An Investigation On Fuel Auto-ignition And Its Impact On Combustion Processes

Auto-ignition is the basic combustion phenomenon controlled by chemical kinetics of fuels,which plays an important role in flame propagation and combustion mode transition,etc.Under the trend of engine downsizing and supercharging,the brake mean effective pressure required in modern spark-ignited engines is continuously increasing,which makes it increase obviously for the possibilities of abnormal combustion,such as engine knock and super-knock.Essentially,these abnormal combustion phenomena are hot spots(or cool spots)induced combustion processes followed by intensive combustions and local pressure mutation as well as pressure wave(or shock wave)propagation.It has been widely studied for knocking combustion and end-gas auto-ignition in spark-ignited engines so far,however,it is still unclear for the combustion process and corresponding mechanism starting from auto-ignition initiation to eventual knocking combustion.This is the key scientific problem hindering our clear understanding on knocking combustion formation.Therefore,based on combustion theory,this thesis aims to deeply investigate and analyze auto-ignition characteristics of different fuels and its impact on combustion processes under engine operation conditions.Research results will help to understand the generation mechanism of abnormal combustions in combustion engines,which then provides important theoretical support and engineering guidance for the optimization and regulation of practical combustion engines.First,focused on knocking combustion of gasoline engines,the relation between auto-ignition and knocking combustion has been preliminarily investigated.Based on three-dimensional numerical simulations,a calculation model for knocking combustion of a gasoline engine has been established,which is then utilized to analyze the mechanism of pressure oscillation during knocking combustion and the relation between end-gas auto-ignition and engine knock.Finally,the effects of exhausted gas recirculation(EGR),intake supercharging and compression ratio and their synergism on engine knock have been discussed.The results show that the established three-dimensional calculation model is able to accurately calculate knocking combustion,including cylinder pressure,pressure oscillation and its frequency distributions.Compared with other sources for pressure generation,end-gas auto-ignition reaction contributes the most to pressure oscillation.The relation between end-gas auto-ignition and knocking combustion can be accurately obtained based on pressure oscillation behaviors.Both supercharging and compression ratio can greatly improve engine dynamic property,however,this will result in pressure oscillation in combustion chamber and thus heavy engine knock.The introduction of appropriate levels of cooled EGR could effectively decrease knocking intensity without obvious impacts on engine dynamic property,thanks to the good capability of EGR in controlling combustion process.Second,chemical mechanism controlling low temperature auto-ignition and high temperature auto-ignition has been discussed based on fuels? auto-ignition characteristics,followed by the analysis on the effects of fuel property,fluid parameters and their inhomogeneities on auto-ignition process of different fuels.Combined with combustion chemistry theory,empirical models have been put forward to quickly and accurately calculate two-stage ignition delay time.The assumptions of original Livengood-Wu integration have been analyzed and then validated its feasibility for predicting the two-stage ignition timing in HCCI engines.The results show that affected by negative temperature coefficients(NTC),it exhibits obvious two-stage ignition behavior for large hydrocarbon fuels.The octane number of primary reference fuel has much to do with low temperature chemical kinetics,but it seems that only octane number is not able to accurately measure ignition delay time.H2 and NOx as added gases show distinct effects on low temperature auto-ignition and high temperature auto-ignition.There are close relations between low temperature auto-ignition and initial parameters,which give the insights in putting forward Arrhenius empirical models for two-stage ignition delay time based on combustion chemistry.The inhomogeneity in the temperature,pressure and equivalence ratio have important influence on auto-ignited flame propagation,but there are different mechanisms on auto-ignited flame propagation for different parameters.Livengood-Wu integration may be not critically limited to zero-oder chemical reactions,and the Livengood-Wu integral method is able to accurately predict two-stage ignition timing at some range operating conditions of HCCI engines whether it is located in compression stroke or expansion stroke.Third,experimental investigation has been performed to study the effects of end-gas auto-ignition on initial flame propagation characteristics and combustion process based on an optical constant-volume bomb and controllable heating system.The results show that the self-designed controllable heating system is able to stabilize heating process,which then realizes the controllable auto-ignition process for mixtures at different conditions.Compared with initial flame front initiated by spark-ignition,auto-ignition flame front is greatly affected by the auto-igniting reaction progress of adjacent mixture and the auto-ignited flame speed can be up to over 100 m/s.In comparison to normal combustion,end-gas auto-ignition can dramatically increase pressure rise rate,maximum out-break pressure and decrease combustion duration,which enhance the whole combustion process.However,the intensive chemical reactions of end-gas auto-ignition can induce local pressure mutation and result in pressure oscillation phenomenon.The higher the initial pressure is,the more intensive the pressure oscillation is.Four,based on one-dimensional numerical simulation,effects of single-and two-stage auto-ignition as well as added gases on stable flame front structure,flame speed and combustion mode have been deeply investigated.Then the effects of single hot-spot and multiple hot-spots on transient combustion process have been compared and analyzed,followed by the deep investigations in the important significance of flame propagation,auto-ignition and pressure wave interactions on auto-ignition generation and combustion mode transitions during knocking combustion.The results show that initial auto-ignition reaction progress has importance influence on flame front speed and combustion mode,and as the increases of auto-ignition reaction progress,the dominance of fluid transportation become weak while the effect of chemical reactions becomes strong,which makes flame speed increase obviously and combustion model transitions from deflagration to spontaneous ignition.Flame propagation is also affected by NTC properties,such that it exhibits cool flame and hot flame fronts at the same time,and affected by the initial auto-ignition reaction progress and NTC properties,it shows acceleration phenomenon for flame front propagation.H2 can obviously inhabit low temperature auto-ignition and promote high temperature auto-ignition,and combustion mode transitions can be also suppressed at some combustion conditions.There is no essential difference between single hot-spot and multiple hot-spots auto-ignition,but the later can induce the interactions of multiple flame front and pressure wave propagation and result in local pressure mutation.Knocking combustion is a complicated combustion process,involving flame propagation,auto-ignition and pressure wave interactions.As the increase of initial temperature,flame speed is increased obviously and auto-ignition location transfers from the end near-wall region to the location of main flame front.It shows that pressure wave disturbance from combustion processes can create thermodynamic inhomogeneities which then promote the end-gas auto-ignition,and meanwhile,the interaction of pressure wave and flame front structure can also promote the local mixture auto-ignition in flame preheat zone.Subsequently,the interaction of auto-ignition flame front and pressure results in the combustion mode transitions from deflagration to detonation.Both auto-ignition and flame propagation are affected by NTC chemistry,such that two-stage combustion behaviors are observed.As the initial temperature increases in NTC regime,auto-ignition position transitions from the location ahead of flame front to end-wall region,showing obvious variations in combustion modes and knocking intensity.The auto-ignition initiated from NTC regime can lead to supersonic detonation waves,and different knocking intensity can be induced even for the auto-ignition events located in the same combustion mode.Furthermore,it seems less severe for knocking intensity induced by thermal explosion than that of developing detonation.