Failure analysis of an overhead valve train system in urban buses

摘要

This work is focused on the study of the failures identified in the valve train system of two different types of engines, one diesel and one fed by compressed natural gas, both pertaining to a fleet of urban buses. The studied failure has been reported in 20 different units, the affected elements being both the cams (material removal at the cam nose) and the tappets (hard plate removal and strong tappet core deformation) and requiring the vehicles to be out of service until being repaired. In order to determine the root cause of the failure, the performed analyses have covered the lube oil characteristics, the kinematic chain status, highlighting the review of the camshaft modal response and the behaviour of the valves springs, and, finally, the verification of the selection regarding both the material and heat treatment of the camshaft.