Study On The Evolution Of Mass,Surface Fuctional Groups And Oxidation Activity Of Diesel Soots During Post-injection Combustion Process

Post-injection strategy is one of the effective approaches to control the exhaust particulates of diesel engines.Associated with the use of post-injection strategy,the physicochemical characteristics of diesel soots concurrently change during the combustion process.For better understanding of the evolution mechanism of diesel soots and reducing the particulate pollutions further,it is necessary to shed light on the physicochemical characteristics of in-cylinder soots when using post-injection strategy.In this dissertation,a total cylinder sampling system was employed to obtain in-cylinder soots samples generated under different post-injection strategies.The evolution mechanism of soots mass concentrations,surface functional groups and the oxidation activity of in-cylinder soots was subsequently studied using the analytical method such as Soxhlet extraction,Fourier transform infrared spectroscopy,X-ray photoelectron spectroscopy,and thermogravimetric analysis.The major achievements of this dissertation are listed as follows:According to the variations of in-cylinder soots mass concentration,the post-injection combustion process could be divided into four phases:preparatory phase of post-injection combustion,early phase of post-injection combustion,middle phase of post-injection combustion and late phase of post-injection combustion.During preparatory phase of the post-injection combustion,soots mass concentration is hardly influenced by the post-injection fuel.Then the soots mass concentration increases rapidly and reaches the maximum value,owing to large amount of soots produced by the post-injection fuel in the early phase of post-injection combustion.At last,it declines in the middle and late phase of post-injection combustion.In the case of identical total injection fuel mass per cycle,the use of close post-injection strategy can effectively reduce both soots and PM mass emissions with respect to single-injection case,whereas the use of far post-injection strategy results in marginal decrease in soots emissions and a slight increase in PM emissions.The relative content of the aliphatic C-H groups on the in-cylinder soots surface,which was evaluated by the normalized peak height ratio(I_C-H/I_C=C),generally decreases during post-injection combustion process.The concentrations of both C-OH and C=O groups present bimodal distributions during the close post-injection combustion process.However,the concentrations of both C-OH and C=O groups generally decreases throughout the far post-injection combustion process,except for slight increases in the middle phase of post-injection combustion.During the post-injection combustion process,the apparent activation energies of diesel soots generally increase during the post-injection combustion,and vary in the range of 126.348¹79.132kJ/mol.It is found that aliphatic C-H groups on the soots surface serve as more important factors governing the soots oxidation activity than the oxygenated surface groups.