This paper shows the technical trends and future aspects of piston rings for 4-stroke and 2-stroke marine diesel engines. The combination of cast iron and chromium plating is the current standard specification of piston ring for 4-stroke diesel engine. On the other hand, for 2-stroke diesel engine, the combination of cast iron and thermal spray coating, which is applied in accordance with the liner type, is also adopted. In addition, differences in piston ring specification for marine diesel, for example base materials, surface coatings or shape of cross section compared to the small bore 4 stroke diesel engines for trucks, buses and off-high way application are shown. From the viewpoint of technical development, it is assumed that the combination of steel material and PVD coating, likewise heavy duty trucks, will be applied to piston ring for 4-stroke marine diesel engines in order to achieve higher durability in the near future.
To reduce friction loss between piston rings and cylinder liner is important for achievement of high efficient engine. In this report,the ways of reduction friction of piston rings and cylinder liner are introduced. Effects of shape, coatings and texturing of peripheral of piston rings, surface characteristics of cylinder liner, and engine oil on friction are described. When friction of piston rings and cylinder liner is reduced, lubricating oil consumption is needed to be unchanged or decrease. Therefore in order to design of piston rings, both low friction and low lubricating oil consumption are needed to be taken into consideration.
The application of dry process technology to piston rings is introduced. The transition of surface treatment technology from the hard chromium plating to DLC coating is explained with the background about improving the performance of piston ring to respond the social requirement such as the improving fuel economy. Finally, the possibility of the applied technology to the future engine components by using the super-lubrication phenomenon by ta-C coating under the environmentally friendly lubricant is discussed.
In recent years, friction reduction of piston rings is said to be the limit. This is because the LOC increases as the friction force decreases. Piston ring material technology has evolved. The spread of PVD and DLC has progressed. On the other hand, research on frictional force has not progressed much. As a result of relying solely on tension reduction to reduce frictional force, LOC deteriorated. This explanation examines recent evolution of piston rings. Based on these, It is summarized that the limitations of the current technology and issues for further friction reduction.