Brake-specific fuel consumption explained

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,

BSFC = (r)/(P)

where:

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

The above values of r,

\omega , and \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

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 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

See main article: article and 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 (g/(kW⋅h)) for diesel fuel and jet fuel, 43.9 MJ/kg (g/(kW⋅h)) for gasoline.

! kW !! HP ! Year! Engine! Type! Application! lb/(hp⋅h)! g/(kW⋅h)! Efficiency

48disp=tableNaNdisp=table	1989	Rotax 582	gasoline, 2-stroke	Aviation, Ultralight, Eurofly Fire Fox	425g/kW.h^[1]	%
431hp	1987	PW206B/B2	turboshaft	Helicopter, EC135	0.553disp=tableNaNdisp=table^[2]	%
572hp	1987		turboshaft	Helicopter, Bell 427	0.537disp=tableNaNdisp=table	%
670hp	1981	Arrius 2B1/2B1A-1	turboshaft	Helicopter, EC135	0.526disp=tableNaNdisp=table	%
17.8PS	1897	Motor 250/400^[3]	Diesel, four-stroke	Stationary industrial Diesel engine	324g/kW.h	%
1100hp	1960	PT6C-67C	turboshaft	Helicopter, AW139	0.49disp=tableNaNdisp=table	%
515disp=tableNaNdisp=table	1991	Mazda R26B^[4]	Wankel, four-rotor	Race car, Mazda 787B	286g/kW.h	%
1285hp	1989		turboshaft	Helicopter, Tiger	0.46disp=tableNaNdisp=table	%
84.5disp=tableNaNdisp=table	1996		gasoline, turbo	Aviation, Light-sport aircraft, WT9 Dynamic	276g/kW.h^[5]	%
118hp	1942		gasoline	Aviation, General aviation, Cessna 152	NaNlb/hp.h	%
456disp=tableNaNdisp=table	1988	Honda RA168E	gasoline, turbo	Race car, McLaren MP4/4	272g/kW.h	31.6%
	1973	GE T700	turboshaft	Helicopter, AH-1/UH-60/AH-64	^[6]	%
	1995	PW150	turboprop	Airliner, Dash 8-400		%
2412hp	1984	RTM322-01/9	turboshaft	Helicopter, NH90	0.42disp=tableNaNdisp=table	%
63disp=tableNaNdisp=table	1991	GM Saturn I4 engine	gasoline		250g/kW.h^[7]	%
150disp=tableNaNdisp=table	2011	Ford EcoBoost	gasoline, turbo	Cars, Ford	245g/kW.h^[8]	%
400hp	1961	Lycoming IO-720	gasoline	Aviation, General aviation, PAC Fletcher	NaNlb/hp.h	%
	1989	GE T408	turboshaft	Helicopter, CH-53K		%
7000disp=tableNaNdisp=table	1986	Rolls-Royce MT7	gas turbine	Hovercraft, SSC	243.2g/kW.h^[9]	%
2000disp=tableNaNdisp=table	1945	Wright R-3350 Duplex-Cyclone	gasoline, turbo-compound	Aviation, Commercial aviation; B-29, Constellation, DC-7	0.38lb/hp.h^[10]	%
57disp=tableNaNdisp=table	2003	Toyota 1NZ-FXE	gasoline	Car, Toyota Prius	225g/kW.h^[11]	%
134disp=tableNaNdisp=table	2013	Lycoming DEL-120	Diesel four-stroke	MQ-1C Gray Eagle^[12]	0.36lb/hp.h	%
8251disp=tableNaNdisp=table	2005	Europrop TP400	turboprop	Airbus A400M	0.35lb/hp.h^[13]	%
550disp=tableNaNdisp=table	1931	Junkers Jumo 204	diesel two-stroke, turbo	Aviation, Commercial aviation, Junkers Ju 86	NaN155^[14]	%
36000disp=tableNaNdisp=table	2002		turboshaft	Marine propulsion	207g/kW.h^[15]	%
2340disp=tableNaNdisp=table	1949		Diesel-compound		0.34lb/hp.h^[16]	%
165disp=tableNaNdisp=table	2000	Volkswagen 3.3 V8 TDI	Diesel	Car, Audi A8	205g/kW.h^[17]	%
2013disp=tableNaNdisp=table	1940	Deutz DZ 710	Diesel two-stroke	Concept Aircraft engine	0.33lb/hp.h^[18]	%
42428disp=tableNaNdisp=table	1993	GE LM6000	turboshaft	Marine propulsion, Electricity generation	200.1g/kW.h^[19]	%
130disp=tableNaNdisp=table	2007	BMW N47 2L	Diesel	Cars, BMW	198g/kW.h^[20]	%
88disp=tableNaNdisp=table	1990	Audi 2.5L TDI	Diesel	Car, Audi 100	198g/kW.h^[21]	%
66disp=tableNaNdisp=table	1992	VAG 1.9TDI 66kw	Diesel 4-stroke	Car, Audi 80, VW Golf/Passat	197g/kW.h^[22]	%
368disp=tableNaNdisp=table	2017	MAN D2676LF51	Diesel 4-stroke	Truck/Bus	191g/kW.h^[23]	%
620disp=tableNaNdisp=table		Scania AB DC16 078A	Diesel 4-stroke	Electricity generation	190g/kW.h^[24]	%
1200disp=tableNaNdisp=table	early 1990s	Wärtsilä 6L20	Diesel 4-stroke	Marine propulsion	189.4g/kW.h^[25]	%
375disp=tableNaNdisp=table	2019	MAN D2676LF78	Diesel 4-stroke	Truck/Bus	184g/kW.h^[26]	%
3600disp=tableNaNdisp=table		MAN Diesel 6L32/44CR	Diesel 4-stroke	Marine propulsion, Electricity generation	172g/kW.h^[27]	%
4200disp=tableNaNdisp=table	2015	Wärtsilä W31	Diesel 4-stroke	Marine propulsion, Electricity generation	165g/kW.h^[28]	%
34320disp=tableNaNdisp=table	1998		Diesel 2-stroke	Marine propulsion, Electricity generation	160g/kW.h^[29]	%
27060disp=tableNaNdisp=table		MAN Diesel S80ME-C9.4-TII	Diesel 2-stroke	Marine propulsion, Electricity generation	154.5g/kW.h^[30]	%
34350disp=tableNaNdisp=table		MAN Diesel G95ME-C9	Diesel 2-stroke	Marine propulsion	154.5g/kW.h^[31]	%
605000disp=tableNaNdisp=table	2016	General Electric 9HA	Combined cycle gas turbine	Electricity generation	NaNg/kW.h (eq.)	62.2%^[32]
640000disp=tableNaNdisp=table	2021	General Electric 7HA.3	Combined cycle gas turbine	Electricity generation (proposed)	NaNg/kW.h (eq.)	63.9%^[33]

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

2,050 kW Pratt & Whitney Canada PW127 turboprop (1996)^[34]

Mode	Power	fuel flow	SFC	Energy efficiency
Nominal idle (7%)	192hp	3.06kg/min	NaN{{#expr:3.06*60/192}}	%
Approach (30%)	825hp	5.15kg/min	NaN{{#expr:5.15*60/825}}	%
Max cruise (78%)	2132hp	8.28kg/min	NaN{{#expr:8.28*60/2132}}	%
Max climb (80%)	2192hp	8.38kg/min	NaN{{#expr:8.38*60/2192}}	%
Max contin. (90%)	2475hp	9.22kg/min	NaN{{#expr:9.22*60/2475}}	%
Take-off (100%)	2750hp	9.9kg/min	NaN{{#expr:9.9*60/2750}}	%

See also

• Fuel economy in automobiles
• Energy-efficient driving
• Fuel management systems
• Marine fuel management
• Thrust specific fuel consumption

Further reading

• 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
• Exemplary maps for commercial car engines collected by ecomodder forum users

Notes and References

1. Web site: Operator Manual for 447/503/582 . Rotax . Sep 2010 . 2018-06-08 . https://web.archive.org/web/20170722033156/http://docusearch.flyrotax.com/files/pdf/d04495.pdf . 2017-07-22 . dead .
2. Gas Turbine Engines . Aviation Week . January 2008 .
3. Günter Mau: Handbuch Dieselmotoren im Kraftwerks- und Schiffsbetrieb, Vieweg (Springer), Braunschweig/Wiesbaden 1984,, p. 7
4. Shimizu . Ritsuharu . Tadokoro . Tomoo . Nakanishi . Toru . Funamoto . Junichi . SAE Technical Paper Series . Mazda 4-Rotor Rotary Engine for the Le Mans 24-Hour Endurance Race . SAE International . 1992-02-01 . 1 . 0148-7191 . 10.4271/920309 . 4.
5. Web site: Operator Manual for 914 series . Rotax . Apr 2010 . 2018-06-08 . https://web.archive.org/web/20170611004034/http://docusearch.flyrotax.com/files/pdf/d06153.pdf . 2017-06-11 . dead .
6. Global Research . GE turbines and small engines overview . . https://arpa-e.energy.gov/2019-integrate-annual-meeting . Peter deBock . ARPA-e INTEGRATE meeting . September 18, 2019.
7. Web site: Are Airplane Engines Inefficient? . Michael Soroka . March 26, 2014.
8. Web site: Advanced Gasoline Turbocharged Direct Injection (GTDI) Engine Development . Ford Research and Advanced Engineering . May 13, 2011.
9. Web site: MT7 Brochure . Rolls-Royce . 2012 . 2018-07-09 . https://web.archive.org/web/20170420204819/https://www.rolls-royce.com/~/media/Files/R/Rolls-Royce/documents/customers/marine/mt7-brochure.pdf . 2017-04-20 . dead .
10. Web site: Wright R-3350 "Cyclone 18" . Kimble D. McCutcheon . 27 October 2014 . dead . https://web.archive.org/web/20160801001759/http://www.enginehistory.org/Wright/Wright%20R-3350.pdf . 1 August 2016 .
11. Book: http://www.sae.org/technical/papers/2004-01-0064 . Development of New-Generation Hybrid System THS II - Drastic Improvement of Power Performance and Fuel Economy . . 8 March 2004. 10.4271/2004-01-0064 . SAE Technical Paper Series . Muta . Koichiro . Yamazaki . Makoto . Tokieda . Junji . 1 .
12. Web site: 22 October 2013 . GA-ASI's Improved Gray Eagle Flies Over 45 Hours Non-Stop . 2024-07-20 . General Atomics . en.
13. 27–29 July 2015 . 10.2514/6.2015-4028 . 51st . AIAA/SAE/ASEE Joint Propulsion Conference . A composite cycle engine concept with hecto-pressure ratio . Kaiser . Sascha . Donnerhack . Stefan . Lundbladh . Anders . Seitz . Arne.
14. inter-action association, 1987
15. Web site: Marine Trent . Civil Engineering Handbook . 19 Mar 2015.
16. Web site: Napier Nomad . Flight . 30 April 1954.
17. The new Audi A8 3.3 TDI quattro: Top TDI for the luxury class . Audi AG . July 10, 2000.
18. Web site: Jane's Fighting Aircraft of World War II . Bracken Books . London, UK . 1989.
19. Web site: LM6000 Marine Gas Turbine . General Electric . 2016 . dead . https://web.archive.org/web/20161119115601/http://www.geaviation.com/engines/docs/marine/datasheet-lm6000.pdf . 2016-11-19 .
20. Web site: BMW 2.0d (N47) . Auto-innovations . June 2007 . fr.
21. Book: http://www.sae.org/technical/papers/900648 . The New Audi 5-Cylinder Turbo Diesel Engine: The First Passenger Car Diesel Engine with Second Generation Direct Injection . . 1 February 1990 . 10.4271/900648 . SAE Technical Paper Series . Stock . Dieter . Bauder . Richard . 1 .
22. Web site: Realizing Future Trends in Diesel Engine Development . Society of Automotive Engineers/VAG.
23. Web site: MAN TGX 2019 . MAN Truck & Bus.
24. Web site: DC16 078A . Scania AB.
25. Web site: Wärtsilä 20 product guide . Wärtsilä . 14 February 2017 .
26. Web site: MAN TGX 2019 . MAN Truck & Bus.
27. Web site: Four-Stroke Propulsion Engines . https://web.archive.org/web/20160417234415/http://marine.man.eu/docs/librariesprovider6/4-Stroke-Engines/2015-four-stroke-propulsion-engines.pdf . dead . 2016-04-17 . Man Diesel . 2015 .
28. Web site: The new Wärtsilä 31 engine . Wärtsilä Technical Journal . 20 October 2015 .
29. Web site: RTA-C Technology Review . Wärtsilä . 2004 . dead . https://web.archive.org/web/20051226062109/http://www.wartsila.com/Wartsila/docs/en/ship_power/media_publications/brochures/product/engines/rtac_tr.pdf . December 26, 2005 .
30. Web site: MAN B&W S80ME-C9.4-TII Project Guide . Man Diesel . May 2014 . 2016-06-15 . 2016-08-09 . https://web.archive.org/web/20160809092926/http://marine.man.eu/applications/projectguides/2stroke/content/printed/S80ME-C9_4.pdf . dead .
31. Web site: MAN B&W G95ME-C9.2-TII Project Guide . Man Diesel . May 2014 . 16.
32. Here's Why The Latest Guinness World Record Will Keep France Lit Up Long After Soccer Fans Leave . Tomas Kellner . . 17 Jun 2016 . 14 April 2017 . 16 June 2017 . https://web.archive.org/web/20170616021542/http://www.gereports.com/bouchain/ . dead .
33. Web site: GE Unveils New H-Class Gas Turbine—and Already Has a First Order . October 2, 2019.
34. Web site: ATR: The Optimum Choice for a Friendly Environment . https://web.archive.org/web/20160808173542/http://web.fc.fi/data/files/ATR_TheOptimumChoice.pdf . dead . 2016-08-08 . PW127F engine gaseous emissions . . June 2001 .