Subtopic: Power efficiencies and measurement, engine performance curves
Question Statement:
The brake power of a four-cylinder engine is 30 kW with all cylinders firing and 20 kW with any one cylinder cut. The mechanical efficiency of the engine in percent is
Options:
(A) 60
(B) 67
(C) 75
(D) 83
Official Answer Key:
C
Step-by-Step Solution & Explanation
This is the Morse test principle. With one cylinder cut, the engine still overcomes the same friction, but that cylinder contributes no indicated power; the drop in measured brake power approximates the indicated power of the cut cylinder:
IP of one cylinder ≈ BP(all firing) − BP(one cut) = 30 − 20 = 10 kW.
Total indicated power (4 identical cylinders) = 4 × 10 = 40 kW.
Mechanical efficiency = BP/IP = 30/40 = 75%. Answer: (C).
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