Optical Diagnostic On Soot Formation Characteristics Of Ternary Hydrogenated Catalytic Biodiesel/Methanol/N-octanol Blends In Low Temperature Combustion Mode In Optical Engine

With its excellent power performance and high energy density,the internal combustion engine maintains a major power position in the fields of heavy engineering machinery and ship transportation.However,it faces the challenge of burning pollutants such as soot,NOx and CO2.The combination of advanced combustion concept and renewable clean alternative fuel can promote clean and efficient combustion of the engine,reduce pollutant emissions and achieve carbon neutrality.Methanol can be prepared from hydrogen produced by electricity from photovoltaic or wind power and converted with recycled carbon dioxide,making it a potential carbon neutral alternative fuel for internal combustion engines.However,methanol has a low octane value and a high latent heat of evaporation,making it prone to ignition difficulties and combustion instability when used alone.Hydrogenarate Catalytical Biodiesel(HCB),as a new generation of biomass alternative fuel,has the advantages of high cetane number,high fuel activity and higher thermal value.It is very suitable for co-use with methanol to achieve the complementary advantages of the two fuels.At present,the combustion and soot formation characteristics of the blended fuels in the engine are still unclear.Therefore,in this paper,the effects of methanol ratio and injection strategy on combustion and soot formation characteristics under the advanced low temperature combustion mode were studied by two-color method,which provided a theoretical basis for the study of engine applicability of methanol / HCB blended fuels.In this paper,two blended fuels M15(15 % methanol,25 % n-octanol and 60 % HCB)and M25(25 % methanol,25 % n-octanol and 50 % HCB)were prepared by using n-octanol as a cosolvent,and pure hydrogenated catalytic biodiesel M0(100 % HCB)was used as a reference fuel.A four-stroke optical engine was applied to three fuels.High-speed two-color method and combustion image velocimetry technology were used to obtain the temperature field,soot KL value distribution and soot velocity field information of three fuels under two advanced combustion modes of partially premixed combustion(PPC)and reactivity controlled compression ignition(RCCI).The information of cylinder pressure and heat release rate were obtained by combustion analyzer,and then the effects of different methanol blending ratios and injection strategies on combustion and soot generation characteristics under different engine operating conditions were analyzed.The results are as follows :(1)The combustion phase of three fuels under different injection strategies was studied.It is concluded that under the single injection condition,the peak cylinder pressure of M0 and M15 increases first and then decreases with the advance of direct injection timings.The mixing of methanol reduces the activity of blended fuel,resulting in the combustion phase of M15 lagging behind M0 at the same direct injection time.When the CA50 is close to the top dead center,the combustion capacity is high,the combustion duration of the two fuels is the shortest,and the combustion is more rapid.In the double injection mode,with the delay of the first injection time,the peak cylinder pressure of M15 increases.With the delay of the second injection time,the peak cylinder pressure decreases and the combustion phase moves backward.For M15 and M25,better ignition performance and larger flame area can be obtained by reducing the interval of two injection times or the ratio of two injection close to 50 % and 50 %.However,with the continuous reduction of the interval of two injection times and the increase of the second injection ratio,the engine operating conditions gradually become more intense.(2)The temperature field and soot KL value distribution under PPC and RCCI combustion modes were studied.The results show that the flame temperature of M15 and M25 is always less than M0 in the early stage of combustion under all injection strategies due to the evaporation and heat absorption of methanol in the early stage of combustion under PPC mode single injection and double injection strategies.In the middle and late stage of combustion,the flame temperature is greater than M0 due to the higher oxygen content and more uniform fuel distribution of blended fuel.With the increase of methanol ratio,the unique physical and chemical properties of methanol significantly reduced the formation of soot.In RCCI mode,the combustion temperature of M15 and M25 is more stable,and the soot generation is slightly lower than that of PPC mode.(3)The in-cylinder velocity field and flame oscillation were studied by combustion image velocimetry and image post-processing.It is concluded that in the single injection mode,the vortex velocity of M0 is less than that of M15,and the heat release is more intense due to the high activity of HCB fuel.M0 is more prone to have flame oscillation and the amplitude is greater than M15.In the double injection mode,the vortex velocity of M15 is still greater than that of M0,but compared with the single injection mode,the velocity of both fuels decreases.Under the double injection strategy,the fuel distribution is more uniform,the heat release process is stable,and the flame oscillation amplitude of M0 and M15 decreases.