Measuring and comparing taxi emissions with privately owned vehicles in an urban environment using a remote sensing device

摘要

Diesel engines represent a significant fraction of the light duty vehicle fleet in the UK and Europe. Their emission characteristics differ vastly from Petrol fuelled vehicles and the lean burn characteristics of diesel engines that make them so desirable for fuel consumption also lead to many problems relating to the emissions of pollutants. The presence of excess oxygen from the lean burn in the exhaust plume compared to traditional petrol engines operating at stoichiometric air-fuel mix levels means that the reduction reactions used in Three-Way Catalysts (TWCs), so successful at removing the vast majority of pollutants from petrol engines, are comparatively ineffective for Oxidised Nitrogen (collectively NOX) compound removal in lean burn engines. Diesel engines also emit a complex particulate matter component in their exhaust plumes containing a volatile organic fraction as well as solid organic material, often with one adsorbed onto the other. All of these factors mean that control systems for diesel emissions are far more complicated than for petrol (Johnson, 2006). The short-term health impacts of exposure to air pollution are significant, with research consistently showing the negative impacts of exposure. Vehicle emissions are also cited as a significant contributor to air pollution (WHO, 2005). Exposure to species contained in diesel emissions such as NO2, particulate matter, Benzene and Polycyclic Aromatic Hydrocarbons are of special concern and therefore effective control systems are required. A NOX Storage and Removal (NSR) catalyst aims to store the NOX as a nitrate under lean burn conditions and periodically remove it under rich burn conditions managed by the engine control unit (Matsumoto, Ikeda, Suzuki, Ogai, & Miyoshi, 2000). During lean burn periods the NO is oxidized over a Platinum catalyst to create NO2. The resultant NO2 is captured by forming compounds containing Barium, for example BaO is often used alongside Pt. During the short rich burning periods of operation, the NOx is released from the Barium and reacts with hydrocarbons to produce N2, H2O and CO2 (Olsson & Fridell, 2002). Furthermore, an Oxidation catalyst (DOC) is used to remove CO and HC in the exhaust. The reactions in the DOC also remove the volatile organic fractions adsorbed onto the solid organic soot compounds (Walker, 2004). Adsorption of molecules onto the catalyst occurs during periods of lean burn engine operation and the molecules are removed during short periods of rich burn operation, regenerating the catalyst and allowing it to work at full efficiency again. The rich burn process is typically controlled by the engine control unit (Lapuerta, Hernandez, & Oliva, 2014). Catalysts in Diesel vehicles suffer from a lack of self-heating, means that even if a catalyst reaches light-off temperature it may later cool to a point that it is no longer effective at removing pollution from the exhaust gas (Herreros, Gill, Lefort, Tsolakis, Millington, & Moss, 2014). The number and contribution of these cold start and cold operation emission sources is not currently fully understood.