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Piston rings are open-ended rings that are installed with minimum clearance in special grooves made on the outer surface of the piston. Piston rings are the sealing element of the CPG, using which it is possible to achieve the necessary sealing of the combustion chamber in the device of piston engines.

Diagram of the device of the piston group and piston rings
There are two types of piston rings:

  • compression piston rings;
  • oil scraper piston rings;

Modern gasoline and diesel engines usually have 3 rings:

  1. the first (upper) compression ring;
  2. second (lower) compression ring;
  3. oil scraper ring;

High-performance gasoline units can only have 1 compression ring, while You can install 3 compression rings on the diesel engine piston.

A compression piston ring allows for a sliding tight connection and an effective labyrinth-type seal between the piston and the cylinder walls.
Due to compression piston rings, it is possible to prevent excess exhaust gases from escaping from the combustion chamber. The permissible amount of escaping gases on an internal combustion engine with a working CPG is considered to be up to 1%.
Piston rings are also responsible for regulating the engine oil that remains on the cylinder walls to lubricate the rings and pistons themselves and prevent lubricant from entering the combustion chamber. The specified function is performed by the oil scraper and also partially by the second compression ring.
Another function of the piston rings is to cool the pistons, which is achieved by removing heat from the piston by transferring excess heat to the cylinder walls.
The rings in the engine cylinders work in tough conditions, as they constantly experience severe mechanical and thermal stress. For this reason, special requirements are imposed on the manufacturing material, the piston rings’ design, and the method of their attachment to the piston. Piston rings are made of cast iron or resilient alloy steel. For improved wear resistance, an additional coating is applied to the surface of the compression ring during production. The material for such a coating is chromium or molybdenum.

The piston rings have a special lock. The piston ring lock is actually a cut. Thanks to this lock, the elastic ring can expand and contract like a spring.

In other words, the piston ring is not a circle but an arcuate shape with a gap between the ends in the cut area. After the piston ring is installed in the cylinder, it is compressed. The clearance in the lock area is reduced to 0.1-0.5 mm, which is determined by the engine’s design. In this case, the cut size does not allow the piston ring to break off from the seat on the piston.

The size of the piston ring clearance for each engine model is a strictly defined parameter. An increase in the clearance of the piston rings leads to depressurization and the breakthrough of gases from the combustion chamber. The result is a loss of engine power. Reducing the piston ring clearance can cause the ring to become seized in the engine cylinder after thermal expansion.
Seizure of the piston rings causes breakage of the rings themselves, the formation of scoring on the cylinder mirror and other damage. Also, some manufacturers offer special piston rings without backlash.

The oil scraper rings are installed under the compression rings and have a more complex design. The oil scraper ring has the shape of a box with two edges and internal slots. The edges act as a scraper, using which excess oil is removed from the cylinder walls. Through the slots in the oil scraper ring, the lubricant is directed to the piston drain holes.

Among the most common faults are wear, destruction and loss of mobility of the piston rings. Detonation and overheating of the engine often leads to accelerated wear and destruction; loss of mobility occurs due to the coking of the piston rings. This is also called ring seating, with the oil scraper rings usually going first.

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