Causes:

Polishing or glazing of cylinder liners in medium-speed engines primarily occurs due to the burning of residual fuel, which leaves unburnt carbon deposits around the topland of the piston. These abrasive carbon deposits remove the lubricating oil film, leading to increased wear. Additionally, as the liner surface becomes polished, it develops a glazed texture that prevents the lubricating oil from adhering properly, resulting in metal-to-metal contact and further abrasion.

Other causes include:

• The use of incorrect grades of lubricating oil, such as high TBN oil, which may leave behind unused chemicals that burn to form abrasive ash.
• Incorrect running-in procedures for newly installed liners and pistons, leading to improper surface adjustment.

Effects:

• Excessive wear of the liner, reducing its service life.
• Blowpast, where combustion gases escape past the piston rings.
• Piston and liner seizure due to overheating and lack of lubrication.
• Breakage of piston rings caused by increased friction and wear.
• Loss of engine power due to poor sealing and combustion inefficiency.
• Increased lubricating oil consumption due to reduced film adhesion.
• Formation of hot spots in the liner, potentially leading to crankcase explosions.

An anti-polishing ring (APR) is a metal ring with an inner diameter slightly smaller than the liner's inner diameter but larger than the piston topland. The APR is designed to scrape off carbon deposits from the piston topland as it reciprocates.

The APR is fitted in a recess machined at the top of the liner. After the piston is inserted into the liner, the ring is pressed into the slot, ensuring a snug fit. The cylinder head is installed above the APR, holding it securely in place during operation. The APR is a clearance fit and can be replaced when it shows signs of wear.

As the piston reciprocates, the anti-polishing ring acts as a scraper, removing carbon deposits and other abrasive particles from the piston crown's top surface. This prevents these particles from directly contacting the cylinder liner. By preventing the buildup of abrasive material and ensuring the maintenance of a lubrication film between the piston and cylinder, it significantly reduces liner wear. It also protects the top part of the liner from the high temperatures of combustion, decreasing thermal stress. The ring essentially forms a protective barrier between the combustion chamber and the most vulnerable part of the liner.