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The piston ring design using low tangential force to reduce friction loss has become a general trend in engine development - especially in passenger car engine - in recent years. This trend emphasizes the importance of stable sealing function that does not require much tangential force. Thus, it is important to grasp the correct pressure distribution between the ring and the cylinder. Reliable and accurate calculation to understand the pressure distribution has been an issue for many years among the engineers. In this report, we propose a new calculation method based on the piston ring contour in a free condition. Generally, the piston ring contour is calculated to achieve its correspondence to a given pressure distribution. By analyzing inversely, the pressure distribution is calculated when a ring contour is given. But as is well known, the solution is often unstable in inverse analyses. We introduced the idea of the method of least squares to avoid this problem.

The gas nitrided stainless steel pistoring (referred to as the gas nitrided ring in the following) was placed in mass production in 1983 to improve the durability of small diesel engines and leaded gasoline engines. Since their introduction, gas nitrided rings with their stable performance have replaced many chromium-plated rings with low durability and thermal-sprayed rings with weak coating strength. As a result of their superior wear resistance, the application of gas nitrided rings has expanded to a variety of engines1). After a steady increase in production over the past 10 years, gas nitrided rings account for more than 20 percent of the total piston ring production at our plant today. Gas nitriding contributed significantly to the change of the base piston ring material from cast iron to steel in Japan. During this period, however, requirements for piston rings have grown more sophisticated as a result of improvements in engines.