Brake-specific fuel consumption

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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. In traditional units, it measures fuel consumption in pounds per hour divided by the brake horsepower, lb/(hp⋅h); in SI units, this corresponds to the inverse of the units of specific energy, kg/J = s2/m2.

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 term "brake" here as in "brake horsepower" refers to a historical method of measuring torque (see Prony brake).

Calculation

The brake-specific fuel consumption is given by,

<math> \text{BSFC}= \frac{r}{P} </math>

where:

<math> r </math> is the fuel consumption rate in grams per second (g/s)
<math> P </math> is the power produced in watts where <math>P = \tau \omega</math> (W)
<math>\omega</math> is the engine speed in radians per second (rad/s)
<math>\tau</math> is the engine torque in newton metres (N⋅m)

The above values of r, <math>\omega</math>, and <math>\tau</math> 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

Relation to 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)

Operating values and as a cycle average statistic

BSFC [g/(kW⋅h)] map
Main article: Consumption 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)

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 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<ref>"Operator Manual for 447/503/582" (PDF). Rotax. Sep 2010. Archived from the original (PDF) on 2017-07-22. Retrieved 2018-06-08.</ref> 19.3%
321 431 1987 PW206B/B2 turboshaft Helicopter, EC135 0.553 336<ref name=GTEnginesAWJan2008>"Gas Turbine Engines" (PDF). Aviation Week. January 2008.</ref> 24.4%
427 572 1987 PW207D turboshaft Helicopter, Bell 427 0.537 327<ref name=GTEnginesAWJan2008/> 25.1%
500 670 1981 Arrius 2B1/2B1A-1 turboshaft Helicopter, EC135 0.526 320<ref name=GTEnginesAWJan2008/> 25.6%
13.1 17.8 1897 Motor 250/400<ref name="Mau_1984_7">Günter Mau: Handbuch Dieselmotoren im Kraftwerks- und Schiffsbetrieb, Vieweg (Springer), Braunschweig/Wiesbaden 1984, ISBN 978-3-528-14889-8, p. 7</ref> Diesel, four-stroke Stationary industrial Diesel engine 0.533 324 26.2%
820 1,100 1960 PT6C-67C turboshaft Helicopter, AW139 0.490 298<ref name=GTEnginesAWJan2008/> 27.5%
515 691 1991 Mazda R26B<ref name="Shimizu Tadokoro Nakanishi Funamoto 1992 p. 4">Shimizu, Ritsuharu; Tadokoro, Tomoo; Nakanishi, Toru; Funamoto, Junichi (1992-02-01). Mazda 4-Rotor Rotary Engine for the Le Mans 24-Hour Endurance Race. SAE International. p. 4. doi:10.4271/920309. ISSN 0148-7191.</ref> Wankel, four-rotor Race car, Mazda 787B 0.470 286 28.7%
958 1,285 1989 MTR390 turboshaft Helicopter, Tiger 0.460 280<ref name=GTEnginesAWJan2008/> 29.3%
84.5 113.3 1996 Rotax 914 gasoline, turbo Aviation, Light-sport aircraft, WT9 Dynamic 0.454 276<ref>"Operator Manual for 914 series" (PDF). Rotax. Apr 2010. Archived from the original (PDF) on 2017-06-11. Retrieved 2018-06-08.</ref> 29.7%
88 118 1942 Lycoming O-235-L gasoline Aviation, General aviation, Cessna 152 0.452 275<ref>O-235 and O-290 Operator's Manual (PDF), Lycoming, Jan 2007, p. 3-8 version-L</ref> 29.8%
456 612 1988 Honda RA168E gasoline, turbo Race car, McLaren MP4/4 0.447 272<ref>Honda Formula One Turbo-charged V-6 1.5L Engine, SAE, 1989-02-01</ref> 31.6%
1,770 2,380 1973 GE T700 turboshaft Helicopter, AH-1/UH-60/AH-64 0.433 263<ref name=GE18sep2019>Peter deBock (September 18, 2019). GE turbines and small engines overview (PDF). ARPA-e INTEGRATE meeting. Global Research. General Electric.</ref> 31.1%
3,781 5,071 1995 PW150 turboprop Airliner, Dash 8-400 0.433 263<ref name=GTEnginesAWJan2008/> 31.1%
1,799 2,412 1984 RTM322-01/9 turboshaft Helicopter, NH90 0.420 255<ref name=GTEnginesAWJan2008/> 32.1%
63 84 1991 GM Saturn I4 engine gasoline Cars, Saturn S-Series 0.411 250<ref name=Soroka>Michael Soroka (March 26, 2014). "Are Airplane Engines Inefficient?".</ref> 32.8%
150 200 2011 Ford EcoBoost gasoline, turbo Cars, Ford 0.403 245<ref>"Advanced Gasoline Turbocharged Direct Injection (GTDI) Engine Development" (PDF). Ford Research and Advanced Engineering. May 13, 2011.</ref> 33.5%
300 400 1961 Lycoming IO-720 gasoline Aviation, General aviation, PAC Fletcher 0.4 243<ref>IO-720 Operator's Manual (PDF), Lycoming, October 2006, p. 3-8</ref> 34.2%
5,600 7,500 1989 GE T408 turboshaft Helicopter, CH-53K 0.4 240<ref name=GE18sep2019/> 33.7%
7,000 9,400 1986 Rolls-Royce MT7 gas turbine Hovercraft, SSC 0.3998 243.2<ref>"MT7 Brochure" (PDF). Rolls-Royce. 2012. Archived from the original (PDF) on 2017-04-20. Retrieved 2018-07-09.</ref> 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<ref>Kimble D. McCutcheon (27 October 2014). "Wright R-3350 "Cyclone 18"" (PDF). Archived from the original (PDF) on 1 August 2016.</ref> 35.5%
57 76 2003 Toyota 1NZ-FXE gasoline Car, Toyota Prius 0.370 225<ref>Muta, Koichiro; Yamazaki, Makoto; Tokieda, Junji (8 March 2004). "Development of New-Generation Hybrid System THS II - Drastic Improvement of Power Performance and Fuel Economy". SAE Technical Paper Series. Vol. 1. Society of Automotive Engineers. doi:10.4271/2004-01-0064.</ref> 36.4%
8,251 11,065 2005 Europrop TP400 turboprop Airbus A400M 0.350 213<ref name="HectoPressure">Kaiser, Sascha; Donnerhack, Stefan; Lundbladh, Anders; Seitz, Arne (27–29 July 2015). A composite cycle engine concept with hecto-pressure ratio. AIAA/SAE/ASEE Joint Propulsion Conference (51st ed.). doi:10.2514/6.2015-4028.</ref> 39.6%
550 740 1931 Junkers Jumo 204 diesel two-stroke, turbo Aviation, Commercial aviation, Junkers Ju 86 0.347 211<ref>inter-action association, 1987</ref> 40%
36,000 48,000 2002 Rolls-Royce Marine Trent turboshaft Marine propulsion 0.340 207<ref>"Marine Trent". Civil Engineering Handbook. 19 Mar 2015.</ref> 40.7%
2,340 3,140 1949 Napier Nomad Diesel-compound Concept Aircraft engine 0.340 207<ref>"Napier Nomad". Flight. 30 April 1954.</ref> 40.7%
165 221 2000 Volkswagen 3.3 V8 TDI Diesel Car, Audi A8 0.337 205<ref>"The new Audi A8 3.3 TDI quattro: Top TDI for the luxury class" (Press release). Audi AG. July 10, 2000.</ref> 41.1%
2,013 2,699 1940 Deutz DZ 710 Diesel two-stroke Concept Aircraft engine 0.330 201<ref>"Jane's Fighting Aircraft of World War II". London, UK: Bracken Books. 1989.</ref> 41.9%
42,428 56,897 1993 GE LM6000 turboshaft Marine propulsion, Electricity generation 0.329 200.1<ref>"LM6000 Marine Gas Turbine" (PDF). General Electric. 2016. Archived from the original (PDF) on 2016-11-19.</ref> 42.1%
130 170 2007 BMW N47 2L Diesel Cars, BMW 0.326 198<ref>"BMW 2.0d (N47)" (in français). Auto-innovations. June 2007.</ref> 42.6%
88 118 1990 Audi 2.5L TDI Diesel Car, Audi 100 0.326 198<ref>Stock, Dieter; Bauder, Richard (1 February 1990). "The New Audi 5-Cylinder Turbo Diesel Engine: The First Passenger Car Diesel Engine with Second Generation Direct Injection". SAE Technical Paper Series. Vol. 1. Society of Automotive Engineers. doi:10.4271/900648.</ref> 42.6%
66 89 1992 VAG 1.9TDI 66kw Diesel 4-stroke Car, Audi 80, VW Golf/Passat 0.324 197<ref>"Realizing Future Trends in Diesel Engine Development" (PDF). Society of Automotive Engineers/VAG.</ref> 42.8%
368 493 2017 MAN D2676LF51 Diesel 4-stroke Truck/Bus 0.314 191<ref>"MAN TGX 2019" (PDF). MAN Truck & Bus.</ref> 44.1%
620 830 Scania AB DC16 078A Diesel 4-stroke Electricity generation 0.312 190<ref>"DC16 078A" (PDF). Scania AB.</ref> 44.4%
1,200 1,600 early 1990s Wärtsilä 6L20 Diesel 4-stroke Marine propulsion 0.311 189.4<ref>"Wärtsilä 20 product guide" (PDF). Wärtsilä. 14 February 2017.</ref> 44.5%
375 503 2019 MAN D2676LF78 Diesel 4-stroke Truck/Bus 0.302 184<ref>"MAN TGX 2019" (PDF). MAN Truck & Bus.</ref> 45.8%
3,600 4,800 MAN Diesel 6L32/44CR Diesel 4-stroke Marine propulsion, Electricity generation 0.283 172<ref>"Four-Stroke Propulsion Engines" (PDF). Man Diesel. 2015. Archived from the original (PDF) on 2016-04-17.</ref> 49%
4,200 5,600 2015 Wärtsilä W31 Diesel 4-stroke Marine propulsion, Electricity generation 0.271 165<ref>"The new Wärtsilä 31 engine". Wärtsilä Technical Journal. 20 October 2015.</ref> 51.1%
34,320 46,020 1998 Wärtsilä-Sulzer RTA96-C Diesel 2-stroke Marine propulsion, Electricity generation 0.263 160<ref>"RTA-C Technology Review" (PDF). Wärtsilä. 2004. Archived from the original (PDF) on December 26, 2005.</ref> 52.7%
27,060 36,290 MAN Diesel S80ME-C9.4-TII Diesel 2-stroke Marine propulsion, Electricity generation 0.254 154.5<ref>"MAN B&W S80ME-C9.4-TII Project Guide" (PDF). Man Diesel. May 2014.</ref> 54.6%
34,350 46,060 MAN Diesel G95ME-C9 Diesel 2-stroke Marine propulsion 0.254 154.5<ref>"MAN B&W G95ME-C9.2-TII Project Guide" (PDF). Man Diesel. May 2014. p. 16.</ref> 54.6%
605,000 811,000 2016 General Electric 9HA Combined cycle gas turbine Electricity generation 0.223 135.5 (eq.) 62.2%<ref>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.</ref>
640,000 860,000 2021 General Electric 7HA.3 Combined cycle gas turbine Electricity generation (proposed) 0.217 131.9 (eq.) 63.9%<ref>"GE Unveils New H-Class Gas Turbine—and Already Has a First Order". October 2, 2019.</ref>

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)<ref name=PW127F>"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.</ref>
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

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Further reading

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