Failure analysis of an overhead valve train system in urban buses

• Published in: Engineering failure analysis Vol. 96; pp. 455 - 467
• Main Authors: Vélez Godiño, José Antonio, García, Miguel Torres, Aguilar, Francisco José Jiménez-Espadafor, Guerrero, Daniel Palomo
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.02.2019
• ISSN: 1350-6307; 1873-1961
• DOI: 10.1016/j.engfailanal.2018.11.003

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. •The failure was reported in a fleet of urban buses (>20 units affected).•The affected elements are both the cams and the tappets.•The analyses revealed an insufficient average hardness at the cams nose.•Some measures have been identified to avoid the failure.