Next generation lubricating oils for transportation sector require higher durability in operation, compatibility with new engine technologies, aftertreatment devices and fuel economy (FE) characteristics, thus contributing to the reduction of CO2 emissions, both in passenger cars and heavy-duty vehicles.
The current paper aims to highlight the impact of dispersant technologies for their main properties, that are preventing sludge and deposits formation on engine surfaces, and in regard to the effect on frictional properties of lubricating oils, through a multi-step activity.
Oil contamination by soot is a big concern not only for diesel but also for new generation of direct injection gasoline (GDI) engines. The presence of soot leads to oil thickening that heavily impacts on friction coefficient thus enhancing the role of dispersant in controlling soot and related viscosity increase and, indirectly, fuel consumption for long running periods.
After an introduction on dispersant technologies, the focus of the paper moves on an innovative lab-scale methodology based on optical microscopy for the direct assessment of the dispersing capacity of oil. This methodology was firstly applied to a SAE Grade 15W-40 heavy duty (HD) oils contaminated with surrogate soot, where different dispersant systems were evaluated. In parallel, in order to simulate the soot effect on GDI oils, the aforementioned methodology was also applied on 0W-12 oils. Finally, the different dispersant were evaluated on engine test scale carrying out the standard ASTM engine test Mack T8 extended at 300 hours running (MT8E) achieved a ranking in terms of soot handling properties.
The second part of the paper deals with the investigation of the frictional properties of dispersants, both in GDI and HD engines. After a brief introduction on lubrication and tribology fundamentals, the authors considered an experimental activity based on a tribological test by using the Mini Traction Machine (MTM). For GDI engines, 0W-12 oils were tested at three different temperatures in order to evaluate the friction coefficient (COF) profile of oils as fresh. Afterwards, the authors considered the effect of the oil aging on friction losses, mainly consisting in soot loading and oxidation, through a two-step evaluation process: firstly, testing by MTM, the drained oil that achieved the low performance in soot handling at the Mack T8E. Secondly, from this oil, different candidates samples containing increasing amount of soot were derived and tested by MTM as fresh and after a laboratory ageing procedure: a performance comparison between aged oil vs fresh oils with and without soot was done in order to assess the effect of both soot content and ageing on frictional properties of lubricating oil.
The described experimental steps demonstrated that dispersant characteristics of engine oil play an important role not only in maintaining engine cleanliness but also in controlling the increase in friction during operation that influences fuel economy of finished product.