Pf is large when one of two surfaces is perfectly smooth, and it is small when two surface have same rms roughness values. The influence of variance ratio on Pf is comparatively small. Fig.9 shows the effects of the ring profile factor a on the film thickness and friction characteristics of the oil ring. The ring profile factor a is defined as the second coefficient of a quadratic equation since the barelled surface of the ring may be approximated by a quadratic equation. With the decrease in the a value, i.e., the roundness of ring surface, Fmax and Pf become large because the asperity contact increases and the cyclic fluctuation of film thickness decreases. The roughness amplitudes of sliding two surfaces decrease with the progress of running-in. The ring surface is rounded off as both sides of surface wear off. The friction characteristics widely vary with changes of σ and a values during the running-in process.
Relations between the engine speed and the film thickness and friction characteristics are investigated for different engine loads. The calculation results are shown in Fig.10. The engine load affects the oil film temperature and the gas pressure in combustion chamber. Fmax increases with the decrease in the engine speed and the increase in the engine load, and then the lubrication condition becomes severe. This results from the increase in the ring load and the diminished viscosity of the lubricant due to the rise of oil film temperature as the engine load increases. Pf reduces as the engine speed decreases and the engine load increases since the viscous friction force at the midstroke falls. As to monograde lubricants of SAE viscosity grade, 10W, 30 and 50, the Pf value is small when the lubricant viscosity is low. As a means of reducing the friction loss, the low viscosity lubricant is effective. However, the large friction spike at the beginning of the expansion stroke is an important problem to be solved when low viscous lubricants are employed.
