What are the piston rings functioning?

Piston rings are components assembled into the piston, which receives the combustion force inside the engine.

・The main functions of piston ring are gas sealing, oil consumption control, and heat transfer.

・Typically, three piston rings are assembled per piston.

・The Piston ring is an one of springs, creating a pressure against the cylinder wall for sealing by being compressed when inserted into cylinder.

・Piston rings are used in engines for motorcycles, passenger cars, trucks, buses, construction/agricultural machines, generators and so on.

What do piston rings do?

・Piston rings ensure airtightness by contacting the cylinder wall while the piston moves up and down.

・Piston rings prevent the leakage of air, fuel, and combustion gases during the compression and expansion strokes.

・Piston rings suitably control the consumption of engine oil that protects various components.

・Piston rings transfer the heat caused by combustion gases from the piston to the cylinder wall.

The functions of piston rings contribute to reducing exhaust emissions and improving fuel efficiency.

What are the functions of each of the three piston rings?

・The TOP ring primarily ensures gas sealing and heat transfer.

・The 2ND ring supports the functions of both the TOP ring and the oil control ring.

・The oil ring appropriately manages the amount of oil between the cylinder and the piston ring.

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In what kind of environment do piston rings operate?

・In diesel engines, the piston rings are exposed to combustion pressures of 25 MPa, equivalent to the force felt at a depth of 2500 meters underwater.

・They move back and forth at high speeds, akin to performing repeated side jumps at an average speed of 70 km/h.

・The TOP ring operates particularly in high temperatures, close to 300°C.

※While the specific environments may differ depending on the engine and its application, piston rings work in the harsh conditions inside the engine.

What technologies are applied in the piston rings?

・Traditionally, materials like cast iron and chrome plating were used, but currently, steel with PVD (Physical Vapor Deposition) or DLC (Diamond-Like Carbon) treatments is the main specification.

・Surface treatment is a crucial technology and both PVD and DLC ensure durability and reliability with thick layer that are not found in other industries.

・The outer circumference shape that contacts the cylinder wall is manufactured with micron-level machining techniques to create curved surfaces.

NPR-Riken Piston Rings

RIKEN CORPORATION and Nippon Piston Ring Co., Ltd., both are pioneers in piston ring industry, have merged to establish NPR-RIKEN CORPORATION. Our rings represent a blend of the traditions, know-how, and latest technologies of both companies. We take pride in providing piston rings that our users can confidently use and trust.
Although the shift towards electric vehicles (EVs) is progressing, there remains a significant role for engines, and development towards carbon neutrality continues. We are committed to advancing piston ring development as a united team to contribute to the upcoming carbon-neutral society.
We are actively developing hydrogen engines. By retrofitting existing diesel engines into hydrogen engines, we aim to convert existing trucks and buses into hydrogen-powered vehicles, fostering a society where hydrogen can be utilized easily and sustainably.

Surface Treatment Technology

The outer circumference of piston rings is in constant contact and sliding against the cylinder wall. Therefore, the surface coating of the piston ring’s outer circumference must exhibit high wear resistance, anti-scuffing properties, and low friction. In the past, chrome plating and nitriding had been the mainstream methods, but recently, harder coatings with lower friction such as PVD (Physical Vapor Deposition) and DLC (Diamond-Like Carbon) have been utilized.

■PVD（Physical Vapor Deposition）

■DLC（Diamond Like Carbon）

DLC coatings are high-performance coatings that excel in hardness, wear resistance, anti-scuffing and low friction properties. Compared to traditional surface treatment coatings, DLC coatings demonstrate superior wear resistance, leading to enhanced shape retention and prolonged maintenance of low friction performance. We have a wide range of hydrogen-free DLC coatings that are suitable for various sliding environments.

The operating environment for piston rings is extremely harsh. Therefore, evaluations using actual engines have always been important. Recently, to enhance development efficiency, we have been utilizing various rig test and simulations that mimic the engine's operating conditions. Simulation, in particular, has become crucial in recent years. We have developed our original simulation technology to aid in the design and phenomenon analysis of piston rings by leveraging the engine evaluation results we have accumulated over time, along with visualization and ring behavior analysis data from within the engine.