| Methanol has been considered as a promising alternative fuel for combustion engines as its wide range of sources,large-scale production and unique physical and chemical characteristics to achieve clean combustion.However,it is a significant challenge to use it directly in compressed ignition（CI）engines due to its ignition issues caused by the high latent heat of evaporation and lower cetane number,especially for low load conditions.Hydrogenated catalytic biodiesel（HCB）,as a new generation of high-quality biofuels,has become a hot research topic of alternative fuels in recent years,because it can solve the problems in practice compared with the acid methyl ester biodiesel.Thanks to its characteristics of oxygen-free,high calorific value and high cetane number,HCB can achieve complementary advantages with methanol.In this research,the blends of methanol and HCB was proposed to be used in CI engines using n-octanol as co-solvent to provide a new solution to realize the replacement of traditional petroleum in CI engines and carbon neutrality.Thus,the fundamental research on droplet evaporation,spray and combustion characteristics of the ternary blends was carried out to promote its efficient and clean application in CI engines.The tested fuels include two ternary mixtures with different methanol percentage,which are denoted as M15（15%methanol,17%octanol and 68%HCB）and M25（25%methanol,17%octanol and 58%HCB）,and neat HCB（denoted as M0）.The evaporation study on droplet morphology,square diameter and bubble rate of all tested fuels droplet was carried out within a constant volume chamber by hanging method coupled with a high-speed microscopic imaging technique.The spray,combustion and soot characteristics study on spray morphology,ignition delay,lift-off length,non-reacting and reacting liquid length,in-flame soot was performed within a high-pressure combustion chamber equipped with a single-hole injector by two type optical diagnostics,the one is high-speed Schlieren combined with OH*chemiluminescence,another is Diffuse back illumination（DBI）extinction imaging combined with Natural luminosity.The results are as follows:1)The stability of the ternary blends was significantly enhanced using n-octanol as co-solvent,which can last more than 6 months without layer separation,and the OH*functional group increased obviously with alcohols addition.The amount and droplet size of dispersed phase in blends increased with higher methanol addition,which makes the micro explosion of droplet happen easier.2)Since the lower boiling point,the alcohols are easy to be vaporized when being heated.The alcohols bubble expanded obviously as increasing methanol ratio and broke through the oil film,which resulted in an enhanced puffing and micro explosion performance.Furthermore,the diameter of child-droplet induced from puffing of M25was an order of magnitude larger than that of M15.The average evaporation rate of droplets increases with the higher ambient temperature.When the ambient temperature was below 800 K,the evaporation rate decreases with the methanol addition.However,the evaporation rate of the blends was higher than neat HCB caused by the violent micro explosion of droplet when the ambient temperature rose to 850 K.3)With constant ambient oxygen concentration,the stoichiometric fuel mass fraction(Zst)increased with methanol addition in blends as the high oxygen content of methanol,which contributes to a longer ignition delay.And the ignition delay correlation has been built by Zst based on the experimental data.With non-reacting,the spray penetration of all tested fuels is well consistent,it is mainly attributed to the momentum of the nozzle outlet.While the spray of M0 presents a faster penetration under reaction,following by M15 and M25,which is mainly attributed to spray expansion caused by the shorter ignition delay of M0.In addition,the lift-off length correlation has been built by Zst and cetane number,it was found the cetane number played a significant role than Zst on lift-off length.And there is an approximate linear relationship between the ignition delay and lift-off length for all three fuels.4)The liquid length decreased with the methanol addition.And there is an overlapping area between the fuel liquid phase and flame for all fuels under all operating conditions,also the lift-off length is shorter than liquid length in-flame.Combustion enhanced fuel evaporation and made liquid length shorter after ignition,especially for M0.Lift-off length and oxygen concentration strengthened by alcohols addition,which further shortened the averaged equivalence ratio at the cross-section of lift-off length and weakened the reciprocal effect between liquid length and combustion.Besides,the less rich fuel-air mixture formed through droplets vaporization in-flame and soot oxidation rate enhanced with methanol addition,both contributed less soot production. |
