Question 5 DDE01 - Designated Duty Engineer - Unlimited HP

The Correct Answer is C **Why option C ("Worn piston compression rings") is correct:** The crankcase pressure on a large diesel engine usually runs in a vacuum due to a connection with the engine air intake system or dedicated ventilation (like a crankcase breather/separator) which draws air out of the crankcase. The engine is experiencing a gradual reduction in this vacuum (becoming less negative or trending toward atmospheric pressure or even positive pressure). Worn piston compression rings allow excessive amounts of high-pressure combustion gas (known as "blow-by") to escape past the piston and into the crankcase during the power stroke. This continuous influx of combustion gases overwhelms the engine's ability to maintain the crankcase vacuum, causing the pressure to rise (become less negative). This is the classic symptom of increasing blow-by due to worn rings or liners. **Why the other options are incorrect:** * **A) Burned cylinder exhaust valve:** A burned exhaust valve reduces engine power and causes high exhaust temperatures. It primarily affects the combustion chamber's ability to hold pressure during the power stroke and could lead to high turbocharger inlet temperatures, but it does not directly increase the gas flow (blow-by) into the *crankcase* via the piston/liner interface. * **B) Dribbling injector needle valve:** A dribbling (leaking) injector causes poor fuel atomization, resulting in incomplete combustion, excessive smoke, and potentially washed-down cylinder liners. While poor combustion can lead to increased blow-by over a long period due to liner washing, the immediate and most direct cause of a rising crankcase pressure trend is physical leakage past the rings (blow-by), not just poor injection quality. * **D) Leaking crankcase handhole cover:** A leaking handhole cover would allow atmospheric air to leak *into* the crankcase. This would reduce the vacuum maintained by the crankcase ventilation system, making the pressure less negative. However, this is typically a sudden leak (or a leak that doesn't significantly change over time once it starts). The question specifies a **gradual** condition occurring **over time**, which is characteristic of mechanical wear (like worn rings) leading to a steadily increasing amount of blow-by gas.