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,

where: is the fuel consumption rate in grams per second (g/s) is the power produced in watts where (W) is the engine speed in radians per second (rad/s) is the engine torque in newton metres (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

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 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

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 1989 Rotax 582 gasoline, 2-stroke Aviation, Ultralight, Eurofly Fire Fox % 1987 PW206B/B2 turboshaft Helicopter, EC135 % 1987 PW207D turboshaft Helicopter, Bell 427 % 1981 Arrius 2B1/2B1A-1 turboshaft Helicopter, EC135 % 1897 Motor 250/400Günter Mau: Handbuch Dieselmotoren im Kraftwerks- und Schiffsbetrieb, Vieweg (Springer), Braunschweig/Wiesbaden 1984, , p. 7 Diesel, four-stroke Stationary industrial Diesel engine % 1960 PT6C-67C turboshaft Helicopter, AW139 % 1991Mazda R26B Wankel, four-rotor Race car, Mazda 787B % 1989 MTR390 turboshaft Helicopter, Tiger % 1996 Rotax 914 gasoline, turbo Aviation, Light-sport aircraft, WT9 Dynamic % 1942 Lycoming O-235-L gasoline Aviation, General aviation, Cessna 152 % 1988 Honda RA168E gasoline, turbo Race car, McLaren MP4/4 31.6% 1973 GE T700 turboshaft Helicopter, AH-1/UH-60/AH-64 % 1995 PW150 turboprop Airliner, Dash 8-400 % 1984 RTM322-01/9 turboshaft Helicopter, NH90 % 1991 GM Saturn I4 engine gasoline Cars, Saturn S-Series % 2011 Ford EcoBoost gasoline, turbo Cars, Ford % 1961 Lycoming IO-720 gasoline Aviation, General aviation, PAC Fletcher % 1989 GE T408 turboshaft Helicopter, CH-53K % 1986 Rolls-Royce MT7 gas turbine Hovercraft, SSC % 1945 Wright R-3350 Duplex-Cyclone gasoline, turbo-compound Aviation, Commercial aviation; B-29, Constellation, DC-7 % 2003 Toyota 1NZ-FXE gasoline Car, Toyota Prius % 2013 Lycoming DEL-120 Diesel four-stroke MQ-1C Gray Eagle % 2005 Europrop TP400 turboprop Airbus A400M % 1931 Junkers Jumo 204 diesel two-stroke, turboAviation, Commercial aviation, Junkers Ju 86 inter-action association, 1987 % 2002 Rolls-Royce Marine Trent turboshaft Marine propulsion % 1949 Napier Nomad Diesel-compound Concept Aircraft engine % 2000 Volkswagen 3.3 V8 TDI Diesel Car, Audi A8 % 1940 Deutz DZ 710 Diesel two-stroke Concept Aircraft engine % 1993 GE LM6000 turboshaft Marine propulsion, Electricity generation % 2007 BMW N47 2L Diesel Cars, BMW % 1990 Audi 2.5L TDI Diesel Car, Audi 100 % 1992 VAG 1.9TDI 66 kW Diesel 4-stroke Car, Audi 80, VW Golf/Passat % 2017 MAN D2676LF51 Diesel 4-stroke Truck/Bus % Scania AB DC16 078A Diesel 4-stroke Electricity generation % early 1990s Wärtsilä 6L20 Diesel 4-stroke Marine propulsion % 2019 MAN D2676LF78 Diesel 4-stroke Truck/Bus % Scania DC13 541A 283 kW Diesel 4-stroke Industrial % MAN Diesel 6L32/44CR Diesel 4-stroke Marine propulsion, Electricity generation % 2015 Wärtsilä W31 Diesel 4-strokeMarine propulsion, Electricity generation % 1998 Wärtsilä-Sulzer RTA96-C Diesel 2-stroke Marine propulsion, Electricity generation % MAN Diesel S80ME-C9.4-TII Diesel 2-stroke Marine propulsion, Electricity generation % MAN Diesel G95ME-C9 Diesel 2-stroke Marine propulsion % 2016 General Electric 9HA Combined cycle gas turbine Electricity generation (eq.) 62.2% 2021 General Electric 7HA.3 Combined cycle gas turbine Electricity generation (proposed) (eq.) 63.9%

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) Mode Power fuel flow SFC Energy efficiency Nominal idle (7%) % Approach (30%) % Max cruise (78%) % Max climb (80%) % Max contin. (90%) % Take-off (100%) %

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

References

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