Experimental And Numerical Studies On The Soot Formation Mechanism

The particle matter is one of the major pollutant emissions of diesel engine.Exploring the control strategies to reduce the soot emissions, developing moredetailed and generalized soot formation mechanisms, and models have very importanttheoretical and practical significance. In the current investigation, experimental andnumerical studies have been conducted to explore the soot emission reduction controlstrategies and to develop more sophisticated soot formation mechanisms and models.Experiments were conducted to investigate the soot emission reduction controlstrategies. It is found that the fuel economy and emissions of diesel engine can begreatly improved by applying boosting, EGR, post injection and n-butanol additive.By applying multi-injection coupling EGR, combined with n-butanol oxygenatedadditive, it is possible to reduce the NO_x and soot emissions simultaneously whilestill maintaining good fuel economy, thus it is one of the most competitive controlstrategies to realize high efficiency and clean diesel combustion.A reduced n-heptane/n-butanol/PAH chemical kinetics mechanism has beendeveloped, and experimental data from shock tube ignition delay, premixed flamespecies concentrations and HCCI combustion were taken to validate the proposedmechanism. The simulation results with CFD coupled the reduced mechanism showthat the predicted spray liquid and vapor penetrations, the lift-off length, theconcentration and distribution of soot volume fraction, as well as soot formationregions, agree quite well with the experimental results in constant volume n-heptanespray combution cases. Simulations were also conducted to study the effects ofboosting, EGR, post injection and n-butanol additive on combustion and sootemissions. Results show that increasing the intake pressure can reduce the overallin-cylinder equivalence ratio and enhance the soot oxidation, thus effectively reducethe soot emission; the soot formation rate increases while the soot oxidation ratedecreases as the EGR rate increases, which results in much more soot emissions,especially under high EGR conditions; post injection is an effective measure to reducethe soot emission, and this can be attributed to the higher temperature caused by thepost injected fuel to accelerate the soot oxidation process, and also the enhancedin-cylinder air motion which can improve the combustion during the late combustion phase, the in-cylinder air utilization can also be improved by splitting single injectioninto double injections; soot emission can be greatly reduced by addition of n-butanol.By blending n-butanol into a non-oxygenated hydrocarbon fuel, air entrainment isenhanced by prolonging the lift-off length and the overall equivalence ratio is reducedby introducing extra available oxygen through the n-butanol molecule. The oxygenatom in the n-butanol molecule can also effectively reduce the carbon remaining inthe form of soot precursors by forming the stable C-O bond. The simulation resultsconfirm that the combination of post injection and n-butanol additive has the potentialto greatly reduce the soot emissions.The proposed n-heptane/PAH mechanism has been further extended to formulatean n-heptan/toluene/PAH mechanism, and the mechanism has also been extensivelyvalidated with experimental data from shock tube ignition delay, premixed flamespecies concentrations and HCCI combustion. Simulations were conducted bycouping the reduced mechanism with the KIVA CFD code to investigate thecombustion process and soot emissions of diesel and n-heptane/toluene blended fuelsin direct injection diesel engine. Results show that the proposed mechanism has theability to predict the in-cylinder pressure and heat release of various fuels underdifferent EGR rate conditions, the soot emissions of various fuels were also wellcaptured by the reduced mechanism. The effects of toluene on combustion and sootformation processes in the condition similar to that of a typical diesel engineoperating condtions have also be studied. The effects of toluene in the blended fuelson the distributions of soot volume fraction, equivalence ratio, flame temperature, OHand A4species concentrations show that the toluene molecule structure and chemicalkinetics characteristic play dominat role in the soot formation process compared to thephysical properties.It should be pointed out that the proposed mechanisms can be easily extended to ann-heptane/iso-octane/n-butanol/toluene/PAH mechanism; the reaction pathways ofbenzene and PAH formation have been well validated. These indicate that it will bevery helpful for the development of multicomponent diesel surrogate mechanism andfor the accurate predictions of PAH formation and soot emissions with such asophisticated surrogate mechanism.