Study On Characteristics Of String Cavitation In Diesel Injector Nozzles And Effects On Fuel Injection And Spray

Today's increasingly prominent energy-depletion problems have led to the current trend of developing with energy saving and environmental protection in the field of internal combustion engines.With the application of the new energy-saving and emission-reduction technologies,the demand for fuel injection systems is increasing.The cavitating flow in diesel engine nozzles will affect working stability of the injection system,the injector service life,fuel atomization and combustion characteristics.As a special type of cavitation,the string cavitation's characteristics and effects on the diesel engine working characteristics are still unclear.So,in this study,the string cavitation characteristics and its influences were investigated under the steady state.Combining the global research status in this study,the origin of string cavitation,development characteristics of the string cavitation,scale effects on the string cavitation and the effects of string cavitation were systematic experimental studied by using transparent nozzles with different geometric structures and different enlargement ratios.Based on this,the study analyzed the effects of string cavitation on the diesel engine working characteristics,and the method of how to utilize the string cavitation properly was explored.The main contents of this paper are as follows:(1)The origin and the induction factors of the string cavitation were systematic studied and the factors influencing the string cavitation developing were well investigated by using the visual test bench.For the first time,the method of defining the string cavitation intensity from two dimensions of space and time was proposed.According to the axial rotation characteristics of the string cavitation,an analytical method for characterizing three-dimensional cavitation volume by two-dimensional image in experiments was proposed.The proposed methods facilitate the quantitative analysis of the characteristics and effects of the string cavitation.(2)For the first time,the string cavitation inside the nozzles with different nozzle entrance positions,different tapered holes and different rounded corners were studied systematically.The interaction between geometrically induced cavitation and string cavitation was analyzed.The structural features were obtained which will facilitate the stable operation of the fuel injection system(3)The characteristics of string cavitation inside the nozzle with different scaled-up ratios were analyzed comparatively and the differences of the string cavitation formation,the inception of string cavitation and the developments of string cavitation were obtained in this study.The study has preliminary investigated the scale effects on string cavitation.(4)A pressure fluctuation test system in scaled-up nozzles was built up.Based on the pressure fluctuation test system and the high speed cameras,the influence of string cavitation on the pressure fluctuation in injection system was studied.It is concluded that the string cavitation in nozzle is one of the important reasons for causing the pressure fluctuation in the entire injection system.(5)The study analyzed the reason of the occurrence of different string cavitation forms in the nozzle,and the similarities and differences among different string cavitation were indicated.The effects of different string cavitation forms on circulation characteristics and spray performance were compared.The mechanism of string cavitation affecting the fuel atomization process and spray angle was revealed,which provides a reasonable explanation for the sudden increase on the spray angle during single injection cycle,and provides a reference for how to use string cavitation to optimize the spray performance.This research contributes to the study of string cavitation in diesel engine nozzle,provides theoretical guidance and rich experimental data for the optimization of nozzle structures and improvement of spray performance in diesel engine injection system and proposes a method of using string cavitation to optimize the spray performance of the pre-injection and improve the combustion efficiency under multiple injection strategy.