The Combustion Simulation And Knock Prediction Of GDI Engine

Due to the outstanding performance of the GDI engine in a power and torqueand the obvious advantage in reducing fuel consumption and pollutant emissions, itbecome a major topic in the research and development of engine. However, theimprovement of gasoline engine fuel economy has been limited to the knockphenomenon. In this paper, CFD simulation computation method was applied toevaluate the main factors of knocking resources by analyzing the relationship betweenthe variety parameter of double ignition combustion chamber and the incidence ofknocking.It is significant to verify the reliability of simulation model before doingtheoretical analysis. Based on our self-developed single cylinder thermodynamicmachine test bench, the cylinder pressure data as the main parameter of modelvalidation was acquisition.After been verified, the computational models were used to simulate the processof normal combustion and knocking combustion. Analysis of the two conditions wascarried out from the three-dimension output dates, which included the effect ofparameters (eg. specific location and time) to combustion process. When the ignitionadvance angle is large enough, it shows that firstly the propagation of flame surfacedensity and flame speed was same. As time go on, flame speed would be faster. Thepropagation of flame in cylinder was not along the direction of the flame front.Sometimes combustion had also been occurred inside all of the chamer before theignition flame reached the side. Then the so-called Knock happened. Compared withthe knock-combustion, normal combustion would lead the delay of the flame growdue to the delay of ignition advanced angle. Then combustion is not concentrated inthe compression top dead center region, so the combustion process would besmoother.The results for the combustion simulation model, including autoignitionmodule, were analyzed, and the concept of knock predictive parameters–knockfactor was introduced, and the relationship between equivalent ratio, cylindertemperature, combustion chamber structure and knock factor was analyzed fromdifferent aspects. 1. The mixed gas is the most likely to be ignited and spontaneous combustion,when the ratio approaches1. According to the simulation results, the distribution ofknock factor is rather high when the ratio approaches1.2. Temperature decreases radially from the middle of two spark plugs to the sidesafter7°AC crank angle, as the piston reaches TDC on the compression stroke. Knockfactor is high in the place where the temperature is high in chamber. But the relation isnot clear because the trend is not very obvious with variation of temperature.3. It exist a close relationship between the distribution of knock factor andstructure of combustion chamber. Knock factor is rather high in the complex structureregion including narrow, sharp region and the area near the edge of the exhaust valve.4. Knock will easily occur when reducing advance angle of ignition. Change thesparking time from13°CA BTDC to6°CA BTDC. The simulation results showthat the maximum knock factor decreased by6times.The absolute knock factor is a relative concept. It is convenient to predict thepossibility of occurrence of knocking in different conditions and space, which willoffer guidance for the design and development of the engine.