Effect of ambient humidity and hygroscopy on the lubricity of diesel fuels

The lubricity of a selection of multicomponent fuels and pure substances was measured on a high frequency reciprocating rig at different vapor pressures. The objective was to evaluate the effect of air humidity on the wear scar diameter. Specific correction factors depending on the chemical composition of the fuel are proposed to normalize the mean wear scar diameter. The humidity correction factor recommended by the European standard shows an overestimation for most of test fuels, except biodiesel fuels which require higher correction factor. The effect of the water content in the fuel on the wear scar was also studied, showing significant direct correlations for all test fuels. An indirect approach for the estimation of the fuel hygroscopy, based on the combination of the effects of humidity and water content of the fuel on the wear scar, has confirmed that the air humidity affects wear through the humidification of the fuel. The results obtained by the proposed method show that paraffinic fuels are more hydrophilic than diesel fuels but they are less hydrophilic than biodiesel and alcohol fuels, which is consistent with the literature.