| Due to the advantages of improving dynamic property and higher fuel economy,Homogeneous Charge Compression Ignition(HCCI)combustion mode is becoming a research hotspot nowadays.Comparing to PFI,HCCI model has a much lower particle emission.However,developing detailed oxidation mechanisms and generalized soot formation mechanisms in HCCI combustion mode combustion from gasoline engines have very important theoretical and practical significance.The main results and conclusions of this dissertation are listed asfollows:(1)A kinetic mechanism for four-component gasoline surrogate consisting of iso-octane,n-heptane,toluene and 1-hexene is developed including types of cross reactions.The mechanism comprises 1392 species and 6019 reactions.Validation results show good agreement with experimental observations for pure components and their various mixtures.Effects of cross reactions on the ignition delays are discussed under HCCI conditions.It is found that three types of cross reactions take effect for the acceleration of ignition delays in shock tube and HCCI engine.And accelerate effect of these cross reactions is more pronounced at high pressures,lowtemperaturesand fuel rich conditions.(2)A comparison study of various mixtures of four-component gasoline surrogates has been carried out,and the results indicate that ignition delay times of gasoline surrogates exhibit strong RON dependence at high temperature regime,and strong octane sensitivity dependence at intermediate temperature regime.Thus,a computational methodology to determine the optimal surrogate composition to match the fuel properties such as H/C,RON and MON has been proposed in the text.(3)A multi-stage mechanism reduction strategy for performing reductions of hydrocarbon fuel is proposed based on DRG,R-DRG,DRGEP,PFA,and CSP methods.A reduced chemical model was developed from the four-component gasoline surrogate consisting of optimal ratio of iso-octane,n-heptane,toluene and 1-hexene by using this multi-stage mechanism reduction strategy,with the scale of mechanism reduced to 20% of the original.Validation of the reduced mechanism showed good agreement with the detailed mechanism for auto-ignition,extinction phenomena and laminar flame speed over a wide range of conditions.However the reduced mechanism is not suitable for HCCI engine application,which indicates that the reduction strategy of guaranteeing the ignition delays error cannot ensure its accuracy when applied to numerical simulation in different fields.(4)A chemical kinetic mechanism is presented to predict the formation ofpolycyclic aromatic hydrocarbons(PAHs)up to six aromatic rings forC1-C2 smaller hydrocarbon molecular.The new mechanism is validated by comparing ignition delay times,laminar flame speeds,concentration of major PAH speciesin methane and ethane premixed laminar flames with experimental data.The good agreements between simulation and experimental observations indicate that the newly developed mechanism can be used as a basis for further PAH and soot study.A detailed soot model is proposed including particle nucleation,hydrogen abstraction from particle,surface growth,coagulation and particle oxidation.The model is validated in methane flame experiment and shock tube experiment for n-heptane,and the results show that the model can correctly reproduce the trend and sharp of measurement curves.This soot model couples with oxidation mechanism for four-component gasoline surrogate,PAH generated model and perfect stirred reaction model,to simulate the process of gasoline fule oxidation and soot formation at top dead center(TDC)of a HCCI engine.The effects of temperature,pressure and fuel-air equivalence ratio on the soot particles formation is discussed,andthe time-dependent change of number density,size distribution and volume fraction of soot particle is also analyzed.(5)The combustion and emission characteristics of a HCCI enginefuelled with gasoline and methanol dissociated gas are investigated by engine bench experiments and numeral simulation.The results indicate that,an increase in methanol substituted ratio has slightly lowered the dynamic performance;increased the maximum cylinder pressure and heat release rate,advanced the ignition time and the peak cylinder pressure corresponding time.And some explanationsof this improvement are given fromaview point of reaction kinetics. |
