Study On Spray And Combustion Characteristics Of Diesel/Methyl Heptanoate Blend Fuels Based On The Constant Volume Combustion Chamber

Due to its advantages of high efficiency,wide power range,and strong practicality,traditional internal combustion engines are still the mainstream configuration of automobiles.Improving the dependence of traditional internal combustion engines on non renewable fossil fuels,solving the problem of increasing exhaust emissions year by year,and finding renewable and clean alternative energy sources is the necessary way to alleviate the current situation of fossil energy shortage in China,respond to the"dual carbon"call,and build an environmentally friendly society.As a renewable energy source,biodiesel has good low-temperature starting performance and good lubrication function compared to traditional fossil fuels,effectively reducing engine wear and extending engine life.At the same time,it has low sulfur content and generates low SO₂ content,which also has a significant improvement effect on engine exhaust emissions.Due to the main component of biodiesel being methyl esters with larger molecular sizes,the combustion model is complex and the viscosity is high,Poor oxidation stability may lead to poor fuel evaporation and atomization effect.As a fatty acid methyl ester with appropriate carbon chain length,methyl heptanoate has simple chemical structure and similar physical and chemical properties to biodiesel.It has high oxygen content,releases a large number of hydroxyl groups when burning at high temperature,and has good oxidation stability.It is considered to be a good biodiesel derivative,which can be blended with traditional diesel as an excellent clean and renewable energy to improve engine emissions.In this paper,methyl heptanoate is blended with diesel oil（recorded as D100,MH10,MH20）according to the volume fractions of 0%,10%,and 20%respectively.A visual study of methyl heptanoate blended with fuel is carried out in a constant volume combustion bomb.The background pressure is 4MPa,and the background temperature is set at 500K and 800K respectively during spray and combustion,The whole process of spray and combustion of three fuels under different injection pressures（80MPa,100MPa,120MPa,140MPa）is captured by high-speed camera.Based on Matlab software,the background difference method is used to extract the spray liquid phase contour,and the inter frame difference method is used to extract the spray gas phase boundary.The macro characteristics of spray are summarized.The macro parameters of liquid phase,gas-liquid phase spray penetration,spray cone angle and spray area after blending methyl heptanoate are analyzed,and the influence of blending methyl heptanoate on the spray atomization effect is compared.The results show that after blending methyl heptanoate,the oil beam becomes narrower and longer,the liquid and gas-liquid penetration distances of spray increase,and the liquid penetration distance increases with the increase of blending volume fraction.The gas-liquid phase hinders the atomization effect of methyl heptanoate,and the MH10 penetration distance is larger;The liquid spray cone angle of blended fuel slightly decreased with the increase of injection pressure.The average drop rate of MH10liquid spray cone angle increased from 3.96%to 7.07%,and the average drop rate of MH20liquid spray cone angle increased from 5.05%to 9.60%.The gas-liquid spray cone angle showed the rule of MH10>MH20>D100.With the increase of injection pressure,the proportion of gas spray cone angle increased;After blending,the liquid and gas-liquid spray areas are increased,and the blending area of spray is expanded.It can be seen that methyl heptanoate can improve the atomization effect of spray to some extent.Under a constant ambient temperature of 800K in a constant volume bomb,the combustion development images of three fuels under different injection pressures were captured using the direct camera method.The combustion parameters of each fuel were obtained through Matlab processing to explore the degree of improvement of combustion performance by blending fuels.The results show that the cetane number of the blended fuel increases,and the ignition delay period and combustion duration of the fuel are shortened after blending methyl heptanoate.With the increase of injection pressure and blending ratio,the shortening rate of the ignition delay period of the fuel decreases,and the shortening rate of the combustion duration increases;When the blended fuel is burned,the combustion temperature increases,and the flame lift length,KL factor,and self luminescence intensity all decrease.As the injection pressure increases from 80MPa to 140MPa,the total KL proportion gradually decreases from 69.63%to13.76%,and the KL proportion of MH20 decreases from 37.36%to 3.52%.This is due to the release of a large number of hydroxyl groups during the combustion process of methyl heptanoate.The increase in oxygen atom content is conducive to promoting the oxidation of carbon atoms in the high-temperature region;The total luminous intensity decreases with the increase of methyl heptaneate blending ratio,and as the injection pressure increases,the proportion of total luminous intensity of MH10 gradually decreases from 76.72%to 31.64%,while the proportion of total luminous intensity of MH20 gradually decreases from 47.61%to12.16%.MH20 exhibits a good effect in suppressing carbon smoke,therefore,blending methyl heptanoate can improve emissions after fuel combustion and reduce carbon smoke generation.