Mechanism And Suppression Of Low-speed Pre-ignition In Trubocharged Direct-Injection Spark-ignition Engines

Intake boosting and direct-injection are effective technical means in saving energy cost and reducing emissions for gasoline spark-ignition engines.However,with utilization of intake boosting,a new form of abnormal combustion was observed at low engine speed and high engine load condition,so called low-speed pre-ignition(LSPI).LSPI can lead to different intensive knocking combustion,including non-knock,normal-knock and super-knock.Among the pre-ignition induced knock,super-knock can deteriorate engine performance and become very destructive due to its extremely high combustion peak pressure and intensive pressure oscillation,such as spark ceramic fragmented,valves melt and cylinder head breakdown.LSPI usually occurs sporadically and intermittently,therefore,conventional knocking suppression strategy is no longer suitable for LSPI.LSPI has become one of the biggest obstacle for boosted direct-injection spark-ignition engines.Previous research works have revealed that engine lubricant oil is one of the widely accepted and well-known root causes of LSPI,but there is limited information about its quantitative analysis and optical investigation,especially that the mechanism of how the pre-ignition eventually leading to different intensive knocking combustion is unknown,therefore,there is still a lack of effective LSPI suppression strategy.In this thesis,focus was given on the LSPI abnormal combustion in boosted direct-injection gasoline engines.Based on a thermodynamic engine and an optical engine,experimental research combining with theoretical analysis,formation of LSPI and mechanism of pre-ignition caused different knocking combustion were systematically investigated,and on this basis,LSPI suppression strategies of double-injection stratified combustion and ethanol-gasoline dual-injection were proposed,LSPI frequency and knock intensity were effectively suppressed and engine performance was improved.Firstly,based on the thermodynamic engine experiments,a new pre-ignition frequency calculation method was proposed,effects of fuel injection strategy on lubricant induced pre-ignition and quantitative correlations between lubricant and pre-ignition were revealed.LSPI frequency was usually equal to super knock frequency,while previous work indicated LSPI can cause different knocking combustion,thereby it’s not accurate to calculate LSPI frequency only based on the in-cylinder peak pressure.Due to this problem,index of start of combustion was used to identify pre-ignition in this thesis,and LSPI frequency calculation method was proposed which was based on the outlier detection.A sample which consists by the start of combustion of continuous cycles was statistically analyzed,comparing with the values of normal combustion cycles,those of pre-ignition cycles can be considered as outliers.The number of potential outliers were calculated by a function of median and the median absolute deviation,and thereby the LSPI frequency can be accurately calculated.This method not only improves the accuracy of LSPI frequency calculation,but also can be used in varied engines and test conditions,which has good versatility.According to this method,impacts of the key fuel spray injection parameters on the LSPI frequency and intensity were quantitatively studied,including different injection modes,different injector types and injection timing.It was found that cooling effect of fuel direct-injection is helpful to decrease LSPI frequency,20% decrease in LSPI frequency was found when comparing with PFI,and the corresponding knock intensity slightly dropped.Increase of spray penetration distance or delay of fuel injection timing resulted in more severe fuel impingement on cylinder liner,increasing the probability of lubricant entering into combustion chamber,eventually leading to increase of the pre-ignition frequency.However,the pre-ignition induced knock intensity was not significantly changed with fuel impingement.A lubricant oil direct-injection system was designed,thereby repeatable and controllable LSPI engine test condition was achieved.Through adjusting the injecting timing,injecting quantity and the diluted ratio of the detached lubricant into the combustion chamber,the formation conditions of LSPI was studied.Experiment results showed that the spontaneous ignition of the lubricant oil inside the combustion chamber was not only controlled by the auto-ignition chemical process,but also controlled by the droplet broken and evaporated physical process.When the release timing was retarded to a critical level,and the lubricant quantity was decreased or diluted to a critical level,lubricant releasing into the combustion chamber failed to result in any pre-ignition.Then,on the basis of an optical engine,optical investigation of lubricant induced pre-ignition was designed,pre-ignition process and pre-ignition caused different knocking combustion was revealed.Lubricant oil was injected into the high-temperature zone of the combustion chamber to deliberately induced LSPI,it was found that the auto-ignition frequency of the injected lubricant can be up to 75%.Among them,the frequency of lubricant auto-ignition occurs prior to the spark-ignition was approximately 20%.The pre-ignited flame-front of lubricant auto-ignition propagated much faster than that of normal spark-ignited flame-front,leading to more intensive heating and compression to the end-gas mixture and resulting higher tendency of end-gas auto-ignition.When pre-ignition caused knocking combustion,“hot-spot” autoignition was observed either near cylinder wall or close to the pre-ignited flame front,which consumed the remained unburned gas immediately and produced fast heat release,producing high-frequency pressure oscillation inside the combustion chamber.Optical results also confirmed that high frequency oscillation can eject lubricant into combustion chamber,which was thought to be the root cause of the super knock “self-healing”.At last,combing with theoretical calculation,the correlations between knock intensity with energy density and chemical reactivity index was quantitatively built up,and the technical means to control combustion mode was proposed.On this basis,the suppression strategies of LSPI were experimentally studied in a real engine.Based on the experimental data and the thermal analysis,unburned gas pressure and temperature was obtained and calculated,respectively.Different combustion modes developed by end-gas auto-ignition under different initial condition were calculated.Calculation results showed that,super-knock belonged to developing detonation,while normal-knock and non-knock was sub-sonic deflagration.Distinguishing different knocking mode by using two parameters of energy density and chemical reactivity index,it was found that either decreasing the energy density or extending the auto-ignition time,super-knock combustion had tendency to transfer to normal knock combustion and non-knock combustion.Utilization of double-injection stratified combustion and ethanol gasoline dual-injection can realize the decrease of energy density and chemical reactivity,respectively.These two strategies can both largely decrease more than 80% LSPI frequency,and eliminate pre-ignition caused knocking combustion,meanwhile improving engine performance.