Evaluation of extreme pressure lubrication characteristics of bio-based palm kernel methyl ester against hydrogen engine oil

Abstract

This study evaluates the extreme-pressure (EP) lubrication behaviour of palm kernel methyl ester (PKME) and a viscosity-modified blend containing ethylene–vinyl acetate (PKME + EVA), benchmarked against commercial hydrogen engine oil (H2EO). PKME was synthesised via transesterification of palm kernel oil and characterised using FTIR and ¹H NMR, confirming ester formation and compatibility with EVA. Incorporation of 4wt% EVA significantly improved the rheological properties of PKME, increasing the kinematic viscosity at 100°C from ∼3 to ∼9 mm² s⁻¹ and raising the viscosity index to ∼185, compared with ∼135 for H2EO. Four-ball extreme-pressure testing showed that PKME + EVA achieved lower coefficients of friction under low-to-moderate loads (40–70 kg), with up to 27% friction reduction relative to H2EO at 70 kg. However, PKME + EVA reached its weld point at 110 kg, whereas H2EO sustained loads up to 140 kg. Wear morphology analysis indicated comparable and more uniform wear features for PKME + EVA relative to PKO and PKME at low loads, though higher surface damage occurred under severe conditions. Overall, EVA acts as a viscosity modifier that enhances mid-load tribological performance of PKME, while highlighting the need for additional EP additives for high-load applications.