Synergistic and Competitive Effects between Zinc Dialkyldithiophosphates and Modern Generation of Additives in Engine Oil

The increasing demand for low-viscosity engine oil has underscored the role of zinc dialkyldithiophosphates (ZDDP) as a conventional anti-wear and antioxidant additive. It is essential to investigate the influence of modern additives such as cyclopropanecarboxylic acid (CPCa) and Ni nanoparticles on the tribological performance of ZDDP for practical commercial oil application. According to the experimental results, Ni nanoparticles formed a protective film that exhibited a synergistic effect with ZDDP. A significantly higher concentration of sulphur in the tribofilm was detected compared to ZDDP by itself, which was responsible for a 27.6% lower wear loss. Meanwhile, a competitive effect between CPCa and ZDDP resulted in a dramatic increase in friction and unstable anti-wear performance. This was demonstrated by a localized formation of the ZDDP tribofilm on the wear surfaces after the friction test. These results have highlighted the synergistic and competitive effects of emerging additives (CPCa and Ni nanoparticles) in the ZDDP tribofilm formation between the sliding steel contacts. This further suggests a new approach to increase the efficiency of ZDDP’s tribological performance at cold start-up processes.

The depletion of ZDDP additives within marine lubricants and associated cylinder liner wear in RNLI lifeboat engines

Previous work of authors indicated the wear of cylinder liners in marine engines of RNLI lifeboats due to the intense lubricant degradation identified by inductively coupled plasma and Fourier Transform Infrared spectroscopy techniques. In this paper, further analysis carried out to evaluate the effects of lubricant degradation on the actual cylinder liners installed in the Trent Class Lifeboat engines is presented. Surface characterisation of actual cylinder liner’s bore surface showed maximum wear near the top dead centre region compared to rest of the piston stroke. Wear in this region of the cylinder liner surface is controlled primarily by the protective film forming anti-wear additives in the lubricant which limit the direct surface contact between the piston rings and cylinder liner. The condition of zinc dialkyldithiophosphates anti-wear additives was analysed using the nuclear magnetic resonance spectroscopy. Tribology analysis was conducted to evaluate the tribological and boundary film forming performance of zinc dialkyldithiophosphates additives by simulating cylinder liner–piston ring contact near the top dead centre. To further understand the wear mechanisms of the cylinder liner, wear debris analysis (Analytical Ferrography) of lubricant samples was performed. Results revealed the depletion of phosphorus containing zinc dialkyldithiophosphates anti-wear additives as a function of the lubricant’s duty cycle within the marine engines and its effect on the tribological and boundary film forming performance of lubricants. Wear debris analysis showed the generation of ferrous debris potentially from the cylinder liners as a result of reduced anti-wear protection from the depleted zinc dialkyldithiophosphates additives during the tribological contact with piston rings and piston skirt region. These findings are useful to understand the lubricant degradation mechanisms which affect the functionality of cylinder liners, therefore allowing to plan the engine maintenance strategies.