Thrust-specific fuel consumption
Thrust-specific fuel consumption is the fuel efficiency of an engine design with respect to thrust output.
TSFC may also be thought of as fuel consumption per unit of thrust. It is thus thrust-specific, meaning that the fuel consumption is divided by the thrust.
TSFC or SFC for thrust engines is the mass of fuel needed to provide the net thrust for a given period e.g. lb/ or g/. Mass of fuel is used, rather than volume 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 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 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, 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/ ; this means the engines transferred 5.98 million foot pounds per pound of fuel, equivalent to an SFC of 0.50 lb/ 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 | Specific Impulse | Effective exhaust velocity | Specific Fuel Consumption | |
| SI | =X seconds | =9.8066 X N·s/kg | =9.8066 X m/s | =101,972 g/ / |
| Imperial units | =X seconds | =X lbf·s/lb | =32.16 X ft/s | =3,600 lb/ |
Typical values of SFC for thrust engines
| Model | SL thrust | SL SFC | cruise SFC | Weight | Layout | cost | Introduction | ||
| GE GE90 | 8.4 | 39.3 | 1+3LP 10HP 2HP 6LP | 11 | 1995 | ||||
| RR Trent | 4.89-5.74 | 36.84-42.7 | 1LP 8IP 6HP 1HP 1IP 4/5LP | 11-11.7 | 1995 | ||||
| PW4000 | 4.85-6.41 | 27.5-34.2 | 1+4-6LP 11HP 2HP 4-7LP | 6.15-9.44 | 1986-1994 | ||||
| RB211 | 4.30 | 25.8-33 | 1LP 6/7IP 6HP 1HP 1IP 3LP | 5.3-6.8 | 1984-1989 | ||||
| GE CF6 | 4.66-5.31 | 27.1-32.4 | 1+3/4LP 14HP 2HP 4/5LP | 5.9-7 | 1981-1987 | ||||
| D-18 | 5.60 | 25.0 | 1LP 7IP 7HP 1HP 1IP 4LP | 1982 | |||||
| PW2000 | 6 | 31.8 | 1+4LP 11HP 2HP 5LP | 4 | 1983 | ||||
| PS-90 | 4.60 | 35.5 | 1+2LP 13HP 2 HP 4LP | 1992 | |||||
| IAE V2500 | 4.60-5.40 | 24.9-33.40 | 1+4LP 10HP 2HP 5LP | 1989-1994 | |||||
| CFM56 | 4.80-6.40 | 25.70-31.50 | 1+3/4LP 9HP 1HP 4/5LP | 3.20-4.55 | 1986-1997 | ||||
| D-30 | 2.42 | 1+3LP 11HP 2HP 4LP | 1982 | ||||||
| JT8D | 1.77 | 19.2 | 1+6LP 7HP 1HP 3LP | 2.99 | 1986 | ||||
| BR700 | 4.00-4.70 | 25.7-32.1 | 1+1/2LP 10HP 2HP 2/3LP | 1996 | |||||
| D-436 | 4.95 | 25.2 | 1+1L 6I 7HP 1HP 1IP 3LP | 1996 | |||||
| RR Tay | 3.04-3.07 | 15.8-16.6 | 1+3LP 12HP 2HP 3LP | 2.6 | 1988-1992 | ||||
| RR Spey | 0.64-0.71 | 15.5-18.4 | 4/5LP 12HP 2HP 2LP | 1968-1969 | |||||
| GE CF34 | 21 | 1F 14HP 2HP 4LP | 1996 | ||||||
| AE3007 | 24.0 | ||||||||
| ALF502/LF507 | 5.60-5.70 | 12.2-13.8 | 1+2L 7+1HP 2HP 2LP | 1.66 | 1982-1991 | ||||
| CFE738 | 5.30 | 23.0 | 1+5LP+1CF 2HP 3LP | 1992 | |||||
| PW300 | 4.50 | 23.0 | 1+4LP+1HP 2HP 3LP | 1990 | |||||
| JT15D | 3.30 | 13.1 | 1+1LP+1CF 1HP 2LP | 1983 | |||||
| FJ44 | 3.28 | 12.8 | 1+1L 1C 1H 1HP 2LP | 1992 |
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.
| Turbofan | efficiency |
| GE90 | 36.1% |
| PW4000 | 34.8% |
| PW2037 | 35.1% |
| PW2037 | 33.5% |
| CFM56-2 | 30.5% |
| TFE731-2 | 23.4% |