Experimental And Numerical Studies On The Soot Particle Formation Mechanism Inthe Gasoline Direct Injection Engine

The soot particle is one of the major pollutant emissions ofthe gasoline direct injection?GDI?engine.Exploring the control strategies for reducing the soot emissions would require the development ofchemical reaction kinetics mechanism of polycyclic aromatic hydrocarbons?PAHs?andsoot formation model for gasoline surrogate fuels.In the present investigation,engine experiments have been conducted to explore the PAHs and soot particle emissions generated from a GDIengine,and chemical reaction kinetics and numerical simulationshave been exploredto develop more sophisticated PAHs and soot formation mechanisms and models.Experiments were conducted to investigate the chemical characteristics of PAHsand soot particulates emitted from aGDIengine.The microscopic morphology of the soot particulate and the correlations between PAHsspecies and the primary carbon particles were studied under different engine operating conditions using a new purpose-built sampling system.The obtained extracts of PAHs samplings and soot samplewere analyzed both qualitatively and quantitatively using gas chromatograph–massspectrometry?GC–MS?and field-emission transmission electron microscopy?FE-TEM?.The vapor-phase and particulate-bound PAHs exist in the GDI engine exhaust emissions.PAHs with two and three rings are present nearly entirely in the gas phase PAHs,whereas five or more larger six rings are predominantly adsorbed on soot particles.The intermediate 4-rings PAHs exist in the two typephase ofPAHs.A2 Naphthalene is the most abundant polyaromatic hydrocarbon that was detected in the exhaust vapor-phase PAHs,which was approximately two or three orders of magnitude higher than the PAHsadsorbed on soot particles.The variation trend of soot particles was similar to that of the PAHs,a comprehensive analysis of these results reveals that the larger ring PAHs?A4^A6?are the key precursors that lead to the formation of soot particles in a GDI engine.PAHs and soot particle emissions could be significantly reduced by increasing the injection pressure,introducing the exhaust gas into the cylinders and delaying the spark timing.Particulate-bound PAHs?A4-A6?were suitable for estimating soot emissions from the GDI engine.A semi-detailed TRF-PAHs chemicalmechanism is developed to predict the PAHs formation contains219 species and 1229 reactions for the toluene reference fuel?TRF?.Extensive validations were performed for the measured ignition delay times and laminar flame speeds,especially for gasoline/air and toluene reference fuels/air mixtures under different engine conditions.The results has shown that the semi-detailed TRF-PAHs mechanism could predict the auto-ignition chemistryand flame propagation occurring in different conditions that coveredlean,stoichiometric and rich mixtures under low and high pressure conditions.The TRF-PAH mechanism can capture the oxidation process and oxidation products?CO and CO₂?in n-C₇H₁₆ and i-C₈H₁₈ flames.The calculated small chemical species?C₂H₂,C₃H₄ and C₃H₆?and A1 profiles were also reproduced well.The predicted synergisticeffectalso shows good agreement with the measurements in the two opposed flowflames?soot formation flame?SF?and soot formation/oxidation flame?SFO??.The main reaction paths related to the ignition delay times,laminarflame speed,benzene formation/ consumptionand reactions that cause the "synergistic effect? were analyzed.A reducedkinetic mechanism with an emphasis on coupling CFD simulation to predict the PAHs formation containing 85 species and 232 reactions was developed using toluene reference fuel that was typically observed in gasoline fuel surrogates?referred to as the P.AN mechanism?.The P.AN mechanism was validated by comparing the ignition delay times,premixed laminar flame speeds,major species profiles and PAHs concentrations with the results of a shock tube,a flat flame adiabatic burner and GDI engine data.The simulation results indicate that the new P.AN mechanism gives reliable prediction for auto-ignition,flame propagation and PAHs.The present mechanism can be used to simulate combustion andpredict the PAHs emissions from gasoline fuels in 3D CFD simulations.Finally,a mathematical soot growth model coupled with the reduced P.AN mechanism was developed based on the method of moment.The PAHs was treated as the soot precursor species,and soot inception started with pyrene?A4?.Then,soot particles underwent PAHs condensation,soot surface growth by C₂H₂,particle coagulation and soot surface oxidation processes via oxygen?O₂?and hydroxyl radicals?OH?.The obtained experimental soot emissions were well predicted by this soot model.The distribution of particle number density was consistent with the distribution of A4 and C₂H₂,and the growth and distribution of soot mass was determined by the particle surface growth due to C₂H₂,the nuclear reaction of A4 and the particle surface oxidation reaction.