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

where:

is the fuel consumption rate in grams per second (g/s)
is the power produced in watts where
is the engine speed in radians per second (rad/s)
is the engine torque in newton meters (N⋅m)

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:

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.

kWhp Year Engine Type Application lb/(hp⋅h) g/(kW⋅h) efficiency
48 64 1989 Rotax 582 gasoline, 2-stroke Ultralight aviation 0.699 425[1] 19.3%
321 431 1987 PW206B/B2 turboshaft EC135 helicopter 0.553 336[2] 24.4%
427 572 1987 PW207D turboshaft Bell 427 helicopter 0.537 327[2] 25.1%
500 670 1981 Arrius 2B1/2B1A-1 turboshaft EC135 helicopter 0.526 320[2] 25.6%
820 1,100 1960 PT6C-67C turboshaft AW139 helicopter 0.490 298[2] 27.5%
958 1,285 1989 MTR390 turboshaft Tiger helicopter 0.460 280[2] 29.3%
84.5 113.3 1996 Rotax 914 gasoline, turbo general aviation 0.454 276[3] 29.7%
93 125 1942 Lycoming O-235 gasoline General aviation 0.43 262[4] 31.4%
1,799 2,412 1984 RTM322-01/9 turboshaft NH90 helicopter 0.420 255[2] 32.1%
63 84 1991 GM Saturn I4 engine gasoline Saturn S-Series cars 0.411 250[5] 32.5%
150 200 2011 Ford EcoBoost gasoline, turbo Ford cars 0.403 245[6] 33.5%
300 400 1961 Lycoming IO-720 gasoline General aviation 0.4 243[7] 34.2%
7,000 9,400 1986 Rolls-Royce MT7 gas turbine SSC hovercraft 0.3998 243.2[8] 34.7%
2,000 2,700 1945 Wright R-3350 Duplex-Cyclone gasoline, turbo-compound Bombers, airliners 0.380 231[9] 35.5%
57 76 2003 Toyota 1NZ-FXE gasoline Toyota Prius car 0.370 225[10] 36.4%
550 740 1931 Junkers Jumo 204 diesel 2-stroke, turbo Bombers, airliners 0.347 211[11] 40%
36,000 48,000 2002 Rolls-Royce Marine Trent turboshaft Combat ships 0.340 207[12] 40.7%
2,340 3,140 1949 Napier Nomad Diesel-compound planned (aircraft) 0.340 207[13] 40.7%
165 221 2000 Volkswagen 3.3 V8 TDI Diesel Audi A8 car 0.337 205[14] 41.1%
2,013 2,699 1940 Deutz DZ 710 Diesel two stroke none (aircraft) 0.330 201[15] 41.9%
42,428 56,897 1993 GE LM6000 turboshaft Ship, electricity 0.329 200.1[16] 42.1%
130 170 2007 BMW N47 2L Diesel BMW cars 0.326 198[17] 42.6%
88 118 1990 Audi 2.5L TDI Diesel Audi 100 car 0.326 198[18] 42.6%
620 830 Scania AB DC16 078A Diesel 4-stroke electric generator 0.312 190[19] 44.4%
1,200 1,600 early 1990s Wärtsilä 6L20 Diesel 4-stroke Ship 0.311 189.4[20] 44.5%
3,600 4,800 MAN Diesel 6L32/44CR Diesel 4-stroke Ship, electricity 0.283 172[21] 49%
4,200 5,600 2015 Wärtsilä W31 Diesel 4-stroke Ship, electricity 0.271 165[22] 51.1%
34,320 46,020 1998 Wärtsilä-Sulzer RTA96-C Diesel 2-stroke Ship, electricity 0.263 160[23] 52.7%
27,060 36,290 MAN Diesel S80ME-C9.4-TII Diesel 2-stroke Ship, electricity 0.254 154.5[24] 54.6%
34,350 46,060 MAN Diesel 12G95ME-C9 Diesel 2-stroke Ship 0.254 154.5[25] 54.6%
605,000 811,000 2016 General Electric 9HA turbine, combined cycle electricity generation 0.223 135.5 (eq.) 62.2%[26]

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)[27]
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%

See also

References

Notes
  1. ↑ "Operator Manual for 447/503/582" (PDF). Rotax. Sep 2010.
  2. 1 2 3 4 5 6 "Gas Turbine Engines" (PDF). Aviation Week. January 2008.
  3. ↑ "Operator Manual for 914 series" (PDF). Rotax. Apr 2010.
  4. ↑ O-235 and O-290 Operator's Manual (PDF), Lycoming, Jan 2007, p. 3-8
  5. ↑ Michael Soroka (March 26, 2014). "Are Airplane Engines Inefficient?".
  6. ↑ "Advanced Gasoline Turbocharged Direct Injection (GTDI) Engine Development" (PDF). Ford Research and Advanced Engineering. May 13, 2011.
  7. ↑ IO-720 Operator's Manual (PDF), Lycoming, October 2006, p. 3-8
  8. ↑ "MT7 Brochure" (PDF). Rolls-Royce. 2012.
  9. ↑ Kimble D. McCutcheon (27 October 2014). "Wright R-3350 "Cyclone 18"" (PDF). Archived from the original (PDF) on 1 August 2016.
  10. ↑ "Development of New-Generation Hybrid System THS II - Drastic Improvement of Power Performance and Fuel Economy". Society of Automotive Engineers. 8 March 2004.
  11. ↑ inter-action association, 1987
  12. ↑ "Marine Trent". Civil Engineering Handbook. 19 Mar 2015.
  13. ↑ "Napier Nomad". Flight. 30 April 1954.
  14. ↑ "The new Audi A8 3.3 TDI quattro: Top TDI for the luxury class" (Press release). Audi AG. July 10, 2000.
  15. ↑ "Jane's Fighting Aircraft of World War II". London, UK: Bracken Books. 1989.
  16. ↑ "LM6000 Marine Gas Turbine" (PDF). General Electric. 2016. Archived from the original (PDF) on 2016-11-19.
  17. ↑ "BMW 2.0d (N47)" (in French). Auto-innovations. June 2007.
  18. ↑ "The New Audi 5-Cylinder Turbo Diesel Engine: The First Passenger Car Diesel Engine with Second Generation Direct Injection". Society of Automotive Engineers. 1 February 1990.
  19. ↑ "DC16 078A" (PDF). Scania AB.
  20. ↑ "Wärtsilä 20 product guide" (PDF). Wärtsilä. 14 February 2017.
  21. ↑ "Four-Stroke Propulsion Engines" (PDF). Man Diesel. 2015. Archived from the original (PDF) on 2016-04-17.
  22. ↑ "The new Wärtsilä 31 engine". Wärtsilä Technical Journal. 20 October 2015.
  23. ↑ "RTA-C Technology Review" (PDF). Wärtsilä. 2004. Archived from the original (PDF) on December 26, 2005.
  24. ↑ "MAN B&W S80ME-C9.4-TII Project Guide" (PDF). Man Diesel. May 2014.
  25. ↑ "MAN B&W G95ME-C9.2-TII Project Guide" (PDF). Man Diesel. May 2014. p. 16.
  26. ↑ Tomas Kellner (17 Jun 2016). "Here's Why The Latest Guinness World Record Will Keep France Lit Up Long After Soccer Fans Leave" (Press release). General Electric.
  27. ↑ "ATR: The Optimum Choice for a Friendly Environment" (PDF). Avions de Transport Regional. June 2001. p. PW127F engine gaseous emissions. Archived from the original (PDF) on 2016-08-08.
Bibliography

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