Abstract: Six used diesel engine oils from RFWT engine test were studied on changes in physical and chemical properties and were characterized by 31P-NMR. The ZDDP degradation was investigated by NMR spectroscopy and the depth etchings on the surface of RFWT test pieces was analyzed by X - ray photoelectron spectroscopy (XPS). The results showed that the available phosphorus from ZDDP, nZDDP, and dithiophosphates, the decomposition product, in early stage is the key effective anti-wear component. The lower the ratio of effective phosphorus to intermediate phosphorus, the lower the wear is. XPS surface analysis revealed a thicker anti-wear film on surface of lower wear test part by ZDDP and its degradation products serves as anti-wear protection. The film contains phosphate, sulfide and sulphate.

Key words: RFWT engine test, diesel engine oil, wear, soot, ZDDP