Research On The Mechanisms Of Pre-ignition Phenomenon And Knock Process Of Ethanol-gasoline Blends Turbocharged Direct Injection Engines

The development of automotive industry for many years has shown a fact that the turbocharged direct injection（DI）technology can be used as an important method to increase the power of engines.Adding a proportion of ethanol fuel to gasoline,meanwhile,is used as an efficient method for improving the combustion performance of engines obviously and reducing the harmful emissions effectively.However,the addition of ethanol fuel may increase the probability of low speed pre-ignition（LSPI）and super knock of the engines,especially for the turbocharged DI engines,which will significantly affect the normal running and even cause irreparable damage of engines.The effects of engine oil injection time and oil injection quantity on the LSPI and super knock are studied by the test simulating above phenomena.The research focus of this paper is placed on the LSPI and the subsequent super knock process induced by oil droplets formed in the engine cylinder.Therefore,this paper uses the method of injecting oil into the cylinder directly in order to create a reliable LSPI phenomenon in the course of the test.The results indicate that both the LSPI and super knock are suppressed when the oil injection time too early or too late.And there are greater potentials of LSPI and super knock with the increase of oil injection quantity.In addition,the impact of different ethanol blending rates（0%,10%,20%,50%,85%）on the knock strength of rapid compression machine（RCM）was also studied in the RCM test for knock process of ethanol-isooctane blends.The results show that the knock intensities of RCM decreases with the increase of ethanol blending rate and there is no knock in RCM chamber at all when the ethanol content reaches 50%.During the work for numerical simulation about the effect of ethanol blending rate on LSPI and knock process,the analytical results are in good agreements with the test data by comparing.Thus,the correctness of the calculation models and methods selected in this paper is proved.Then the geometric model and grid model based on a downsized turbocharged DI engine are established for numerical analysis in this paper.The LSPI phenomenon and knock process induced by oil droplets formed in cylinder are simulated at different engine speeds(1200 r·min^-1,1600 r·min^-1)in turbocharged DI engine with pure gasoline fuel and blends of10%,20%,30%,50%and 85%ethanol rates（E0,E10,E20,E30,E50,E85）by the method of injecting oil droplets into chamber directly.The results demonstrate that there are super knock processes in turn when the fuel is pure gasoline and the blends of 10%or 20%ethanol rates under the 1200r·min^-11 engine speed.When the ethanol blending rate is up to 30%,even if the LSPI phenomenon occurs and leads to the subsequent knock process,the pressure increase rate was obviously decreased.And there is no super knock phenomenon at this time.When the ethanol rate is higher than 50%,the LSPI phenomenon disappears in cylinder.Moreover,the LSPI phenomenon and super knock process will be restrained because the larger engine speed will led to the enhancement of the turbulent flow in cylinder.There are super knock processes in turn when the fuel is pure gasoline or blends with 10%ethanol rate under the 1600r·min^-1engine speed.Although the LSPI phenomenon and subsequent knock process occurs in chamber when the ethanol rate reaches 20%,the pressure increase rate was obviously reduced and this phenomenon is conventional knock.The LSPI which could not lead to knock occurred in the engine using 30%ethanol rate fuel and only normal combustion occurred when the ethanol rate is higher than 50%.The results of different ethanol blending rates show that the pre-ignition may not lead to knock but the super knock must be caused by LSPI.With the increase of ethanol blending rate,even if the LSPI phenomenon occurs in the engine cylinder,it may only causes regular knock or not.