Question 4 OSE02 - Assistant Engineer - OSV

The Correct Answer is A ### Explanation of Correct Option (A) - 360 rpm The engine type described is a main propulsion diesel engine, which operates on the four-stroke cycle principle (Intake, Compression, Power, Exhaust). In a four-stroke engine, the piston completes four strokes (two up and two down) for every power stroke, meaning the crankshaft completes two full revolutions ($720^\circ$) to complete one full thermodynamic cycle. The function of the camshafts is to open and close the intake and exhaust valves in synchronization with the piston movement. Since all four strokes must occur once per cycle, and the crankshaft turns twice per cycle, the camshafts (which control the valves) only need to turn once per cycle. Therefore, the relationship between crankshaft speed ($\text{N}_{\text{crank}}$) and camshaft speed ($\text{N}_{\text{cam}}$) in a four-stroke engine is: $$\text{N}_{\text{cam}} = \frac{\text{N}_{\text{crank}}}{2}$$ Given the crankshaft speed is 720 rpm: $$\text{N}_{\text{cam}} = \frac{720 \text{ rpm}}{2} = 360 \text{ rpm}$$ ### Explanation of Incorrect Options **B) 720 rpm:** This would be the rotational speed of the camshafts if the engine were operating on a two-stroke cycle, where the crankshaft completes only one revolution per cycle. However, modern propulsion diesel engines of the illustrated type typically use the four-stroke cycle, or if two-stroke, the camshafts would often control fuel injection but not all valve timing in the same 1:1 ratio. In standard four-stroke operation, 720 rpm for the camshafts is double the required speed. **C) 1440 rpm:** This speed is double the crankshaft speed. It is not mechanically correct for controlling valve timing in either a two-stroke or four-stroke diesel engine. The camshaft cannot logically turn faster than the crankshaft in this configuration. **D) Not enough information is given to determine camshaft rpm:** Sufficient information is provided. Standard marine diesel engines of this configuration operate on the four-stroke principle, which dictates a fixed 2:1 reduction ratio between the crankshaft and the camshafts. The type of engine (four-stroke cycle) is inferred from standard engineering practice for this application, making the calculation possible.