Brake specific fuel consumption

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Brake specific fuel consumption (BSFC) is a measure of an engine's efficiency. It is the rate of fuel consumption divided by the rate of power production. BSFC is specific for the piston engine known as the reciprocating engine. The general term is specific fuel consumption (SFC). There is also thrust specific fuel consumption (TSFC) for turbine and rocket engines.

[edit] Metric lower heating value method

To calculate, use the formula BSFC = f/((0.10472s)(0.001τ))

where f is fuel consumption in grams per hour (1 g/s = 3600 g/h)
s is engine speed in rpm
τ is engine torque in newton meters (N·m)

The resulting measure BSFC is in g/(kW·h)
To calculate the actual efficiency of the engine requires that you know the energy density of the fuel being used.
Different fuels have different energy densities:

The lower heating value is used for energy density.
The lower heating value for vehicle fuels are:
Certification gasoline = 18640 BTU/lb = 12.04 kW·h/kg
Regular gasoline = 18917 BTU/lb = 12.22 kW·h/kg
Diesel fuel = 18500 BTU/lb = 11.95 kW·h/kg

Thus a diesel engine's efficiency = 1/(BSFC*.0119531)
and a gasoline engine's efficiency = 1/(BSFC*.0122225)

[edit] Mass based U.S. units method

BSFC is commonly expressed in pounds of fuel per horsepower hour, in the U.S.
(Smaller number is better efficiency.)

  • BSFC = (observed fuel flow) / (observed brake horsepower)
  • Units are observed.
  • Fuel flow is pounds per hour
  • Observed BHP is used because it is impractical to correct fuel flow. Correcting BHP for weather, as is typically done to compare engine output (SAE J-1349), is inappropriate for BSFC. Using corrected BHP would only lead to the next question which is, what would the fuel flow be under the corrected weather conditions, requiring yet another data correction creating more ambiguity in the data.

Note: A realistic value for a passenger car engine is 0.53 pounds of fuel consumed to produce each horsepower for each hour of operation. This example is at maximum power output -- wide-open throttle at optimum engine speed. A reciprocating engine achieves maximum efficiency at torque-peak speed and wide-open throttle. Under the best conditions, a typical passenger car gasoline engine might achieve BSFC ≈ 0.44 lb/(hp·h). Efficiency is worse under all other operating conditions.

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