GATE 2023 AG – Question 53
A four-stroke engine uses diesel at 45 MJ/kg and BSFC260 g/(kW·hour),or B20 at 42.1 MJ/kg and 310 g/(kW·hour). Brake power is 20 kW. Find change in brake thermal efficiency when B20 replaces diesel, to two decimals.
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Correct answer: 3.15 to 3.25 OR 0.3 to 0.3
Explanation
Brake efficiency is $\eta_b=3.6/(BSFC\cdot HV)$ with BSFC in kg/(kW·hour) and HV in MJ/kg. Diesel gives $3.6/(.260\cdot45)=.3076923$; B20 gives $3.6/(.310\cdot42.1)=.2758409$. Efficiency therefore **decreases3.19 percentage points**. Brake power cancels. Preserve the complete official answer string, including both accepted alternatives:3.15–3.25 OR.3–.3; the second interval is an official grading provision, not a derived efficiency difference.