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This study investigates the wear characteristics of a variable valve timing (VVT) system used to vary the phasing of the inlet valve events on a medium speed marine diesel engine. The running-in properties of critical components within the system are examined. The effect of surface finish and surface hardness upon wear is examined. It was found that in order to prevent excessive wear between the roller and tappet follower then the roller should be harder than the tappet. Tappet and roller hardness values of 60 and 70 Rockwell Hardness ‘C’ (HRC) respectively were found to be satisfactory.

A program is described here that analyzes the behaviour of roller-follower valve gear. The motion is computed from a kinematic analysis; the instantaneous radius of curvature and the pressure angle are determined, so that the contact force and stresses can be evaluated at the cam/follower interface. The validation of the software is discussed. Also analyzed, is the hydrodynamic lubrication of the roller-follower journal bearing. A case study is described in which journal and bearing wear are attributed to inappropriately sited oil feed holes. It is shown that there is a region where no hydrodynamic pressure develops, thus providing an appropriate location for the oil feed holes. The assumptions in the hydrodynamic analysis are discussed.

A Variable Valve Timing system has been designed and rig tested for application on a high speed marine diesel engine. It has been demonstrated that inlet valve phasing is an appropriate way of reducing the flow of exhaust gas residuals, particularly at light loads, into the inlet manifold during valve overlap. The system described controls the position of the roller follower by means of an eccentric to allow the inlet valve events to be retarded by up to 40° crank angle. A kinematic model has been constructed to examine the tappet motion, and comparisons are made with measured values. Modelling of the lubrication regime at the roller, cam and tappet contact points is presented, to examine the system wear characteristics. Roller follower speed is measured to examine the prevailing lubrication regime. Rig results show that the VVT system is reliable and that the desired phasing of the valve events is obtained.