To comply with stringent emission regulations, the deployment of hybridized powertrains is continuously expanding. However, architectures such as plug-in and parallel hybrids intrinsically reduce the overall runtime of the internal combustion engine (ICE). Because the battery state-of-charge (SOC) dictates intermittent engine activation, this operational strategy inevitably induces frequent cold-start events. This study investigates the thermal dynamics of commercial catalyst geometries (300–1200 cpsi, 2–8 mil) via 1D numerical simulations under real-world driving conditions. Without active heating, high-thermal-mass substrates unexpectedly outperform ultra-thin-wall variants by buffering against convective quenching during prolonged idling. However, integrating start–stop functionality halts cold exhaust flow, elevating mean temperatures and marginalizing geometric disparities. Evaluating electrically heated catalysts (EHCs) reveals that discrete preheating is highly inefficient due to rapid heat dissipation. Conversely, continuous closed-loop heating coupled with start–stop functionality sustains operational temperatures for over 90% of the cycle. Under continuous heating, substrate geometry ceases to dictate thermal performance; instead, it governs electrical efficiency. Low-thermal-mass monoliths minimize cumulative energy demand to 213 kJ (versus 277 kJ for high-mass variants), incurring a negligible CO₂ penalty. Consequently, future hybrid architectures must integrate lightweight EHCs to ensure sustainable emission control.

This study investigates the geometric parameters of commercially available or recently published models of catalyst substrates for passenger vehicles and provides a numerical evaluation of their influence on thermal behavior for use in urban areas. To ensure the plausibility of the results, a range of scenarios have been meticulously designed to replicate real operational conditions. To ensure a reliable basis for comparison, all geometries were standardized to the same gas-solid exchange surface by adapt the total monolith length. The simulation experiments conducted revealed that the primary role in question is played by the mass of the monolith and its internal surface area, while the heat transfer coefficient plays a secondary role. The necessity of adapting the geometry to the deployment scenario was demonstrated by comparing the performance of different scenarios. The employment of contemporary technologies, such as start-stop systems and automatic catalytic converter preconditioning, has also been factored. The study's findings suggest that geometries characterized by high cell density and low wall thickness, in conjunction with modern materials and intelligent engine control, are ready to become the norm for future ICE applications. This claim is particularly salient in the context of the requirements of mobile exhaust gas purification systems.