Question 43 AEL01 - Assistant Engineer - Limited

The Correct Answer is C **Why option C ("Sea water") is correct:** The question refers to "Illustration MO-0003," which typically depicts a marine diesel engine, often focusing on the cooling system components. The part labeled "W" in standard marine engine illustrations (especially those showing cooling systems) is often the **jacket water cooler** or a similar primary heat exchanger. In large marine engines, the primary cooling medium (fresh jacket water) is almost universally cooled by an external medium, which is **sea water** (or raw water) drawn from outside the vessel. The sea water flows through the heat exchanger ("W") to dissipate the heat from the engine's cooling system before being discharged overboard. **Why the other options are incorrect:** * **A) Lube oil:** Lube oil is used to lubricate and cool moving parts within the engine itself, and it has its own separate cooler. While lube oil is cooled, it is usually the sea water that cools the lube oil, not the lube oil cooling the main engine jacket water cooler ("W"). * **B) Convection:** Convection is a mode of heat transfer (heat transfer via fluid movement). While the engine radiates heat via natural convection and the cooling system relies heavily on forced convection, it is not the *substance* or *medium* used to cool the part "W." The actual medium used for cooling is sea water. * **D) Air:** While some very small auxiliary marine engines (or automotive engines) may be air-cooled, large marine propulsion engines depicted in standard illustrations like MO-0003 are liquid-cooled. Air is insufficient and impractical for cooling the immense heat load generated by the main jacket water system of a large marine diesel engine.