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 the 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 to ensure oils are qualified to meet the specific demands of 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 which focused on oil emulsification and water entrainment, this work focuses on four underexplored yet industry-relevant aspects:
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Piston cleanliness: Under typical PHEV operating conditions, and after accounting for variations in driving-cycle characteristics, additive technologies incorporating salicylate detergents and elevated treat rates demonstrate substantial effectiveness in reducing piston deposit formation.
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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.
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Oil Degradation: For REEVs equipped with ICE featuring exhaust gas recirculation (EGR) system, particularly those of the non-plug-in type, high-performance engine oils with enhanced resistance to nitration can help to ensure adequate protection and long-term durability.
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Lubricant compatibility with Gasoline Particulate Filter (GPF): The reduced ICE engagement in modern PHEVs and REEVs leads to lower ash accumulation, thereby enabling the potential use of higher-ash engine oils in GPF-equipped hybrid vehicles without compromising filter durability performance.