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Fuel economy has become the dominant criterion in the design of new automobiles. The globally enacted targets for fleet average emissions pose true challenges to automobile manufacturers. Increasing fuel economy requires enhancements both in hardware as well as in lubricant performance. As a key component of the lubricant, poly(alkyl methacrylate) PAMA viscosity index improvers have been identified as crucial design element due to their multiple modes of action. In their original application, they serve the well-known mechanism of polymer coil expansion at high temperatures and collapse at low temperatures. They help to flatten the viscosity/temperature relationship of the lubricant and allow for reduced low temperature viscosities and reduced internal friction, which directly translates into fuel economy. In addition to this bulk application, interfacial tribological phenomena contribute significantly to efficiency and fuel economy.

Recent work in the literature has suggested that the poor performance in the ASTM Sequence VI fuel efficiency test may result from an inappropriate balance of ZDDP antiwear agents. This paper describes a study of the friction behavior of ZDDPs. Two recently-developed techniques have been employed to chart the variation of film thickness and friction of ZDDP-containing oils from the EHD through to the boundary lubrication regimes. It is shown that some secondary ZDDP additives give significantly higher friction coefficient in the mixed EHD/boundary regime at temperatures above 80 °C. This behavior is very similar to that seen using formulated ASTM Sequence VI reference oils. Film thickness measurements show that the onset of high friction seen with the ZDDPs correlates with the formation of a solid-like reaction film in the lubricated contact.