Late Cycle Soot Oxidation in Diesel Engines

Sammanfattning: Diesel engines are the most common engine technology used in transportation. Its widespread use comes from its inherent high efficiency and its relative low cost. It suffers from pollutant emission issues, however, that need detailed understanding of their origins to meet the strictest current and upcoming emission standards. The work presented in this thesis focuses on the study of the mechanism at the origin of soot particle emissions and gives indications on which parameters can effectively reduce them.There are two competing in-cylinder processes, soot formation and soot oxidation, governing soot emissions from conventional combustion in diesel engines. To this day, it is still often believed that inhibiting the formation of soot reduces its emissions, despite several studies showing poor relationship between soot formation and emission in most conditions encountered in diesel engines. The results presented in this thesis aims at highlighting the importance of soot oxidation in the late cycle over the soot formation process. An effort was realized to identify the parameters governing its oxidation and how it results in lower emission levels. The study of those parameters has been realised through the use of in-cylinder sampling of gases and optical measurements. The optical measurements were realised in a heavy-duty diesel engine using a Bowditch design with a specifically designed three-valve cylinder head. This cylinder head had an optical access located at the top of the combustion chamber, allowing the study of processes occurring in the late cycle, when the piston is moving down into the cylinder. The diagnostic chosen for this study is called laser extinction, a line of sight technique based on the absorption of light by the soot particles present in the flame. The data collected gave information on the evolution of the soot concentration and could be used to characterise the soot oxidation rate in the late cycle. The in-cylinder sampling measurements were realized in order to gather information on the soot particle characteristics.The results showed a strong correlation between the soot oxidation rates and the emission levels, underlining the importance of improving the oxidation rate rather than the soot formation in order to reduce soot emissions. It was shown that increasing the mixing of gases in the cylinder by means of increased turbulences could lead to a negative impact on the soot oxidation rates that were passed on to the soot levels in the exhaust. A variation of temperature of gases showed little to no impact on the soot oxidation rates and did not seem to effectively impact the emission levels. An increase of the gas density in the cylinder of gases lead to improved oxidation rate that also reduced emission levels. Of the injection related parameters studied, the injection pressure and the injector hole size had a strong impact on the soot oxidation rates even long after the end of the injection process. The most important parameter identified that affected the soot oxidation rates was the oxygen concentration. This parameter was studied in detail and it showed that a reduction of oxygen concentration lead to a slower maturation of soot during the combustion. While this feature produced more reactive soot particles prone to be more easily oxidized, it was revealed to be concerning a limited amount of particles in the cylinder. Moreover, it was largely overruled by the reduction of OH produced in the flame. Using simulation tools, it was shown that lowering the oxygen concentration reduced the flame temperature, which in turn reduced the OH concentration. The reduction of OH correlated strongly with the reduction in oxidation rates observed in the optical measurements, to a greater extent than just the drop of oxygen concentration.