Study On The Evolution Of Physicochemical Characteristics And Oxidation Mechanism For Diesel Particulates During Postinjection Combustion Process

Combustion conditions,such as postinjection strategy and initial fuel composition,play an important role on the physicochemical characteristics of in-cylinder diesel particulates.The physicochemical characteristics of particulates can in turn affect the particulate reactivity toward oxidation.For better understanding of the diesel particulates formation mechanism and the reduction of particulate matter?PM?pollutions,therefore,it is necessary to shed light on make detailed studies of the diesel in-cylinder particulates produced under different postinjection strategies?close and far postinjection,for instance?.In this dissertation,a total cylinder sampling system,fueled with low-sulfur diesel fuel and pure biodiesel fuel,was used to acquire soot samples in the postinjection combustion process.The evolution history of soot mass concentration,nanostructure,surface functional groups of in-cylinder soot was investigated using the analytical methods,such as digital image processing,Raman spectrum,Fourier transform infrared spectroscopy,X-ray photoelectron spectroscopy,and thermogravimetric analysis.Furthermore,the oxidation mechanism of in-cylinder soot under the postinjection combustion was discussed.The major work and achievements of this dissertation are listed as follows:According to the variations of in-cylinder soot mass,the postinjection combustion process can be divided into four phases:preparatory phase,early combustion phase,middle combustion phase and late combustion phase.In the preparatory phase,soot mass concentration is hardly influenced by the postinjection fuel;in the early combustion phase,the soot mass concentration increases and reaches the maximum value,owing to large amount of soot produced by the postinjection fuel.At last,it declines rapidly and moderately in the middle and late combustion phases,respectively.In the case of identical total injection fuel mass per cycle,the use of close postinjection strategies can effectively reduce both soot and PM mass concentrations with respect to single-injection case,whereas the use of far postinjection strategy results in a marginal decrease in soot emissions and a slight increase in PM emissions.Moreover,under the same postinjection strategies,biodiesel fuel produces lower soot and PM mass concentrations than diesel fuel.The morphology of in-cylinder particulates is independent on fuel identity and postinjection strategy,and the aggregates are in the form of fractal-like geometry.During the postinjection combustion process,the fractal dimension of in-cylinder diesel and biodiesel particulates vary in the range of 1.52¹.89 and 1.60¹.99,respectively,and the fractal dimensions of in-cylinder particulates first increases in the preparatory phase,then declines in the early combustion phase,and increases again in the middle and late combustion phases.The minimum value of fractal dimension resides in the range from early combustion phase to middle combustion phase.In the case of identical fuel mass per cycle,the fractal dimensions obtained from postinjection combustion process are relatively larger relative to single-injection case,indicating a more compact morphology.In the same postinjection combustion phase,the fractal dimensions for biodiesel fuel are higher than those for diesel fuel.The sizes of primary particles generated by diesel fuel show a near Gaussian shape distribution with the mean value of primary particle size in the range of 17.3²2.1nm,and the mean values of primary particle size decrease as the postinjection combustion proceeds.Compared with diesel primary particles,biodiesel primary particles are smaller in size ranged from 16.0 to 20.1nm throughout the postinjection combustion process.The mean fringe length?L_f?for in-cylinder particulates exhibits an increase in the preparatory combustion phase,then a decrease in the early combustion phase and an increase again in the middle and late combustion phases.In contrast,the mean fringe separation distance?S_f?and tortuosity?T_f?show contrary trends in terms of changes.Compared with diesel soot,biodiesel soot generally possesses the smaller L_f and the larger S_f and T_f in the same postinjection combustion process.In addition,the trends in terms of changes in the ratios of I_G/I_D1 in Raman spectra follow those in the fringe length throughout the combustion process,evidencing the rationality of the image processing results.In this paper,the normalized peak height ratio(I_C-H/I_C=C)evaluates the aliphatic C-H groups relative content on soot surface,which exhibits a decrease in the preparatory phase,then an increase in the early combustion phase,and a decrease again in the middle and late combustion phases.At the same time,the concentrations of both C-OH and C=O groups show bimodal distributions during close postinjection combustion history,with the two peaks in the period from preparatory phase to early combustion phase,and from middle combustion phase to late combustion phase,respectively.For the far postinjection combustion history,the distributions of both C-OH and C=O group concentrations are unimodal,with the peak in the period form the middle combustion phase to late combustion phase.Moreover,throughout the postinjection combustion process,the relative concentrations of aliphatic C-H,C-OH and C=O groups on biodiesel soot surface are higher than those on diesel soot surface.During the postinjection combustion process,the apparent activation energies generally increase for both diesel and biodiesel soot,and vary in the range of128.56¹74.21 kJ/mol and 121.35¹63.44 kJ/mol,respectively.Soot nanostructure and aliphatic C-H groups do duty for the more important factors governing the soot oxidation reactivity compared to the soot morphology and oxygenated surface groups.Soot oxidation rate was efficiently improved by use of a postinjection strategy,especially for the close postinjection strategy.In-cylinder soot oxidation complies with the external and internal burning modes.In the case of identical total injection fuel mass per cycle,the use of postinjection strategy makes the soot more vulnerable to be oxidized in internal burning mode.Fuel formulation exerts a considerable effect on soot oxidation rate.Biodiesel soot has higher oxidation rate and more apparent internal burning mode than diesel soot.