Development of urea-SCR system for heavy-duty commercial vehicles (Improvement of SCR catalyst and proposal for new SCR concept)

Some manufacturers of heavy-duty commercial vehicles have adopted Urea-SCR system that can significantly reduce NO_X exhaust gas emissions by using a catalyst for the purpose of meeting the Japanese New Long-term Diesel Emissions Regulation. This system can satisfy the demand for excellent fuel economy as well as reduced emissions. With engine modification, combustion efficiency is improved, thereby fuel economy is enhanced and engine-out emissions of PM are also reduced simultaneously. Though NO_X emissions increase considerably as a resultant trade-off, a Urea-SCR catalyst is used as an after-treatment technology to reduce the NO_X level. As global exhaust emissions regulations will be further strengthened in the near future, along with newly revised fuel consumption standards, the adoption of DPF together with the enhanced catalytic conversion efficiency for the Urea-SCR system is indispensable. Moreover, miniaturization of the after-treatment system is also an important issue. In order to enhance conversion efficiency of the Urea-SCR system, the improvement of (i) catalyst performance itself, (ii) urea dosing system, and (iii) the exhaust gas flow, etc. are required. At first, we focused on improved catalyst performance itself that could have bigger effect on conversion efficiency. The catalyst materials were improved to increase adsorption capability of the NH₃ as a reducing agent, resulting in increased NO_X and NH₃ reaction rates on the catalyst. Furthermore, capacity of the SCR catalyst was increased and oxidation performance in the oxidation catalyst was optimized, which led to enhanced NO_X conversion efficiency of the system not only in a steady cycle but also in a transient cycle. In addition, the new concept of 'the Urea-SCR system with the DPF function' was proposed for the miniaturization of the system.