Solving air quality problems in cities by retrofitting: Case studies showing real-world driving performances

Urban air quality has been widely discussed and debated over the last decades. It started mainly with the focus on fine particles and concern was in major cities in certain developing countries. Lately, more and more weight has been put on nitrogen oxides (NO_x), also in European cities. Public and political pressure has been increasing over the last years, while additional studies on the current situation of air quality together with health risks have been published. There are multiple sources of nitrogen oxides (NO_x) and the whole picture is complex in nature. However, traffic has been shown to be one of the main reasons. As it is known today, older generation diesel vehicles emit a great deal of NOx. It is believed that the situation would improve while a newer fleet is taking place. But, unfortunately the progress has not been as expected there. The current knowledge on the difference between heavy-duty engines laboratory condition homologations, application specific chassis dynamometer tests and lastly real world driving conditions is underlining the importance of improvements in exhaust emission reduction technologies. This study shows the differences between chassis dynamometer and real-world emissions on city busses. Online monitoring and portable measurement technologies together with retrofitting and upgrading the vehicles using state of the art emission reduction technology has been studied for measuring and reducing the harmful exhaust gas compounds, especially NO_x in real-drive conditions in city driving. The retrofitting of buses with an improved emission after treatment systems has proven to be a feasible and quick solution for reducing local NO_x emissions significantly. The comparison of new buses and the retrofitted ones clarifies the potential of the retrofit approach. Several case studies are presented, and some European city retrofit programs are referred to in this study.