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

BSFC [g/kW·h] map

Main article: 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)

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 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.

Power (kW)	Year	Engine type	Application	SFC (lb/hp·h)	SFC (g/kW·h)	Energy efficiency
2,050	1996	Pratt & Whitney Canada PW127 turboprop	ATR 72 regional airliner	0.477	290	29.1%
95	1970	Lycoming O-320 piston, gasoline	General aviation	0.460	280	29.3%
63	1991	GM Saturn I4 engine, gasoline	Saturn S-Series cars	0.411	250	32.5%
150	2011	Ford EcoBoost gasoline, turbo	Ford cars	0.403	245	33.5%
2,000	1945	Wright R-3350 Duplex-Cyclone gasoline, turbo-compound	Bombers, airliners	0.380	231	35.5%
57	2003	Toyota 1NZ-FXE, gasoline	Toyota Prius car	0.370	225	36.4%
550	1931	Junkers Jumo 204 two-stroke diesel, turbo	Bombers, airliners	0.347	211	40%
36,000	2002	Rolls-Royce Marine Trent turboshaft	Combat ships	0.340	207	40.7%
2,340	1949	Napier Nomad Diesel-compound	planned (aircraft intended)	0.340	207	40.7%
165	2000	Volkswagen 3.3 V8 TDI	Audi A8 car	0.337	205	41.1%
2,013	1940	Klöckner-Humboldt-Deutz DZ 710 Diesel two stroke	none (aircraft intended)	0.330	201	41.9%
42,428	1993	General Electric LM6000 turboshaft	Ship, electricity	0.329	200.1	42.1%
130	2007	BMW N47 2L turbodiesel	BMW cars	0.326	198	42.6%
88	1990	Audi 2.5L TDI	Audi 100 car	0.326	198	42.6%
3,600		MAN Diesel 6L32/44CR four-stroke	Ship, electricity	0.283	172	49%
34,320	1998	Wärtsilä-Sulzer RTA96-C two-stroke	Ship, electricity	0.263	160	52.7%
27,060		MAN Diesel S80ME-C9.4-TII two-stroke	Ship, electricity	0.254	154.5	54.6%

Turboprops efficiency are only good at high power, for approach at low power (30% P_max) and especially at idle (7% P_max), SFC increases dramatically :

2,050 kW Pratt & Whitney turboprop PW127 (1996)

Mode	Power	fuel flow	SFC	Energy efficiency
Nominal idle (7%)	192 hp (143 kW)	3.06 kg/min (405 lb/h)	1,282 g/(kW⋅h) (2.108 lb/(hp⋅h))	6.6%
Approach (30%)	825 hp (615 kW)	5.15 kg/min (681 lb/h)	502 g/(kW⋅h) (0.825 lb/(hp⋅h))	16.8%
Max cruise (78%)	2,132 hp (1,590 kW)	8.28 kg/min (1,095 lb/h)	312 g/(kW⋅h) (0.513 lb/(hp⋅h))	27%
Max climb (80%)	2,192 hp (1,635 kW)	8.38 kg/min (1,108 lb/h)	308 g/(kW⋅h) (0.506 lb/(hp⋅h))	27.4%
Max contin. (90%)	2,475 hp (1,846 kW)	9.22 kg/min (1,220 lb/h)	300 g/(kW⋅h) (0.493 lb/(hp⋅h))	28.1%
Take-off (100%)	2,750 hp (2,050 kW)	9.9 kg/min (1,310 lb/h)	290 g/(kW⋅h) (0.477 lb/(hp⋅h))	29.1%

See also

• Fuel economy in automobiles
• Fuel economy-maximizing behaviors
• Fuel management systems
• Marine fuel management
• Thrust specific fuel consumption

References

Notes

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