Brake specific fuel consumption
Brake specific fuel consumption (BSFC) is a measure of the fuel efficiency of any prime mover that burns fuel and produces rotational, or shaft, power. It is typically used for comparing the efficiency of internal combustion engines with a shaft output.
It is the rate of fuel consumption divided by the power produced. It may also be thought of as power-specific fuel consumption, for this reason. BSFC allows the fuel efficiency of different engines to be directly compared.
- 1 The BSFC calculation (in metric units)
- 2 The relationship between BSFC numbers and efficiency
- 3 The use of BSFC numbers as operating values and as a cycle average statistic
- 4 The significance of BSFC numbers for engine design and class
- 5 Examples of values of BSFC for shaft engines
- 6 See also
- 7 References
- 8 External links
The BSFC calculation (in metric units)
To calculate BSFC, use the formula
- r is the fuel consumption rate in grams per second (g/s)
- P is the power produced in watts where
The above values of r, , and may be readily measured by instrumentation with an engine mounted in a test stand and a load applied to the running engine. The resulting units of BSFC are grams per joule (g/J)
Commonly BSFC is expressed in units of grams per kilowatt-hour (g/kW·h). The conversion factor is as follows:
- BSFC [g/kW·h] = BSFC [g/J]×(3.6×106)
The conversion between metric and imperial units is:
- BSFC [g/kW·h] = BSFC [lb/hp·h]×608.277
- BSFC [lb/hp·h] = BSFC [g/kW·h]×0.001644
The relationship between BSFC numbers and efficiency
To calculate the actual efficiency of an engine requires the energy density of the fuel being used.
Different fuels have different energy densities defined by the fuel's heating value. The lower heating value (LHV) is used for internal combustion engine efficiency calculations because the heat at temperatures below 150 °C (300 °F) cannot be put to use.
Some examples of lower heating values for vehicle fuels are:
Thus a diesel engine's efficiency = 1/(BSFC × 0.0119531) and a gasoline engine's efficiency = 1/(BSFC × 0.0122225)
The use of BSFC numbers as operating values and as a cycle average statistic
Any engine will have different BSFC values at different speeds and loads. For example, a reciprocating engine achieves maximum efficiency when the intake air is unthrottled and the engine is running near its torque peak. However, the numbers often reported for a particular engine are a fuel economy cycle average statistic. For example, the cycle average value of BSFC for a gasoline engine is 322 g/kW·h, translating to an efficiency of 25% (math calculation: 1/(322 × 0.0122225) = 0.2540). However, efficiency for that engine can be lower or higher than this average statistic depending on the operating condition. In the case of a production gasoline engine, the most efficient BSFC is approximately 225 g/kW·h, which is equivalent to a thermodynamic efficiency of 36%.
The significance of BSFC numbers for engine design and class
BSFC numbers change a lot for different engine design and compression ratio and power rating. Engines of different classes like diesels and gasoline engines will have very different BSFC numbers, ranging from less than 200 g/kW·h (diesel at low speed and high torque) to more than 1,000 g/kW·h (turboprop at low power level).
Examples of values of BSFC for shaft engines
The following table takes selected values as an example for the minimum specific fuel consumption of several types of engine.
- Turboprop (aircraft engine) values are given for the complete range of power during a flight.
- Ground roll SFC value (at 0.07 Pmax) and especially idle (at rest) SFC values show dramatic increases when compared with cruise value (0.70 Pmax).
For specific engines values can and often do differ from the table values shown below. For comparison, the theoretical work that can be derived from burning octane (C8H18) (based on change in Gibbs free energy going to gaseous H2O and CO2) is 45.7 MJ/kg, corresponding to 79 g/kW·h.
|Power||Year||Engine type||Application||SFC (lb/hp·h)||SFC (g/kW·h)||Energy efficiency (%)|
|2,020 kW||1996||Pratt & Whitney turboprop PW127||Aircraft engine at idle||2,390|
|Aircraft engine at ground roll||1,270|
|Aircraft engine @ 0.30 Pmax||508|
|Aircraft engine @ 0.70 Pmax||328|
|Aircraft engine at Pmax||294|
|Otto cycle gasoline engines||0.45–0.37||273–227||30–36|
|80 kW/l||2011||Ford Ecoboost Downsized turbocharged otto engine||Automobile engine||245|
|2,000 kW||1945||Wright R-3350 Duplex-Cyclone gasoline turbo-compound||Aircraft engine||0.4||243||33.7|
|57 kW||Toyota Prius THS II engine only ||automobile||.362||225||37%|
|68 kW||2008||Revetec Controlled Combustion Engine gasoline engine||Automobile||212||38.6|
|550 kW||1931||Junkers Jumo 204 turbocharged two-stroke diesel||Aircraft engine||0.345||210||39.8|
|36 MW||2002||Rolls-Royce Marine Trent turboshaft||Marine engine||0.345||210||39.8|
|2,013 kW||1940||Klöckner-Humboldt-Deutz DZ 710 Diesel two stroke||Aircraft engine||0.33||201||41.58|
|2,340 kW||1949||Napier Nomad Diesel-compound||Aircraft engine||0.345||210||39.8|
|Diesel engine turbocharged diesels||0.34–0.30||209–178||40–47|
|165 kW||2000||Volkswagen 3.3 V8 TDI||Automobile engine||0.33||205||41.1|
|43 MW||General Electric LM6000 turboshaft||Marine engine, power generation||0.32||199||42|
|88 kW||1990||Audi 2.5 litre TDI||Automobile engine||198||42.5|
|80 MW||1998||Wärtsilä-Sulzer RTA96-C two-stroke||Marine engine||0.268||163||51.7|
|23 MW||MAN Diesel S80ME-C Mk7 two-stroke||Marine engine ||0.254||155||54.4|
- Fuel economy in automobiles
- Fuel economy-maximizing behaviors
- Fuel management systems
- Marine fuel management
- Thrust specific fuel consumption
- ATR, The Optimum Choice, ATR 72-500 trip pattern
- SAE 2004-01-0064 para 2-3-1
- SAE paper 900648
- Man Diesel Se - Marine
- Reciprocating engine types
- HowStuffWorks: How Car Engines Work
- Reciprocating Engines at infoplease
- Piston Engines US Centennial of Flight Commission
- Effect of EGR on the exhaust gas temperature and exhaust opacity in compression ignition engines
- Heywood J B 1988 Pollutant formation and control. Internal combustion engine fundamentals Int. edn (New York: Mc-Graw Hill) pp 572–577
- Well-to-Wheel Studies, Heating Values, and the Energy Conservation Principle