wt% ZDDP to 500 ppm moly may provide balanced wear and friction reduction.52 To have a stable · tribofilm that can support the UMW

wt% ZDDP to 500 ppm moly may provide balanced wear and friction reduction.52 To have a stable tribofilm that can support the UMW regime, four things are needed: ● Sufficient temperature to convert ZDDP and moly present in the oil to an anti-wear film ● The hardness and yield strength of the two surfaces rubbing (piston and piston rings against the cylinder bore) must be similar to maintain the oil film ● The resulting “glassy-plate” anti-wear film results in a reduction in the coefficient of friction, indicating a stable film ● No abrasive foreign object debris such as fractured silicon particles can be present53 Considering the importance of this tribofilm and knowing that 50% of the friction in an internal combustion engine comes from the pistons and rings, selecting the right lubricant for this application is critical. Legacy and performance engines might benefit from a full-SAPS (sulfated ash, phosphorus, and sulfur) oil with increased ZDDP and elevated moly levels, over levels found in OEM specified lubricants. Zinc-free oils tend to form tribofilms more slowly than ZDDP and are more sensitive which surfaces they bind to. Choosing a lubricant designed to minimize wear, but not necessarily provide optimum fuel savings or emissions system protection, may extend cylinder, piston, and ring life.54 The introduction of ash-less and zinc-free oils optimized for emissions systems protection are on the horizon, making choosing an oil that much more difficult. The aforementioned operation of Al-Si engines in colder climates and increased prevalence for scored bores can also be attributed to acid formation in the crankcase at temperatures below 40°C (104°F). Many familiar with failures of BMW V8 and V10 engines with Nikasil were attributed to high sulfur content in U.S. fuels, however Porsche and other manufacturers did not experience similar failures, but sulfur content of fuels have steadily decreased since 2000. Tests carried out by Toyota, with results published in 2000, measured the effect of sulfur in fuel on wear in Al-Si engines. Toyota found North American regular grade (87 octane) fuel to have the highest levels of sulfur at a maximum of 1350 ppm, followed by European fuels at 1000 ppm, and Japanese fuels having nearly zero sulfur content. The quality of North American premium grades of unleaded fuels varied greatly, with a maximum of 650ppm and an average of 175 ppm of sulfur. The study also found that as sulfur content increased, wear rates also significantly increased at coolant temperatures below 40°C (104°F), resulting in three times the wear at 1000 ppm sulfur versus unleaded fuels with 200ppm or less. At low temperatures, acids are created as byproducts of combustion, with a higher concentration of acid associated with high sulfur fuels. It is these acids that break down the lubricant film between the bores and the piston rings, causing abrasive wear associated with the plastic deformation of the aluminum matrix. As expected, ring wear rates saw similar increases with increased sulfur concentration. For fuels with a sulfur content less than 200 ppm, wear rates 52 Influence of temperature and ZDDP concentration on tribochemistry of surface-capped molybdenum sulfide nanoparticles studied by XANES spectroscopy. Bakunin, V. N. et al. Tribology Letters, Vol. 26, No. 1 April 2007. DOI: 10.1007/s11249-006-9180-y 53 The influence of start-stop transient velocity on the friction and wear behavior of a hyper-eutectic Al-Si automotive alloy. J.C. Walker, et al. National Centre for Advanced Tribology at Southampton. 54 On the Mechanism of ZDDP Antiwear Film Formation. Zhang, Jie; Spikes, Hugh. Imperial College London, London, UK. DOI: 10.1007/s11249-016-0706-7 LN Engineering Technical Library. Copyright ©2019 LN Engineering LLC