Experimental Investigation Into the Effects of Diesel Dilution on Engine Lubrication

The dilution of lubricant due to contamination with diesel fuel is an increasingly prevalent, potentially important and poorly understood issue. This study addresses two fundamental questions: 1) How does the change in lubricant rheology due to diesel dilution affect engine lubrication? 2) How is the chemical performance of lubricant components (base oil and performance additives) impacted by diesel dilution under different lubrication regimes (boundary/full film, hydrodynamic/elastohydrodynamic). This is achieved by testing three lubricant samples: 1) neat fully formulated 0W-30 engine oil, 2) fully formulated 0W-30 oil diluted with diesel at a concentration of 15%, denoted “0W-30D”, and 3) neat, fully-formulated 0W-16, with the same base oil components and performance additives as the 0W-30, but blended to give a viscosity equal to that of the diluted an equivalent “0W-30D”. Tribometer tests, including 1) low pressure, low shear viscosity, 2) Ultra-high Shear Viscosity (USV), 3) elastohydrodynamic film thickness, 4) Stribeck friction and 5) boundary friction and wear, are then conducted. To further emulate engine lubrication conditions, Stribeck curve measurements are performed on the three lubricants using a journal bearing test rig, fitted with a connecting-rod and commercial diesel engine shells. Results suggest that diesel dilution only slightly affects chemical additive performance (with friction modifiers being more inhibited than anti-wear additives) but does reduce both viscosity and film thickness. However, care must be taken in using viscometrics to predict dilution behaviour because 1) the pressure viscosity coefficient is also affected by diesel dilution which has implications for elastohydrodynamically lubrication contacts, 2) shear thinning means that viscosity modifier additives effects lose their functions at high shear rates; whereas diesel contamination affects viscosity behaviour throughout the whole shear rate range.