Question 68 AEL01 - Assistant Engineer - Limited

The Correct Answer is C **Why Option C ("Lube oil entering in the air intake manifold") is correct:** When a diesel engine is shut down normally (usually by cutting off the primary fuel supply), the engine should stop because it no longer has the necessary fuel to sustain combustion. If lube oil (or any combustible hydrocarbon vapor, such as crankcase blow-by or oil leaking past a turbocharger seal) enters the air intake manifold, the engine can begin to use this oil mist as an unintended fuel source. This phenomenon is known as "dieseling" or "runaway." Since the engine is no longer relying on the controlled fuel rack, cutting the primary fuel supply will fail to stop the engine, leading to an uncontrolled shutdown condition. **Why the other options are incorrect:** * **A) Supplying high temperature inlet air:** While high inlet air temperature reduces engine power, efficiency, and potentially increases the risk of pre-ignition knock in some engines, it does not provide an independent fuel source and therefore will not prevent the engine from stopping when the primary fuel is cut off. * **B) Maintaining a high exhaust back pressure:** Excessive exhaust back pressure inhibits the engine's ability to expel combustion gases, reducing power output and efficiency, and causing overheating. However, like high inlet temperature, it does not introduce an auxiliary fuel source that would bypass the primary shut-off mechanism. * **D) Carbon buildup on the overspeed pawl:** The overspeed pawl is part of the emergency overspeed protection mechanism, designed to trip the engine (shutting off the fuel) if the engine RPM exceeds a safe limit. Carbon buildup might *prevent* the overspeed system from tripping when necessary, but it does not cause the engine to continue running after a *normal* (manual or automatic) shutdown attempt where the fuel rack is intentionally commanded to the zero fuel position. The normal shutdown procedure operates independently of the overspeed trip mechanism.