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.
The BSFC calculation (in metric units)Edit
To calculate BSFC, use the formula
- is the fuel consumption rate in grams per second (g/s)
- is the power produced in watts where (W)
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 efficiencyEdit
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 statisticEdit
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 peak torque. The efficiency often reported for a particular engine, however, is not its maximum efficiency but a fuel economy cycle statistical average. For example, the cycle average value of BSFC for a gasoline engine is 322 g/(kW⋅h), translating to an efficiency of 25% (1/(322 × 0.0122225) = 0.2540). Actual efficiency can be lower or higher than the engine’s average due to varying operating conditions. 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 classEdit
BSFC numbers change a lot for different engine designs, 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 enginesEdit
The following table takes values as an example for the specific fuel consumption of several types of engines. For specific engines values can and often do differ from the table values shown below. Energy efficiency is based on a lower heating value of 42.7 MJ/kg (84.3 g/(kW⋅h)) for diesel fuel and jet fuel, 43.9 MJ/kg (82 g/(kW⋅h)) for gasoline.
|48||64||1989||Rotax 582||gasoline, 2-stroke||Aviation, Ultralight, Eurofly Fire Fox||0.699||425||19.3%|
|427||572||1987||PW207D||turboshaft||Helicopter, Bell 427||0.537||327||25.1%|
|500||670||1981||Arrius 2B1/2B1A-1||turboshaft||Helicopter, EC135||0.526||320||25.6%|
|84.5||113.3||1996||Rotax 914||gasoline, turbo||Aviation, Light-sport aircraft, WT9 Dynamic||0.454||276||29.7%|
|88||118||1942||Lycoming O-235-L||gasoline||Aviation, General aviation, Cessna 152||0.452||275||29.8%|
|63||84||1991||GM Saturn I4 engine||gasoline||Cars, Saturn S-Series||0.411||250||32.5%|
|150||200||2011||Ford EcoBoost||gasoline, turbo||Cars, Ford||0.403||245||33.5%|
|300||400||1961||Lycoming IO-720||gasoline||Aviation, General aviation, PAC Fletcher||0.4||243||34.2%|
|7,000||9,400||1986||Rolls-Royce MT7||gas turbine||Hovercraft, SSC||0.3998||243.2||34.7%|
|2,000||2,700||1945||Wright R-3350 Duplex-Cyclone||gasoline, turbo-compound||Aviation, Commercial aviation; B-29, Constellation, DC-7||0.380||231||35.5%|
|57||76||2003||Toyota 1NZ-FXE||gasoline||Car, Toyota Prius||0.370||225||36.4%|
|550||740||1931||Junkers Jumo 204||diesel 2-stroke, turbo||Aviation, Commercial aviation, Junkers Ju 86||0.347||211||40%|
|36,000||48,000||2002||Rolls-Royce Marine Trent||turboshaft||Marine propulsion||0.340||207||40.7%|
|2,340||3,140||1949||Napier Nomad||Diesel-compound||Concept Aircraft engine||0.340||207||40.7%|
|165||221||2000||Volkswagen 3.3 V8 TDI||Diesel||Car, Audi A8||0.337||205||41.1%|
|2,013||2,699||1940||Deutz DZ 710||Diesel two stroke||Concept Aircraft engine||0.330||201||41.9%|
|42,428||56,897||1993||GE LM6000||turboshaft||Marine propulsion, Electricity generation||0.329||200.1||42.1%|
|130||170||2007||BMW N47 2L||Diesel||Cars, BMW||0.326||198||42.6%|
|88||118||1990||Audi 2.5L TDI||Diesel||Car, Audi 100||0.326||198||42.6%|
|620||830||Scania AB DC16 078A||Diesel 4-stroke||Electricity generation||0.312||190||44.4%|
|1,200||1,600||early 1990s||Wärtsilä 6L20||Diesel 4-stroke||Marine propulsion||0.311||189.4||44.5%|
|3,600||4,800||MAN Diesel 6L32/44CR||Diesel 4-stroke||Marine propulsion, Electricity generation||0.283||172||49%|
|4,200||5,600||2015||Wärtsilä W31||Diesel 4-stroke||Marine propulsion, Electricity generation||0.271||165||51.1%|
|34,320||46,020||1998||Wärtsilä-Sulzer RTA96-C||Diesel 2-stroke||Marine propulsion, Electricity generation||0.263||160||52.7%|
|27,060||36,290||MAN Diesel S80ME-C9.4-TII||Diesel 2-stroke||Marine propulsion, Electricity generation||0.254||154.5||54.6%|
|34,350||46,060||MAN Diesel 12G95ME-C9||Diesel 2-stroke||Marine propulsion||0.254||154.5||54.6%|
|605,000||811,000||2016||General Electric 9HA||Combined cycle||Electricity generation||0.223||135.5 (eq.)||62.2%|
Turboprop efficiency is only good at high power; SFC increases dramatically for approach at low power (30% Pmax) and especially at idle (7% Pmax) :
|Mode||Power||fuel flow||SFC||Energy efficiency|
|Nominal idle (7%)||192 hp (143 kW)||3.06 kg/min (405 lb/h)||1,282 g/(kW⋅h) (2.108 lb/(hp⋅h))||6.6%|
|Approach (30%)||825 hp (615 kW)||5.15 kg/min (681 lb/h)||502 g/(kW⋅h) (0.825 lb/(hp⋅h))||16.8%|
|Max cruise (78%)||2,132 hp (1,590 kW)||8.28 kg/min (1,095 lb/h)||312 g/(kW⋅h) (0.513 lb/(hp⋅h))||27%|
|Max climb (80%)||2,192 hp (1,635 kW)||8.38 kg/min (1,108 lb/h)||308 g/(kW⋅h) (0.506 lb/(hp⋅h))||27.4%|
|Max contin. (90%)||2,475 hp (1,846 kW)||9.22 kg/min (1,220 lb/h)||300 g/(kW⋅h) (0.493 lb/(hp⋅h))||28.1%|
|Take-off (100%)||2,750 hp (2,050 kW)||9.9 kg/min (1,310 lb/h)||290 g/(kW⋅h) (0.477 lb/(hp⋅h))||29.1%|
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