A study into the engine oil performance on plug-in hybrid electric vehicles through an on-road fleet test under extreme cold environmental condition

In pursuit of reducing carbon emissions and to fulfill the customers’ needs for fuel-saving and environmentally friendly cars, car manufacturers have been increasingly offering different choices of electrified cars to their customers. Among those different powertrain solutions, with a balance of energy source between on-board electricity and fossil fuels, plug-in hybrid electric vehicles (PHEV) are becoming a choice for more and more end users, particularly in regional car markets such as China in recent years. Owing to the diversified vehicle operating conditions, new challenges are brought to the engine oil to protect the hardware from issues such as piston deposit, water/oil emulsification, oil thinning caused by fuel dilution, stop-start bearing wear and corrosion. This technical paper seeks to understand the impact of different operating modes of PHEV on engine oil performance. One key finding is that extreme conditions were needed to accumulate water content in the oil. When the oil temperature is under 10°C and the coolant temperature is under 40°C, water content could be continuously built up in the oil. Conversely, when these conditions are not met, water in the oil can be easily vaporised and released from oil. On the other hand, when the ambient temperature is low (below -20°C), and under low-speed/frequent stop-start driving cycle, fuel can be entrained in oil much more easily to as high as 20% (m/m). Fuel dilution can significantly thin the oil. A SAE 0W-20 oil could effectively end up as a SAE 0W-8 oil under high fuel dilution rate, which promotes wear between friction pairs such as ring-liner and bearing-journal. Finally, it has been observed that wear metals such as iron, aluminum are higher in non plug-in hybrid electric vehicles (HEV) than in PHEV ones, and that the wear level is lowest in internal combustion engine (ICE). This is owing to the decreasing stop-start severity, HEV > PHEV > ICE.