Performance Evaluation of Multi-Additive TMPTO Bio-Lubricant: Friction, Wear, and Diesel Engine Efficiency

Abstract

Abstract. The growing demand for sustainable lubrication solutions has driven the development of bio-based esters such as trimethylolpropane trioleate (TMPTO), which exhibit excellent biodegradability and intrinsic lubricity but require advanced additive systems to perform under severe engine operating conditions. Although laboratory tribology studies have demonstrated that ionic liquids and nanoparticles can enhance the performance of biolubricants, validation under firing-engine conditions remains limited. In this study, a TMPTO-based lubricant optimized with glycerol monooleate, molybdenum dithiocarbamate, and ionic liquid was further fortified with titanium dioxide (TiO2) nanoparticles to evaluate its tribological and engine-level performance. TiO2 nanoparticles were incorporated at concentrations between 0.75 and 1.50 wt%, and tribological behavior was assessed using a high-frequency reciprocating rig to simulate piston ring–cylinder liner contact, together with four-ball extreme pressure testing to determine load-carrying capacity. System-level validation was conducted using a single-cylinder Yanmar TF 120-M diesel engine, where frictional power losses were quantified using Willan's line analysis at different engine speeds. The optimized formulation reduced the coefficient of friction by 41.8% compared with neat TMPTO, while the addition of 1.25 wt% TiO2 produced the lowest friction and wear and shortened the running-in period from 20 to 5 min, attributed to a nano-rolling effect. At 1.50 wt% TiO2, extreme pressure performance was further enhanced, increasing the initial seizure and last nonseizure loads by up to 33% and 30%, respectively, surpassing the performance of a commercial, fully formulated diesel engine lubricant (SAE 0W-30, API CK-4). Engine tests confirmed reduced frictional power losses and fuel consumption, demonstrating the potential of nano-fortified bio-esters as high-performance alternatives to conventional engine oils.