The Effect of Oil Properties on Engine Oil Pump Failure Mechanisms

In recent years, there has been an increasing demand in the automotive industry to improve fuel economy. Poor fuel economy is attributed to the high friction and wear of various engine components. Thus, the automotive industry aims to design fuel efficient vehicles in order to reduce the fuel consumption. A Variable Displacement Vane Pump (hereafter denoted VDVP) is an innovative type of oil pump that has a variable capacity that is adjustable to the engine demand. This type of pump can improve the efficiency of engines. However, high friction and severe wear in the component of this pump is an issue. Oil contamination is one of the factors that influence the friction and wear behaviour of components in VDVP. Many studies have been conducted on the effect of contaminants such as soot on tribological performance of oils. However, the exact mechanism of the oil contaminations is not fully understood yet.
This study aims to investigate the effect of oil contaminants such as soot and diesel on the oil properties and tribological behaviour of oils in the boundary lubrication regime in VDVP. Furthermore, it examines the effect of these contaminants on the oil’s properties during the ageing process.
To achieve this, the tribological performance of various oils has been evaluated using ball-on-disc and pin-on-plate test rigs in the boundary lubrication regime. Different surface analysis techniques were used to understand the effect of each of the contaminants on wear and friction behaviour of contacts. A number of surface analysis techniques were employed in this study such as Scanning Electron Microscopy (SEM), Energy- Dispersive X-ray (EDX), Raman spectroscopy, Fourier Transform InfraRed Spectroscopy (FTIR) and Inductively Coupled Plasma (ICP).
The results of this study indicate that Carbon Black (CB) contamination (soot surrogate) increases the wear of components. It has been proposed that a corrosive-abrasive mechanism is responsible for the high wear observed in fully formulated oil (FFO) when contaminated with CB. The interaction of sulphur- and phosphorus-based additives such as ZDDP with CB resulted in an antagonistic behaviour that enhanced wear. This mechanism is more dominant than the abrasion effect of itself.
This study shows that diesel contamination can reduce the friction and wear under certain condition due to the lubricity of diesel. It also reveals that the contaminants and ageing conditions can change the bulk properties (viscosity and TAN) of the oils. This is found to be dependent on the type of contaminants and ageing duration. When CB is present in the oil during the ageing process, it results in high Total Acid Number (TAN) and severe wear of engine components. These results also reveal that the high wear observed with CB is not solely due to abrasion. Additive depletion and additive adsorption by CB particles during the ageing process are shown to play a significant role in producing high wear.