Question 7 AEL01 - Assistant Engineer - Limited

The Correct Answer is B **Explanation of Correct Answer (B): Pearlitic cast iron with flake graphite** Cylinder liners in large, slow-speed diesel engines (like those used in marine propulsion) are subject to extreme conditions: high temperatures, high pressures, intense friction, and corrosive combustion byproducts (e.g., sulfuric acid condensation). Pearlitic cast iron with flake graphite is the standard material choice for the following reasons: 1. **Wear Resistance:** The pearlitic matrix provides excellent hardness and strength, crucial for resisting abrasive and adhesive wear caused by the piston rings. 2. **Lubrication and Scuffing Resistance:** The flake graphite structure is critical. During operation, graphite acts as a solid lubricant. More importantly, when the liner surface wears slightly, the graphite flakes leave behind microscopic pores that act as oil reservoirs, significantly improving lubrication retention and preventing catastrophic wear (scuffing). 3. **Cost and Manufacturing:** Cast iron is relatively inexpensive and easily cast into the large, complex shapes required for massive engine liners. **Explanation of Incorrect Options:** **A) Unalloyed carbon steel:** This material lacks the necessary hardness, wear resistance, and, critically, the self-lubricating properties (graphite content) required to survive the operating environment of a diesel engine liner. It would suffer rapid wear and scuffing. **C) High alloyed forged steel with chromium:** While steel alloys offer high strength, they are typically difficult and expensive to manufacture into large cylindrical liners. More importantly, forged steel lacks the necessary porosity and graphite structure for effective lubrication retention, making it prone to scuffing and seizure in high-friction environments. **D) Cast steel with chromium:** Cast steel is generally superior to cast iron in terms of ultimate tensile strength but lacks the superior damping characteristics and, fundamentally, the inherent lubricating properties provided by the flake graphite found in specialized cast iron (Option B). Furthermore, depending on the specific application, cast iron often exhibits better machinability and better resistance to thermal cycling.