Question 66 OSE02 - Assistant Engineer - OSV

The Correct Answer is C **Explanation for Option C (Wet-type cylinder liners):** Wet-type cylinder liners are the component most susceptible to cavitation corrosion/erosion in closed, recirculating diesel engine cooling water systems primarily due to their geometry and interaction with the coolant flow dynamics. 1. **Vibration Source:** The liner is subjected to intense, high-frequency lateral vibrations (often called "skirt slap" or "liner vibration") caused by the piston's motion and the combustion forces (especially at the top dead center). These vibrations are transmitted directly to the thin layer of coolant surrounding the liner in the water jacket. 2. **Cavitation Mechanism:** The rapid oscillation of the liner surface creates localized, momentary drops in pressure in the adjacent coolant (when the liner moves away from the fluid). When the pressure drops below the vapor pressure of the water, vapor bubbles (cavities) form. As the liner moves back, or the pressure returns, these bubbles violently collapse (implode) near the metal surface. 3. **Corrosion/Erosion:** The implosion of these vapor bubbles generates immense localized shock waves (micro-jets) that physically hammer the protective passive oxide layer and the underlying metal of the liner's external surface. This continuous process leads to severe pitting and material removal, which is characteristic of cavitation damage. **Explanation of Why the Other Options Are Incorrect:** * **A) Cylinder head cooling water passages:** While cylinder heads experience high thermal loads and potentially high coolant velocity, they are generally much thicker and more structurally rigid than the liners. They do not experience the same destructive high-frequency, lateral vibration pattern induced by the piston motion, making them less prone to vibration-induced cavitation damage. * **B) Cylinder cooling water jackets:** This term refers to the static, outer casing structure surrounding the liner. While the jacket contains the coolant, the primary site of cavitation damage is the moving component (the outer surface of the wet liner) inside the jacket, not the jacket wall itself. The liner is the source of the vibration. * **D) Engine exhaust cooling water jackets:** These jackets (usually around the exhaust manifold or turbocharger casing) experience high thermal loads but are typically robust cast iron structures. The coolant flow here is usually less turbulent and the component is not subjected to the direct, destructive piston-induced high-frequency vibrations that characterize the cavitation damage seen on wet liners.