Brake-specific fuel consumption

Brake-specific fuel consumption 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/; 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.

Calculation

The brake-specific fuel consumption is given by,
where:
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
Commonly BSFC is expressed in units of grams per kilowatt-hour. The conversion factor is as follows:
The conversion between metric and imperial units is:

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 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:
Thus a diesel engine's efficiency = 1/ and a gasoline engine's efficiency = 1/

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/, translating to an efficiency of 25%. 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/, which is equivalent to a thermodynamic efficiency of 36%.
An iso-BSFC map 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

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/ to more than 1,000 g/.

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 for diesel fuel and jet fuel, 43.9 MJ/kg for gasoline.

kW	HP	Year	Engine	Type	Application	lb/	g/	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 250/400	Diesel, four-stroke	Stationary industrial Diesel engine		%	-	-
	1960	PT6C-67C	turboshaft	Helicopter, AW139		%	-	-
	1991	Mazda 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, turbo	Aviation, Commercial aviation, Junkers Ju 86		%	-	-
	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 66kW	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-stroke	Marine 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		62.2%	-	-
	2021	General Electric 7HA.3	Combined cycle gas turbine	Electricity generation		63.9%	-	-

Turboprop efficiency is only good at high power; SFC increases dramatically for approach at low power and especially at idle :

Mode	Power	fuel flow	SFC	Energy efficiency
Nominal idle				%
Approach				%
Max cruise				%
Max climb				%
Max contin.				%
Take-off				%