Question 48 MODE02 - Assistant MODU Engineer

The Correct Answer is A **Explanation for Option A (Correct):** A safety shutdown due to excessive crankcase pressure (often referred to as an "oil mist detector alarm" or high pressure alarm) indicates the potential for a severe internal engine fault, such as a major bearing failure, piston seizure, or scuffing. These faults generate extreme localized heat, causing the lubricating oil to vaporize and form a flammable oil mist (aerosol) within the crankcase. If the engine is stopped, the localized hot spot may still be glowing red hot. If the crankcase is opened immediately, fresh air (oxygen) rushes in and mixes with the volatile oil mist at the ignition point, which can lead to a violent secondary ignition and subsequent crankcase explosion. The mandatory waiting period (typically 2 hours, sometimes less depending on engine size and class rules, but 2 hours is standard for larger diesels) allows the superheated localized components to cool below the oil mist's auto-ignition temperature, thereby eliminating the ignition source and making it safe to ventilate and inspect the crankcase. **Explanation for Incorrect Options:** **B) Opening the crankcase before 2 hours has elapsed may result in crankshaft rotation.** Crankshaft rotation is controlled by the vessel's operational procedures (e.g., turning gear engaged, indicator cocks opened) and starting mechanisms (air supply isolated). Opening the inspection doors of the crankcase has no mechanical bearing on the rotation of the shaft, nor does the duration of the cooling period affect the engine's mechanical state in this manner. **C) Opening the crankcase before 2 hours has elapsed may result in excessively rapid cooling.** While rapid, uneven cooling is generally undesirable for large engine components (potentially causing thermal stress or cracking), this is a secondary concern and not the primary, immediate safety hazard. The 2-hour waiting rule is mandated specifically for the prevention of explosions, which is a life-critical safety risk, not component longevity. **D) Opening the crankcase before 2 hours has elapsed may result in the engine spontaneously restarting.** A large diesel engine cannot spontaneously restart. Restarting requires the deliberate injection of high-pressure starting air into the cylinders. The integrity of the engine's starting system (isolated and secured after shutdown) determines the restart potential, not the status of the crankcase doors or the internal temperature.