The dilution of lubricant due to contamination with diesel fuel is an increasingly prevalent, potentially important
and poorly understood issue. Thisstudy addressestwo fundamental questions: 1) How doesthe 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.