Driven by increasingly stringent emissions regulations, rapid advancements in electrification technologies, and rising consumer demand for fuel-efficient and environmentally sustainable mobility, Plug-in Hybrid Electric Vehicles (PHEVs) and Range-Extended Electric Vehicles (REEVs) have seen substantial growth in global automotive market. These hybrid architectures integrate electric propulsion with internal combustion engines (ICEs), offering extended driving range and operational flexibility. However, the evolution of hybrid powertrain systems introduces distinct operating characteristics—such as frequent engine start-stop events, reduced average engine loads, and extended oil drain intervals—that diverge significantly from conventional ICE vehicle usage profiles.
These changes present new challenges for engine lubricants, which must maintain performance under intermittent engine operation, increased exposure to water and fuel, and fluctuating thermal and environmental conditions. Conventional lubricant formulations, designed for continuous ICE operation, may not sufficiently address the demands of hybrid applications, where concerns such as oil degradation, wear protection, deposit control, and compatibility with aftertreatment systems are increasingly critical. Consequently, there is a growing need to redefine lubricant performance criteria and develop tailored solutions for hybrid powertrains.
This study presents a comprehensive evaluation of engine lubricant performance in PHEVs and REEVs, based on extensive field testing under diverse real-world operating conditions. Unlike prior research focused on oil emulsification and water entrainment, this work focuses on three underexplored yet industry-relevant aspects:
(A)Long-term aging in simulated parked condition: Simulated aging tests with used oils (15,000 km and 30,000 km) over a two-year period showed stable values in key lubricant parameters including kinematic viscosity (KV), total base number (TBN), total acid number (TAN), and oxidation.
(B)High-temperature piston deposit formation: PHEV engines exhibited increased piston deposits due to more frequent stop-start events compared to conventional ICEs.
(C)Gasoline Particulate Filter (GPF) compatibility: Reduced ICE engagement in PHEVs/REEVs resulted in lower ash accumulation, supporting the feasibility of using higher-ash engine oils in GPF-equipped hybrid vehicles.