Types and Functions of Piston Rings

Piston rings are designed and classified based on their function and usability. Their primary role is to seal the chamber where the piston moves, which can be either a combustion chamber in a 2-stroke or 4-stroke engine. Marine engines often have three or more types of rings fitted around the piston.

Piston rings are a crucial component of the piston assembly, with their number and function varying according to the engine type and capacity.

In large 2-stroke engines, compression-type piston rings are used to seal the combustion chamber. Wiper rings are installed below them to remove deposits from the liner and distribute oil evenly on the liner surface.

In smaller marine engines, various types of piston rings are used for specific purposes. For example, the oil scraper ring is employed in 4-stroke engines, particularly trunk-type engines, where the crankcase oil has direct access to the cylinder liner and piston. In this article, we will explore the different types of piston rings used in marine engines.

Types and Functions of Piston Rings

Compression Rings or Pressure Rings

Compression rings, also known as pressure rings, are designed to seal above the piston and prevent gas leakage from the combustion side. These rings are typically located in the first grooves of the piston, though their placement can vary depending on the engine design.

The primary function of compression rings is to seal combustion gases and facilitate the transfer of heat from the piston to the piston walls. They work by controlling the oil layer left by the oil rings, ensuring that the top compression rings receive adequate lubrication. Additionally, they assist the top compression rings with sealing and heat transfer.

Wiper Ring

The wiper ring, also known as a Napier ring or backup compression ring, is installed below the compression ring. Its primary function is to clean excess oil from the liner surface and serve as a support backup ring to prevent gas leakage that might have escaped the top compression ring. Most wiper rings feature a tapered face positioned toward the bottom to provide a wiping action as the piston moves toward the crankshaft.

If the wiper ring is installed incorrectly with the tapered angle facing the compression ring, it can lead to excessive oil consumption. This occurs because the wiper ring will direct excess oil into the combustion chamber.

Oil Control / Scraper Rings

Oil control rings regulate the amount of lubricating oil that moves up or down the cylinder walls. They also ensure that oil is evenly distributed around the circumference of the liner.

Oil is splashed onto the cylinder walls, and these rings, also known as scraper rings, scrape excess oil off the cylinder walls and redirect it back to the crankcase. They prevent oil from passing through the space between the ring face and the cylinder.

Oil control rings feature holes or slots cut into the radial center of the ring to allow excess oil to flow back to the reservoir.

These rings can be one-piece or two-piece. To enhance the contact pressure between the ring and the liner surface, they may have chamfered edges on the outer sides of the lands or facing the combustion chamber. This design improves oil scraping and reduces oil consumption.

Two-piece oil control rings consist of a cast iron or profiled steel ring paired with a coil spring made from heat-resistant spring steel. The coil spring maintains pressure and contact around the entire circumference of the ring.

Piston Ring Material

Cast iron is one of the most commonly used materials for piston rings due to its graphite content in lamellar form, which acts as a lubricant and aids in the smooth sliding motion between the rings and the liner.

Piston rings may also be alloyed or coated, with the specific materials varying depending on the type of ring and its functionality. Common alloying elements for cast iron include chromium, molybdenum, vanadium, titanium, nickel, and copper.

To ensure maximum durability, the material of the piston ring is kept harder than the cylinder liner.