# 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

where:

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

## 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[1] 29.1%
95 1970 Lycoming O-320 piston, gasoline General aviation 0.460 280[2] 29.3%
63 1991 GM Saturn I4 engine, gasoline Saturn S-Series cars 0.411 250[2] 32.5%
150 2011 Ford EcoBoost gasoline, turbo Ford cars 0.403 245[3] 33.5%
2,000 1945 Wright R-3350 Duplex-Cyclone gasoline, turbo-compound Bombers, airliners 0.380 231[4] 35.5%
57 2003 Toyota 1NZ-FXE, gasoline Toyota Prius car 0.370 225[5] 36.4%
550 1931 Junkers Jumo 204 two-stroke diesel, turbo Bombers, airliners 0.347 211[6] 40%
36,000 2002 Rolls-Royce Marine Trent turboshaft Combat ships 0.340 207[7] 40.7%
2,340 1949 Napier Nomad Diesel-compound planned (aircraft intended) 0.340 207[8] 40.7%
165 2000 Volkswagen 3.3 V8 TDI Audi A8 car 0.337 205[9] 41.1%
2,013 1940 Klöckner-Humboldt-Deutz DZ 710 Diesel two stroke none (aircraft intended) 0.330 201[10] 41.9%
42,428 1993 General Electric LM6000 turboshaft Ship, electricity 0.329 200.1[11] 42.1%
130 2007 BMW N47 2L turbodiesel BMW cars 0.326 198[12] 42.6%
88 1990 Audi 2.5L TDI Audi 100 car 0.326 198[13] 42.6%
3,600 MAN Diesel 6L32/44CR four-stroke Ship, electricity 0.283 172[14] 49%
34,320 1998 Wärtsilä-Sulzer RTA96-C two-stroke Ship, electricity 0.263 160[15] 52.7%
27,060 MAN Diesel S80ME-C9.4-TII two-stroke Ship, electricity 0.254 154.5[16] 54.6%

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

2,050 kW Pratt & Whitney turboprop PW127 (1996)[1]
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%

## References

Notes
1. "ATR: The Optimum Choice for a Friendly Environment" (PDF). Avions de Transport Regional. June 2001. p. PW127F engine gaseous emissions.
2. Michael Soroka (March 26, 2014). "Are Airplane Engines Inefficient?".
3. "Advanced Gasoline Turbocharged Direct Injection (GTDI) Engine Development" (PDF). Ford Research and Advanced Engineering. May 13, 2011.
4. Kimble D. McCutcheon (27 October 2014). "Wright R-3350 "Cyclone 18"" (PDF).
5. inter-action association, 1987
6. "Marine Trent". Civil Engineering Handbook. 19 Mar 2015.
7. "Napier Nomad". Flight. 30 April 1954.
8. "The new Audi A8 3.3 TDI quattro: Top TDI for the luxury class" (Press release). Audi AG. July 10, 2000.
9. "Jane's Fighting Aircraft of World War II". London, UK: Bracken Books. 1989.
10. "LM6000 Marine Gas Turbine" (PDF). General Electric. 2016.
11. "BMW 2.0d (N47)" (in French). Auto-innovations. June 2007.
12. "Four-Stroke Propulsion Engines" (PDF). Man Diesel. 2015.
13. "RTA-C Technology Review" (PDF). Wärtsilä. 2004. Archived from the original on December 26, 2005.
14. "MAN B&W S80ME-C9.4-TII Project Guide" (PDF). Man Diesel. May 2014.
Bibliography