Hybrid Electric Vehicle Engine Operation and Engine Oil Degradation: A Research Approach

Hybrid electric vehicles (xHEV) are a critical enabler to fulfil the most recent CO₂ and fuel economy requirements in key markets like North America, China, and Europe [1, 2]. Different levels of hybridization exist; the main differentiator is the power of the electric system and battery capacity. Increased electrical power enables the vehicle to run more often in electric mode and recuperate energy from braking, which enhances the saving potential [3]. Mild (MHEV) and plug-in hybrid vehicles (PHEV) impose different duty cycles on the engine compared to a conventional powertrain, potentially altering the degradation mechanisms of the lubricant, and challenging the basis on which the lubricant should be condemned [4]. The biggest concerns are water and fuel dilution [5], which promote corrosion and can form emulsions [6]. This may result in so-called white sludge formation (a thick and creamy emulsion) which can deposit inside the engine on colder surfaces, potentially blocking pipes and breather hoses [6]. White sludge deposits on the oil filler cap can become visible to the vehicle operator and may be a reason for concern. Many original equipment manufacturers (OEMs), and their customers, need advice in defining the important oil parameters for the oil to be fit for purpose. If oil and additive companies are to respond to these challenges, an increased awareness and understanding of oil degradation in modern vehicle platforms is required. In this work, we have investigated the operating conditions in different hybrid vehicles and their impact on the engine oil. First, a chassis dynamometer (CD dyno) test program was conducted to understand how three different concepts influence engine operation, specifically the engine oil temperature and the number of stop/start events. Second, engine dyno testing was designed to replicate a worst-case scenario, extrapolating some of the observations from CD testing, to investigate the effect of an extreme drive cycle on the engine oil degradation and contamination. Finally, an analysis of the chemical and physical properties of these engine test drain oils, and the resulting impact on wear protection and engine cleanliness, was undertaken to understand the risks associated with worst-case scenario xHEV operation.