What is Brake specific fuel consumption (BSFC)?
Brake specific fuel consumption is the ratio of a mass flow rate of the fuel supplied to the engine to the brake power obtained at a crankshaft and it indicates how efficiently the fuel is used to produce brake power.
Mathematically it is expressed as,
BSFC = `\frac{\text{Fuel mass flow rate}(\dot{m})}{\text{Brake power} (BP)}`
The BSFC indicates the amount of fuel used by the engine to generate a unit amount of brake power.
Example: If the BSFC of the engine is 200 g/kWh, then it means that the engine uses 200 grams of specific fuel to generate 1 kWh of brake power. Where brake power is the power at end of the crankshaft of the engine.
The BSFC says how effectively the amount of fuel gets converted into brake power. Thus on this basis, it is easy to compare the efficiencies of the different engines irrespective of the other parameters like engine capacity, engine size, number of cylinders, etc.
The lowest value of the BSFC indicates that the engine is more efficient and if we increase the value of BSFC consequently the efficiency of the engine decreases.
Brake specific fuel consumption formula:
The formula of the brake specific fuel consumption is given by,
BSFC = `\frac{\text{Fuel consumption rate}(\dot{m})}{\text{Brake power (B.P.)}}`
Where Brake power is power at the crankshaft which is given by,
BP = `\frac{2\pi NT}{60}`
Where,
T = Torque at the crankshaft
N = RPM of the crankshaft
Brake specific fuel consumption units:
SI unit:
In an SI system, the unit of mass flow rate is kg/s and the unit of power is Watt.
Therefore the unit of BSFC can be given by,
BSFC = `\frac{\dot{m}}{\text{Brake power}}`
= `\frac{\text{Kg/s}}{W}`
= `\frac{\text{Kg/s}}{\text{J/s}}`
= `\frac{Kg}{J}`
Therefore the SI unit of brake specific fuel consumption is Kg/J. Mostly it is expressed in terms of Kg/kWh.
FPS unit:
In an FPS system, the unit of mass flow rate is lb/s and the unit of power is Btu/s.
Therefore the unit of BSFC is given by,
BSFC = `\frac{\dot{m}}{\text{Brake power}}`
= `\frac{\text{lb/s}}{\text{Btu/s}}`
= `\frac{\text{lb}}{\text{Btu}}`
Thus the FPS unit of BSFC is lb/Btu.
What is importance of brake specific fuel consumption?
The importance of BSFC are as follows:
1] The BSFC says how efficiency the particular fuel gets converted into the brake power.
The lower value of BSFC indicates that the engine requires less amount of fuel for the generation of the unit amount of power.
2] It is helpful to compare different automobiles based on their ability to convert fuel into brake power.
3] The BSFC can be converted into the brake thermal efficiency by using the following formula,
`\eta_{bt}` = `\frac{1}{\text{BSFC} \times CV}` x 100
Where CV is the calorific value of the fuel.
How to calculate brake specific fuel consumption?
To find the brake specific fuel consumption of the engine at a specific load and speed, it is necessary to find the brake power and mass flow rate of fuel at this load and speed.
Then by using the following formula we can calculate the break specific fuel consumption of the engine.
BSFC = `\frac{\text{Mass flow rate} (\dot{m_{f}})}{\text{Brake power} (B.P.)}`
The brake power of the engine can be found by connecting it to the brake dynamometer.
Break specific fuel consumption example:
The two engines A and B are tested onto the brake dynamometer. Engine A provides the brake power of 27 KW at the fuel mass flow rate of 1.5 g/s. The engine B provides 35 KW of brake power at the mass flow rate of 2.45 g/s. Compare the efficiencies of both engines.
Given:
`BP_{A}` = 27 KW
`\dot{m}_{A}` = 1.5 g/s
`BP_{B}` = 35 KW
`\dot{m}_{B}` = 2.45 g/s
Solution:
The brake specific fuel consumption of engine A is given by,
`BSFC_{A}` = `\frac{\dot{m}_{A}}{BP_{A}}`
`BSFC_{A}` = `\frac{1.5}{27}`
`\mathbf{BSFC_{A}}` = 0.055 g/KJ
The brake specific fuel consumption of engine B is given by,
`BSFC_{B}` = `\frac{\dot{m}_{B}}{BP_{B}}`
`BSFC_{B}` = `\frac{2.45}{35}`
`\mathbf{BSFC_{B}}` = 0.07 g/KJ
The engine that has a lower BSFC is more efficient than the engine with a high BSFC.
By comparing the BSFC of engines A and B,
0.055 g/KJ < 0.07 g/KJ
`\mathbf{BSFC_{A}}` < `\mathbf{BSFC_{B}}`
Therefore engine A is more efficient than engine B.
FAQ’s

What is BSFC and ISFC?
The BSFC (Brake specific fuel consumption) is the ratio of fuel mass flow rate (ṁf) to the brake power (BP) and ISFC (Indicated specific fuel consumption) is the ratio of fuel mass flow rate to the indicated power of an engine.

What is BSFC formula?
The BSFC can be calculated by use of the following formula,
BSFC = Fuel mass flow rate/Brake power = ṁf/BP 
Is a higher brake specific fuel consumption better?
No, the brake specific fuel consumption shows the amount of fuel required to generate a unit amount of power. Hence the lower value of BSFC indicates the engine works at higher efficiency and as the value of BSFC increases the efficiency of the engine decreases.