Optical Study On Spray Combustion And Soot Formation Of Gasoline/Hydrogenated Catalytic Biodiesel Blends

Gasoline compression ignition realizes the rapid mixing of fuel and air before combustion by taking advantage of high octane number and high volatility of gasoline,which guarantees a large proportion of premixed combustion and reduces soot emissions.However,the cold start of gasoline compression ignition is still difficult,and the pressure rise rate is too high under heavy load conditions.Hydrogenated catalytic biodiesel(HCB)has good stability and ignition combustion characteristics because of its oxygen-free molecular structure and high cetane number.In addition,the kinematic viscosity of the biodiesel is high,which can alleviate the lubricity problem well.In this paper,hydrogenated catalytic biodiesel is proposed to be added into gasoline to form different proportions of blended fuel with different properties.Advanced optical testing methods were used to study the spray combustion and soot generation characteristics of different proportions of blended fuel in the constant volume combustion vessel.The main research contents and results are as follows:(1)High frequency back-illumination extinction imaging(HFBEI)and laser Mie scattering were used to measure spray liquid length under combustion conditions.The differences between the two methods were compared and the results were analyzed.The results show that the spray liquid length measured by HFBEI is larger than that measured by laser Mie scattering.The spray liquid length measured by the two methods is close under low ambient temperature and low oxygen concentration.The HFBEI method can show the whole development process of the fuel liquid phase,but the spray liquid length is not accurate due to the interference of soot radiation in most cases.The spray liquid length acquired by the laser Mie scattering is more accurate,but only transient state of the liquid phase at a certain time can be obtained by this method.(2)In this paper,the spray liquid length,ignition delay and flame lift-off length of three gasoline/HCB blends with different blending ratio are measured simultaneously in a constant volume combustion chamber to provide the spray combustion characteristics.It was found that with the increase of the proportion of hydrogenated catalytic biodiesel blended in gasoline,the ignition delay increases,the ignition performance improves,and the cyclic fluctuation of the gasoline compression ignition decreases.In this paper,the difference(D-value)between the lifted-off length and the liquid length is used to characterize the degree of oil-gas mixing.Compared with diesel,it is found that the ignition delay of blended fuel is longer,and there is a certain space for oil-gas mixing between the flame and the spray liquid,which is conducive to the formation of high quality mixtures and the reduction of soot generation.(3)Based on the direct measurement of soot radiation,the soot generation characteristics of blended fuel and diesel were analyzed.Then,the soot generation characteristics of different fuels were quantitatively measured by diffused backillumination extinction method,and compared with the experimental data of diesel.The results show that the initial position of soot is positively correlated with the ignition delay and the length of flame lifted-off length respectively.The ignition delay of G70H30 in large proportion of EGR is too long,which will contribute to low combustion efficiency and large quantities of hydrocarbons.This result indicates that G70H30 is not suitable for large proportion of EGR.However,its combustion and soot formation are excellent under high ambient temperature or high oxygen concentration.The soot production of G50H50 is close to that of diesel,even higher than that of diesel under high temperature and without EGR.From the analysis of soot production,the proportion of gasoline blended with hydrogenated catalytic biodiesel should not be more than 50%.In this paper,hydrogenated catalytic biodiesel is used to improve the ignition performance of gasoline compression ignition.The spray combustion and soot generation characteristics of different proportions of blended fuel are studied.The research results can provide important theoretical basis and guidance for high thermal efficiency and low emission of gasoline/hydrogenated catalytic biodiesel for compression ignition engines.