Impact of post-oxidation phenomena on internal combustion engine performance-emissions and potential scope for future clean engine development: A review analysis

Cold-start emissions account for a disproportionate share of regulated pollutants from spark-ignition (SI) engines, as the three-way catalyst remains inactive below its light-off temperature. Post-oxidation—the secondary oxidation of unburned CO, hydrocarbons (HC), and H 2 in the exhaust manifold—is an exhaust-side strategy that can accelerate catalyst activation, lower emissions, and enhance turbocharger response. Despite substantial research on individual actuation methods, no prior review has systematically compared both approaches under a common framework of controlling parameters. This paper synthesizes experimental, computational fluid dynamics (CFD), one-dimensional modeling, and chemical-kinetic studies published between 1975 and 2025 on post-oxidation actuated by two methods: secondary air injection (SAI) and scavenging via variable valve timing (VVT) overlap. The literature demonstrates that SAI can reduce cold-start HC by 46%–86% and CO by 37%–93%, primarily through near-port oxidation that accelerates catalyst light-off. Scavenging-based post-oxidation can raise exhaust enthalpy sufficiently to increase turbine speed by up to 12,000 rpm and brake mean effective pressure by up to 2 bar, but is confined to moderate speed–load conditions and limited by mixing inhomogeneity. Combined SAI–scavenging operation extends the effective post-oxidation region to lower overlap and load conditions. Across the reviewed studies, exhaust gas temperature, oxygen availability, mixing quality, and residence time are identified as the governing parameters. Key gaps include the absence of transient real-driving validation and limited engine diversity. Post-oxidation enthalpy recovery is identified as a potential—but not yet experimentally validated—pathway for on-board alternative-fuel reforming.