Influence of Fuel Introduction Parameters on the Reactivity and Oxidation Process of Soot from a Gasoline Direct Injection Engine

[摘要] Upcoming regulations, such as Euro 6c, decrease limits on allowable tailpipe particulate number emissions. These regulations may place some current gasoline engine technologies above the legal limit, especially those vehicles using direct fuel injection. Therefore, gasoline particulate filters (GPF) may be necessary for future emissions compliance. The key functionality of the GPF is filtration efficiency and control of this function is dependent on the soot build-up within the filter. The more soot present, the higher the filtration efficiency. As soot will often oxidize during much of the engine operation due to high temperatures and available oxygen, an understanding of soot reactivity and the degree to which reactivity may vary is key for designing after-treatment architecture and explaining low filtration efficiency results in real world situations.In this work, soot was evaluated from four conditions incorporating changes in the fuel injection pressure and timing, and the influence these parameter variations had on soot reactivity was evaluated through well-controlled isothermal and ramped oxidation events. It was determined that soot reactivity and the temperature by which oxidation would commence were quite different for the four samples. Following this, soot parameters known to influence reactivity were investigated. It was determined that nanostructure, volatile organic fraction and surface functional groups were similar between all four samples and though surface area was different, it did not correlate with reactivity. It was determined through estimations of the ash content that soot reactivity was likely influenced by the ash too soot ratio of the particulate matter. Ash is a known catalyst, found to enhance soot oxidation rates. The presence of ash and the interaction of soot and ash were investigated using a scanning transmission electron microscope coupled with energy dispersive x-ray spectroscopy (STEM+EDS).To carry further the investigation, soot samples from the most and least reactive condition underwent partial oxidation and the partially oxidized soots were analyzed to obtain information regarding the oxidation process. During oxidation, the soot particles appeared more amorphous and began to meld into each other, forming larger non-spherical particles.It was determined that beyond a sufficient ash-soot ratio, a ratio likely below 1% and easily reached for a modern gasoline direct injection engine, the soot oxidation process is marked by three distinct phases relating to interactions of ash and soot particles. In the first phase, the oxidation rate is enhanced by the presence of close soot to ash contact. Soot in close contact with the ash will oxidize first, leaving a loose contact between the ash and soot. This loose contact causes the observed distinction in the second stage. In the second stage, despite increases in the ash to soot ratio due to the loss of soot, the oxidation rate decreases due to the loose ash to soot contact. As oxidation proceeds, soot particles begin to meld together, forcing the ash to be in close contact with any remaining soot. This renewed close soot to ash contact causes the observed distinction in the third stage. In the third stage, soot oxidation rates increase until the end as the close soot to ash contact is maintained and the ash to soot ratio increases with oxidation of the soot

[发布机构] University of Michigan

[效力级别] Mechanical Engineering

[关键词] Reactivity of Gasoline Soot from Differing Fuel Introduction Parameters;Mechanical Engineering;Engineering;Mechanical Engineering