Morphology and oxidation reactivity of exhaust particles from diesel engine fueled by N-pentanol-diesel blend

The exhaust particles from diesel engine can be reduced by mixing diesel with n-pentanol, however, the effect of n-pentanol blending fuels on the oxidation reactivity and morphology of diesel exhaust particulates has not been well understood. Therefore, this paper aims to design an experiment and then solve this problem. Experiments were conducted in a high pressure common-rail diesel engine, and three fuels were selected, including pure diesel, DP15 (15% n-pentanol +85% diesel, by vol.), and DP30 (30% n-pentanol +70% diesel, by vol.). In this work, the engine speed and torque were set at 2000 r/min and 0.59MPa, respectively. Tests were performed without any engine modification, but only with fuels change. In the test, the particulate samples were collected from the exhaust pipe of the engine through a vacuum pump, then the particulate matter (PM) characteristics were analyzed by transmission electron microscope (TEM), Raman spectroscopy (RS) and thermogravimetric analysis (TGA). Results showed that the morphologies of soot particles from three fuels were similar. Soot aggregates with numbers of primary particles were observed at low magnification, while a typical “shell-core” nanostructure was presented at high magnification. The “shell” part is mainly composed of parallel microcrystals, representing the order of basic carbon particles, while the “core” part consists of irregular microcrystals, indicating the disorder of basic carbon particles. With the increase in n-pentanol blending ratio, the oxidation process of soot was promoted due to the oxygen content of n-pentanol, while the mixing of n-pentanol reduced the generation of soot precursor, such as pyrene, and thereby the surface growth of soot decreases. Therefore, the primary particle diameters of soot samples slightly decreased from 21.813 nm to 20.030 nm. Meanwhile, as the n-pentanol blending ratio increased, the fringe length of soot samples decreased 0.025 and 0.051 nm, while the fringe tortuosity increased slightly. The decrease of fringe length and the increase of fringe tortuosity indicated that the nanostructure of particles was much more disordered. Results from TEM images demonstrated that there was less graphitic structure in soot samples emitted from blended fuels. Similar to the results obtained from TEM, a higher AD1/AG was also observed for blended fuels than others. Since the AD1/AG is an important parameter to characterize the graphitization degree of exhaust particles, the graphitization degree of particles is higher when the value of AD1/AG is smaller. Therefore, the soot samples from diesel/n-pentanol mixtures showed smaller primary particles in size, and more disorder nanostructure. Meanwhile, the oxidation temperature of the particulate samples (616.9 ℃ in pure diesel, 609.9 ℃ in DP15, and 583.6 ℃ in DP30) decreased with the increase of n-pentanol ratio. There was much higher oxidation reactivity in the exhaust particles from blended fuels than others. The correlation analysis between the oxidation activity and morphology showed that the high oxidation activity of particulate samples in the mixed fuel was related to the disordered nanostructure. Finally, the oxidation activity and diesel fuel increased in the exhaust particles that formed by the mixtures of n-pentanol. This finding demonstrates that the blended fuels in the engine can be used to improve the regeneration performance of DPF, and further reduce the required regeneration temperature.