The paper addresses the issue of modeling the transient thermal behavior of the hot-end of an exhaust system with a close-coupled catalyst using a CFD code. The model considers the different materials used in the construction (up to five) and for the involved heat transfer mechanisms (convection at the fluid-solid interface both on manifold walls and in the channels of the ceramic brick; conduction within the fluid, within the solids and at solid-solid interfaces). Additionally, custom-made routines give a more efficient calculation of the transient ceramic warm-up, avoiding the modeling of all fine geometrical details of the fluid-solid interface. Moreover, catalyst chemical reactions have been ignored. Finally, time dependent boundary conditions from the NEDC cycle have been implemented.
The NEDC cycle (New European Driving Cycle; also called ECE+EUDC cycle) is used, as is well known, to evaluate vehicle fuel economy and emissions in Europe. Different design configurations and materials have been simulated, to evaluate the influence on catalyst light-off time based on the temperature. Specific experimental tests have been carried out to validate the calculation results and the agreement was reasonably good.
The methodology has been developed by investigating the performances of a new component, and the importance of the thermal inertia of the manifold has been evaluated. The results confirm that a thin steel exhaust manifold instead of a thicker cast iron one improves the thermal light-off of the catalytic converter. The final goal of the methodology is to computationally evaluate thermal performances of different designs and modifications.