Instantaneous piston friction under actual engine running conditions, both firing and motoring, was investigated by developing a new experimental method. Some comparative theoretical calculations were made based upon a hydrodynamic and mixed lubrication model.; Various factors that might affect lubrication and friction of the piston assembly were tested and studied. Piston friction was found to be greatly affected by operating temperature. On the other hand, the effect of engine speed change was moderate. Depending upon the lubrication regime, different responses to variables were observed. Piston friction is also affected by the supply of oil to the cylinder wall. With elimination of the oil control ring, the lubrication characteristics are changed and the magnitude of friction is reduced significantly.; Differences between firing and motoring friction are especially evident during the expansion stroke. This is presumed to arise from differences in cylinder pressure. Instantaneous power consumption due to piston friction is relatively high during the compression stroke under motoring conditions. The peak power loss under firing is relatively high during the expansion stroke due to the near coincidence of peak friction point and high piston speed.; A theoretical model was developed for calculating the piston and ring friction. Inter-ring pressure was calculated and used as a boundary condition for the calculation of oil film thickness and friction force. The unsteady one-dimensional Reynolds equation was used to compute oil film thickness that included the squeeze film effect. From the calculated oil film thickness, hydrodynamic friction forces were calculated. A mixed lubrication model was used for parts of the strokes near the dead centers. Some comparisons were made between the calculated results and the experimental measurements.
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