The calculation is made for the monograde lubricant SAE30 using the constant σ value of 0.6 μm. The ho value changes about 1 μm and the film parameter Λo=ho/σ, which indicates the degree of asperity interaction, is less than 3 throughout one cycle. The effect of asperity contact on F is remarkable around the dead centers and the friction spikes appear at the ring reversals. Especially at the beginning of the expansion stroke, the maximum friction force Fmax occurs. The friction force due to the asperity contact FA accounts for the majority of Fmax, since the Λo value is less than 1. On the other hand, the friction force due to the viscous shear stress FH is dominant at the midstroke. Fmax and the frictional mean effective pressure Pf for the piston ring pack are shown in Fig.6. The friction force of top ring have a great influence on the ring pack friction under the present calculation conditions. As to Pf about 60% of the total value is due to the top ring and about 20% is due to the 2nd ring. Taylor et al.13) also conducted analyses of friction loss for the same piston ring pack as the present study, and indicated that 40% of total power loss is due to the top ring and 35% is due to the 2nd ring. The present study, however, somewhat differs from their analyses in which the effects of surface irregularities and squeeze action were not considered. Wakuri et al.11) 19) 20) measured the instantaneous friction force of a piston assembly under firing conditions and showed experimentally that the reduction rate of Pf is 15% when the 2nd ring is removed from the piston ring pack. It seems that that the influence of the 2nd ring on the piston ring pack friction is comparatively small, although there are many complicated factors influencing the friction characteristics of piston ring pack.
Effects of σ on the film thickness characteristic values, hmin, hmean and the friction characteristic values, Fmax and Pf are shown in Fig.7. The increase in o leads to the increase in hmin, hmean, and the decrease in the film parameters Λmin=hmin/σ and Λmin=hmean/σ. The rough surface makes the lubrication conditions severe since the increases in Fmax and Pf are caused by the asperity interaction. The effect of increase in Fmax on Pf are considerably small as the occurrence of Fmax is at the beginning of stroke. It is, however, important to reduce Fmax as the magnitude of Fmax is closely connected with the increase of wear and the occurrence of scuffimg. Fig.8 shows the effects of the ring variance ratio Vrr=σρ2/σ2 at a constant value of σ. Although both of φs, in Eq.(5) and τ H in Eq.(14) are expressed as the function of Vrr, hmin, hmean and Fmax, hardly change.
