This report provides a summary of work undertaken within Workpackage 5 of the European Commission 5th Framework project OSCAR, which has delivered an ‘Assessment System’ to enable users to evaluate road traffic-related air pollution, and to identify suitable impact-reduction options. An important element of the Assessment System, and the overarching goal of Workpackage 5, was an improved understanding of driving characteristics, vehicle operation and exhaust emissions at low speeds. The principal tasks of the Workpackage were to characterise driving patterns in four cities (Athens, Helsinki, London and Madrid), to improve existing emission databases for slow-moving and stationary traffic, and to develop a System module for assessing the effects on emissions of different levels of congestion.
The report reviews existing emission models, and assesses their suitability for congested urban conditions. The review showed that although many emission and air pollution models utilise average speed emission functions, for the latest vehicle technologies average speed alone is not a reliable determinant of emissions on the street level, and some descriptor of driving cycle dynamics is also required. Within OSCAR, vehicle operation patterns, road characteristics and traffic conditions were recorded in each of the four cities using on-board diagnostics (OBD) and a global positioning system (GPS). A large number of descriptive statistics were determined for each driving pattern, and generalised relationships between traffic speed, traffic density and driving pattern parameters were identified. Representative driving cycles were derived using these and other parameters (notably parameters describing engine power output), and exhaust emissions from a sample of 20 vehicles were measured over the driving cycles in the laboratory.
With the exception of average speed, the parameters describing driving pattern dynamics were not found to be strongly related to traffic conditions. In the absence of any other means of quantifying dynamics on the part of the model user, traffic density (vehicles per km) was used to define congestion. For driving cycles having the same average speed, cycle dynamics was found to have a notable effect on emissions for some vehicle types and pollutants, but the effects were not systematic. NOx emissions from petrol vehicles showed little dependence on speed. However, for a given speed range the level of driving dynamics was found to be important, with higher emissions being recorded over cycles having higher dynamics. In contrast, NOx emissions from diesel vehicles exhibited a general dependence on speed, with little contribution due to driving dynamics. Emissions from LPG vehicles were comparable to those from petrol vehicles. PM emissions from diesel vehicles over the OSCAR cycles tended to decrease with decreasing speed, and cycle dynamics appeared to have an effect. Results from TNO’s VERSIT+ model were also used to provide information

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