A diesel engine has a brake thermal efficiency of 30%. If the calorific value of fuel is 40 MJ/kg, then what will be brake specific fuel consumption?  

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JSSC JE Mechanical Re-Exam 23 Oct 2022 Official Paper-II
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  1. 0.4 kg/kWh  
  2. 0.35 kg/kWh 
  3. 0.25 kg/kWh  
  4. 0.3 kg/kWh  

Answer (Detailed Solution Below)

Option 4 : 0.3 kg/kWh  
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Detailed Solution

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Concept:

Brake thermal efficiency is given as:

\(η_b = \frac{B.P}{\dot{m}\;×\;C.V}\) ...(i)

Where, ṁ = mass flow rate of fuel, B.P = Brake Power, C.V = Calorific value

Brake specific fuel consumption is given as:

\(bsfc = \frac{\dot{m}}{B.P}\) ...(ii)

Combining (i) and (ii) we have

\(bsfc = \frac{1}{η_b\;×\;C.V}\) ...(iii)

Calculation: 

Given:

ηb = 30% = 0.3, C.V = 40 MJ/kg = 40000  kJ/kg

Using equation (iii)

\(bsfc = \frac{1}{η_b\;×\;C.V}\)

\(\Rightarrow bsfc = \frac{1}{0.3\;×\;40000} =8.33\times10^-5 \ \rm{kg/kWsec } \)

\(\therefore bsfc=8.33\times10^-5\times3600=0.3\ \rm{kg/kWh} \)

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