52,500–67,500 lbf 234–300 kN	4.66-5.31	27.1-32.4	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	5.9-7	1981-1987
D-18	51,660 lbf 229.8 kN	5.60	25.0		0.570 lb/(lbf⋅h) 16.1 g/(kN⋅s)	9,039 lb 4,100 kg	1LP 7IP 7HP 1HP 1IP 4LP		1982
PW2000	38,250 lbf 170.1 kN	6	31.8	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	4	1983
PS-90	35,275 lbf 156.91 kN	4.60	35.5		0.595 lb/(lbf⋅h) 16.9 g/(kN⋅s)	6,503 lb 2,950 kg	1+2LP 13HP 2 HP 4LP		1992
IAE V2500	22,000–33,000 lbf 98–147 kN	4.60-5.40	24.9-33.40	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
CFM56	20,600–31,200 lbf 92–139 kN	4.80-6.40	25.70-31.50	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	3.20-4.55	1986-1997
D-30	23,850 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
JT8D	21,700 lbf 97 kN	1.77	19.2	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	2.99	1986
BR700	14,845–19,883 lbf 66.03–88.44 kN	4.00-4.70	25.7-32.1	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
D-436	16,865 lbf 75.02 kN	4.95	25.2		0.610 lb/(lbf⋅h) 17.3 g/(kN⋅s)	3,197 lb 1,450 kg	1+1L 6I 7HP 1HP 1IP 3LP		1996
RR Tay	13,850–15,400 lbf 61.6–68.5 kN	3.04-3.07	15.8-16.6	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	2.6	1988-1992
RR Spey	9,900–11,400 lbf 44–51 kN	0.64-0.71	15.5-18.4	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
GE CF34	9,220 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
AE3007	7,150 lbf 31.8 kN		24.0	0.390 lb/(lbf⋅h) 11.0 g/(kN⋅s)		1,581 lb 717 kg
ALF502/LF507	6,970–7,000 lbf 31.0–31.1 kN	5.60-5.70	12.2-13.8	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	1.66	1982-1991
CFE738	5,918 lbf 26.32 kN	5.30	23.0	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
PW300	5,266 lbf 23.42 kN	4.50	23.0	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
JT15D	3,045 lbf 13.54 kN	3.30	13.1	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
WI FJ44-4A	1,900 lbf 8.5 kN				0.456 lb/(lbf⋅h) 12.9 g/(kN⋅s)	445 lb 202 kg	1+1L 1C 1H 1HP 2LP		1992
WI FJ33-5A	1,000–1,800 lbf 4.4–8.0 kN			0.486 lb/(lbf⋅h) 13.8 g/(kN⋅s)		300 lb 140 kg			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]\displaystyle{ \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).

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

• Physics:Specific impulse – Change in velocity per amount of fuel

External links

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