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Similar to single-brick SCR architectures, the multi-brick SCR systems described in this paper require urea injection control software that meets the NOx conversion performance target while maintaining the tailpipe NH3 slip below a given threshold, under all driving conditions. The SCR architectures containing a close-coupled SCRoF and underfloor SCR are temperature-wise more favorable than the under-floor location and lead to significant improvement of the global NOx conversion, compared to a single-brick system. But in order to maximize the benefit of close-coupling, the urea injection control must maximize the NH3 stored in the SCRoF. The under-floor SCR catalyst can be used as an NH3 slip buffer, lowering the risk of NH3 slip at the tailpipe with some benefit on the global NOx conversion of the system. With this approach, the urea injection strategy has a limited control on the NH3 coverage of the under-floor SCR catalyst.

Thermal mass and external insulation of exhaust manifolds and down pipes have always been known as main factors that could significantly influence the inlet gas temperature of catalytic converters. However, heat loss from exhaust gas to the environment is first influenced by the internal gas flow. This indicates the importance of gas-to-wall heat transfer. In the present work, a basic heat-transfer analysis was performed to assess and rank the relative impact of thermal mass, external heat insulation and internal gas flow on the exhaust manifold outlet temperature. For a Euro 4 application, where the light-off time is about 15-30 s, the results show that internal flow plays the most important role in heat loss from the exhaust gases. Thermal mass and external heat insulation start to be important only once the material temperature is high, beyond the first 30 sec. Therefore improving internal flow of the exhaust manifold will be crucial to minimizing light-off time.

The introduction of the catalytic converter into the engine compartment has led to severe requirements for thermal insulation. The converter and the exhaust manifold constitute significant heat sources that increase the underhood air temperature and the direct radiation to surrounding components with limited heat tolerance. In the present study, various methods of converter thermal insulation are investigated, and recommendations for minimizing heat transfer are presented. The basic heat transfer mechanisms from the converter are first reviewed to provide a basis for understanding the most effective means of insulation. Ceramic fiber insulation with various thermal conductivities are evaluated at several different thicknesses. This type of internal insulation, which is often used for catalytic converters, is compared to heat shielding. It is found more effective. Experiment and/or simulation have been used to evaluate the various insulation methods.