As the vehicle speeds increased from the urban (NIER05) to the rural (NIER09) and motorways (NIER12, NIER13), the fuel injection pressure and intake pressure (MAP) increased owing to more engine power being required because of the increased opening rate of the throttle valve. Moreover, the spark timing at the motorway modes exhibited a tendency to be more advanced than at the urban and rural modes because of higher engine speed. As all vehicle test modes were operating at fully warmed-up condition within the lambda closed-loop zone, the stoichiometric A/F was maintained for full utilization of the efficient high catalyst purification except for aggressive transient operation.
The second-by-second exhaust port and catalyst bed temperature profiles, measured with K-type thermocouples, are shown in Fig. 4. The vehicle preconditioning procedure of the NIER mode was carried out following the HWFET mode and for sustaining fully warmed-up conditions of the UCC, the exhaust system temperatures at the mode starting phase were over 400 °C, which are an advantage for higher catalyst efficiency. Similar trends as shown in the vehicle parameters are observed in the exhaust system temperatures. Operation of the vehicle at higher speeds resulted in higher exhaust gas and catalyst bed temperature.