Specific fuel consumption
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Specific fuel consumption, often shortened to SFC, is an engineering term that is used to describe the fuel efficiency of an engine design w/ a mechanical output. It measures the mass of fuel needed to provide a given power for a given period. The common unit of measure is lb/hp·h - that is, pounds of fuel consumed for every horsepower generated during one hour of operation or kg/kW·h in metric units. Therefore a lower number indicates better efficiency. The way SFC for thrust engines (e.g. turbojets, turbofans, ramjets, rocket engines, etc) is measured is slightly different; the mass of fuel needed to provide a given net thrust for a given period e.g. lb/hr/lbf or g/s/kN in S.I. units. This is called specific impulse. However, gas turbines with a mechanical shaft output (i.e. turboprops and turboshafts) have their efficiency measured in terms of specific fuel consumption.
SFC is dependent on engine design, but differences in the SFC between different engines using the same underlying technology tend to be quite small. For instance, typical gasoline engines will have a SFC of about 0.5 lb/hp·h or (0.3kg/kW·h = 83g/MJ), regardless of the design of any particular engine. Generally, SFC within a particular class of engine will decrease when the compression ratio is increased. Increasing overall pressure ratio on jet engines also tends to decrease SFC. Diesel engines have better SFCs than gasoline, largely because they have much higher compression ratios and therefore they can convert more of the heat produced into power.
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. Consequently, although the nominal SFC is a useful measure of fuel efficiency, the Total Fuel Burn is of more importance to the customer.
[edit] Typical Values of SFC
The following table gives the specific fuel consumption of several types of engine. For comparison, the theoretical work that can be derived from burning octane (based on change in Gibbs free energy going to gaseous H2O and CO2) is 45.7 MJ/kg, corresponding to 79 g/kWh.
| Power | date | Engine type | SFC in lb/hp·h | SFC in g/kW·h | Energy efficiency |
|---|---|---|---|---|---|
| Ramjet | 1.0 | 610 | |||
| Turbo-prop | 0.8 | 360 to 490 | |||
| Otto cycle gasoline engine | 0.5 | 300 | |||
| Diesel engine automotive | 0.4 | 230 to 260 | |||
| 2000 kW | 1945 | Wright R-3350 gasoline airplane engine | 0.4 | 243 | |
| 550kW | 1931 | Junkers Jumo 204 Turbocharged Diesel | 210 | ||
| 2340 kW | 1949 | Napier Nomad Diesel-Compound engine | 0.345 | 210 | |
| 165 kW | 2000 | Volkswagen 3.3 V8 TDI car engine | 0.33 | 205 | 41.1% |
| 213 kW | Volvo D7E 290 hp diesel truck engine [1] | 188 | 44.8% | ||
| 43 MW | General Electric LM6000 turboshaft | 42% | |||
| 23 MW | MAN B&W Diesel S80ME-C Mk7 two-stroke marine engine [2] | 155 | 54.4% | ||
| 2005 | idem, with thermo efficiency system | 58,2% | |||
| 80 MW | 1998 | Wärtsilä-Sulzer RTA96-C two-stroke marine engine | 163 | 51.7% | |
| 480 MW | combined cycle GE Energy H-System gas turbines | 60% |
- Turbo-prop. The specific fuel consumption varies a lot with speed, altitude and power percentage. For example, PT6 turbine on Cessna 208 : at low power (parking, ground rolling, descent) sfc may exceed 800 g/kW·h. Cruise sfc is about 400 g/kW·h. A turbine is sfc efficient at high power only.
- Jet engines. The General Electric CF6-80C2B2F medium sized civil turbofan (at Max Power, Sea Level Static, ISA, uninstalled) uses 0.307 lb/hr per lbf of thrust (8.696 g/s/kN). At a normal cruising speed of 913 km/hr, this comes out to 123 g/kWh.
