Fuel Economy Motor Oils: Scientific Rationale and Controversies

Abstract

Since a significant part of energy losses in the internal combustion engine comes from viscous dissipation, the trend has shifted toward low-viscosity oils from SAE 40 and 50 in the 1960s-1980s to current SAE 20 and lower viscosity grades. This transition has been facilitated by availability of high-quality hydroprocessed and synthetic base oils. Use of low viscosity engine oils significantly reduces energy losses in the main bearing and piston/bore systems, while tribological stresses on the valvetrain - especially in flat-tappet cammed engines - may increase. This makes a strong argument for deploying new classes of friction modifiers and antiwear additives. However, development of a balanced formulation is not as straightforward as it appears, and numerous pitfalls may be encountered. One potential hurdle is that certain additives require high treat levels for fully revealing their tribological effect, and such high levels are not acceptable due to potential negative impact on emission control equipment. Finally, there is always a cost factor. Another serious problem is that the definition of ?fuel-economy engine oil? is rather vague, as it depends on choice of reference oil. Nowadays, the assessment of fuel economy is based on the Sequence VIE or VIF tests using a 2012 3.6L GM V6 gasoline engine. It is not unexpected that the results of this test turn to be largely misleading when extrapolated to modern heavily boosted low-displacement engines. Hence, many OEM-specific fuel economy tests also exist. Furthermore, the ?fuel economy? performance of the same oil may change dramatically depending on the driving cycle. For instance, a low viscosity oil may boost fuel economy at cruising speeds (high speed / low load limit) and degrade fuel economy during aggressive city driving (low speed / high load). All the aforesaid circumstances are to be taken into account when trying to harmonize normative performance claims with customer expectations.