Study On The Physicochemical Characteristics Of Exhaust Particulates From Gasoline Direct Injection Engine

With the increasingly strict requirements for vehicle emission regulations, theparticulate emission from gasoline direct injection (GDI) engine has been receivedconsiderable attention in recent years. In this dissertation, series of investigations havebeen performed to shed light on the physicochemical characteristics of particulateemission from GDI engine, including particle number, particle size distribution,microstructure and particle-phase polycyclic aromatic hydrocarbons (PAHs), and theeffect of combustion control parameters on them. Meanwhile, the three-dimensionalnumerical simulation has been adopted to explore preliminarily the formationhistories of particulates and the effect of air/fuel ratio. These research works willfurther reveal the essence of particulates from GDI engine, and provide theoreticalbasis for adopting effective measures to reduce particulate emission. The majorcontributions are listed as follow:1. Except for idle operation condition, exhaust particulates from GDI engine are ofbimodal distribution consisting of nucleation mode and accumulation mode particle,where the particle number with nucleation mode is more than that with accumulationmode. The mass concentration, surface area concentration and the amount ofaccumulation mode particles from GDI engine are generally higher as compared tothose from PFI engine in most operation condition. Furthermore, there is closerelationship between the particulate emission from GDI engine and the combustioncontrol parameters. The total particle number is obviously decreased with adoptinglean air/fuel mixture or delaying spark timing, and the number of accumulation modeparticle is decreased with reasonably optimizing injection time or adopting exhaustgas recirculation (EGR) technology.2. The exhaust particulates from GDI engine are fractal-like agglomeratesconsisting of quasi-sphere primary particles, which mainly compose of carbon (C),oxygen (O) element, and many kinds of trace metal and nonmetal elements. Theseparticles possess typical self-similarity fractal character, and the fractal dimension isin the range of1.36~2.38. Moreover, combustion control parameters have greatinfluence on the fractal dimension, which will increase with using stoichiometricair/fuel mixture or delaying spark timing, and decrease with introducing EGR intocylinder.3. Besides a small amount of disordered and amorphous structure, onion-shellcrystallitic carbon structure is found to be dominant in primary particles. The sizedistribution of primary particles is similar to Gauss distribution with the peak value of particle diameter in the range of20~35nm. Fringe length of primary particles shows aunimodal distribution with the maximum value in the range of0.4~0.6nm, and70%of that is less than1nm. Fringe separation of primary particles is in the range of0.28~0.58nm, with more than70%of that locating in0.34~0.44nm. Tortuosity ofprimary particles spreads from1to2with peak value in the ranger of1.2~1.6.The microstructure of primary particle is closely related to combustion controlparameter. Adopting stoichiometric air/fuel mixture, advancing spark timing andinjection timing and introducing EGR will cause the growth of primary particle.Fringe separation obviously increases while employing lean or rich air/fuel mixtures,delaying spark timing and injection timing and introducing EGR. Meanwhile,Adopting stoichiometric air/fuel mixture, delaying spark timing, advancing injectiontiming and introducing EGR will bring about bigger bending of primary particlemicrocrystal.4. An electron energy loss spectroscopy was employed to estimate the effect ofcombustion control parameter on the graphitization degree of exhaust particulates.The results indicate that adopting stoichiometric air/fuel mixture, delaying sparktiming, advancing injection timing and introducing EGR will decline thegraphitization degree, resulting in an enhanced oxidation activity of particles.5. Particle-phase PAHs were investigated, and the results show that the emissions offour-ring PAHs is highest, followed by five-ring PAHs, and the subtotal emissions ofboth account for more than58.70%of total PAHs. The lowest emissions of PAHs aretwo-ring PAHs, which account for less than7.5%of total PAHs. Furthermore,Adopting lean air/fuel mixture, advancing injection timing or reducing EGR will leadto a decrease in total PAHs emissions.6. A numerical simulation model for soot particles of GDI engine was establishedby AVL-Fire software. The results show that the soot simultaneously undergoes theformation and oxidation process during the flame propagation, and its formation isclosely related to fuel/air equivalent ratio and combustion temperature.