Research On Super-knock Mechanism And Suppression Strategies For High Power Density Gasoline Engine

Turbocharged gasoline direct injection(TGDI)has been one of the main technology areas for high efficiency gasoline engines.However,it could result in random super knock at low-speed high-load condtions,which could damange the engines in a few cycles and is the main obstacle for developing advanced gasoline engines.The mechanism and suppression strategies of super-knock should be investigated in detail.The key combustion process of super knock consists of pre-ignition and detonation combustion.Pre-ignition can induce super knock,while the detonation combustion is the key reason why super knock could damage the engine fiercely.The mechanism and suppression strategies of pre-ignition have been extensively investigated,while the mechanism of preignition to detonation combustion process and the detonation suppression strategies for suppressing super knock thoroughly in gasoline engines are the key investigation duties in the next step.This thesis investigates the mechanism and suppression strategies of detonation combustion in super knock cycles,while these of conventional knock are used as references.Based on turbocharged gasolilne engine and rapid compression machine(RCM)experiments,the effects of pre-ignition timing and initial thermodynamic conditions on knock mode are investigated;the detailed combustion process and initiation modes of super knock are studied;and the quantitative criteria to distinguish non-knock combustion,conventional knock and super knock is summarized and analyzed.Experimental results show that at high boost pressure conditions,supre knock is induced by pre-ignition but pre-ignition itself could result in non-knock,conventional knock and super knock.Ignition delay and energy densiy could be used as the quantitative criteria to distinguish non-knock combustion and super knock from other combustion modes respectively.The detailed combustion process of super knock consists of six steps.They are pre-ignitioin;flame propagation induced by pre-ignition;end-gas auto ignition with rapid heat release;shock wave initiated by rapid auto-ignition;detonation combustion induced by the interaction of shock wave with combustion chamber wall,reaction front and other shock waves;extremely high pressure oscillation triggered by the detonation propagation.The detonation initiation modes of super knock mainly include shock wave reflection inducing detonation(SWRID),near-wall auto-ignition inducing detonation(NWAID),shock wave intersection inducing detonation(SWIID),and shock-reaction front inducing detonation(SRFID).Based on experimental conditions of the engine and RCM,the combustion chamber is simplied as a closed pipe and one dimensionl direct numerical simulation of compressible reaction flow with multi-species is used to study the mechanism of different knock modes.The theoretical suppression strategies of detonation combustion for super knock are also investigated.The combustion mode of super knock is detonation,and conventional knock could be relevant to supersonic or subsonic autoignitive deflagration,while normal combustion could be ascribed as flame propagation.Stratified stoichimotric mixture,diluted or lean combustion under the same energy density,fuels with long ignition delay,end-gas with pure air area and other super knock suppression strategies are proposed and proved by simulation methods.Based on engine and RCM experimental results,new effective conventional knock suppression strategies are proposed and proved,while some theoretical suppression strategies of super knock are proved.Spark-assisted stratified compression ignition with moderate end-gas auto ignition and dual-fuel dual-injection system could suppress conventional knock effectively.Diluted or lean combustion under the same energy density could transfer super knock to conventional knock.Methane,propane and methanol could transfer super knock of isooctane mixture to non-knock combustion or weak conventional knock under the same condition.