Numerical Simulation Of Diesel Engine Working Process And Fuel Injection Strategy Based On Intake Component Design

High atmospheric pressure and oxygen concentration in the high altitude area are low,which affects the intake air quality during the operation of the diesel engine,resulting in the decrease of oxygen concentration,insufficient combustion,soot emission and reduced power and economy.The control of the mixing of different components of the intake gas is an active technique designed to enhance the plateau adaptability of the engine and to achieve efficient and low-emission combustion of the engine in a high altitude environment.This paper is based on the engine combustion model which is built with the CFD software coupled with chemical reaction mechanism.The simplified heptane/toluene/n-hexene mechanism was used as the chemical reaction kinetics of diesel fuel to study the effects of different intake components(O2,H2,CO2 and gaseous H2O)with various volume blending ratios(1.6%,4.8%and 8%)on common-rail diesel engine combustion process when the atmospheric environment(16.2%02+83.8%N2)of 3000 meters in altitude was regarded as reference.The study is aimed at raising theory guidance for inlet optimization design of high thermal efficiency and ultra-low emission engine,and revealing the mechanism of the fonnation of diesel engine combustion process,important active groups(OH,O),soot precursor(acetylene-C2H2,pyrene-A4)and the final product(NO,Soot).The results of the study on the intake components with various volume blending ratios are shown that:compared to others intake components,with O2 addition,ignition timing advances,the proportion of premixed combustion and the peak heat-release-rate reduce,active radicals(OH.O)and NO increase while soot decreases dramatically:With H2 addition,the rate of heat release,in-cylinder pressure,peak combustion temperature and NO increase sharply,meanwhile soot reduces as blending ratio increases;With CO2 addition,it leads to the latest ignition timing,the lowest combustion temperature,in-cylinder pressure and NO,the longest reaction duration of A4 and C2H2,the highest soot as compared to other components;Additionally,NO reduces and soot changes little as blending ratio increases for the addition of H2O.The results of the study on the intake components coupled with injection timing are shown that:for all intake components,when advancing injection timing,active free radicals(OH,O)and their distribution region,as well as NO increase simultaneously.But when injection timing is advanced too early,ignition delay,heat release rate,and the peaks of in-cylinder pressure and temperature decrease,thus reducing NO.Compared with other intake components,when O2 is added into intake charge,O radical concentration increases,soot and the effects of injection timing on its formation reduce;With H2 addition into the intake charge,the peaks of in-cylinder pressure and temperature are the highest,OH distribution is the largest,and soot reduces significantly with the advanced injection timing;With CO2 addition into the intake charge,the peaks of in-cylinder pressure and temperature are the lowest,O and OH radicals are the least.When injection timing is advanced beyond 24°CA BTDC,fuel is injected into the compression clearance volume gradually,which leads to local fuel-rich region.As a result,C2H2 and A4 increase,meanwhile,soot increases greatly when injection timing is further advanced.The results of the study on the intake components coupled with pilot injection are shown that:for all intake components,as compared with single injection,ignition delay for the main injection short ensremarkably(especially for H2 addition)when pilot-main injection interval was kept at 4° CA.As a result,the peak of heat release rate and rate of maximum pressure rise(MPRR)reduce simultaneously.However,the effects of pilot-injection strategy on the main-injection combustion become weaker with increased pilot-main injection interval.Compared with other intake components,when O2 is added into the intake charge,pilot-main injection interval has a little effect on NO and soot because of oxygen-rich combustion.When H2and CO2 are added into the intake charge,as compared with single injection,Soot increases sharply(especially for CO2 addition)because of the increase in the formation of C2H2 and A4 when pilot-main injection interval was kept at 4° CA.However,Soot will reduce gradually due to the prolonged ignition delay with increased pilot-main injection interval.For H2 addition,MPRR decreases while Soot increases with retarding the main-injection timing when pilot-injection fuel mass was kept at 10%of total mass.When pilot-injection fuel mass increase sup to 20%,pilot-injection combustion leads to large amount of heat release by H2 premature combustion,which will weaken the influence of main-injection timing on main-injection combustion.Meanwhile,MPRR still remains much high owing to the advancement of the center of heat release.Therefore,for H2 addition,the pilot-injection proportion should be controlled and main-injection timing should be retarded appropriately.The study on the effect of EGR rate coupled H2 ratio on diesel engine working process,thermal efficiency and energy distribution is based on the establishment of one dimensional thermodynamic simulation model of diesel engine by GT-Power software.After that,the impact of post injection on performance of diesel engine will be studied through the selection of appropriate EGR rate and H2 ratio.The results of the study on the intake of H2 coupled EGR are shown that:at low EGR rate,the incorporation of H2 into the intake gas improves the combustion quality,increases the torque and decreases the effective fuel consumption rate,the Soot reduction degree and the effective thermal efficiency increase with the increase of H,ratio;at high EGR rate,the oxygen concentration in the intake gas is greatly reduced,which leads to the deterioration of the combustion in the cylinder,and offsets the improving effect of combustion quality by H2.With the increase of the H2 ratio,the Soot emission is significantly increased and the effective thermal efficiency is not changed.In addition,EGR reduce NO emissions more significantly at higher H2 blending ratio.The study of post injection shows that:compared to single spray,the use of post-spray strategy reduces the soot emissions to a great extent.As the interval is increased,the post-spray time is delayed,the peak temperature and pressure in the cylinder are not changed and the NO emission changes little,but the exhaust loss increases and the effective thermal efficiency decreases accordingly.With the increase of the post-spray volume,the main spray amount decreases,the peak temperature in the cylinder and the NO emission decreases,the exhaust loss increases,and the effective thermal efficiency decreases.