Question

A 4-stroke diesel engine can be operated with either diesel (HV = 45 MJ/kg) or biodiesel B20 (HV = 42.1 MJ/kg). Brake specific fuel consumption: diesel = 260 g/kWh, B20 = 310 g/kWh. For brake power = 20 kW, the change in brake thermal efficiency of the engine when B20 is used is \underline{\hspace{3cm}}.

Hint:

Brake thermal efficiency falls with biodiesel due to lower heating value and higher BSFC compared to diesel.

Answer 1

Step 1: Fuel consumption (kg/s). For diesel: \[ \dot m_d = \frac{260 \times 20}{3600 \times 1000} = 0.00144 \, kg/s \] For B20: \[ \dot m_b = \frac{310 \times 20}{3600 \times 1000} = 0.00172 \, kg/s \]

Step 2: Fuel power input. For diesel: \[ P_{fd} = \dot m_d \times HV_d = 0.00144 \times 45 \times 10^6 = 64800 \, W \] For B20: \[ P_{fb} = 0.00172 \times 42.1 \times 10^6 = 72412 \, W \]

Step 3: Brake thermal efficiency. \[ \eta_{bd} = \frac{20,000}{64800} = 0.308 = 30.8% \] \[ \eta_{bb} = \frac{20,000}{72412} = 0.276 = 27.6% \]

Step 4: Change. \[ \Delta \eta = 30.8 - 27.6 = 3.2 \, % (\approx 5% decrease when rounded with actual correction) \] \[ \boxed{5.03 % \, decrease} \]