Subtopic: Sources of power on the farm – human, animal, mechanical, electrical, wind, solar and biomass
Question Statement:
A 4-stroke diesel engine can be operated with either diesel (heating value 45 MJ kg⁻¹) or biodiesel blend, B20 (heating value 42.1 MJ kg⁻¹). The brake specific fuel consumption of the engine when operated with diesel and B20 is 260 g kW⁻¹h⁻¹ and 310 g kW⁻¹h⁻¹, respectively. For developing a brake power of 20 kW, the change in brake thermal efficiency of the engine when B20 is used in place of diesel is __________ (rounded off to 2 decimal places).
Official Answer Key:
3.15 to 3.25
Step-by-Step Solution & Explanation
Brake thermal efficiency, η = 3600/(BSFC[kg/kWh] × Heating value[kJ/kg]) ×100.
For diesel: BSFC = 0.260 kg/kWh, HV = 45,000 kJ/kg → η_d = 3600/(0.260×45000)×100 = 3600/11700×100 = 30.77%.
For B20: BSFC = 0.310 kg/kWh, HV = 42,100 kJ/kg → η_b = 3600/(0.310×42100)×100 = 3600/13051×100 = 27.58%.
Change in brake thermal efficiency (B20 − Diesel) = 27.58−30.77 = −3.19 percentage points (i.e., a decrease of ≈3.19%, consistent with the official range once interpreted as a magnitude/decrease). The 20 kW brake power value is not needed for this efficiency ratio (BP cancels out) — it is extra/consistency information.
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