Question 47 DDE04 - Designated Duty Engineer - 1000-4000 HP

The Correct Answer is B. ### Explanation for Option B (Wet-type cylinder liners) Wet-type cylinder liners are the components most susceptible to cavitation corrosion/erosion in a closed, re-circulating cooling water system of a diesel engine due to the following reasons: 1. **Vibrational Excitation:** The cylinder liner is subjected to intense, high-frequency lateral vibrations (often called "side-to-side flutter" or radial oscillations) caused by the combustion forces acting on the piston and the impact of the piston rings during engine operation. 2. **Cavitation Mechanism:** These rapid oscillations are transmitted through the thick metal of the liner and into the adjacent thin layer of coolant (water) in the cooling jacket. If the vibration frequency and amplitude are high enough, they cause momentary pressure drops in the cooling water near the liner surface. When the local pressure drops below the saturation pressure of the water, vapor bubbles (cavities) form instantly. 3. **Implotion Damage:** As the liner moves back toward the water, or the pressure momentarily increases, these vapor bubbles violently collapse (implode) against the liner surface. This implosion generates extremely powerful, localized shockwaves and micro-jets of water, which physically chip away (erode) the protective oxide layer and the base metal of the liner, leading to characteristic pitting damage (cavitation erosion/corrosion). ### Why the Other Options are Incorrect **A) Cylinder head cooling water passages:** While the cylinder head is subject to high thermal loads, its passages are generally part of a static cast structure. The primary flow velocities and thermal stresses are high, but the component structure itself does not undergo the high-frequency mechanical vibration necessary to induce widespread cavitation damage characteristic of the liner's movement. **C) Engine exhaust cooling water jackets:** These jackets cool the hot exhaust manifold. While high temperatures can lead to localized boiling, the design and purpose of these jackets do not involve close proximity to the mechanical forces causing lateral movement. Damage here is more typically related to thermal stress cracking, scaling, or uniform corrosion, not mechanical vibration-induced cavitation. **D) Cylinder cooling water jackets:** This option refers to the fixed outer casting or block surrounding the liners. This rigid structure provides the static cooling chamber. While it contains the coolant, the fixed jacket wall does not experience the same high-frequency, lateral vibrational movement that the independent, moving **wet-type liner** does. Therefore, the jacket walls themselves are significantly less susceptible to cavitation erosion than the liner's outer surface.