Investigation Of Wall Film And Fuel-air Mixture Distribution Characteristics During Spray-wall Impingement Process

In order to realize the HCCI(Homogenous charge compression ignition)combustion of diesel engines,early injection strategy has been widely utilized.However,due to the low in-cylinder pressure and temperature at early injection timing,spray-wall impingement can not be avoided.Spray-wall impingement is a major source of excessive hydrocarbon(HC)and carbon monoxide(CO)emissions to HCCI diesel engine,which has been proved by previous studies.The increasing trend of the emissions caused by spray-wall impingement can be attributed to two aspects.On one hand,it develop over-rich fuel-air mixture in the impingement region,on the other hand,pool fires also take place during the combustion process.Therefore,a better understanding of the wall film and fuel-air mixture distribution charactereristics formed during the spray-wall impingement process is well needed in order to improve the HC/CO emissions of HCCI diesel engine.The in-cylinder spray-wall impingement can be devided into two styles according to the wall condition.One is the the impingement between spray and piston head or combustion chamber which is called dry wall,the other is the impingement between the spray and cylinder wall covering with lubricant oil film which is called wet wall.In this paper,the effects of the impingement parameters and wall conditions on the wall film and fuel-air mixture distribution characteristics of diesel and its blended fuels have been investigated.Finally,the early injection parameters of HCCI diesel engine have been optimized in order to reduce the HC/CO emissions.1.The effects of the impingement parameters and wall conditions on the wall film characteritics of diesel and its blended fuels have been investigated.The investigations of wall film characteritics can be devided into wall film ratio and wall film distribution characteritics(wall film morphology,wall film area and wall film average thickness).The variations of the wall film ratio and wall film distribution characteritics according to different impingement parameters under different wall conditions have been studied and the main impact factors has been analyzed.Finally,the relationships between the wall film ratio/average thickness and the Weber number of the impinged droplet have been obtained.2.The effects of the impingement parameters and wall conditions on the fuel-air mixture distribution charateristics of diesel and its blended fuels have been investigated.The investigation of fuel-air mixture distribution characteritics can be devided into fuel-air mixture distribution morphology and equivalence ratio distribution characteritics.The variations of the fuel-air mixture distribution morphology and equivalence ratio distribution characteritics according to different impingement parameters under different wall conditions have been studied and the main impact factors has been analyzed.3.The effects of the early injection parameters on in-cylinder wall film and fuel-air mixture distribution characterices caused by spray-wall impingement in a HCCI diesel engine have been investigated.The variation of the HC/CO emissions according to the early injection parameters has been explained from the perspectives of residual wall film mass and homogeneity of the fuel-air mixture in the wall-wetting region.Finally,the early injection parameters of HCCI diesel engine have been optimized in order to reduce the HC/CO emissions.