Raman Spectroscopic Analysis Of Diesel Engine Particulates With After-treatment Devices

Diesel soot can be effectively removed from the exhaust by means of after-treatment device, but its regeneration problem has always been a difficult problem. This work focuses on the structure variations in diesel soot during oxidation process and the relation between soot inner structure and its oxidative reactivity in order to provide important basis for making effective equipment.PM sampling was conducted on a CY4102-CE4B engine fueled with ultralow sulphur diesel. PM samples were collected when the engine was equipped and non-equipped with DOC respectively at different operating conditions. Those the original samples were partially oxidized in a thermo-gravimetric analyzer to gain samples at oxidation levels. Afterwards, all the samples were analyzed using Raman spectroscopy and variations in PM structure during oxidation process were discussed. The conclusions are as follows:1. The two characteristic peaks of first order Raman spectrum are located at1580cm-1and1350cm-1respectively. To analyze the structure of PM quantitatively curve fitting was applied to the first order spectrums using Levenberg—Marquardt algorithm. The degree of fitting was evaluated by χ2, and the4L1G fitting method was chosen for its lowest χ2value.2. The chemical heterogeneity decreases as the engine load increases, leading to a more ordered structure, which means that the soot collected at lower engine load are more reactive. The chemical heterogeneity will also decrease when using the fuel with higher quality or equipping DOC. In addition, DOC is sensitive to fuel.3. There is an upward trend in the graphitization degree of PM in general along with the decrease of the amorphous content as oxidation proceeds. The length of C-C atomic bond distance of sp2hybridization also increases gradually during oxidation. Raman spectrum can be used to obtain the average fringe length of carbon materials, while the variation in the spectrums can be an indicator of the anisotropy of soot structure.4. In addition, soot oxidative reactivity is not only related to its structure, but also to the fraction of organic compounds and the surface oxygenated functional groups, thus, the graphitization degree of carbon layers alone cannot dictate the oxidative reactivity. At low load, the carbon layers are at a disordered state, indicating high oxidative reactivity. However, the inner structure of soot changed with the use of DOC, meanwhile, the oxidation mode also changed, thus the graphitization degree of carbon is no long the main factor.