Simulation On Combustion Process Of PCCI Diesel Engine Coupled With DMC/Diodiesel Reaction Mechanism

To reduce the excessive dependence on petrochemical diesel and solve the difficulty of simultaneously reducing NO_x（Nitrogen Oxides）and soot emissions in traditional diesel engines,the chemical reaction mechanism of oxygenated fuel and combustion characteristics of PCCI（Premixed Charge Compression Ignition）diesel engine were evaluated based on simultaneous control of oxygenated fuel characteristics and new combustion mode.A heavy EGR（Exhaust Gas Re-circulation） rate was coupled with the pilot-pilot-main injection strategy to obtain the PCCI mode based on the previous experimental research.The mixing ratio of oxygenated fuel in diesel was adjusted to control the ignition time and combustion activity to achieve the high efficiency and clean combustion for PCCI mode of oxygenated fuel.On the one hand,the Dimethyl Carbonate（DMC）-diesel blending mechanism was developed for PCCI mode.The influence of cross-reactions on radical consumption,low temperature and high temperature oxidation of fuel was analyzed in DMC-diesel blending and biodiesel-diesel blending respectively.On the other hand,based on the method of three-dimensional simulation coupled chemical reaction kinetic model,the effects of DMC and biodiesel with the same mass fraction and oxygen content in mixed fuel on the combustion activity and emission generation mechanism of PCCI mode were investigated from both macro and micro aspects.The specific research results are as follows:（1）The reduced combustion mechanism of DMC and DMC-diesel blending were developed for PCCI mode.The reduced combustion mechanism of DMC consists of 53 species and 243reactions,which can accurately predict the ignition time and main components′concentrations of opposed-flow diffusion flame for DMC under different initial conditions.The reduced combustion mechanism of DMC-diesel blending consists of 73 species and 330 reactions,which can accurately predict the ignition time and main components′concentrations of premixed flame for DMC-diesel blending.（2）Based on CHEMKIN-PRO,the combustion characteristics and main active components of DMC-diesel blending and biodiesel-diesel blending were investigated respectively in a constant volume homogeneous reactor.It was revealed that the ignition time,OH and H₂O₂generation time of D10（adding 10 percent DMC to diesel）were delayed compared with diesel but earlier than DMC.Moreover,in D10,diesel consumption rate decreased but DMC consumption rate slightly increased in the low temperature exothermic period.In D50（adding 10percent DMC to diesel）,the ignition time,OH and H₂O₂generation time were delayed compared with D10 but earlier than DMC.The generation and existence time of H₂O₂increased but OH generation decreased.In B10（adding 10 percent biodiesel to diesel）,the ignition time,OH and H₂O₂generation time were delayed compared with diesel but earlier than biodiesel.In addition,the consumption rate of diesel decreased but the consumption rate of biodiesel slightly increased in B10.In B50,the ignition time,OH and H₂O₂generation time were delayed compared with B10 but earlier than biodiesel.Compared with B10,the diesel consumption rate OH generation decreased but the biodiesel consumption rate increased in B50（adding 50 percent soybean biodiesel to diesel）.（3）The cross-reaction flux in DMC-diesel blending and biodiesel-diesel blending mechanism was simulated respectively by CHEMKIN-PRO.It demonstrated that OH flowed from DMC to diesel at low and high temperature in D10,which was beneficial to the activity of the whole reaction system.However,In D50,the dehydrogenation starting reactions of DMC and diesel competed with each other for OH,which resulted in the decrease of OH generation and reaction system activity.Hence,it is not advisable to add large amounts of DMC to diesel.The dehydrogenation starting reactions of biodiesel and diesel in B10 competed for OH,which was not conducive to the combustion starting of biodiesel-diesel blending.However,the subsequent cracking of C₇H₁₄O₂HO₂into NC₇KET and OH generated a large amount of OH,which increased the activity of biodiesel-diesel blending reaction system.In B50,the contribution rate of OH in the whole reaction system increased with the increase of biodiesel blending ratio,and the reaction system activity increased compared with B10.It can be found that CH₂O→HCO→CO reaction path plays an important role in DMC-diesel blending and biodiesel-diesel blending cross-reaction flows at low and high temperature.Besides,CH₂O is an important cross-reaction bridge between oxygenated fuel and diesel in the mixed fuel system（4）Coupled with DMC-diesel blending and biodiesel-diesel blending mechanism,the combustion process of PCCI diesel engine was simulated by Converge.It can be found that the reaction system activity of DMC-diesel blending and biodiesel-diesel blending increased compared with diesel.Adding 10%DMC to diesel increased the whole reaction system activity.Compared with diesel,the better diffusion of fuel and air in D10 decreased the fuel-air mixing concentration regions,which shortened the time from ignition to CA10 and the combustion duration and caused the heat release center CA50 to move forward.In addition,the in-cylinder pressure and heat release rate increased since the addition of 10%DMC in diesel.Adding 10%biodiesel to diesel only increased the local reaction system activity.Compared with diesel,the fuel-air mixing concentration regions and local high temperature zones increased but the combustible mixture region decreased.Besides,the in-cylinder pressure and heat release rate decreased and the heat release center CA50 moved backward.The local reaction system activity in B50 increased compared with B10.The generation of NO_xincreased slightly after the addition of oxygenated fuel,mainly concentrated in the areas with high temperature and low equivalence ratio.The NO_xgeneration of D10 was higher than B10.The generation of soot decreased after the addition of oxygenated fuel,mainly distributed in the areas with high temperature and high equivalence ratio.The distribution region of soot precursors C₂H₂and C₆H₆was consistent with soot.DMC has better effect on reducing soot emission than biodiesel when the mass fraction of DMC and biodiesel are the same.