Question 5 UFIV01 - Chief Engineer - UFIV

The Correct Answer is B **Explanation for Correct Answer (B): Worn piston compression rings** The crankcase pressure in a large two-stroke diesel engine normally operates under a slight vacuum (negative pressure) maintained by the scavenge air system or a dedicated vacuum pump/exhauster, primarily to prevent oil mist from escaping into the engine room. Worn piston compression rings (and/or worn cylinder liners) lead to increased "blow-by." Blow-by is the combustion gases escaping past the piston rings and into the crankcase. These hot, high-pressure gases counteract the system designed to maintain the vacuum, causing the negative pressure to become less negative (i.e., move toward zero or even positive pressure). Since the condition has developed gradually over time, worn rings/liners are the most likely cause of this continuous, gradual reduction in crankcase vacuum. **Why the other options are incorrect:** **A) Leaking crankcase handhole cover:** A leak in the handhole cover would allow atmospheric pressure (or engine room pressure) air to be drawn *into* the crankcase (if the crankcase is under vacuum). This acts as an air leak, making it harder for the vacuum system to maintain the set negative pressure, thus slightly reducing the vacuum. However, the effect of combustion gas blow-by (Option B) is a far more significant and common cause of positive pressure change than an external air leak when the engine is running under load. **C) Burned cylinder exhaust valve:** A burned exhaust valve would primarily affect combustion efficiency, increasing exhaust gas temperatures, and potentially causing back-pressure in the exhaust system. While it would reduce cylinder compression, leading to less power, the resulting leakage path for combustion gases (blow-by) is still primarily controlled by the piston rings. A burned exhaust valve is not the direct or primary cause of increased pressure *in the crankcase*. **D) Dribbling injector needle valve:** A dribbling injector causes incomplete fuel atomization, leading to poor combustion, increased thermal stress, and high smoke. If the fuel continues to burn down the expansion stroke, it might increase cylinder pressures overall, but it does not create a primary pathway for high-pressure combustion gases to enter the crankcase. The pathway remains the piston rings, which are addressed by Option B.