Brake Specific Fuel Consumption (BSFC) is a measure of fuel efficiency within a

shaft reciprocating engine. 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 reciprocating engines to be directly compared.

Contents

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1   The BSFC calculation (in metric units)

2   The relationship between BSFC numbers and efficiency

3   The use of BSFC numbers as operating values and as a cycle average statistic

4   The significance of BSFC numbers for engine design and class

5   Typical values of BSFC for shaft engines

6   See also

7   References

8   External links

[edit]The BSFC calculation (in metric units)

To calculate this rate, use the formula 

Where:

r is the fuel consumption rate in grams per second (g·s-1)

P is the power produced in watts where P = τω

ω is the engine speed in radians per second (rad·s-1)

τ is the engine torque in newton meters (N·m)

The resulting units of BSFC are grams per joule (g·J−1)

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

[edit]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 fuels

heating value. The lower heating value LHV is used for internal

combustion engines 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 = 18640 BTU/lb = 0.01204 kW·h/g

Regular gasoline = 18917 BTU/lb = 0.0122225 kW·h/g

Diesel fuel = 18500 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)

[edit]The use of BSFC numbers as operating values and as a cycle average statistic

BSFC [g/(kW·h)] 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 torque peak. However, the numbers

often reported for a particular engine are a fuel economy cycle

average statistic. For example, the cycle average value of BSFC

for a gasoline engine is 322 g/(kW·h), translating to an efficiency

of 25%. However, efficiency for that engine can be lower or

higher than this average statistic depending on the operating

condition. 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%.

A fuel island plot of a diesel engine BSFC is shown. The sweet

spot at 206 BSFC has 40.6% efficiency.

[edit]The significance of BSFC numbers for engine design and class

The beauty of BSFC numbers is that they remain similar over a

wide range of engine sizes. These numbers only change for

different engine designs and compression ratios. For example, a

small one cylinder 50 cc four-stroke and a large V8 engine can

both have the same BSFC number. However, engines of

different classes like diesels and gasoline engines will have very

different BSFC numbers.

[edit]Typical values of BSFC for shaft engines

The following table gives the minimum specific fuel consumption

of several types of engine. For comparison, the theoretical work

that can be derived from burning octane (C8H18) (based on

change inGibbs free energy going to gaseous H2O and CO2) is

45.7 MJ/kg, corresponding to 79 g/(kW·h).

Power date Engine typeSFC in lb/(hp·h

)

SFC in g/(kW·h

)

Energy efficienc

y

Turbo-prop 0.8360 to 490

17 to 23%

Otto cycle Gasoline Engines

.45 to .37

273 to 227

30 to 36%

Diesel engine Turbocharged Diesels

.34 to .30

209 to 178

40 to 47%

2000 kW

1945

Wright R-3350 gasoline-compound airplane engine

0.4 243 33.7%

57 kWToyota Prius THS II engine only [1] 225 37%

68 kW2008

REVETEC X4 Gasoline aircraft/auto engine[2]

212 38.6%

550 kW1931

Junkers Jumo 204 Turbocharged Diesel

210 39.8%

36 MWRolls-Royce MT30 turboshaft

210 39.8%

2340 kW

1949

Napier Nomad Diesel-compound engine

0.345 210 39.8%

165 kW2000

Volkswagen 3.3 V8 TDI car engine

0.33 205 41.1%

43 MWGeneral Electric LM6000 turboshaft

42%

88 kW1990

Audi 2.5 litre TDI[3] 198 42.5%

213 kW Volvo D7E 290 hp diesel truck

188 44.8%

engine[citation needed]

80 MW1998

Wärtsilä-Sulzer RTA96-C two-stroke marine engine

163 51.7%

23 MW

MAN B&W Diesel S80ME-C Mk7 two-stroke marine engine [4]

155