Studies On The Microscopic Physical Properties And Oxidation Characteristics Of Diesel Exhaust Particles

Fuel composition and engine operating conditions play a critical role on themicroscopic physical properties of diesel exhaust particles and their oxidationcharacteristics, which have close inherent connections with each other. Consequently,the investigation of the evolution law of aforementioned properties for diesel exhaustparticles from various fuels and operating conditions is of great theoretical value tounderstand the mechanism of particle formation and to optimize the diesel particulateemission control technologies. In this study, a common-rail Euro IV diesel engine andparticulate matter sampling system were applied to obtain particulate samples. Thenmicroscopic physical properties and oxidation characteristics of these samples weresystematically studied using the analytical methods like transmission electronmicroscopy (TEM)， digital image processing, Raman spectrum, and thermogravimetric analysis (TGA). Major conclusions from this dissertation are as follows:1. It can be seen from TEM images that both primary particles and branchedchains of exhaust particles from Euro IV ultra-low sulfur diesel (E4) and a blend ofE4and biodiesel (with the latter at a proportion of20%v/v, B20) are more than thosefrom coal-to-liquid diesel(FT), which is synthesized by an indirect route. Theincrements of engine speed and torque not only increase the numbers of primaryparticles and branched chains, but also the size of the whole cluster, as well as theaggregation level.2. The fractal dimensions of exhaust particles, which in a range of1.47-1.96, havea positive relationship with torque and a negative correlation with engine speed. Thefractal dimensions of particles from three kinds of fuels, in descending order, are:E4> B20>FT. Besides, all three fuels show a unimodal distribution of primaryparticle diameters, and the diameter distribution range exhibits a similar trend to thatof fractal dimensions with the change of torque and speed. The mean diameters ofprimary particles fall within the scope of15.48~25.80nm with an order of: E4> B20>FT.3.Under various operating conditions, the average values of the crystallitedimension, fringe separation distance, and tortuosity of exhaust particles originatedfrom three fuels fall in the ranges of0.863~1.092nm、0.341~0.375nm and1.420~1.513, respectively, and the values of these parameters all exhibit a unimodal distribution. The crystallite dimension increases with the increasing of torque anddecreases with the increment of speed, while both the fringe separation distance andtortuosity show an opposite trend. The study shows that fuel differentials only have aneffect on the values of microstructure characteristic parameters, but exhibit noinfluence on the their variation trends with torque and speed.4. The graphitization degree of exhaust particles increases with torque whiledecreases with speed for all three fuels. The IG/IDratios of three kinds of particlesfollow a trend of FT>E4>B20.5.The initial oxidation temperature(Ti)、the maximum oxidation rate temperature(Tmax)、the50%particle mass loss temperature (T1/2) and the burnout temperature (Th)for three kinds of particles vary in the ranges of394.0~504.0℃、482.5~608.5℃、471.5~581.0℃and514.0~622.5℃, respectively. The characteristic temperatures ofoxidation process generally present a trend of FT>E4>B20. Moreover, the particleswith chaotic and amorphous internal carbon layer structure exhibit lowercharacteristic temperatures than those with a high graphitization degree.