Question 43 DDE01 - Designated Duty Engineer - Unlimited HP

The Correct Answer is A. **Explanation for why Option A is correct:** When a large diesel engine, particularly one operating under heavy load like those on push boats, experiences a safety shutdown due to excessive crankcase pressure (often referred to as an "overspeed/overshoot" or "high mist" trip), it is usually because of severe overheating within the engine's bearings or pistons (a "hot spot"). This overheating causes engine oil to vaporize, creating a flammable oil mist mixed with air within the crankcase atmosphere. This mixture is too rich to ignite spontaneously while the engine is running and pressurized. However, the internal components remain extremely hot even after shutdown. If the crankcase is opened too soon, a sudden influx of fresh, cool air (oxygen) will mix with the superheated oil mist, potentially reaching the explosive concentration limits (stoichiometric ratio) near the residual hot spots. This can lead to a violent and extremely dangerous crankcase explosion (a secondary explosion), which is why maritime safety regulations mandate a waiting period (typically 2 hours) to allow the internal temperatures to fall below the auto-ignition point of the lubricating oil before inspection. **Explanation for why other options are incorrect:** * **B) Opening the crankcase before 2 hours has elapsed may result in crankshaft rotation.** This is incorrect. The engine has been tripped and is shut down. The investigation involves opening inspection doors, not engaging the starting mechanism. There is no direct link between opening the crankcase doors and spontaneous crankshaft rotation, especially if standard safety procedures (e.g., locking the engine) are followed. * **C) Opening the crankcase before 2 hours has elapsed may result in the engine spontaneously restarting.** This is incorrect. A diesel engine requires fuel, air compression, and a starting mechanism to restart. Once the engine has shut down (especially due to a safety trip), it cannot spontaneously restart simply because a crankcase door is opened. * **D) Opening the crankcase before 2 hours has elapsed may result in excessively rapid cooling.** While opening the engine does cause cooling, the main concern is not the speed of cooling itself. The concern is the immediate interaction of fresh air with the existing hot oil mist and hot components, leading to the potential for an explosion, which far outweighs any moderate metallurgical stress from rapid cooling. The waiting period is specifically designed to mitigate explosion risk, not cooling speed.