Discussion
A 3-cylinder 4-stroke diesel engine with a bore 73 mm and stroke 78 mm is tested at 2200 rpm. When a braking torque 55 N.m is applied, the indicated mean effective pressure is found to be 1050 kPa. The mechanical efficiency of the engine, in %, is __________. (Rounded off to 2 decimal places)
Answer: C
The mechanical efficiency is 67.21%.
Brake power from the torque:
\[ \begin{aligned} BP &= \frac{2\pi N T}{60} = \frac{2\pi \times 2200 \times 55}{60} \\ &\approx 12.67\ \text{kW} \end{aligned} \]
Indicated power from the mean effective pressure, with one power stroke every two revolutions in a 4-stroke engine:
\[ \begin{aligned} A &= \tfrac{\pi}{4}(0.073)^{2} = 4.185 \times 10^{-3}\ \text{m}^{2} \\ IP &= p_{i} L A \cdot \tfrac{N}{2 \times 60} \cdot n \\ &= 1050 \times 0.078 \times 4.185 \times 10^{-3} \\ &\quad \times 18.33 \times 3 \approx 18.85\ \text{kW} \end{aligned} \]
\[ \eta_{m} = \frac{12.67}{18.85} = 0.6721 = 67.21\% \]
Forgetting the factor 2 for a 4-stroke engine doubles the indicated power and halves the efficiency, and the other listed values do not follow from consistent working.
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