Gasoline engine valve train design evolution and the antiwear requirements of motor oils

Abstract

The ability of motor oils to control valve train wear adequately is ensured by the evaluation of new formulations in a variety of standard industry tests. The most critical of these tests have single-overhead-cam valve train systems with pivoted (rocker- or finger-type) followers. Although many vehicles in service have these types of valve train, there is a strong trend towards the use of roller-follower and direct-acting tappet (DAT) systems in recently introduced passenger car engine designs. Roller designs virtually eliminate sliding between the cam and follower and in gasoline engines reduce wear and friction to low levels. Practical experience indicates that DAT systems also give lower wear and friction than pivoted systems do. The parameters affecting wear in the pivoted-follower valve trains of the Sequence VE and Peugeot TU-3 tests are analysed in comparison with equivalent DAT systems. The friction and wear benefits observed with DAT systems are shown to be inherent in the geometry and kinematics of this type of valve train. The consequences of this finding are discussed in relation to the requirements for antiwear protection provided by future motor oils.