Question 5 MODE01 - Chief MODU Engineer

The Correct Answer is B **Explanation for Option B (Correct Answer):** Trunk-type diesel engine pistons are subject to very high thermal loads, particularly on the piston crown. The primary, most effective, and necessary path for dissipating this heat is through the piston skirt/lands to the cylinder liner walls. These cylinder walls are continuously cooled by circulating water (or sometimes oil/air). When the piston is at the bottom of its stroke, a significant amount of heat is transferred via conduction and subsequent radiation/convection across the small oil film gap between the piston skirt and the water-cooled cylinder wall. This continuous transfer to the water-cooled cylinder walls is the principal mechanism for maintaining the piston within acceptable operating temperature limits. **Explanation of Incorrect Options:** * **A) radiated through the engine block:** While radiation occurs, it is a secondary and relatively minor cooling mechanism compared to conduction through the cylinder walls. The bulk of the heat is transferred before it reaches the outer surface of the block for radiation. * **C) conducted through the piston crown:** Heat is generated on the piston crown, but the phrase implies the heat is removed *through* the crown itself to an external sink, which is inaccurate. The heat must be conducted *away* from the crown, primarily down the sides (skirt/lands) to the cylinder wall, which acts as the heat sink. * **D) transferred to escaping exhaust gases:** The exhaust gases are significantly hotter than the piston surface (during the power stroke) or are at lower temperatures during the intake/compression strokes. While some heat is transferred to the gases, this is heat that is generally trapped within the combustion chamber and is expelled, not a dedicated cooling mechanism for the piston structure itself. In fact, excessive heat transfer to the exhaust gases (before expulsion) would make the engine less thermally efficient. The primary goal is to cool the piston structure via the cylinder walls.