Diesel soot (carbon) contamination of engine oil appears to be the cause of higher valve train wear in diesel engines than in gasoline engines with similar metallurgy. Four-ball wear studies to determine the mechanism by which carbon reduces the antiwear benefit of zinc dithiophosphates (ZDP's)indicate that the higher wear is due to preferential adsorption of the ZDP decomposition products by the soot thus reducing the formation of the antiwear surface coating. Adsorption of undecomposed ZDP, changes in ZDP decomposition reactions and abrasive removal of the surface coating were eliminated as significant contributors to higher wear.
