Question 33 MODE01 - Chief MODU Engineer

The Correct Answer is B **Explanation of Option B (Crankcase explosion):** The crankcase of a diesel engine contains lubricating oil that is atomized into fine mist and heated by the engine components. If the crankcase ventilation system fails or if blowby (combustion gases leaking past the piston rings) is excessive, these oil vapors and combustion gases accumulate, mixing with air to form a highly flammable atmosphere. An ignition source—such as an overheated bearing, piston, or other hot spot caused by friction or mechanical failure—can ignite this mixture. Because the crankcase is a sealed or semi-sealed volume, the rapid burning of the oil vapor mixture leads to a sudden and significant pressure increase, resulting in a **crankcase explosion**. *** **Explanation of Why Other Options are Incorrect:** **A) Reduced lubrication:** While poor ring sealing (excessive blowby) and the underlying mechanical wear that causes it may eventually lead to oil degradation, the accumulation of oil *vapors* itself does not directly reduce the flow or pressure of the liquid oil pumped through the lubrication system. The direct hazard associated with the accumulation of the flammable vapor mixture is the explosion risk. **C) Combustion knock:** Combustion knock (or "diesel knock") is a phenomenon that occurs inside the combustion chamber (cylinder head) due to issues like delayed ignition, poor fuel quality (low cetane), or incorrect injection timing. It is entirely unrelated to conditions or vapor accumulation within the engine crankcase. **D) Poor fuel economy:** Poor fuel economy is usually a consequence of inefficient combustion, incorrect timing, or excessive friction caused by underlying mechanical issues (like worn rings causing the excessive blowby). While the accumulation of vapors is a *symptom* of the mechanical wear that causes poor economy, the primary danger and direct problem resulting from the **accumulation of the vapor mixture** is the risk of detonation (explosion), not the engine's fuel efficiency.