Study On Oxidation And Fragmentation Of Soot Particles During Diesel Combustion Course

Improvement of the in-cylinder soot oxidation is one of effective solutions to reduce diesel particle emissions.Therefore,it is of great importance to study the incylinder soot oxidation mechanism for reducing the soot emissions from diesel engine.Based on an automated total cylinder sampling system and the combustion simulation,soot oxidation behavior was investigated,and the factors that affected the oxidation behavior were analyzed in this work.Furthermore,the oxidation-induced fragmentation of soot particles was also studied based on the evolution of soot physicochemical properties.Finally,the correlation between the soot fragmentation behavior and the incylinder combustion conditions was estimated.The major work and achievements of this dissertation are listed as follows:The combustion process of single-main-injection case can be divided into a soot formation-dominated phase and a soot oxidation-dominated phase.For the main-plus post-injection(M*P)case,the combustion process can be divided into four consecutive phases: main-soot formation-dominated phase,main-soot oxidation-dominated phase,post-soot formation-dominated phase and co-oxidation phase of main-soot and postsoot.Herein,the main-soot and post-soot denote the soot particles generated from the main-injection and post-injection combustion courses,respectively.The addition of post injection increases the bulk specific oxidation rate of incylinder soots during the co-oxidation phases.The improvement of in-cylinder mixing condition induced by post injection is not the primary factor for the soot oxidation enhancement.The increased temperature by post-injection combustion preferentially promotes the oxidation of post-soot,and then exerts a positive effect on the main-soot oxidation.The addition of post injection leads to a higher bulk oxidative reactivity of the M*P soots,which in turn facilitates the soot oxidation reactions.This behavior presents an additional explanation for the reduction of soot emissions by post injection.For the in-cylinder soots from the single-main injection case,there is a well linear correlation the sp³/sp² hybridization ratio and the fringe length.However,the addition of post injection weakens this correlation,suggesting that the structural non-uniformity of the soot surface and the inner core.Such non-uniformity indicates the occurrence of internal burning in certain parts of the M*P soots.In the electron microscopy images,the presence of soot particles with hollow interiors(the characteristic feature of internalburning regime) provides a further evidence for the occurrence of internal burning.As the soot oxidation further proceeds,these hollow particles evolve to ones with extended and flatter graphene layers,polygonal outer-shape and significant-reduced primary particle size.Associated with the soot oxidation,the bridges between the primary particles can be attacked by oxidant species such that the structure weakens and the aggregates subsequently break apart.This process is responsible for the oxidation-induced aggregate fragmentation.The aggregate fragmentation rate shows a distinctly decline with the decreased in-cylinder O₂ concentration,whereas no significant correlation is found between the aggregate fragmentation rate and the concentration of OH radicals.The soot particles that are oxidized in internal-burning regime is likely to collapse into small ones or fragments.This process is called primary-particle fragmentation.The potential of primary-particle fragmentation is related to the combination of the concentration of O₂ and OH radicals,the in-cylinder temperature and the soot structure.When using the lower injection pressure,the addition of post injection can elevate the potential of primary-particle fragmentation.