It is commonly accepted that future powertrains will be based to a large extent on hybrid architectures, in order to optimize fuel efficiency and reduce CO₂ emissions. Hybrid operation is typically achieved with frequent engine start-and-stops during real-world as well as during the legislated driving cycles. The cooling of the exhaust system during engine stop may pose problems if the substrate temperature drops below the light-off temperature. Therefore, the design and thermal management of after-treatment systems for hybrid applications should consider the 3-dimensional heat transfer problem carefully. On the other hand, the after-treatment system calculation in the concept design phase is closely linked with engine calibration, taking into account the hybridization strategy. Therefore, there is a strong need to couple engine simulation with 3d aftertreatment predictions.In this paper, we perform measurements in flow-through catalytic substrates and wall-flow DPF systems to monitor the thermal losses after engine shut-off. In parallel, we use an existing 3-dimensional modeling platform (axisuite®/Exothermia SA), which is enhanced with a heat transfer submodel to account for free convection and radiation from the front and rear substrate faces. The model predictions are compared with experimental data under various operating conditions.The validated model is used to predict the performance of a diesel after-treatment system of a hybrid vehicle application. The performance of the after-treatment system in terms of DOC conversion efficiency is studied in detail with the help of 3-dimensional modeling during engine start-stop operation. The importance of heat losses is illustrated and the potential implications for exhaust system thermal management are discussed.