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

Abstract The growing demand for sustainable lubrication solutions has accelerated the development of bio-based esters such as trimethylolpropane trioleate (TMPTO), which offer excellent biodegradability and intrinsic lubricity but require advanced additive systems to perform under severe engine operating conditions. In this study, a TMPTO-based lubricant formulated with glycerol monooleate, molybdenum dithiocarbamate, and an ionic liquid was further fortified with titanium dioxide (TiO2) nanoparticles to evaluate its tribological and engine-level performance. TiO2 was incorporated at concentrations between 0.75 and 1.50 wt.%, and tribological behavior was assessed using a high-frequency reciprocating rig simulating piston ring-cylinder liner contact, alongside four-ball extreme pressure testing to determine load-carrying capacity. System-level validation was conducted on a single-cylinder diesel engine, where frictional power losses were quantified using Willans line analysis across different engine speeds. The optimized formulation reduced the coefficient of friction by 41.8% relative to 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 minutes, attributed to a nano-rolling effect. At 1.50 wt.% TiO2, extreme pressure performance was further enhanced, increasing the initial seizure and last non-seizure loads by up to 33% and 30%, respectively, surpassing a commercial formulated lubricant. 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.