Tribological Performance of an Engine Mineral Oil Blended with a Vegetable Oil under Approached Long-Term Use Conditions

It has widely reported that tribological performance of engine mineral oils (EMOs) can be improved by blending them with vegetable oils (VOs) in certain concentrations. Nonetheless, bio-oils are more susceptible to oxidation than EMOs by thermal ageing, which could be a drawback when they are used in engines comprising high variations of temperature. In this paper, a comparative analysis of tribological performance of an EMO and a blend made of 80%vol. of EMO and 20%vol. of a VO in fresh and aged conditions is given. The VO selected for the blend was Jatropha oil since various advantages reported in literature. EMO and B20 were thermally aged in laboratory approaching actual oxidation and additives depletion caused in EMO used in a car for 7500 km. The effects of ageing on the oils were evaluated by means of oxidation (PAI value), Zinc dialkyldithiophosphates (ZDDPs) depletion and viscosity. The tribological performance of the oils was determined by measuring the friction coefficients and wear rates generated in samples from engine cylinder liners in a pin-on-disk tester under boundary lubrication conditions. The ageing caused increased viscosity in B20 contrary to EMO that presented a slight decrease. The friction coefficients of B20 were lower than EMO in fresh and aged states. Moreover, the wear rate caused by fresh EMO and B20 were similar; however, ageing caused an increased wear rate by EMO but a decreased rate by B20 meaning that B20 exhibited better tribological performance than EMO under boundary lubrication in fresh and aged conditions.