In four-stroke engines there are usually three types of rings:
Sealing compression ring, Compression-scraper ring and scraper ring.
The sealing ring is located in the groove that is closest to the piston head. This ring seals the combustion chamber, preventing combustion gases from passing from the chamber into body of the engine. When fuel-air mixture ignites, exhaust gas pressure acts on the piston, forcing it to move towards the crankshaft.
The compression-scraping ring is located between the sealing and scraping ring.
Its purpose is to stop gases that get through the sealing ring and scrape excess oil from the cylinder surface. For this purpose, it has a special, working-surface shape.
The scraper ring is the ring located in the lowest position. The oil ring is used to scrape excess oil from the cylinder wall during movement of the piston. Then oil returns to the engine block through slots in the ring and the piston. In two-stroke engines, this ring is not necessary as oil is supplied along with fuel and its combustion is intentional.
The piston rings seal the combustion chamber, transmit heat from the piston to the cylinder, and control oil consumption. In order for the ring to perform its task, it must be able to elasticize under high temperature.
For this purpose, the ring inserted into the cylinder must have a slit in the lock. Size of the lock slot depends on diameter of the ring and the material from which the ring was made.
The ring has a well-defined shape in order to adhere exactly to the cylinder walls, which would provide required tightness. Appropriate shape of the ring is obtained through ovalisation.
Another important factor is the pressure that the ring exerts on the cylinder walls. This force usually depends on elasticity of the material from which the ring is made. Most rings are made of gray cast iron. Cast iron easily adapts to the cylinder walls and can also be easily covered with other materials in order to increase their durability.
The compression ring is closest to the combustion chamber, being exposed to the highest amount of corrosive substances and the highest operating temperature. Through compression rings is transferred up to 70% heat from the piston to the cylinder. Typically, this ring has a rectangular or spherical shape.
Spherical ring has curved working surface in order to allow for lubrication of the ring and the cylinder wall. In addition, its curved surface reduces the possibility of disappearing of oil layer caused by excessive ring pressure on the cylinder.
The compression-scraper ring is the next ring if we count them going from the bottom of the piston. Its task is to create a layer of oil with constant thickness, which makes it possible to lubricate the compression ring. Some rings have a certain cut, thanks to which they scrape oil better.
Incorrect mounting of the compression-scraping ring results in higher oil consumption, as the ring presses oil into the combustion chamber instead of scraping it towards the crankshaft.
Scraper ring (oil)
Scraper ring has two narrow working surfaces, between which there are oval or circular holes, through which excess oil flows into the engine block. Oil rings are usually made of appropriately shaped casts, and in many of them spreading spring is used in order to increase pressure on the cylinder walls.
How to mount rings onto the piston
An important factor determining proper and long-lasting operation of the piston rings is their correct assembly. Dimensions of piston rings are adjusted to original pistons of specific engines. Size of slot on ring lock after inserting into the cylinder, changes and depends on material of which the ring is made. You should observe the principle that rings with nominal size should be assembled on nominal-sized pistons and oversized rings on corresponding oversized pistons.
Rings should be assembled on new pistons that guarantee correct operation in the engine. During assembly on used pistons, pay attention to cleaning grooves in the piston thoroughly before inserting the rings. You should use special tweezers, whereas experienced professionals can perform assembly without tools. Do not open the mounted rings too much – only to the extent necessary to put the ring onto the piston. Opening too much might cause cracking or permanent ring deformation and such deformed rings do not ensure proper cylinder tightness.
Special attention should be paid to rings marked with the designation “TOP” or “G” and one should insert them into the piston grooves as these markings suggest. Piston rings should be assembled into the piston grooves starting from the lowest ring on the piston, i.e. the oil ring. If oil ring has a spring, remove it from the ring and insert into the groove. Then open the ring and put it on the spring in the piston groove. If the ring is located below the piston pin, assembly of rings should be started with this ring. After assembling all the rings, turn them in the piston grooves so that locks of the adjacent rings are aligned with each other.