Study On Evaporation And Auto-ignition Characteristics Of Lubricating Oil Droplet And Lubricant/Gasoline Blended Droplet In GDI Gasoline Engine

Super-knock has become the major obstacle for further increasing power density and reducing fuel consumption of gasoline direct injection(GDI)engine.The view is widely accepted that the auto-ignition of lubricating oil and lubricating oil/gasoline blended droplet in cylinders is one of super-knock origins in GDI engine.The droplet is the basic component for liquid fuel combustion,thus its evaporation characteristics and auto-ignition have important research significance.It is hard to develop a test prototype to study the spontaneous combustion process of lubricating oil in cylinder,thus the evaporation phenomenon and auto-ignition characteristics of lubricating oil and lubricating oil/gasoline blended droplets were investigated using a homemade experimental apparatus.In this paper,a single-drop suspension evaporation setup was designed.The K-type thermocouple was adopted to suspend droplet and study the evaporation and combustion characteristics of lubricating oil droplet under normal pressure and high temperature environment.The evaporation characteristics,combustion process and temperature changes were recorded by high-speed photography camera and temperature acquisition system.The effects of droplet initial diameter,ambient temperature and blended ratio on droplet evaporation and auto-ignition characteristics were studied.The results show that the pure and blended droplets have distinct evaporation and auto-ignition characteristics.The whole evaporation process of lubricating oil droplet can be divided into three periods,including initial heating period,rapid evaporation period and slow evaporation period.However,the evaporation of lubricating oil/gasoline blended droplet is mainly characterized by the fluctuation evaporation phase.The evaporation phenomenon of lubricating oil droplet is relatively stable,and the evaporation process of blended droplet is violent,including bubble generation,nucleation,puffing,and incomplete micro-explosion phenomena.The normalized squared diameter during droplet combustion process can be roughly divided into three phases: ignition delay period(initial evaporation period),combustion period,and extinction period.Under the same ambient temperature,as the proportion of gasoline in the blended droplet increases,the ignition delay time gradually decreases.When the ambient temperature increases,the droplet ignition delay decreases,the maximum expansion degree and the frequency of droplets micro-explosion increase.The evaporation and auto-ignition of droplet tend to form hot spot,which may result in anabnormal combustion in GDI engine.