1985-10-01

Incipient Scuffing Detection by Ferrography in a Diesel Valve Train System 852124

Diesel soot contaminants may promote valve train scuffing in a passenger car diesel engine by interfering with the antiwear oil additives and metals surfaces.Incipient scuffing in a simulated valve train system has been identified by oil ferrographic analysis and SEM evaluation of the large wear particles produced.The mechanism of scuffing has been related to the combined effect of soot contaminants and antiwear additive (ZnDTP) depletion under high load and boundary lubrication regime. Friction and wear results in a TNO (pin-and-ring) tribometer with several lubricants, at various Zn DTP and soot concentration, have confirmed the close relation between carbon level and the phenomenon of scuffing.Ferrography has shown to be a very valid technique in the evaluation of incipient engine failure at Fiat, and represents actually a powerful tool for a better understanding of the wear mechanism leading to scuffing in a diesel valve train system.Specifically, the Wear Severity Index and SEM analysis of scuffed particles give a precise indication of the failing components, always associated with a sharp increase of the coefficient of friction.Due to the high local (flash) temperature developed, the lubricant and the soot contaminants may interact with metal surfaces, through carbon and hydrogen embrittlement, with consequent fatigue failure and scuffing of the diesel valve train components under high, cyclic loads.

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