Question 15 MODE02 - Assistant MODU Engineer

The Correct Answer is A. ### Why Option A is Correct The purpose of an oil mist detector (OMD), typically installed in the crankcase of large main propulsion diesel engines, is to detect the presence of an explosive oil mist atmosphere. This mist is generated when small amounts of lubricating oil come into contact with a localized hot spot. The most common cause of such a hot spot is a mechanical failure resulting in extreme friction, such as a **seizing or severely overheated bearing** (main bearing, connecting rod bearing, or crosshead bearing). Detecting this mist is a critical early warning sign of impending catastrophic engine failure, allowing the operator to shut down the engine before a crankcase explosion occurs. Therefore, the detector warns of a possible overheated bearing. ### Why Other Options Are Incorrect **B) excessively high crankcase vacuum:** Crankcase pressure (or vacuum) is typically monitored by a separate pressure gauge or sensor. While excessive pressure relief is a function of the crankcase relief valves, detecting the pressure level itself is not the primary function of the oil mist detector. **C) low cylinder oil pressure:** Low cylinder oil pressure (or low system lube oil pressure) is monitored by dedicated pressure sensors and alarms on the engine's main lubricating system. The oil mist detector only focuses on airborne mist concentration within the crankcase space. **D) excessive carbon buildup in the lube oil:** The quality and contamination level (including carbon content) of the lube oil are determined by laboratory analysis of oil samples taken periodically, or sometimes by continuous condition monitoring sensors. The oil mist detector is a safety device for detecting thermal events, not a diagnostic tool for oil contamination.