Effects Of Mixture Formation On Particulate Exhaust Simulation In A GDI Engine

With the development of the automobile industry, the energy crisis andenvironmental pollution problems are more and more prominent. Gasoline directinjection (GDI) engine is similar to diesel with fuel directly injects into the cylinder,making gasoline engine has good power, economy and emission performance, andbecoming the main development direction of automobile gasoline engine. But GDIengine injects fuel into the cylinder directly that makes the fuel inject into the cylinderwall and the piston crown easily and shorten mixture formation time, likely to causelocalized too thick phenomenon. So the GDI engine produces more particulateemissions than conventional port injection (PFI) engines. Due to the GDI enginesparticulate emissions size is smaller and more harmful to human health, so the EUemission regulations Ⅵ has made a restrictions for both of the GDI enginesparticulates quantity and quality. Therefore, we need do further research for thegeneration of GDI engine particulates.Soot is the main part of the particulates that generates in the cylinder. In thispaper, the AVL_FIRE software was used to study the effect of injection duration,injection timing, double injection with the proportion of the fuel and the secondinjection timing, speed, load, EGR rate and inlet temperature on the mixtureformation and soot generation in a GDI engine. The following conclusions weredrawn with the study that how the parameters affect the mixture formation and sootgeneration on the GDI engine:1、Under the strategy of single injection, both fuel injection duration is too shortand injection timing is too early can increase the amount of fuel to touch the cylinderwall and the piston. This makes the liquid fuel increases at the ignition timingmoment. This results the mean pressure and temperature decreases, and the sootgeneration increases. The fuel injection duration is too long and injection timing is toolate will shorten the time of fuel atomize and mix with air, the mixed gas qualitydeterioration, the mean pressure and temperature decreases, and the soot generationincreases. Consideration of the mixture distribution, the pressure, combustion temperature and soot generation, the perfect injection duration is35°CA and theperfect injection timing is80°CA under the2000r/min,50%load condition.2、Under the strategy of double injection, with the fuel distribution proportiondecreases from5:1to2:1, the second injection increases, the mixture qualitydeterioration. This leads the in-cylinder mean pressure, heat release rate decrease andcombustion temperature decrease. The soot mass fraction increases dramatically. Withthe second injection timing delayed from270°CA to300°CA, the in-cylinder meanpressure and heat release rate decreases. The soot mass fraction increases.3、With the engine speed increases from1500r/min to4000r/min, the in-cylinderair movement enhanced. This promotes the fuel and air mix, but the mix time wouldbe shorted as the speed is too large. So at3000r/min, the soot mass fraction is thelowest. As the engine load increases, the cycle fuel mass would be increase, and themixed gas quality deterioration, and the soot mass fraction increases. At100%load,the soot mass fraction is five times of the50%load.4、With the EGR rate increases from0to25%, the mixture quality is improved,the peak of mean pressure and heat release rate decreased, and soot mass fraction ofthe peak value decreases by one order of magnitude. Control the EGR rate less than10%, can decrease the soot mass fraction significantly with little drop of the cylinderpressure, heat release rate and temperature. As the intake air temperature increasesfrom300K to320K, the fuel evaporation rate gets faster, and the soot mass fractiondecreases. It can reduce the soot mass fraction with appropriate of improving theintake air temperature.