# Brake specific fuel consumption

Brake specific fuel consumption

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.

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 correctedFact|date=May 2008."
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 valueFact|date=May 2008.

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 near its torque peakFact|date=May 2008. 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.

ignificance 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.

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 (based on change in Gibbs free energy going to gaseous H2O and CO2) is 45.7 MJ/kg, corresponding to 79 g/(kW·h).

*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). [http://adg.stanford.edu/aa241/propulsion/largefan.html] 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).

ee also

*Fuel economy in automobiles
*Specific fuel consumption (thrust) - jet engine fuel efficiency

References

[http://www.tc.gc.ca/programs/environment/climatechange/subgroups1/vehicle_technology/study2/Final_report/image/Final_29.gif] Typical brake-specific fuel consumption map.

Wikimedia Foundation. 2010.

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