Numerical Analysis About Effect Of Multiple Injections On Diesel Engine With The Common Rail Injection System

Diesel engines has been applied broadly as a higher thermal efficiency power machine along with the energy crisis,increasing environmental pollution. In order to reduce emissions of diesel engines in NOx and PM to meet the increasingly stringent emissions rules, the breakthrough lies in optimizing combustion process of diesel engine. Diesel combustion process is a complex and changeable physical, chemical process. It's difficult to study it through experiments. With computational fluid dynamic (CFD) software ,it can be simulated and got more information of fuel injection and combustion, acquired a deep understanding of engine combustion mechanism of using multiple injections with the Common Rail Injection System.The combustion process of YD4A95-C diesel engine equipped with High Pressure Common Rail System(HPCR) was simulated with a CFD software AVL FIRE in 1860 r / min, 100% load case with different multiple injections. In the modeling process, as the intake flow process of diesel engine is less affected, the model can be simplified. Chamber is modeled with Pro/E, using FIRE grid generation tool to generate hexahedral-based non-structural grid of the chamber, then to create moving grid of piston-moving process. Through adjustments to the fuel injection and diesel combustion parameters, I completed model validation based on the actual measured data.This paper studies the effects of multiple injections strategy on emissions of NOx and Soot during the combustion process. It was investigated using a"main injection"strategy with different main injection advance angles; using a"pilot injection + main injection"strategy with different pilot injection angles, pilot injection volume and fixed main injection advance angle; using a"twice pilot injections + main injection"strategy with different pilot injection angles and different interval angle of the two pilots; using a"main injection +after injection"strategy with different after injection volume and after injection angle.The fuel injection advance angle is an important factor controlling the ignition delay period. If the advance angle of injection is adjusted earlier, the density of the combustion chamber is small and its temperature is low, when the fuel is injected into the combustion chamber. Therefore, it needs long time for atomization and ignition, then the ignition delay period will become longer, then the maximum combustion temperature, pressure, heat release rate are all increased. The higher temperature results in higher NOx emission, while the decreasing of the energy released in the diffusion period results in lower Soot emission. Conversely, advancing injection timing, results in reduced soot but increased NOx emissions. Pilot injection can reduce NO, Soot as well as noise. However, the injection timing and volume is sensitive, so the pilot parameters required a careful choice. A proper pilot injection has an effect to shorten the ignition delay period and in-cylinder pressure.With a smaller pilot injection volume, retarding the pilot injection advance angle can increase NO formation while decrease Soot. With a moderate pilot injection advance angle, the ignition delay period can be shortened, thereby reduced the volume of premixed combustion reducing the peak heat release rate (20 ~ 30%) and hence NO, as well as combustion Noise. There are two different combustion result of different pilot parameters; if pilot injection advance angle is small, there are only cold flame reactions; if pilot injection advance angle is large enough, there is a pre-ignition. This pre-ignition will result in a sharp increase of NO and a sharp decrease of Soot.The increasing of pilot injection volume will result in a decline of Soot and an enhance of NO. However, the overusing of pilot injection will result in a strong pre-ignition leading a sharp increasing temperature to hence NO formation dramatically, while the Soot emission will be quite low.The twice pilot injections strategy is useful in reducing Soot other than using a pilot injection by simply dividing it. With the increase of the interval angle, the Soot emission will be reduced, however, no obvious change can be found of the production of NO. With a large pilot injection advance angle, pre-ignition would happen leading to in-cylinder temperature rising early, so that NO production increased; generally with a increased advance injection angle, twice pilot injections strategy increase NO emission other than small advanced single pilot strategy.After injections in different volumes and after injection angles make little effect on emission, Soot slightly decreased, NO slightly increased in this operation point, while the combustion efficiency decreased.It's a preliminary study of diesel engine combustion process in YD4A95-C equipped with HPCR system by using CFD software AVL FIRE. The simulation of multiple injections strategy of HPCR system can provide a prediction for the future optimization of Diesel engine.