The decomposition products of zinc dialkyldithiophosphate in an engine and their interaction with diesel soot

Abstract

Japanese valve train wear engine tests were carried out using oils containing a common zinc dialkyldithiophosphate (ZnDTP). The ZnDTP decomposition products formed in the oils during the engine tests were analysed. It was found that the ZnDTP decomposition results in the formation of zinccontaining compounds and phosphorus-containing intermediate products such as tetraalkylthioperoxidiphosphate. During an engine test, the ZnDTP decomposes rather quickly, whereas the phosphorus-containing intermediate products were detected over a long period. The adsorption test results indicate that diesel soot adsorbs the zinc compounds but that the soot adsorbs little of the phosphorus compounds. From the valve train wear test results and four-ball wear test results it was found that the oils tested in the engine keep their antiwear performance after the ZnDTP has decomposed and that this performance is mainly attributable to the phosphorus-containing compounds.