Real Driving Emissions in Congested Traffic: A Comparison of Cold and Hot Start

Air quality NO₂ and PM exceedances in cities are common, where congested traffic occurs and the monitoring station is at the roadside. This work investigated real world emissions for a Euro 4 SI vehicle on a congested road by a roadside air quality monitoring station that exceeds European air quality standards for NOx and PM. The PEMS used was the Temet FTIR with Horiba OBS pitot tube exhaust mass flow sensor and gas sampler. Twenty nine hot start repeat journeys were made at different times of the day, so that a range of congestion conditions were encountered. Eight cold starts for the same journeys were compared with the hot start results and shown to give significantly higher emissions. The CO₂ and other emissions were strongly correlated with the average journey velocity which is inversely proportional to congestion. All these emissions were higher than on the NEDC, due to the lower mean velocity and not to any malfunction of the emissions control equipment. The WLTC and proposals for the EU RDE tests do not include congested traffic driving and the RDE does not include cold start. Cold start into congested traffic is a reality in city driving and the emissions are much worse for cold start journeys as the cold start is much longer due to the lower average speeds.