Optimization of the selective catalytic reduction structure of a vehicle based on the improvement in the uniformity of the nitrogen oxides distribution

Abstract

As the sensor probe occupies a small proportion of the cross-sectional area of the pipe, the concentration of nitrogen oxides measured by the sensor may not reflect the average concentration of nitrogen oxides at the measured section when the uniformity of the nitrogen oxides is poor. The uniformity of the nitrogen oxides at the measured section is influenced by the structure of the selective catalytic reduction system of the vehicle, and particularly by the structure of the variable cross-section behind the catalyst. In this paper, a model of the selective catalytic reduction system is established to explore the rules for influencing the uniformity of the nitrogen oxides. The conclusion shows that the uniformity of the nitrogen oxides is influenced by five structure factors, which are as follows: the nozzle location; the structure of the mixing chamber; the exhaust velocity; the pipe-diameter ratio of the variable-cross-section structure; the distance between the measured section and the catalyst and the catalyst outlet. When the nozzle is further away from the catalyst inlet and the mixing chamber is designed as a cone-shaped guide plate, the uniformity of the nitrogen oxides is better. When the relative flow velocity is greater than 4.5–7.5 s⁻¹, the uniformity of the nitrogen oxides is positively related to this factor; otherwise, the uniformity stays invariant. When the pipe-diameter ratio of the variable cross-section is 0.34, the uniformity of the nitrogen oxides is the best. The distance between the measured section and the catalyst outlet is logarithmically related to the uniformity of the nitrogen oxides. Based on the laws for the parameters influencing the uniformity of the nitrogen oxides, the structure of selective catalytic reduction in a vehicle can be optimized to improve the objectivity of the measurements, thereby guaranteeing the accuracy of the vehicle’s on-board diagnostic system, in which the nitrogen oxides sensor is applied.