2021

Söderena, Petri; Kuutti, Hannu; Pellikka, Ari-Pekka

The on-road NOx emissions monitoring project on Euro VI diesel city buses aimed at investigating the effect of aging on emissions performance of typical city buses. The project run between the years 2017-2020, and was jointly funded by the following partners: City of Helsinki, Helsinki Region Environmental Services HSY, Helsinki Region Transport, Traficom, TØI The Institute of Transport Economics and VTT Technical Research Centre of Finland. <br/>Altogether four diesel city buses were chosen for the project. Those buses were driven during the project between 260 000 and 360 000 km depending on the bus. A comprehensive testing program included periodic (twice a year) chassis dynamometer and on-road PEMS testing campaigns. In addition, each of the buses were equipped with continuous NOx concentration monitoring devices. Chassis dynamometer testing was performed with Braunschweig and WHVC cycles. On-road testing was done on two routes representing typical city bus operation at HSL traffic and on a route fulfilling Euro VI ISC requirement. <br/>Regarding energy consumption, no effect of aging was observed. In addition, engine-out emission measurement during the project start and final measurement showed that engine-out NOx emission decreased only 2.4 % - 9.3 % with buses A, C and D. These findings suggest that combustion- wise engines were not changed. However, depending on the bus, different degree of degradation in NOx emissions control was observed. During the first measurements at project start, the buses showed NOx emissions less than 0.3 g/kWh, but at the final measurements the emissions were increased even up to a level of 2.0 g/kWh. However, not all buses suffered degradation of emissions performance. Bus C had low NOx emissions throughout the project both on chassis dynamometer and on-road. Especially in on-road testing high variation in NOx emissions (between less than 0.1 g/kWh and 3.0 g/kWh) was observed depending on the test route, bus and time of testing. Continuous NOx concentration monitoring showed that cold ambient temperature (below 0 °C) may increase the NOx emissions by a factor of 2 - 4 compared to summer conditions. No change in PM, PN, CO or NMHC emission was measured during the project. The Euro VI EAT configuration for those emission components i.e. DOC (CO, NMHC) and DPF (PM, PN) seem to be rather robust against aging.<br/>