Brake-specific fuel consumption

Template:Short description 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,

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

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 Template:Convert 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

Template:Main article 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
Template:Convert	1989	Rotax 582	gasoline, 2-stroke	Aviation, Ultralight, Eurofly Fire Fox	Template:Convert	19.3%
Template:Convert	1987	PW206B/B2	turboshaft	Helicopter, EC135	Template:Convert	24.4%
Template:Convert	1987	PW207D	turboshaft	Helicopter, Bell 427	Template:Convert	25.1%
Template:Convert	1981	Arrius 2B1/2B1A-1	turboshaft	Helicopter, EC135	Template:Convert	25.6%
Template:Convert	1897	Motor 250/400	Diesel, four-stroke	Stationary industrial Diesel engine	Template:Convert	26.2%
Template:Convert	1960	PT6C-67C	turboshaft	Helicopter, AW139	Template:Convert	27.5%
Template:Convert	1991	Mazda R26B	Wankel, four-rotor	Race car, Mazda 787B	Template:Convert	28.7%
Template:Convert	1989	MTR390	turboshaft	Helicopter, Tiger	Template:Convert	29.3%
Template:Convert	1996	Rotax 914	gasoline, turbo	Aviation, Light-sport aircraft, WT9 Dynamic	Template:Convert	29.7%
Template:Convert	1942	Lycoming O-235-L	gasoline	Aviation, General aviation, Cessna 152	Template:Convert	%
Template:Convert	1988	Honda RA168E	gasoline, turbo	Race car, McLaren MP4/4	Template:Convert	31.6%
Template:Cvt	1973	GE T700	turboshaft	Helicopter, AH-1/UH-60/AH-64	Template:Cvt	31.1%
Template:Cvt	1995	PW150	turboprop	Airliner, Dash 8-400	Template:Cvt	31.1%
Template:Convert	1984	RTM322-01/9	turboshaft	Helicopter, NH90	Template:Convert	32.1%
Template:Convert	1991	GM Saturn I4 engine	gasoline	Cars, Saturn S-Series	Template:Convert	32.8%
Template:Convert	2011	Ford EcoBoost	gasoline, turbo	Cars, Ford	Template:Convert	33.5%
Template:Convert	1961	Lycoming IO-720	gasoline	Aviation, General aviation, PAC Fletcher	Template:Convert	%
Template:Cvt	1989	GE T408	turboshaft	Helicopter, CH-53K	Template:Cvt	33.7%
Template:Convert	1986	Rolls-Royce MT7	gas turbine	Hovercraft, SSC	Template:Convert	34.7%
Template:Convert	1945	Wright R-3350 Duplex-Cyclone	gasoline, turbo-compound	Aviation, Commercial aviation; B-29, Constellation, DC-7	Template:Convert	%
Template:Convert	2003	Toyota 1NZ-FXE	gasoline	Car, Toyota Prius	Template:Convert	36.4%
Template:Convert	2013	Lycoming DEL-120	Diesel four-stroke	MQ-1C Gray Eagle	Template:Convert	38.5%
Template:Convert	2005	Europrop TP400	turboprop	Airbus A400M	Template:Convert	39.6%
Template:Convert	1931	Junkers Jumo 204	diesel two-stroke, turbo	Aviation, Commercial aviation, Junkers Ju 86	Template:Convert	%
Template:Convert	2002	Rolls-Royce Marine Trent	turboshaft	Marine propulsion	Template:Convert	40.7%
Template:Convert	1949	Napier Nomad	Diesel-compound	Concept Aircraft engine	Template:Convert	%
Template:Convert	2000	Volkswagen 3.3 V8 TDI	Diesel	Car, Audi A8	Template:Convert	41.1%
Template:Convert	1940	Deutz DZ 710	Diesel two-stroke	Concept Aircraft engine	Template:Convert	%
Template:Convert	1993	GE LM6000	turboshaft	Marine propulsion, Electricity generation	Template:Convert	42.1%
Template:Convert	2007	BMW N47 2L	Diesel	Cars, BMW	Template:Convert	42.6%
Template:Convert	1990	Audi 2.5L TDI	Diesel	Car, Audi 100	Template:Convert	42.6%
Template:Convert	1992	VAG 1.9TDI 66kw	Diesel 4-stroke	Car, Audi 80, VW Golf/Passat	Template:Convert	42.8%
Template:Convert	2017	MAN D2676LF51	Diesel 4-stroke	Truck/Bus	Template:Convert	44.1%
Template:Convert		Scania AB DC16 078A	Diesel 4-stroke	Electricity generation	Template:Convert	44.4%
Template:Convert	early 1990s	Wärtsilä 6L20	Diesel 4-stroke	Marine propulsion	Template:Convert	44.5%
Template:Convert	2019	MAN D2676LF78	Diesel 4-stroke	Truck/Bus	Template:Convert	45.8%
Template:Convert		MAN Diesel 6L32/44CR	Diesel 4-stroke	Marine propulsion, Electricity generation	Template:Convert	49%
Template:Convert	2015	Wärtsilä W31	Diesel 4-stroke	Marine propulsion, Electricity generation	Template:Convert	51.1%
Template:Convert	1998	Wärtsilä-Sulzer RTA96-C	Diesel 2-stroke	Marine propulsion, Electricity generation	Template:Convert	52.7%
Template:Convert		MAN Diesel S80ME-C9.4-TII	Diesel 2-stroke	Marine propulsion, Electricity generation	Template:Convert	54.6%
Template:Convert		MAN Diesel G95ME-C9	Diesel 2-stroke	Marine propulsion	Template:Convert	54.6%
Template:Convert	2016	General Electric 9HA	Combined cycle gas turbine	Electricity generation	Template:Convert (eq.)	62.2%
Template:Convert	2021	General Electric 7HA.3	Combined cycle gas turbine	Electricity generation (proposed)	Template:Convert (eq.)	63.9%

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)

Mode	Power	fuel flow	SFC	Energy efficiency
Nominal idle (7%)	Template:Convert	Template:Convert	Template:Convert	6.6%
Approach (30%)	Template:Convert	Template:Convert	Template:Convert	%
Max cruise (78%)	Template:Convert	Template:Convert	Template:Convert	%
Max climb (80%)	Template:Convert	Template:Convert	Template:Convert	%
Max contin. (90%)	Template:Convert	Template:Convert	Template:Convert	%
Take-off (100%)	Template:Convert	Template:Convert	Template:Convert	%

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