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Concept
Brake-specific fuel consumption
Summary
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.
In traditional units, it measures fuel consumption in pounds per hour divided by the brake horsepower, lb/(hp⋅h); in SI units, this corresponds to the inverse of the units of specific energy, kg/J = s2/m2.
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 term "brake" here as in "brake horsepower" refers to a historical method of measuring torque (see Prony brake).
To calculate BSFC, use the formula
where:
is the fuel consumption rate in grams per second (g/s)
is the power produced in watts where (W)
is the engine speed in radians per second (rad/s)
is the engine torque in newton metres (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
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 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.
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