Specific fuel consumption (thrust)
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Specific fuel consumption, often shortened to SFC, or TSFC is an engineering term that is used to describe the fuel efficiency of an engine design with respect to thrust output. It allows the efficiency of different sized engines to be directly compared.
SFC for thrust engines (e.g. turbojets, turbofans, ramjets, rocket engines, etc) is the mass of fuel needed to provide the specific net thrust for a given period e.g. lb/(h·lbf) - pounds of fuel per hour-pound of thrust, or g/(s·kN) in S.I. units - 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 vary more or less inversely with speed, which in turn means that the fuel consumption per mile can be a more appropriate comparison metric for aircraft that travel at very different speeds.
This figure is inversely proportional to specific impulse.
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[edit] 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.
It should be noted that 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. Consequently, 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 M2 with its engines giving an SFC of 1.195 lb/(lbf·h) (see below); this is equivalent to an SFC of 0.51 lb/(lbf·h) for an aircraft flying at M0.85, which would be highly competitive with even modern engines. However, Concorde ultimately has a less aerodynamically efficient (due to being supersonic the lift to drag ratio is far lower) and heavier airframe. In general the Total Fuel Burn of a complete aircraft is of far more importance to the customer.
[edit] 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 | =(101972/X) g/kN·s |
| English units | =X seconds | =X lbf·s/lb | =32.16 X ft/s | =(3600/X) lb/lbf·h |
[edit] Typical values of SFC for thrust engines
| Engine type | scenario | SFC in lb/(lbf·h) | SFC in g/(kN·s) | Isp in s | Effective exhaust velocity (m/s) |
|---|---|---|---|---|---|
| NK-33 rocket engine | vacuum | 10.9 | 309 | 330 | 3240 |
| SSME rocket engine | Space Shuttle vacuum | 7.95 | 225 | 453 | 4423 |
| Ramjet | M1 | 4.5 | 127 | 800 | 7877 |
| J-58 turbojet | SR-71 at M3.2 (wet) | 1.9 | 53.8 | 1900 | 18,587 |
| Rolls-Royce/Snecma Olympus 593 | Concorde M2 cruise | 1.195[2] | 33.8 | 3012 | 29,553 |
| CF6-80C2B1F turbofan | Boeing 747-400 cruise | 0.605[2] | 17.1 | 5950 | 58,400 |
| General Electric CF6 turbofan | sea level | 0.307 | 8.696 | 11,700 | 115,000 |
Data from: [2]
[edit] References
[edit] See also
- Energies per unit mass
- Specific impulse
- Vehicle metrics
- Brake specific fuel consumption - fuel efficiency of shaft engines
