The Stribeck curve as a suitable characterization method of the lubricity of biodiesel and diesel blends

The adequacy of Stribeck curves for the characterization of the lubricity of biodiesels, B20 blends and diesel fuel is demonstrated. Ball-on-disk tests in the speed range 2-1570 mm/s were performed to obtain the Stribeck plots and the results are compared to those from the conventional HFRR (high frequency reciprocating rig) ball-on-disk method (ASTM (American Society for Testing and Materials) D6079). Contrarily to the HFRR method, in the Stribeck tests very clear ball wear marks are seen without significant wear of the flat counterbody. These characteristics provide more confidence in the lubricity assessment of fuels. Moreover, in the Stribeck method higher lubricity at 60 °C is revealed for all biodiesels and the respective blends, related to the formation of protective oxide tribolayers. When evaluated by the HFRR test method, such temperature effect on the lubricity performance is not identified. From the point of view of the energy loss of the system, or friction response, lubricity is also better depicted by the Stribeck test method. The friction coefficient plots reveal that the major difference among the fuels occurs in the low velocity range, or in the start-stop stage of moving components, where the poorest lubricity is attained with the neat diesel fuel and the best lubricity with the animal fat biodiesel.