Research On The Mechanism Of Knock And Combustion Mode Transition Based On A Visual Rapid Compression Machine

High strengthening of internal combustion engine is the main technical direction to reduce emissions and improve thermal efficiency.However,knock phenomenon is the bottleneck to break through the thermal efficiency limit of internal combustion engine.Due to the extremely destructive effect of knock on the engine,the mechanism of high-strengthen engine knock and its combustion mode transition is not very clear.Based on an optical rapid compression machine,the knocking characteristics and its influencing factors under high-strengthen engine operating conditions were studied.The research can improve the theory of knocking combustion and provide important guidance for knocking control of high-strengthen engines.Firstly,the effects of initial thermodynamic conditions and wall temperature on knocking characteristics were studied.The results show that,with the increase of initial temperature or initial pressure,or equivalent ratio approaching 1,the autoignition timing is advanced and the knocking intensity is enhanced.Based on the effective energy density,the influence weights of different initial thermodynamic conditions on knock were quantified,and the strong correlation between knock intensity and effective energy density was found.Further comparison between SI and CI cases shows that the primary flame can significantly increase the temperature and pressure of end-gas,thus promoting the local autoignition;meanwhile,the increase of thermodynamic state also promotes the generation of the secondary autoignition and DDT.In addition,the increase of wall temperature can also advance the autoignition timing and enhance the knocking intensity.However,for different fuels,the effects of wall temperature and energy density on the knocking intensity are different.Secondly,the influence of fuel properties on autoignition and knocking characteristics was studied,focusing on the NTC phenomenon controlled by low temperature reaction and the important role of chemical reactivity of mixture.The results show that,for isooctane,increasing the initial pressure can accelerate the autoignition reaction wave and enhance the knocking intensity;whereas for methane,even if the higher thermodynamic conditions,the combustion phase is still delayed,and the end-gas autoignition always manifest as subsonic deflagration which does not cause the pressure oscillations.The analysis based on thermodynamic state and mixture reactivity shows that,the thermodynamic state of isooctane at autoignition timing is lower than that of methane,but it has stronger reactivity.Under the same reaction flow conditions,isooctane with high reactivity tends to induce supersonic deflagration and detonation.Finally,the differences in knocking characteristics between laminar and turbulent conditions were initially explored.The results show that turbulence disturbance can promote the mixing of near-wall cold flow and hot mixture gas in the cylinder,reduce the overall temperature of mixture gas thereby the reactivity of mixture,thus increase the ignition delay time and effectively suppress the generation of knock.The velocity of autoignition reaction wave in turbulent condition is much lower than that in laminar condition.Under relatively lower thermodynamic conditions,the secondary autoignition does not cause DDT,but the combustion process is completed by slow multi-point autoignition.With the increase of thermodynamic conditions,the mixture reactivity increases,leading to strong secondary autoignition and promoting the transition of combustion mode,thus enhancing the knocking intensity.