Brake specific fuel consumption

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This article is about specific fuel consumption. For the BSFC Awards, see Boston Society of Film Critics. For the fuel consumption related to thrust for jet engines, see specific fuel consumption (thrust).

Brake specific fuel consumption (BSFC) is a measure of an engine's efficiency. It is the rate of fuel consumption divided by the power produced. BSFC is specific for shaft engines such as reciprocating engine.

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[edit] The BSFC Calculation (in metric units)

To calculate BSFC, use the formula BSFC = Fuel_rate / Power
Where:

Fuel_rate is the fuel consumption in grams per hour (g/hr)
Power is the power produced in Kilowatts where kW = w * Tq / 9549.27
w is the engine speed in rpm
Tq is the engine torque in newton meters (N·m)
Note: The Power in the BSFC calculation is not weather corrected[citation needed].


The resulting units of BSFC are g/(kW·h)
The conversion between metric and U.S. units is:

BSFC_US(Lbs/(HP*Hr)) * 608.277 = BSFC_METRIC(g/(kW·h))
BSFC_METRIC(g/(kW·h)) * 0.001644 = BSFC_US(Lbs/(HP*Hr))


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 lower heating value[citation needed].

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)

A typical cycle average value of BSFC for a gasoline engine is 322 g/(kW·h). This means the average efficiency of a gasoline engine is only 25%. A reciprocating engine achieves maximum efficiency when the intake air is unthrottled and the engine is running at its torque peak[citation needed]. Efficiency is lower at other operating conditions. For a gasoline engine, the most efficient BSFC is approximately 236 g/(kW·h) or an efficiency of 37%. As seen above, lower values of BSFC mean higher engine efficiency. Diesel engines are generally more efficient than gasoline engines and can have a BSFC as low as 155 g/(kW·h) (partly because of the higher calorific value for diesel fuel) and around 55% efficiency.

[edit] Significance of BSFC

SFC is dependent on engine design, but differences in the BSFC between different engines using the same underlying technology tend to be quite small. For instance, typical gasoline engines will have an SFC of about 0.5 lb/(hp·h) or (0.3 kg/(kW·h) = 83g/MJ), regardless of the design of any particular engine. Generally, SFC within a particular class of engine will decrease when the compression ratio is increased. Diesel engines have better SFCs than gasoline, largely because they have much higher compression ratios and therefore they can convert more of the heat produced into power.

In practical applications, other factors are usually highly significant in determining the fuel efficiency of a particular engine design in that particular application.

[edit] Typical values of BSFC for shaft engines

The following table gives the specific fuel consumption of several types of engine. For comparison, the theoretical work that can be derived from burning octane (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 date Engine type SFC in lb/(hp·h) SFC in g/(kW·h) Energy efficiency
Turbo-prop 0.8 360 to 490
Otto cycle gasoline engine 0.5 300
Diesel engine automotive 0.4 230 to 260
2000 kW 1945 Wright R-3350 gasoline-compound airplane engine 0.4 243
57 kW Toyota Prius THS II engine only [1] 236
68 kW 2008 REVETEC X4 Gasoline aircraft/auto engine[2] 212 38.6%
550 kW 1931 Junkers Jumo 204 Turbocharged Diesel 210
2340 kW 1949 Napier Nomad Diesel-compound engine 0.345 210
165 kW 2000 Volkswagen 3.3 V8 TDI car engine 0.33 205 41.1%
43 MW General Electric LM6000 turboshaft 42%
88 kW 1990 Audi 2.5 litre TDI[3] 198 42.5%
213 kW Volvo D7E 290 hp diesel truck engine[citation needed] 188 44.8%
80 MW 1998 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 54.4%
  • Turbo-prop. The specific fuel consumption varies with speed, altitude and power SFC. For example, PT6 turbine on Cessna 208: at low power SFC may exceed 800 g/(kW·h). Cruise sfc is about 400 g/(kW·h). A turbine is SFC efficient at high power only.
  • Jet engines. The General Electric CF6-80C2B2F medium sized civil turbofan (at cruise) uses 0.605 lb/h per lbf of thrust (17.1 (g/s) /kN).[5] At a normal cruising speed of 913 km/h, this comes out at an effective BSFC of 440 g/(kW·h). Note that the figures are not directly comparable to propeller engines above as the jet engine figures allow for propulsive efficiency, whereas the above numbers are 'at the shaft' (see propulsive efficiency).

[edit] See also

[edit] References

  1. ^ SAE 2004010064
  2. ^ http://www.revetec.com/pdf/REVETEC%20X4V2%20Engine%20Evaluation%20Report_final.pdf
  3. ^ SAE paper 900648
  4. ^ Man Diesel Se - Marine
  5. ^ http://adg.stanford.edu/aa241/propulsion/largefan.html

[edit] External links