Influence of Used Oil on Piston Ring Friction

The influences of the used oil on the piston ring friction were evaluated by using a pin-on-disk type reciprocative sliding test equipment and an actual operating diesel engine in which the floating liner method was adopted. The major results are: (1) Because the used oil which contains highly concentrated soot is inferior in oil film spreading characteristics, the lubricating property is significantly lowered by reducing the oil supply rate; (2) Even though the kinematic viscosity of the used oil is increased by the flocculation of soot, the effective viscosity on the sliding surface of the used oil is approximately equal to that of the fresh oil due to the de-flocculation of soot. The friction mean effective pressure obtained for the used oil is not different from that for the fresh oil; and (3) Because the oil starved lubrication is dominant for the piston ring pack, the used oil raises the lubrication severity at the beginning of the power stroke, thereby causing the boundary friction to increase.