SCR exhaust system for passenger cars: parameters, methodologies, specific issues

摘要

Aftertreatment of nitrogen oxides (deNO{sub}x) emitted by Diesel powered passenger cars is or will be soon required to fulfil the most recent emission standards, first on the North American market. Two deNO{sub}x systems are in competition, the NO{sub}x storage catalyst (NO{sub}x-Trap) and the "Selective Catalytic Reduction" system (SCR) using a reductant (urea solution) injected directly into the exhaust system via a dedicated injector. Reliability of SCR system has already been proved on the heavy duty market in Europe where a large number of vehicles are equipped with this technology since 2005. However, integration of the SCR in the exhaust system of a Diesel powered passenger car requires a basic work on the exhaust system design and the right setup of the various emission control devices. The objective of the presentation is to review the specific issues to be resolved to successfully adapt the SCR system on Diesel passenger cars and the related solutions. The relative position of the aftertreatment devices and the various possible options are discussed. System heat management requirements are described, with a focus on solutions to improve the system efficiency during light-off and heating-up phases (injection/vaporization/mixing quality/chemical reactions). Material choice to withstand urea induced corrosion is presented. Last part of the presentation explains how the mixing device operates and shows characterizations and validations, based on prediction and experimental results.