Analysis of Lubrication of a Piston Ring Package. (Effect of Oil Starvation on Oil Film Thickness).

Theoretical analysis of mixed lubrication has been carried out for a piston ring package of an automotive reciprocating engine as a method to elucidate better specifications of rings for reducing friction loss. The starved inlet boundary conditions have been employed for oil film on each ring based upom experimental results obtained through scanning laser-induced-fluorescence oil film thickness measurement. With consideration of oil starvation, the predicted oil film thickness at compression rings was in better agreement with that measured than the former prediction under fully flooded inlet conditions. Film thickness at the 1st compression ring under full load firing conditions was predicted to be smaller than that during motoring, which also agreed with the experimental results, due to the different inlet conditions assumed for the oil film at the 2nd compression and upper rail of the oil ring under different operating conditions. The predicted oil film thickness was much smaller than that measured at both rails of the oil ring, which seems to have been caused mainly by limitations of the spatial analysis of the LIF measurement. Effects of the change of the oil ring surface profile due to tilting during sliding, of vescosity variation and of cylinder bore distortion were not negligible, but were not considered in the analysis.