An oil ring is an important part of an engine located on the bottom of a piston. It allows oil to pass through the piston ring and lubricate the cylinder wall. This part is made of three pieces: A top and bottom ring made of a thin steel ring surrounding a perforated and wavy spacer that allows the oil to pass through.
One of the most critical components of any piston engine is the oil ring. If this ring is installed incorrectly or fails at any time, the engine can be ruined in a matter of seconds. The oil passing through an oil ring not only lubricates the piston as it travels up and down the cylinder bore, it also cools it. This cooling property is what allows a piston engine to operate without heating up and seizing.
A typical piston has several rings installed on it. The top and middle rings are usually referred to as the compression rings. These rings are applied with a gap of a fraction of a centimeter. The rings are rotated so the the gaps are at a 45- to 90-degree location from each other. This allows the piston to build the compression needed for the engine to produce power.
Piston rings are made of cast iron or steel and may be coated with a substance to promote ring seal within the cylinder wall. The rings are much harder than the aluminum piston and take the wear of the cylinder wall much better. The oil ring provides the necessary cooling and lubrication to combat the harsh environment with the engine.
Diesel engines typically operate at much higher compression rates than gasoline cousins and as such, the top ring in a diesel engine will usually be chrome plated. This allows the ring to survive the greater compression and higher heat of the diesel. The oil ring in a diesel engine is designed to leave an oil film a few micrometers thick on the cylinder wall. This oil film not only keeps the rings cool and lubricated, it also protects the piston from the high heat.
The oil ring serves multiple duties within a piston engine. It not only lubricates and cools the engine, it maintains the piston's position within the cylinder bore. If the piston were allowed to rock back and forth, the cylinder would quickly become egg-shaped and worn. Proper placement of the oil ring prevents this from happening and allows the engine to operate for many years.