Diesel soot contaminants may promote valve train wear and scuffing, in passenger car diesel engines, by interfering with the reactivity of oil additives on metal surface.
This is the result of field test and experimental work in a “TNO” tribometer on valve train materials, at various level of reactivity to Zn DTP.
It was found that compatibility between cam and tappet materials depends not only on proper mechanical properties, but on their chemical composition and reactivity to oil additives, provided favorable environmental conditions.
Particularly, high concentration of alloy elements (Ni, Cr) in low carbon steel may result in passivation layers, which inhibit both reactivity to phosphating and Zn DTP.
Besides, the presence of diesel soot contaminants may further reduce metal reactivity to Zn DTP, either through the formation of competitive chemisorption layers with the detergent additives and/or coordination products and micelles with the dispersants and the antiwear additive.
As a result of diesel soot interference, apparently, less antiwear film is formed on the metal surface and severe metal-to-metal contacts and adhesion arise, leading to early engine failure.