Question 63 DDE01 - Designated Duty Engineer - Unlimited HP

The Correct Answer is C ### Explanation for Option C (Wet-type cylinder liners) Wet-type cylinder liners are the components most susceptible to cavitation corrosion/erosion in a diesel engine cooling system for a specific, mechanical reason: vibration. 1. **Vibration Source:** As the piston moves through the cylinder during the combustion cycle, it exerts lateral (side) forces against the cylinder wall (piston slap/thrust). In wet-type liners, this high-frequency vibration is transmitted through the relatively thin liner wall to the coolant boundary layer on the outer surface. 2. **Pressure Fluctuation:** This rapid, mechanical vibration causes instantaneous, localized pressure drops in the adjacent coolant. 3. **Bubble Implosion:** When the localized pressure drops below the coolant's vapor pressure, micro-bubbles form. As the pressure immediately rebounds (due to the oscillating vibration), these bubbles violently collapse (implode) directly against the liner surface. 4. **Damage:** The resulting shock waves and high-speed micro-jets physically erode the metal surface, leading to characteristic pitting damage known as cavitation erosion. Since the liners are thin and experience constant, rapid vibration, they are the primary target for this type of failure. *** ### Explanation of Incorrect Options **A) Engine exhaust cooling water jackets:** These jackets are primarily subject to thermal stresses and general corrosion due to high temperatures. While coolant flow turbulence exists, they are rigidly fixed components and do not experience the high-frequency lateral vibration necessary to induce the severe pressure cycling that causes localized cavitation erosion on the scale seen on liners. **B) Cylinder head cooling water passages:** Cylinder heads experience high heat flux and thermal cycling. The cooling passages here are susceptible to scaling, blockage, and generalized corrosion. However, the head casting is thick and rigid; it does not undergo the rapid, high-amplitude mechanical vibration that causes cavitation damage to the extent seen on wet liners. **D) Cylinder cooling water jackets:** This option refers to the large, thick-walled cast iron structure of the engine block itself. While the coolant flows within this jacket, the jacket walls are massive and stable. If the engine uses wet liners (the susceptible component), the liner vibrates *within* the jacket space, meaning the liner is the component that suffers the localized damage, not the rigid outer jacket wall.