Numerical Study On The Evaporation And Ignition Characteristics Of Lubricating Oil Droplet In Engine Cylinder Condition

Alternative fuels have been getting increasing attention as concerns due to the severeenergy and environment problems.The thermal efficiency and mean effective pressure of advanced gas engine are much higher than those of gasoline engines,and are closing to those of advanced diesel engines.Meanwhile,premixed lean burn gas engines present outstanding performance of low harmful emissions to meet the increasingly stringent harmful emission control regulations.Therefore,more and more attention is being paid on the development of gas engines.However,the further improvement of overall performance of gas engines was impeded by abnormal combustions.The pre-ignition caused by the auto-ignition of lubricating oil droplets is the most threatening abnormal combustions.Engine developers have to reduce the compression ratio of gas engines to decrease the compression temperature and pressure to avoid the auto-ignition of lubricating oil droplets.This results in a lower thermal efficiency of the engines.Meanwhile,since auto-ignition of lubricating oil droplets likely happens in high load operating conditions,it prevents the further increase in mean effective pressure of the gas engine.Therefore,understanding the behaviors of lubricating oil droplets,including the heating up,evaporation,mixing,and auto-ignition,are crucial for the suppression of abnormal combustion in natural gas engines.The evaporation and auto-ignition behaviors of isolated lubricating oil droplets in the natural gas engine conditions were numerically investigated.A one-dimensional multi-component model was developed,including the finite heat and mass transfer in both liquid and gas phases,the transient model with global reaction for the chemical term,and the high pressure phase equilibrium for gas-liquid interface.The present model was validated using the experimental measurements for the evaporation rate of binary-component droplets of n-heptane and n-decane,and the ignition delay time of n-decane and n-hexadecane droplets under various ambient temperatures and pressures.The good levels of agreement were obtained.The model was extended to predict the evaporation and auto-ignition behaviors of lubricating oil droplets.N-triacontane and n-hexadecane were selected as the physical and chemical substitutions of lubricating oils,respectively.Based on the simulation results,the following conclusions are drawn.As the droplet diameter increases,the start-up of evaporation is delayed and the mass fraction is improved apparently once the evaporation starts.And as the pressure increases,the mass fraction decreases due to the fact that the high pressure reduces the mass transfer number from the phase equilibrium.Additionally,the oilvapor around the large oil droplet under high temperature condition diffuses farther than that under low temperature condition.In the case of droplet auto-ignition,the ignition delay periods of lubricating oil droplets are 3.0-5.0 times longer than those of n-hexadecane droplets.The ignitable diameters of lubricating oil droplets are reduced as the ambient temperature and pressure increase.Under low temperature and pressure conditions,with an increase in droplet size,the ignition delay declines slightly first and then increases.However,under high temperature and high pressure conditions,the ignition delay increases monotonously as the droplet size increases.Additionally,high ambient pressure and temperature can reduce the ignition radius.Note that the ignition radius decreases with an increase in droplet size under low ambient temperatures,whereas,it reverses under high ambient temperatures.