China 6 will be implemented from Jul 1st 2020, containing two norms, China 6a and China 6b. China 6a brings an introduction of the WLTC and a PN limit of 6 x 1011 #/km. Emissions limits of THC, CO and NOx, in China 6a are similar to China 5. Overall China 6a sets a similar emissions challenge to Euro 6d. In areas where China 6b is introduced before national roll out, we will see a further tightening in gaseous emissions to around half of China 6a levels for THC, CO and NOx. This therefore presents a very challenging situation for emissions control system design, particularly because of both the stringent gaseous requirement in China 6b and the PN requirement.
In this paper, one development stage vehicle, equipped with a 1.5 L Gasoline Direct Injection (GDI) engine, was chosen for investigation. Firstly, after a calibration modification, 3 TWC systems with variable PGM loading were evaluated. The results showed the systems with a PGM loading higher than 40 g/ft3 for the front brick, plus a rear brick with 10 g/ft3, can meet the engineering target under both fresh and engine aged conditions. In addition, a coated Gasoline Particulate Filter (cGPF) which coated with an active catalytic coating containing Platinum Group Metals (PGM) was also introduced to further reduce particle emission. Compared to the TWC system, the cGPF system maintained a comparable TWC performance, and in addition achieved 56% and 92% filtration efficiency under both fresh and aged conditions respectively. This helped the test vehicle to meet China 6b requirement successfully. Finally, soot deposition and regeneration in the cGPF were also investigated with variable vehicle start temperatures. We found that the soot deposition increased significantly with temperature lower than -10 °C, and even reached 0.33 g after 6 km urban driving at -30 °C. The results also showed that passive regeneration by oxidation of soot could reach 75% combustion efficiency if the driving mileage extended to 32 km. This was observed even with a start temperature of -20 °C. However, in order to cope with the significant soot deposition in the cGPF under the condition of frequent starts at low temperature, followed by just short mileage driving, an active regeneration was also investigated. The results showed that active regeneration could accelerate the soot combustion and protect the cGPF from potential failure caused by instantaneous drastic soot burning.