Brake Specific Fuel Consumption
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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)

To calculate BSFC, use the formula

${\displaystyle BSFC={\frac {r}{P}}}$

where:

${\displaystyle r}$ is the fuel consumption rate in grams per second (g/s)
${\displaystyle P}$ is the power produced in watts where ${\displaystyle P=\tau \omega }$ (W)
${\displaystyle \omega }$ is the engine speed in radians per second (rad/s)
${\displaystyle \tau }$ is the engine torque in newton metres (N?m)

The above values of r, ${\displaystyle \omega }$, and ${\displaystyle \tau }$ 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:

Certification gasoline = 18,640 BTU/lb (0.01204 kW?h/g)
Regular gasoline = 18,917 BTU/lb (0.0122222 kW?h/g)
Diesel fuel = 18,500 BTU/lb (0.0119531 kW?h/g)

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

BSFC [g/(kW?h)] map

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%.

An iso-BSFC map (fuel island plot) of a diesel engine is shown. The sweet spot at 206 BSFC has 40.6% efficiency. The x-axis is rpm; y-axis is BMEP in bar (bmep is proportional to torque)

## The significance of BSFC numbers for engine design and class

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 engines

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.

kW hp Year Engine Type Application lb/(hp?h) g/(kW?h) efficiency
48 64 1989 Rotax 582 gasoline, 2-stroke Aviation, Ultralight, Eurofly Fire Fox 0.699 425[1] 19.3%
321 431 1987 PW206B/B2 turboshaft Helicopter, EC135 0.553 336[2] 24.4%
427 572 1987 PW207D turboshaft Helicopter, Bell 427 0.537 327[2] 25.1%
500 670 1981 Arrius 2B1/2B1A-1 turboshaft Helicopter, EC135 0.526 320[2] 25.6%
820 1,100 1960 PT6C-67C turboshaft Helicopter, AW139 0.490 298[2] 27.5%
958 1,285 1989 MTR390 turboshaft Helicopter, Tiger 0.460 280[2] 29.3%
84.5 113.3 1996 Rotax 914 gasoline, turbo Aviation, Light-sport aircraft, WT9 Dynamic 0.454 276[3] 29.7%
88 118 1942 Lycoming O-235-L gasoline Aviation, General aviation, Cessna 152 0.452 275[4] 29.8%
1,799 2,412 1984 RTM322-01/9 turboshaft Helicopter, NH90 0.420 255[2] 32.1%
63 84 1991 GM Saturn I4 engine gasoline Cars, Saturn S-Series 0.411 250[5] 32.5%
150 200 2011 Ford EcoBoost gasoline, turbo Cars, Ford 0.403 245[6] 33.5%
300 400 1961 Lycoming IO-720 gasoline Aviation, General aviation, PAC Fletcher 0.4 243[7] 34.2%
7,000 9,400 1986 Rolls-Royce MT7 gas turbine Hovercraft, SSC 0.3998 243.2[8] 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[9] 35.5%
57 76 2003 Toyota 1NZ-FXE gasoline Car, Toyota Prius 0.370 225[10] 36.4%
8,251 11,065 2005 Europrop TP400 turboprop Airbus A400M 0.350 213[11] 39.6%
550 740 1931 Junkers Jumo 204 diesel 2-stroke, turbo Aviation, Commercial aviation, Junkers Ju 86 0.347 211[12] 40%
36,000 48,000 2002 Rolls-Royce Marine Trent turboshaft Marine propulsion 0.340 207[13] 40.7%
2,340 3,140 1949 Napier Nomad Diesel-compound Concept Aircraft engine 0.340 207[14] 40.7%
165 221 2000 Volkswagen 3.3 V8 TDI Diesel Car, Audi A8 0.337 205[15] 41.1%
2,013 2,699 1940 Deutz DZ 710 Diesel two stroke Concept Aircraft engine 0.330 201[16] 41.9%
42,428 56,897 1993 GE LM6000 turboshaft Marine propulsion, Electricity generation 0.329 200.1[17] 42.1%
130 170 2007 BMW N47 2L Diesel Cars, BMW 0.326 198[18] 42.6%
88 118 1990 Audi 2.5L TDI Diesel Car, Audi 100 0.326 198[19] 42.6%
620 830 Scania AB DC16 078A Diesel 4-stroke Electricity generation 0.312 190[20] 44.4%
1,200 1,600 early 1990s Wärtsilä 6L20 Diesel 4-stroke Marine propulsion 0.311 189.4[21] 44.5%
3,600 4,800 MAN Diesel 6L32/44CR Diesel 4-stroke Marine propulsion, Electricity generation 0.283 172[22] 49%
4,200 5,600 2015 Wärtsilä W31 Diesel 4-stroke Marine propulsion, Electricity generation 0.271 165[23] 51.1%
34,320 46,020 1998 Wärtsilä-Sulzer RTA96-C Diesel 2-stroke Marine propulsion, Electricity generation 0.263 160[24] 52.7%
27,060 36,290 MAN Diesel S80ME-C9.4-TII Diesel 2-stroke Marine propulsion, Electricity generation 0.254 154.5[25] 54.6%
34,350 46,060 MAN Diesel G95ME-C9 Diesel 2-stroke Marine propulsion 0.254 154.5[26] 54.6%
605,000 811,000 2016 General Electric 9HA Combined cycle Electricity generation 0.223 135.5 (eq.) 62.2%[27]
640,000 860,000 2021 General Electric 7HA.3 Combined cycle Electricity generation (proposed) 0.217 131.9 (eq.) 63.9%[28]

Turboprop efficiency is only good at high power; SFC increases dramatically for approach at low power (30% Pmax) and especially at idle (7% Pmax) :

2,050 kW Pratt & Whitney Canada PW127 turboprop (1996)[29]
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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