Dual Line Exhaust Design Optimisation to Maximize SCR Catalyst Efficiency thru Improved Ammonia Distribution

The SCR after treatment system is already in production for passenger car engines with a single exhaust system. In this case, the exhaust system has to be designed very carefully to optimize the Ammonia distribution on the catalyst and therefore the DeNOx potential. The application to V8 engines with two turbochargers delivering the gas into two separated DOC & DPF units is an additional challenge.

This paper describes the different optimization steps of such an exhaust system and the tools used during this work. After a design phase to integrate the SCR system in the exhaust geometry, a first CFD study was conducted to evaluate the performance of the basic system using one or two urea injectors. An optimization of the connection of the two tubes, directly in front of the SCR catalyst, has been designed using further CFD calculations as well as a marker gas SF₆ on a cold flow bench. After further biphasic CFD calculations simulating the complete hot side of the exhaust system, prototypes were built to compare selected designs to the reference. Measurement of NOx distribution and NH₃ slip on engine test bench were run to confirm the trends founded during the precedent steps.

A good correlation between engine test results, flow bench measurement and two phase flow simulation has been observed. The DeNOx performance of the proposed design shows a big improvement in comparison to the reference and makes it possible to use a single injector, making it at the same time more economical and more efficient.