Question 41 3AE01 - Third Assistant Engineer

The Correct Answer is C ### Why Option C ("During initial starting") is correct: The period of initial starting (or "cranking") is the most critical time for any bearing in a diesel engine primarily because of **hydrodynamic lubrication failure** and **boundary lubrication conditions**. 1. **Lack of Oil Film:** When the engine is shut down, the pressurized oil supply ceases, and the hydrodynamic wedge (the pressurized oil film that separates the journal from the bearing surface) collapses. Upon startup, the crankshaft rotates before the oil pump can establish full system pressure and supply an adequate volume of oil to all bearings. 2. **Metal-to-Metal Contact:** This brief period results in the bearing operating under boundary lubrication, meaning there is significant metal-to-metal contact (or contact mediated only by microscopic oil films or additive layers). This leads to the highest rates of friction, wear, and heat generation, making it the most vulnerable operational state for the bearing surfaces. ### Why the other options are incorrect: **A) During periods of low load:** Under low load, the engine RPM is typically stable, and the oil pump maintains adequate pressure and flow. While the loads are lower, the established hydrodynamic oil film is present, preventing direct metal contact. Wear is significantly lower than during startup. **B) During first warm up:** Once the engine has successfully started, the oil pressure has built up, and the hydrodynamic film is generally established. The warm-up phase involves gradual thermal expansion and viscosity changes, which are manageable and less damaging than the zero-pressure conditions experienced during the initial turn-over. **D) After load reversals:** Load reversals (such as the transition from the power stroke to the exhaust stroke) certainly place high, rapidly changing stresses on the bearings. However, during these periods, the engine is running at full speed and pressure, ensuring the continuous presence of the hydrodynamic oil film. While stress is high, the critical separation of the moving parts is maintained, unlike the dry-start conditions.