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Engine oils contain additives that provide wear protection to prolong engine life. In a previous study using direct acting mechanical bucket valve train components, we found that aged oil provided better wear protection and friction reduction under certain circumstances. To understand this effect further, friction and wear performance of fresh and laboratory-aged oils with 0.1% phosphorus was studied with ball-on-flat and cylinder-on-flat rigs. Test durations were chosen according to the electrical contact resistance (ECR) values observed between the contacting surfaces. Anti-wear films were characterized primarily by UV and visible Raman spectroscopy, and results were corroborated by Auger electron and infrared spectroscopies. The greatest compositional differences occurred between films formed by fresh and aged oils. The degree of ECR response or the length of oil aging generally did not affect the type of component observed in the films.

High concentrations of diesel fuel can accumulate in the engine oil, especially in vehicles equipped with diesel particle filters. Fuel dilution can decrease the viscosity of engine oil, reducing its film thickness. Higher concentrations of fuel are believed to accumulate in oil with biodiesel than with diesel fuel because biodiesel has a higher boiling temperature range, allowing it to persist in the sump. Numerous countries are taking actions to promote the use of biodiesel. The growing interest for biodiesel has been driven by a desire for energy independence (domestically produced), the increasing cost of petroleum-derived fuels, and an interest in reducing greenhouse gas emissions. Biodiesel can affect engine lubrication (through fuel dilution), as its physical and chemical properties are significantly different from those of petrodiesel. Many risks associated with excessive biodiesel dilution have been identified, yet its actual impact has not been well quantified.

Engine oil plays an important role in improving fuel economy of vehicles by reducing frictional losses in an engine. Our previous investigation explored the friction reduction potential of next generation engine oils by looking into the effects of friction modifiers and dispersant Inhibitor packages when engine oil was fresh. However, engine oil starts aging the moment engine start firing because of high temperature and interactions with combustion gases. Therefore, it is more relevant to investigate friction characteristics of aged oils. In this investigation, oils were aged for 5000 miles in taxi cab application.