Question 47 3AE01 - Third Assistant Engineer

The Correct Answer is D. **Why Option D is Correct:** Crankcase explosions require two primary components: a flammable atmosphere and an ignition source. In large diesel engines, the crankcase contains lubricating oil that is naturally agitated and aerosolized. If a localized overheating (known as a 'hot spot') occurs—caused by excessive friction from a failed bearing, piston seizure, or a loose connecting rod bolt—the lubricating oil near that spot quickly vaporizes. This vapor forms a flammable mixture (often referred to as 'oil mist') with the air inside the crankcase. If the hot spot reaches the auto-ignition temperature of this oil mist (typically 450°C to 500°C), the mist ignites, leading to a rapid pressure rise and subsequent explosion. Therefore, the core mechanism of the explosion is the **ignition of vaporized lube oil in the crankcase**. **Why the Other Options are Incorrect:** * **A) The splashing of lubrication oil by the crankshaft:** Splashing is a normal operating function that creates a fine aerosol of oil, which is necessary for lubrication and cooling. While this aerosol is the potential fuel source for an explosion, the splashing itself does not provide the *ignition* mechanism necessary to start the event. * **B) The dilution of crankcase oil with particles of combustion:** Combustion particles (soot, carbon) dilute the oil and reduce its lubricating quality, leading to wear and potential hot spots over time. However, the presence of these particles does not directly cause the explosive ignition of the oil mist. * **C) Broken fuel lines spraying oil on the crankcase:** While a broken fuel line (usually high-pressure heavy fuel oil or diesel) can cause extreme risks (such as engine room fires if sprayed onto hot surfaces), the resulting spray inside the crankcase is usually less volatile than the highly refined lube oil mist created by excessive heat. More importantly, this event is more likely to cause a severe fire outside the engine or damage inside the crankcase due to hydraulic pressure, but it is not the primary mechanism associated with the typical 'hot spot'-induced crankcase explosion.