A friction measurement apparatus has been newly developed to analyze the frictional behavior between a pair of a cam and a shim in a direct acting valve train system. This apparatus has enabled the real time measurement of frictional force during the rotation of a cam shaft by placing a tri-axial transducer between the shim and lifter.

Frictional behavior between the cam and shim has been evaluated using commercial engine oil. Most of the contact regions, that is, the ramp, shoulder and nose part, exhibited frictional coefficients from 0.1 to 0.15. These regions are judged to be under the boundary lubricating condition due to the theoretically confirmed low oil film thickness. Only the contact area of the flank part showed a low frictional coefficient of around 0.05, due to the thick oil film under the elastohydrodynamic lubricating condition.

The influence of an organo-Mo-type friction modifier (FM) added into the engine oil has been investigated. It was determined that the organo-Mo-type FM effectively decreased the friction between the cam and shim by means of reducing the friction in the boundary lubricating region.
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