Synergistic thermal activation of Al 2 O 3 –TiO 2 additives to resolve performance and emission trade-offs in gasoline direct injection (GDI) engines

Abstract

This study mechanistically defines a novel physicochemical synergy between Al₂O₃ and TiO₂ nanoparticles used as fuel additives in a Gasoline Direct Injection (GDI) engine, resolving their inherent performance-emission trade-offs. Using Response Surface Methodology (RSM), we first quantified the distinct roles and conflicts of the additives. Al₂O₃ acted as a physical combustion enhancer, leveraging its high thermal conductivity to improve fuel vaporization, which yielded significant gains in engine torque (up to 10.9%) and power (up to 5.0%). However, this physical enhancement comes at a critical cost: severely elevated in-cylinder temperatures, as proven by a 16.3% increase in thermal NO_x emissions. Conversely, TiO₂ acts as a chemical catalyst, promoting late-stage oxidation to effectively reduce incomplete combustion products, including carbon monoxide (CO) and unburned hydrocarbons (HC) by 12.5% and 17.3%, respectively. The central discovery of this study, visible only through multivariate analysis, is that these two mechanisms are powerfully synergistic. We demonstrate that the primary drawback of Al₂O₃ (high temperature) serves as the primary enabler for TiO₂ (thermal activation), exponentially accelerating its catalytic efficiency according to the Arrhenius principle. This “physicochemical activation” where the physical problem solves the chemical one, is validated by the net thermodynamic gain: the energy recovered from improved combustion completeness (reduced CO/HC) outweighed the increased thermal losses (evidenced by NO_x). This net positive balance was quantified as a 3.1% reduction in the specific fuel consumption (SFC). Multi-response optimization confirmed this mechanism, identifying the 2500–3500 rpm range not merely as a statistical optimum, but as the critical “sweet spot” in which in-cylinder temperatures are sufficiently high to unlock this synergistic pathway. This study provides a new framework for designing multi-additive packages based on synergistic thermal activation.