Experimental Study On The Effects Of Combustion Boundary Conditions On Auto-ignition And Knock

With the enhancement in engine downsizing,the possibility of a knocking phenomenon(e.g.conventional knock and super-knock)is significantly increased in the gasoline engine under low-speed and heavy-load conditions.Because knocking combustion is very destructive to the engine,the knocking problem has become a major technical bottleneck that limits the further improvement of thermal efficiency in downsizing gasoline engines.In essence,these abnormal combustion phenomena are intense combustion processes accompanied by strong pressure oscillations induced by auto-ignition at hotspots(or cold-spots)in the closed space.Therefore,based on experimental tests and theoretical analysis,this thesis has carried out researches on the key factors affecting auto-ignition and knocking within the engines’ operating conditions and corresponding mechanism.The research results help to understand the mechanism of engine knocking combustion,and provide significant theoretical support and engineering guidance for combustion optimization and knock control of practical internal combustion engines.Firstly,for the new visualized rapid compression machine developed by ourselves,the ignition system,high-pressure fuel injection system,mixed gas heating system and synchronous test system were designed and developed,and repeatability tests for nonreactive compression,normal combustion,and knock combustion process were carried out.The experimental results show that the visualized rapid compression machine can accomplish the combustion processes under different combustion boundary conditions,and has good experimental repeatability,which can fully meet the experimental requirements for research on auto-ignition and knocking issues.Secondly,based on the self-developed visualized rapid compression machine experimental platform,the effects of different initial thermodynamic states of nheptane/air and iso-octane/air premixed gas on the auto-ignition and knocking were studied.Based on the effective energy density parameters,the effect weights of different influence parameters,including initial temperature,pressure and equivalence ratio,on the knock intensity were quantified.The correlation among effective energy density,auto-ignition modes and knock intensity was analyzed.The results show that with the increase of initial temperature and pressure and the equivalent ratio approaching 1,the auto-ignition timing is gradually advanced,and knock intensity is significantly enhanced then super detonation occurs.Meanwhile,the initial pressure and the equivalent ratio have a more significant effect on the super-knock than that of the initial temperature,and the knock intensity of n-heptane is stronger than that of isooctane.Besides,super-knock corresponds to the “developing detonation” combustion mode,but the “developing detonation” zone in the original auto-ignition modes’ prediction map needs to be optimized based on actual fuels.Finally,the temperature gradient and the concentration stratification in the cylinder were realized by changing the wall temperature and by the high-pressure common rail direct injection system respectively to initially explore their effects on auto-ignition and knock.The results show that there is a constraint relationship between the influence of effective energy density and temperature gradient(wall temperature)on the knock intensity.Auto-ignition triggered by hot spot often occurs in the boundary layer near the wall,and the larger temperature gradient in the near-wall boundary layer can suppress the occurrence of strong knocking.The fuel temperature and the background gas temperature and its pressure have a great impact on the concentration gradient,and the larger concentration gradient can suppress the knock to a certain extent.