The Experimental And Simulated Investigation On The Auto-ignition Of The High Octane Number Fuel

With the crisis of the oil resources, the downsizing of the gasoline engine is apromising tendency. And the highly turbocharged and direct injection is involved in it.When the brake mean effective pressure(BMEP) get up to2.5MPa, or even to3MPa,the temperature and pressure condition of the combustion chamber will be higher,what may lead the auto-ignition of high octane number fuel. This phenomenon iscalled pre-ignition(PI). Pre-ignition will lead to a rapid combustion process of the fuelin the combustion chamber, and develop a pressure wave with destructive effect. PIbecomes a limitation of improving the performance of the internal combustion engine.Investigating the essence of it is significant to avoid PI occurring and achieve theenergy conservation and emissions reduction of engines.In this study, an experiment research was carried out to investigate thecharacteristics of the pressure wave of high octane number fuel auto-ignition with amodified high compression ratio diesel engine. After that a study of the boundarycondition of the hotspot triggering detonation of the combustible mixture around itwas conducted based on hotspot auto-ignition and detonation theory. Finally, asimulation based on the propagation law of detonation and shock wave in cylinderand the effects of the different engine operation conditions on the pressure andtemperature fields was conducted with a simplified combustion chamber model.The result shows that, the auto-ignition probability of the high octane number fuelis higher as the mixture getting richer. And the abnormal auto-ignitions happenintermittently. When the auto-ignition happens before the top dead center (TDC), thericher the mixture is, the later the ignition time will be, and lead to an increasing ofthe amplitude and frequency of the pressure oscillation. Under the same concentrationof the mixture, when auto-ignition occurs in compression stroke, the maximumcylinder pressure, maximum amplitude of pressure oscillation (MAPO) and themaximum rate of pressure rise are higher than auto-ignition occurring in theexpansion.10%of mass burned fraction (MBF10) is the most appropriate indicator toevaluate the intensity of the super knock of the high octane number fuel caused by PIwith the linear correlation analysis.With the hotspot theory and simulation, result shows that, under propertemperature gradient, the auto-ignition wave speed and pressure wave speed are close in hotspot. As the coupling effect of the two waves, it will become a detonation wave.And the probability of this condition is getting higher with the increasing of theBMEP. The CFD software can capture the strong discontinuity of the detonation andshock wave precisely, and match the ZND model of detonation. The extremely highpressure and temperature will be formed at the center and the sharp angles of thecombustion chamber caused by the detonation and shock waves. When excessive aircoefficient is about0.9, the detonation and shock waves cause the highest pressureand temperature peak values. And departure of this exceed air coefficient will reducethe peak values. EGR can reduce the peak values in the dangerous location effectivelytoo. Eliminating the geometric shape angles can release the aggregation effect of thedetonation and shock waves in cylinder, and decrease the risk of the engine damage.