Real World Driving: Emissions in Highly Congested Traffic

The emissions from vehicles in real world driving are of current concern, as they are often higher than on legislated test cycles and this may explain why air quality in cities has not improved in proportion to the reduction in automotive emissions. This has led to the Real Driving Emissions (RDE) legislation in Europe. RDE involves journeys of about 90km with roughly equal proportion of urban, rural and motorway driving. However, air quality exceedances occur in cities with urban congested traffic driving as the main source of the emissions that deteriorate the air quality. Thus, the emissions measured on RDE journeys may not be relevant to air quality in cities. A Temet FTIR and Horiba exhaust flow measurement system was used for the mass emissions measurements in a Euro 4 SI vehicle. A 5km urban journey on a very congested road was undertaken 29 times at various times so that different traffic congestion was encountered. Each journey was split into ten sections in order that the location and traffic conditions of the highest emissions could be determined. It was found that low speed stop-start traffic has much higher emissions than for freely moving traffic and most of the higher emissions on the longer 5km journeys occurred in relatively short sections of slow moving stop/start traffic. The journey used passed a roadside air quality monitoring station that exceeded the EU NO₂ and PM standards on an annual basis and it was located by the most congested part of the route, where the traffic emissions are shown in this work to be at their highest.