The primary purpose of lubrication is to prevent metal-to-metal contact between the liner and piston rings by forming an oil film. However, maintaining effective lubrication in a large slow-speed engine is challenging due to the following factors:

• The piston speed varies throughout the cycle. At Top Dead Center (TDC), the piston speed is zero, making it difficult to maintain an effective oil film.
• High temperatures at TDC, where combustion occurs, lead to oil film breakdown.
• Residual fuel oils often contain impurities such as ash, sediments, and catalytic fines, which can break the oil film and cause abrasive wear.
• Sulfur in the fuel forms sulfur oxides during combustion, which react with moisture to form sulfuric acid, leading to corrosive wear.
• High thermal and mechanical stresses inside the cylinder make it difficult for the lubricant to maintain its properties.
• Ash and other deposits absorb lubricant, leading to unlubricated metal-to-metal contact, further breaking the oil film.
• Ensuring optimal distribution of lubricating oil to all areas of the liner and piston assembly is difficult, especially at high loads or under uneven conditions.

Cloverleafing presents as corrosive wear on the cylinder liner surface, specifically around the lubricating oil quills. The wear pattern is characteristically shaped like a cloverleaf.

Causes of cloverleafing:

• It stems from the formation of sulfuric acid
• Sulfur in the residual fuel reacts with oxygen to form sulfur oxides during combustion.
• These sulfur oxides combine with moisture or condensate to form sulfuric acid, which corrodes the liner surface.
• The lower part of the liner, where jacket cooling water temperature is relatively low, is more prone to this type of corrosion, especially near the oil quills.

(ii) Micro-Seizure:

Micro-seizure looks like abrasive wear, with characteristic axial marks on the liner surface.

Causes of Micro-seizure:

• Caused by localized contact between the liner and piston rings due to lubrication failure.
• The breakdown of the oil film results in localized heat generation and micro-welding between the liner and piston rings.
• As the piston moves, these welded areas tear, leaving behind characteristic axial marks on the liner surface.

• The oil should have a viscosity that enables the formation of an effective oil film but doesn't impede flow. SAE 50 is a common choice, with a viscosity typically between 16.3 mm²/s and 21.9 mm²/s at 100°C.
• A high VI is essential, as it minimizes viscosity changes with temperature fluctuations, maintaining effective lubrication throughout the engine's operating range. A VI above 100 is generally desirable.
• The TBN should be high enough to neutralize the acidic combustion byproducts from the sulfur in the fuel. A TBN of around 70 is often specified for engines using high-sulfur fuels; however, this will depend on the specific sulfur content of the fuel.
• The oil must have low volatility to prevent ignition during operation.
• The oil needs good detergency to keep the liner surface clean, preventing piston ring sticking and minimizing abrasive wear.