Impact of Viscosity Index Improvers (VII) on the formation of piston deposits in fuel economy engine oils

In the recent years, the achievement of fuel economy through lower viscosity engine oil has been a topic of wide discussions among experts in the industry. Along this journey of engine oil evolution, new classes of Viscosity Index Improver (VIIs) have been developed in order to meet the challenges arising from either hardware re-engineering, environmental protection or both. In relation to this, the continuous tightening of the CO₂ emission level from the authorities has made the situation even tougher for many OEMs and formulators worldwide. While the fuel economy performance in an engine has been intensively investigated, little has been published on the durability aspects of these VIIs nor other aspects such as cleanliness and piston deposits. In this paper, we will present no-harm test results for deposit formation comparing novel comb polymers and conventional hydrocarbon VIIs such as OCPs. The formation of coke like deposits has been studied on low viscosity engine oils in both engine and bench tests. The results from the TDI engine test are compared with test results on the same fluids run in various screening tests such as Micro Coking test (MCT), Panel Coking test (PCT) and Komatsu Hot Tube test (HTT) and certain of these tests will be discussed in more detail.