Question 48 3AE01 - Third Assistant Engineer

The Correct Answer is D **Explanation for D (Crankcase explosion):** The accumulation of oil vapors (oil mist) in the crankcase of a diesel engine, combined with the presence of air (oxygen) and an ignition source (such as a hot spot on a bearing or piston crown), creates a potentially explosive mixture. Under normal operating conditions, crankcase pressure is typically slightly negative or neutral, and ventilation removes these vapors. However, if ventilation fails or excessive blow-by occurs, the concentration of the oil mist can reach the flammability limits. If a mechanical failure generates extreme heat (like a severely failing bearing), it can act as the ignition source, leading to a catastrophic crankcase explosion. Diesel engines are typically equipped with explosion relief valves to mitigate this risk, highlighting that explosion is the specific hazard associated with the accumulation of oil vapors and hot spots. **Why other options are incorrect:** * **A) Reduced lubrication:** While poor ventilation and excessive heat *can* degrade oil quality over time, the direct result of simply accumulating *vapors* (which are gaseous or aerosolized oil) is not immediate reduced lubrication. Reduced lubrication is caused by low oil levels, contamination, or shear thinning, not the presence of mist itself. * **B) Poor fuel economy:** Oil vapor accumulation and resulting crankcase pressure issues do not directly or significantly impact the combustion efficiency or fuel metering in a way that would cause "Poor fuel economy." Fuel economy issues are primarily related to injection timing, injector condition, air-fuel ratio, and engine load. * **C) Combustion knock:** Combustion knock (or detonation, though true detonation is rare in diesels) is a phenomenon related to uncontrolled or premature ignition of the fuel/air mixture *within the combustion chamber*. It is caused by issues like improper injection timing, low-cetane fuel, or excessive compression. Oil vapors accumulating in the separate crankcase compartment do not cause combustion knock.