Kiminobu Hirata

Development of Urea-SCR System for Heavy-Duty Commercial Vehicles

In Japan there is currently a strong social demand for exhaust emissions reduction from heavy-duty diesel engines. Therefore, new Long-Term Regulation will come into effect in October 2005, setting the NOx standard at 2.0 g/kWh and the PM standard at 0.027 g/kWh. At the same time, customers always demand exceptional fuel economy from heavy-duty commercial vehicles. A urea-based Selective Catalytic Reduction (SCR) system was developed to satisfy both these demands, and will be introduced in the fall of 2004. The operating conditions of vehicles in Japan are different from those in the US and Europe. Basically, average vehicle speeds are significantly lower. To improve the low temperature SCR performance, an oxidation catalyst was located upstream of the SCR, and an additional oxidation catalyst was located downstream of the SCR for emergency NH3 slip. The muffler size with all three catalysts was similar to a conventional muffler. The fuel and urea water solution consumption of the system was studied and compared with the fuel consumption of a similar system equipped with EGR and a Diesel Particulate Filter (DPF) unit. The properties of the urea water solution used here were not well known at the time of this project. Therefore, the specifications and corrosion characteristics of relevant materials were investigated and suitable materials were selected. Urea water solutions freeze at around -11 deg C, so a thawing system was also developed. Vehicle tests were carried out at various conditions, during which no major problems arose.