Brake-specific fuel consumption

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. 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 BSFC calculation (in metric units)

To calculate BSFC, use the formula

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

\omega

is the engine speed in radians per second (rad/s)

\tau

is the engine torque in newton meters (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

The relationship between BSFC numbers and 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 (0.0122222kW·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)

The use of BSFC numbers as 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)

The significance of BSFC numbers for engine design and class

BSFC numbers change a lot for different engine design 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 of values of BSFC for shaft engines

The following table takes selected values as an example for the minimum specific fuel consumption of several types of engine.

Turboprop (aircraft engine) values are given for the complete range of power during a flight.
Ground roll SFC value (at 0.07 P_max) and especially idle (at rest) SFC values show dramatic increases when compared with cruise value (0.70 P_max).^[1]

For specific engines values can and often do differ from the table values shown below. For comparison, the theoretical work that can be derived from burning octane (C₈H₁₈) (based on change in Gibbs free energy going to gaseous H₂O and CO₂) is 45.7 MJ/kg, corresponding to 79 g/kW·h.

Power	Year	Engine type	Application	SFC (lb/hp·h)	SFC (g/kW·h)	Energy efficiency (%)
2,020 kW	1996	Pratt & Whitney turboprop PW127	Aircraft engine at idle		2,390
			Aircraft engine at ground roll		1,270
			Aircraft engine @ 0.30 P_max	.83	508	22.82
			Aircraft engine @ 0.70 P_max	.538	328	24.165
			Aircraft engine at P_max	.48	294	27
		Turbo-prop		0.8	360–490	17–23
		Otto cycle gasoline engines		0.45–0.37	273–227	30–36
80 kW/l	2011	Ford Ecoboost Downsized turbocharged otto engine^[2]	Automobile engine	0.40278	245
2,000 kW	1945	Wright R-3350 Duplex-Cyclone gasoline turbo-compound	Aircraft engine	0.4	243	33.7
57 kW		Toyota Prius THS II engine only ^[3]	automobile	.362	225	37
550 kW	1931	Junkers Jumo 204 turbocharged two-stroke diesel	Aircraft engine	0.345	210	39.8
36 MW	2002	Rolls-Royce Marine Trent turboshaft	Marine engine	0.345	210	39.8
2,013 kW	1940	Klöckner-Humboldt-Deutz DZ 710 Diesel two stroke	Aircraft engine	0.33	201	41.58
2,340 kW	1949	Napier Nomad Diesel-compound	Aircraft engine	0.345	210	39.8
		Diesel engine turbocharged diesels		0.34–0.30	209–178	40–47
165 kW	2000	Volkswagen 3.3 V8 TDI	Automobile engine	0.33	205	41.1
43 MW		General Electric LM6000^[4] turboshaft	Marine engine, power generation	0.32	199	42
105-160 kW	2007	BMW N47 2.0 litre variable geometry turbocharging^[5]	Automobile engine		198-204
88 kW	1990	Audi 2.5 litre TDI^[6]	Automobile engine		198	42.5
80 MW	1998	Wärtsilä-Sulzer RTA96-C two-stroke	Marine engine	0.268	163	51.7
23 MW		MAN Diesel S80ME-C Mk7 two-stroke	Marine engine ^[7]	0.254	155	54.4

References

Notes

^ ATR, The Optimum Choice, ATR 72-500 trip pattern
^ http://www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2011/adv_combustion/ace065_rinkevich_2011_o.pdf#23
^ "SAE 2004-01-0064 para 2-3-1". Sae.org. Retrieved 2014-05-22.
^ http://www.geaviation.com/engines/marine/pdfs/datasheet_lm6000.pdf
^ http://www.auto-innovations.com/site/dossier5/BMWn47t28print.html
^ "SAE paper 900648". Sae.org. Retrieved 2014-05-22.
^ Man Diesel Se - Marine

Bibliography

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

External links

Exemplary maps for commercial car engines collected by ecomodder forum users

[1] ATR, The Optimum Choice, ATR 72-500 trip pattern

[2] ttp://www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2011/adv_combustion/ace065_rinkevich_2011_o.pdf#23

[3] "SAE 2004-01-0064 para 2-3-1". Sae.org. Retrieved 2014-05-22.

[4] ttp://www.geaviation.com/engines/marine/pdfs/datasheet_lm6000.pdf

[5] ttp://www.auto-innovations.com/site/dossier5/BMWn47t28print.html

[6] "SAE paper 900648". Sae.org. Retrieved 2014-05-22.

[7] Man Diesel Se - Marine

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