Which piston ring has a coiled expander?

The piston ring that features a coiled expander is the oil control ring. This ring plays a crucial role in managing the lubrication within the engine by controlling the amount of oil that is allowed to move to the combustion chamber.

The design of the oil control ring typically includes a coiled expander that helps keep the ring in contact with the cylinder wall effectively. This contact is vital for scraping excess oil off the cylinder walls, preventing excessive oil consumption and keeping the combustion process clean and efficient.

In contrast, the upper and intermediate rings primarily serve to create a seal between the piston and the cylinder walls to maintain compression and prevent blow-by of gases. These rings do not require a coiled expander for their functioning. Therefore, it is specifically the oil control ring that incorporates this unique design to fulfill its lubrication control function efficiently.