Investigation of PM Formation and Evolution in Plumes Emitted by Heavy-Duty Diesel Vehicles: Wind Tunnel Study.

摘要

Stringent emission regulations have forced drastic technological improvements in diesel aftertreatment systems, particularly in reducing Particulate Matter (PM) emissions. Studies that have led to these technological advancements were made in controlled laboratory environments and are not representative of real-world emissions from these engines or vehicles. In addition, formation and evolution of PM from these engines are extremely sensitive to overall changes in the dilution process. In light of this, the study of the exhaust plume of a heavy-duty diesel vehicle operated inside a subsonic environmental wind tunnel can give us an idea of the dilution process and the representative emissions of the real-world scenario. The wind tunnel used for this study is capable of accommodating a full-sized heavy-duty truck and generating wind speeds in excess of 50mph. It was specifically designed and built by West Virginia University (WVU) to characterize the exhaust plume emitted from heavy-duty vehicles. A three-dimensional gantry system allows spanning the test section and sample regions in the plume with accuracy of less than 5mm. The investigation involves three different heavy-duty Class-8 diesel vehicles equipped with aftertreatment technologies, representative of legacy and modern truck fleets in the USA. The three vehicles investigated are representative of three emission regulation standards, namely a US-EPA 2007 compliant, a US-EPA 2010 compliant, and a baseline vehicle without any aftertreatment technologies as a pre US-EPA 2007, respectively. The testing procedure includes three different vehicle speeds: idling, 20mph, and 35mph. The vehicles were tested on WVU's medium-duty chassis dynamometer, with the load applied to the truck reflecting the road load equation at the respective vehicle test speeds. Wind tunnel wind speed and vehicle speed were maintained in close proximity to one another during the entire test. Results show that the cooling and dilution of the exhaust takes place within 2m from the exhaust stack. The rate of cooling and dilution are greatest in early stages of the dilution process (within 0.15m from the exhaust stack) for the areas with high turbulence intensity (TI), where strong mixing phenomena occurs and the nucleation mode of the PM is formed. On the other hand, the core of the plume observes a slower cooling and dilution rate. This difference is reflected in the PM formation and evolution of these two distinct regions, as shown by the particle size distributions and number concentrations. Eventually, further downstream those heterogeneous areas due to the turbulence mix together to form a homogenous particle size distribution across the entire plume. The content of PM volatile and solid compound in the exhaust set the condition for the nucleation to occur, but the TI has the active role to generate nanoparticles. Using MARS as modeling tool of the plume enhanced the obtained notions, confirming the initial heterogeneity of the plume, further becoming homogenous. In addition, providing further detail of early stages of the plumes showing that a DPF-equipped truck generates a nucleation mode quickly adsorbed by the background PM (0.15m from the exhaust stack).