Development of an improved urea-selective catalytic reduction-diesel particulate filter system for heavy-duty commercial vehicles

Many heavy-duty commercial vehicles are now equipped with urea-selective catalytic reduction (SCR) systems, which can reduce NO_x emissions sufficiently to meet the requirements of legislation such as Japan's New Long-term Diesel Emissions Regulations. However, in order to meet even stricter exhaust emissions regulations (and fuel consumption standards) due to be imposed in many parts of the world in the near future, urea-SCR systems with greater catalytic efficiency combined with diesel particulate filters (DPFs) will be needed. Therefore, in the study presented here the scope for enhancing the efficiency of a urea-SCR system was explored by optimizing the urea dosing system and injection strategies, and the gas flow in the exhaust pipe. However, since improving the catalysis parameters could have the greatest overall effect on conversion efficiency, work focused on modifying the catalyst materials to increase their adsorption capacity for the NH₃ reducing agent, and thus increase the collision frequency between NO_x and NH₃ absorbed on the surface of the catalyst. In addition, the oxidation parameters of the oxidation catalyst were optimized, which enhanced the NO_x conversion efficiency of the system, not only in a steady cycle but also in a transient cycle. Following these adjustments, a DPF-plus-SCR system with the new catalytic material delivered 90 per cent conversions of NO_x and particulate matter to N₂ and CO₂ respectively, in the JE05 test cycle. In addition, a new concept, a miniaturized 'urea-SCR with DPF function system' was proposed and tested, which delivered 90 per cent NO _x conversion rates and 90 per cent reductions in particulate matter emissions in the JE05 test cycle.