The Simulation And Analysis Of The Soot Generation Process On475G GDI Engine

As more petroleum resources are used and worse the atmospheric environmentare getting, high-efficiency and clean combustion are becoming the main researcharea of the whole automotive industry. To the gasoline engines, the gasoline directinjection technology is now the key one of all the research because of its betterperformance and emissions compared with the PFI gasoline engines and carburetorones. But because of the fuel injection system similar to the diesel engines, thissystem contributes to the more particulate emissions than the traditional PFI engines.As the particle matter from a GDI engines is smaller, more in numbers and moreharmful to the human bodies, the developed governments are introducing the relatedregulations to limit the GDI engines’ PM emissions. So how to reduce the PMemissions is becoming a key issue in the research of GDI technology. In this paper,the author built a model of a475G GDI engine and changed some related parametersto see how the soot generation changed. The main contents and results of this paperare shown below:Generally, soot mass fractions show a unimodal trend of first dramatically risethen drop at the early phase of the combustion process, and in the later phase of thisprocess it stabilized; and the soot is bimodal in number density, as in the initialcombustion process it increases significantly then decreases to the valley and in thelater phase it goes up slightly to reach another peak. We find that the trend of sootdiameter size is similar to the soot number density, and its second peak is the valley ofnumber density, which leads to the continuous going-down trend of the soot massfraction.When the ignition timing advanced, the soot mass fraction and number densitypeaks kept growing and the crank angle the peaks occurred kept moving forward, andthe valleys and second peaks of the soot number density first went up then at the pointof27°CA BTDC declined; and the ranges of the more-soot domains increased at17°CA BTDC then declined.As the injection timing delayed, both the soot mass fraction and number densityshowed a trend of going up then decreasing, the least soot emission occurred at theinjection timing of310°CA BTDC; And the domains generally reduced as the injection timing delayed, but the domains occurred at the right part of the combustionchamber when the fuel injected late and when the injection timing was early moresoot generated at both the left-nose part and the right of the chamber.With the intake temperature going high, the peaks of both the soot mass fractionand the soot number density rose after the first reduction，and the valleys and thesecond peaks first dropped then grew, the turning point was at the30℃point; And tothe different intake temperatures, the more-soot domains occurred at the placesmentioned above and the ranges decreased with the temperatures going up.Considering about the mixture concentration, the soot generation decreased whenthe concentration of the mixture came down, but the soot number density changedfrom bimodal of the rich mixture to the unimodal of the lean ones; and the whole sootgeneration was higher in both the two parts in the chamber.From the whole simulations, we found that the places where more soot generatedwere always occurred at the left-nose domain and the upper right part near thecylinder head, and the ranges of these two domains were reducing with thecombustion process continuing, it could also be seen that the places the more sootgenerated were just the places where the mixture concentration was high, thisphenomenon told us that the rich mixture would generate more soot and the sootwould oxidative decompose in the late combustion process.This paper also analyzed the velocity fields when the spark ignited, and got theresults that at that time in the combustion chamber existed strong clockwise tumble,the air velocity decreased from above the piston part to the cylinder head, but thevelocity was low at the left-nose and upper right parts, that contributed to inadequatemixing of air and fuel which was why in these two parts formed rich mixture and thengenerated more soot.