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Aiming at the improvement in piston lubrication and the reduction of piston friction loss under this study, piston friction forces of cylinders with different surface roughness and treatment methods have been measured by means of a floating liner method, and the piston surface conditions have been also observed. As a result, it is found that the piston lubrication can be markedly improved by reducing the cylinder surface roughness. It is also verified that the deterioration in lubrication can be reduced even if some low viscosity oil is used, and the effect on the friction loss reduction becomes greater by reducing the piston surface roughness. On the other hand, it is found that many small vertical flaws are generated on the cylinder surface by reducing the surface roughness. In order to cope with this problem, etching and DLC (Diamond Like Carbon) coating have been tested as the surface treatments. As a result, it is confirmed that DLC coating is effective for the above.

The reduction of piston ring friction forces, which account for high percentages of the total engine friction loss, is vital for the simultaneous attainments of lower fuel consumption, higher engine power and speed. The authors et al. noted a three-piece type oil control ring in this study, and strived for the development of an oil control ring with a narrow width and a low tangential force. A new three-piece type oil control ring with a small tolerance on tangential force and a width of 1.2 to 1.5 mm has been successfully developed by studying the effect of such a ring on the lubricating oil consumption, while providing a spring function by press-forming a wire rod having a particular sectional shape.