Common Rail Diesel Injection Research Articles

Investigation of the enhanced cavitation model considering turbulence effects of fuel flow in the injector nozzle holes

The cavitation model is essential in simulating cavitation flow characteristics of injector nozzles, with its accuracy directly impacting simulation precision. This paper developed a quantitative relationship between critical cavitation pressure, turbulent kinetic energy, and shear stress, and introduced a modified cavitation model that considers turbulence effects. A simulation study was conducted on the fuel flow characteristics inside the nozzle of a high-pressure common rail diesel injector with seven nozzle holes. The results indicate that compared to the original model, the cavitation development calculated by the modified model is enhanced at different injection pressures. In terms of axial distribution, cavitation inside the nozzle hole increases, but the impact on cavitation of different intensities varies, with the development of moderate to low-intensity cavitation being promoted, while strong cavitation is inhibited. In terms of radial distribution, the intensity of cavitation on the upper wall inside the nozzle hole increases, which is caused by the higher mass transfer rate near the upper wall at the nozzle hole inlet. As the injection pressure increases from 120 to 240 MPa, the change rate of cavitation volume ratio inside the nozzle holes calculated by the original and modified models increases from 5.1% to 9.7%. It is necessary to use the modified model for the calculations of gas/liquid two-phase flow within the nozzle hole, especially at high injection pressure.

The Effect of Fuel Quality on Cavitation Phenomena in Common-Rail Diesel Injector—A Numerical Study

Plastic is one of the most widely used materials worldwide. The problem with plastic arises when it becomes waste, which needs to be treated. One option is to transform plastic waste into synthetic fuels, which can be used as replacements or additives for conventional fossil fuels and can contribute to more sustainable plastic waste treatment compared with landfilling and other traditional waste management processes. Thermal and catalytic pyrolysis are common processes in which synthetic fuels can be produced from plastic waste. The properties of pyrolytic oil are similar to those of fossil fuels, but different additives and plastic stabilizers can affect the quality of these synthetic fuels. The quality of fuels and the permissible particle sizes and number density are regulated by fuel standards. Particle size in fuels is also regulated by fuel filters in vehicles, which are usually designed to capture particles larger than 4 μm. Problems can arise with the number density (quantity) of particles in synthetic fuels compared to that in fossil fuels. The present work is a numerical study of how particle size and number density (quantity) influence cavitation phenomena and cavitation erosion (abrasion) in common-rail diesel injectors. The results provide more information on whether pyrolysis oil (synthetic fuel) from plastic waste can be used as a substitute for fossil fuels and whether their use can contribute to more sustainable plastic waste treatments. The results indicate that the particle size and number density slightly influence cavitation phenomena in diesel injectors and significantly influence abrasion.

Obtaining the Synthetic Fuels from Waste Plastic and Their Effect on Cavitation Formation in a Common-Rail Diesel Injector

The presented paper addresses two significant issues of the present time. In general, the studies of the effect of synthetic fuels on cavitation formation and cavitation erosion prediction in the nozzle tip of common-rail diesel injectors were addressed. The first problem is plastic waste, which can have a significant negative environmental impact if not treated properly. Most plastic waste has high energy value, so it represents valuable material that can be used in resource recovery to produce various materials. One possible product is synthetic fuel, which can be produced using thermal and catalytic pyrolysis processes. The first issue addressed in the presented paper is the determination of fuel properties since they highly influence the fuel injection process, spray development, combustion, etc. The second is the prediction of cavitation development and cavitation erosion in a common-rail diesel injector when using pyrolytic oils from waste plastic. At first, pyrolytic oils from waste high- and low-density polyethylene were obtained using thermal and catalytic pyrolysis processes. Then, the obtained oils were further characterised. Finally, the properties of the obtained oils were implemented in the ANSYS FLUENT computational program and used in the study of the cavitation phenomena inside an injection nozzle hole. The cavitating flow in FLUENT was calculated using the Mixture Model and Zwart-Gerber-Belamri cavitation model. For the modelling of turbulence, a realisable k–ε model with Enhanced Wall Treatment was used, and an erosion risk indicator was chosen to compare predicted locations of cavitation erosion. The results indicate that the properties of the obtained pyrolytic oils have slightly lower density, surface tension and kinematic viscosity compared to conventional diesel fuel, but these minor differences influence the cavitation phenomenon inside the injection hole. The occurrence of cavitation is advanced when pyrolytic oils are used, and the length of cavitation structures is greater. This further influences the shift of the area of cavitation erosion prediction closer to the nozzle exit and increases its magnitude up to 26% compared to diesel fuel. All these differences have the potential to further influence the spray break-up process, combustion process and emission formation inside the combustion chamber.

CRDI Engine Emission Prediction Models with Injection Parameters Based on ANN and SVM to Improve the SOOT-NOx Trade-Off

Artificial Neural Network (ANN) and Support Vector Machine (SVM) have been widely used to solve non-linear problems. In the current study, based on 112 groups of experimental data, ANN and SVM models were established and compared to improve the trade-off relationship between SOOT and NOx emissions of a Common Rail Diesel Injection (CRDI) engine fueled with Fischer-Tropsch (F-T) diesel under different operating conditions and injection parameters. The model parameters for the different predictive targets were selected by evaluating the mean square error (MSE) and determination coefficient. Compared to the number of network iterations, the number of implied nodes had a greater effect on the MSE of the ANN model. Compared to the penalty parameter, the width coefficient had a weaker impact on the SVM performance. A comparative analysis showed that the SVM had better predictive accuracy and generalization ability than the ANN, with a maximum error not exceeding five percent and a determination coefficient of over 0.9. Subsequently, the optimal SVM model was combined with the NSGA-II algorithm to determine the optimal injection parameters for the CRDI engine, resulting in solutions to simultaneously decrease the SOOT and NOx emissions. The optimized injection parameters resulted in a 3.7–7.1% reduction in SOOT emission and a 1.2–2.6% reduction in NOx emissions compared to the original engine operating conditions. Based on limited experimental samples, SVM is inferred to be a useful tool for predicting the exhaust emissions of engines fueled with F-T diesel and can provide support for optimizing injection parameters.

Multi-objective optimization of CRDI engine characteristics powered with Mahua methyl ester/diesel blend: a Taguchi fuzzy approach.

An investigation has been conducted on a CRDI (common rail diesel injection) engine utilizing a blended fuel using mahua methyl ester to explore the influence of fuel injection strategies (i.e., FIP, CR, and FIT) on the engine characteristics. It is observed that specific fuel consumption (BSFC) and nitrogen oxide emission (NOx) increased with a decrease in brake thermal efficiency (BTE) in the case of MME100 at a base FIP of 500bar. The volatility and viscosity (physical properties) of the MME fuel blend influenced the combustion phenomena resulting in a reduction in engine performance. The higher viscosity of the methyl ester blend restricts the fuel atomization process during the injection. Fuel injection pressure (Pinj) was raised from 500 to 1000bar in increments of 250bar in the first phase to maximize the use of the MME blend. High injection pressures (1000bar) have higher BTE (12.33%) and better combustion characteristics (2.86%) than lower fuel injection pressures (500bar). The concentrations of smoke (3.62%), CO (15.6%), and HC (2.8%) were reduced as injection pressure was raised, owing to the better formation of the mixture and improved spray atomization. Better results were obtained by optimizing and confirming operational parameters such as compression ratio, injection pressure, and injection time. This optimum value of 1000bar injection pressure, injection timing of 27° bTDC, and a compression ratio of 18: 1 is utilized as the operating parameter values in the subsequent experiments to improve performance and decrease emissions while utilizing MME biodiesel blends in a CRDI engine.

Thermal effects on Common Rail injection system hydraulic performance

The effect of the fuel temperature on the hydraulic performance of a Common Rail diesel injector has been investigated with an integrated experimental-numerical approach. An experimental campaign pertaining to single and double injections has been performed for fuel tank temperatures ranging from 28°C to 68°C. In general, an augment in the injected mass has been observed for increasing values of the fuel tank temperature. Moreover, the interaction between the main and after injection changes with the temperature and the dwell time threshold for fusion-free injections increases with the fuel temperature. The temperature at the injector nozzle has been measured and compared with that obtained with a thermo-fluid dynamics simple model, showing that the real temperature and the estimated one correlate well. The influence of the fuel temperature on the internal injector dynamic has been explored by means of a validated 1D numerical model of the injector thermo-fluid dynamics. The main direct effect of the temperature variation concerns the needle lift, which reaches a larger peak value for a higher fuel temperature: this explains the general increment in the injected mass and the augmented value of the injection fusion threshold for the main-after injections. The obtained results could allow more accurate open-loop control strategies for the injected mass, which include thermal effects, to be implemented.

Influence of split injection strategies and Trade-off study on the CRDI engine characteristics powered with waste cooking Oil/Diesel blend

An investigation has been conducted on a CRDI (common-rail diesel injection) engine utilizing a blended fuel with waste cooking oil (WCO20) to explore the influence of fuel injection strategies (i.e., split injection strategy with different fuel injection pressure) on the engine characteristics. It is observed that specific fuel consumption (BSFC) and nitrogen oxide emission (NOx) increased with a decrease in brake thermal efficiency (BTE) in the case of WCO20 at a base FIP of 40 MPa. The volatility and viscosity (physical properties) of the WCO20 fuel blend influenced the combustion phenomena resulting in a reduction in engine performance. The higher viscosity of the methyl ester blend restricts the fuel atomization process during the injection. Fuel injection pressure (Pinj) was raised from 40 to 100 MPa in increments of 25 MPa in the first phase to maximize the use of the WCO20 blend. High injection pressures (100 MPa) have higher BTE (12.33%) and better combustion characteristics (2.86 %) than lower fuel injection pressures (40 MPa). The concentrations of smoke (3.62 %), CO (15.6 %), and HC (2.8 %) were reduced as injection pressure was raised, owing to the better formation of the mixture and improved spray atomization. When used as a pilot injection before the main injection (BMI), 5% WCO20 reduces HC and CO emissions and NOx (7.82%) and smoke emissions while compromising only a marginal amount of thermal efficiency in the second phase.

Study of fuel spray characteristics using a high-pressure common rail diesel injection system by the method of Schlieren

The supercritical atmosphere created by the increase in temperature and pressure in the diesel engine cylinder is the key factor that causes changes in the fuel atomization process. In this study, the differences in spray morphology of n-heptane under different ambient temperature and pressure conditions were investigated using a constant volume combustion bomb combined with the Schlieren method. Based on the Peng-Robinson equation of state (PR equation), several transport characteristics models are constructed to predict the transport characteristics of n-heptane under different operating conditions. During the experiment, the injection pressure and fuel temperature were kept constant (80 MPa, 300 K), the ambient pressure was varied from 2 to 5 MPa, and the fuel temperature was changed from 400 to 800 K, spanning from subcritical to supercritical state. Three different spray regions have been identified based on the fuel temperature and spray geometry: liquid phase region, gas-liquid interface mixing layer, gas phase region. The results show that in a subcritical environment (400 K, 2 MPa), the fuel injection and atomization process is dominated by evaporation. The degree of heat exchange between the fuel and the medium gas is very low, while a small amount of phase change occurs. When the fuel is in low (600 K, 3 MPa) and medium supercritical (600 K, 4 MPa) ambient atmosphere, turbulence becomes the dominant factor in the fuel atomization process. The mixing layer at the gas-liquid interface at the edge of the liquid jet increases significantly. When the fuel is in a high supercritical (700 K, 4 MPa; 800 K, 5 MPa) atmosphere, the jet takes on a shape similar to that of a gas jet, and the mixing layer at the gas-liquid interface becomes the main part of the spray projection area.

The effect of HDPE and LDPE pyrolytic oils on cavitation formation in a common-rail diesel injector

Plastic production and usage increase every year due to its practicality, adaptability, and low-cost production. The problem with plastic arises when it becomes waste and needs to be treated. Most of the plastic we use is produced from petrochemical material that can be used in resource recovery processes like pyrolysis to produce various materials. One of the pyrolysis process products is pyrolytic oil, whose properties are similar to conventional fuels. Minor differences in fuel properties can influence the injection process, in-nozzle flow condition, spray formation and break-up, the combustion process, etc. The presented paper aims to study the influence of pyrolytic oil‘s properties on cavitation formation in the injection nozzle of a common-rail injector. First, the pyrolytic oils were obtained from waste high- and low-density polyethylene using a pyrolysis reactor. Afterwards, the oils were characterized and implemented in the AVL FIRE computation program for studying their influence on cavitation formation in the injection nozzle hole. The obtained results indicate slight differences in fuel properties that influence cavitation formation and spread in the injection hole, which further influence conditions at the exit of the injection hole. The lower lower viscosity of pyrolytic oils influences lower friction in the fuel nozzle flow. The lower densyt and viscosity of pyrolytic oils promotes cavitation formation, advance time of it appearance at injection hole exit and influence the shorter presence of cavitation in the needle closing phase.

Optimization of combustion parameters for CRDI small single cylinder diesel engine by using response surface method

Limited fossil fuel’s reservoir capacity and pollution caused by them are the big problem today in the world. The small diesel engine, working with a conventional fuel injection system was the major contributor to this. The current study represented a statistical investigation of such a small diesel engine. A mechanical fuel injection system of the small diesel engine was retofitted with a simple version of the electronic common rail diesel injection (CRDI) system in the present study. The effect of combustion parameters such as compression ratio (CR), injection pressure (IP) and start of injection timing (IT) was considered in the study. The study was performed to optimize these parameters with respect to performance and emission aspects. The reduction in parameters such as carbon monoxide (CO), nitrogen oxides (NOx), smoke and hydrocarbon (HC) from engine exhaust gases were considered in the emission aspect. Improve brake thermal efficiency (BTE) and fuel economy was considered in the performance aspect. The response surfaced method (RSM) was used to optimise these combustion parameters. The regression equations were obtained for measurable performance and emission parameters using the RSM model. The surface plots derived from the regression equations were used to analyse the effect of considered combustion parameters. Diesel injected at a pressure 600 bar, with retarded injection timing 15° crank angle (CA) before top dead center (bTDC) and compression ratio set at 15 was found to be optimum for this CRDI small diesel engine. The further validation of optimum parameters was done by conducting a confirmatory test on the engine. The maximum error in prediction was found to be 2.7%, which shows the validation of the RSM model.

Investigation on Primary Breakup of High-Pressure Diesel Spray Atomization by Method of Automatic Identifying Droplet Feature Based on Eulerian–Lagrangian Model

To investigate primary breakup close to an injector, this paper presents both experimental and numerical research on high-pressure common-rail diesel injection. We propose a new method named SD-ELSA model to realize automatically identifying droplet features for high-pressure diesel spray based on the classic ELSA (Eulerian Lagrangian Spray Atomization) model; this method is suitable for varied injection operation conditions. The SD-ELSA first identifies the liquid bulk due to breakup of the continuous phase in near field, and then converts the Eulerian liquid bulk into Lagrangian particles to complete the calculation of the total spray atomization. The SD-ELSA model adopts two key criteria, i.e., the sphericity (S) and the particle diameter (D); the qualified liquid mass is transformed into Lagrangian particle, realizing the coupling of the Eulerian–Lagrangian model. The SD-ELSA model illustrates the total diesel spray atomization process from the breakup liquid column to the droplets.

Effect of injection system parameters on overall performance of a small diesel engine

AbstractLimited fuel capacity and pollution are major concerns in the automobile industry. The combustion, performance, and emission characteristics are important considering the engine's overall performance. In the present paper, an experimental investigation was carried out on a small diesel engine to analyze the effect of injection system parameters on all measurable characteristics. The conventional diesel engine injection system was retrofitted with a common rail diesel injection (CRDI) system. The injection system parameters such as injection pressure, start of pilot injection timing, the start of main injection timing, and quantity of fuel injection percentage during the pilot and the main injection period were considered in the study. With the use of the CRDI system, it was observed that mean effective pressure and cylinder pressure were increased compared to the conventional diesel engine. There was a decrease in the peak value of the HRR and RoPR curve and simultaneously an increase in the width of the curve. MGT inside the combustion cylinder was also decreased. The retrofitted CRDI system shows higher thermal efficiency with good fuel economy as well as found a considerable reduction in carbon monoxide (CO) and smoke percentage. Overall, the considered injection system parameters deliver a significant improvement to the overall attributes of the small diesel engine.

Droplet diameter measurement near a nozzle exit of a common-rail Diesel injector using PDA

Droplet diameter measurement near a nozzle exit of a common-rail Diesel injector using PDA

Gasoline spray characteristics using a high pressure common rail diesel injection system by the method of laser induced exciplex fluorescence

The liquid and vapor phases of gasoline sprays were investigated using a common rail diesel injection system in a constant volume vessel by laser induced exciplex fluorescence (LIEF) and compared with diesel sprays. The effects of injection pressure and ambient pressure on gasoline sprays were measured under evaporating conditions. The results reveal that: the liquid phase penetration has a little change as the fuel is injected, while the vapor phase penetration increases constantly with obvious branch-like structures generated at the downstream vapor phase. The spray volume and mass of entrained gas both increase roughly linearly versus the mass of injected fuel. Compared with diesel sprays in similar conditions, gasoline sprays display a significantly shorter liquid penetration, while a smaller difference between the vapor penetration lengths of gasoline and diesel sprays is observed. The maximum equivalence ratio of gasoline sprays is still more than 2 at selected conditions. As the ambient pressure increases, the penetration lengths of liquid and vapor phases both reduce, and the mean equivalence ratio also decreases with a drop of spray volume. Conversely, the mass of entrained gas rises continually. With the increase of injection pressure, the liquid phase penetration length maintains almost no change. However, the vapor penetration length shows a declined trend during the injection pressure rise at iso-injected fuel mass condition. The spray volume and mass of entrained gas give a slight reduction when the injection pressure enhances. Oppositely, the mean equivalence ratio presents an increased tendency with similar mass of injected fuel.

Influence of deforming of common rail diesel injectors parts on the fuel injection process

Experimental studies have revealed a significant impact of deformation of Сommon Rail injector parts on the fuel supply process. High pressures alter the structure of the fuel supply cy-cle. Theforward front of the fuel supply cycle begins with the stage of unloading the deformed parts of the injector. The rear front of the fuel supply cycle ends with the stage of deformation of the injector parts. The calculated and experimental determination of cyclic fuel supply gave similar results. The developed method of determining the duration of the injection cycle stages creates a basis for experimental verification of mathematical models. Keywords: injector, Common Rail, diesel, fuel system, electronic control, needle, fuel injection

Insights into near nozzle spray evolution, ignition and air/flame entrainment in high pressure spray flames

Experimental investigations were performed to study the dynamic processes related to early development of high pressure sprays and their effects on ignition, local soot formation and its entrainment as well as spray to spray interactions in a six-hole common rail diesel injector. These spray-flame measurements were performed using ultra-high speed imaging technique in an optically accessible constant volume chamber maintained under reactive high pressure environment. The acquired back scattered light from fuel droplets and the broadband natural soot luminosity images and their analysis have revealed new insights on how the radial dispersions of fuel sprays during its early development at high injection pressures affect the ignition, flame stabilisation and soot formation in diesel spray flames.Analysis of the high speed data have shown that radial spreading also called as bulge in this paper occurs during the very early stages of fuel spray development closer to nozzle. The cloud of finely dispersed fuel droplets trapped within these bulges loses its moment and evaporates quickly relative to liquid core of the spray to form a locally ignitable mixture and this initiates ignition from these locations. Local flame kernels formed from these local ignition sites tend to develop into small pockets of sooting flame, which subsequently moves counter to the main spray towards the nozzle and gets entrained into the liquid core of spray. At times, these sooting pockets are also transported to into the next spray when the bulges are large. The sooting flames that are transported towards the nozzle and into the liquid spray core are quenched upon its interaction with the core of liquid spray. Quenched flames are rich in unburnt hydrocarbon and soot particles and they have the potential to significantly impact the pollutant formation through altering the equivalence ratio within the core of fuel spray. Wide variations in the sizes were observed on the formation of bulges, and it also varied between sprays from different orifices of a multi-hole injector. These anomalies are mainly related to the complexity of nozzle sac flow affecting the early spray development.

Effect of the In-Cylinder Back Pressure on the Injection Process and Fuel Flow Characteristics in a Common-Rail Diesel Injector Using GTL Fuel

The presented paper aims to study the influence of mineral diesel fuel and synthetic Gas-To-Liquid fuel (GTL) on the injection process, fuel flow conditions, and cavitation formation in a modern common-rail injector. First, the influence on injection characteristics was studied experimentally using an injection system test bench, and numerically using the one-dimensional computational program. Afterward, the influence of fuel properties on internal fuel flow was studied numerically using a computational program. The flow inside the injector was considered as multiphase flow and was calculated through unsteady Computational Fluid Dynamics simulations using a Eulerian–Eulerian two-fluid approach. Finally, the influence of in-cylinder back pressure on the internal nozzle flow was studied at three distinctive back pressures. The obtained numerical results for injection characteristics show good agreement with the experimental ones. The results of 3D simulations indicate that differences in fuel properties influence internal fuel flow and cavitation inception. The location of cavitation formation is the same for both fuels. The cavitation formation is triggered regardless of fuel properties. The size of the cavitation area is influenced by fuel properties and also from in-cylinder back pressure. Higher values of back pressure induce smaller areas of cavitation and vice versa. Comparing the conditions at injection hole exit, diesel fuel proved slightly higher average mass flow rate and velocities, which can be attributed to differences in fluid densities and viscosities. Overall, the obtained results indicate that when considering the injection process and internal nozzle flow, GTL fuel can be used in common-rail injection systems with solenoid injectors.

Evaporating spray characteristics of methanol-in-diesel emulsions

The main objective of this study is to investigate spray characteristics of methanol-in-diesel emulsions from a common rail diesel injection system under evaporating conditions using Mie-scattering and shadowgraphy techniques. The spray characteristics were measured in an optically-accessible constant volume chamber filled with nitrogen at a pressure of 50 bar and temperature of 900 K. Injection pressures of 500 bar, 1000 bar, and 1500 bar were investigated. Three surfactants (Span-80, Tween-80 and 1-dodecanol) were used to make stable emulsions containing up to 25 wt% of methanol in diesel. Interestingly, the liquid-length of the methanol-in-diesel emulsion spray with 10 wt% of methanol using a mixture of Span-80 and Tween-80 as a surfactant is higher by around 25% as compared to that of the diesel spray, even though methanol is far more volatile than diesel. This is attributed to the higher boiling point of the surfactant used, as confirmed by experiments with a low boiling point surfactant, i.e., 1-dodecanol. Further, microemulsions of methanol-in-diesel were produced by using 1-dodecanol as surfactant, which facilitated increasing the percentage of methanol in the microemulsion to up to 25 wt%. The evaporating spray liquid-length is observed to be insensitive to the methanol percentage in the emulsion when 1-dodecanol is used as the surfactant. Vapor penetration of the emulsions were similar to those of the diesel sprays at similar injection pressures. Overall, the current study provides measurements of detailed spray characteristics of methanol-in-diesel emulsions at engine-relevant conditions.

Modelling and experimental investigation of high – pressure common rail diesel injection system

The high-pressure Common Rail (HPCR) injection system was originally introduced for diesel engines to both reduce pollutant emissions and enhancement of performance. HPCR separates fuel pressurization and injection processes from each other. The high injection pressure generated by the common rail system provides better atomisation and evaporation of fuel spray, resulting in improved air inlet and fuel jet mixing, which is advantageous for lowering soot emission. In this paper, a mathematical common rail injection system model has been presented. A Simulink/Matlab code was developed to execute this simulation. This work does not only seek to validate the presented numerical model but to have more insight for understanding the overall common rail injection system diesel engine performance under different operating conditions. Some simulation results are illustrated to highlight modelling capability.The engine used is an HCCI turbocharged diesel engine, 2776 cc, 4-stroke, and 4-cylinder, water-cooled with overhead valve mechanism. The common rail pressure, fuel consumption, start and duration of each injection through one engine cycle are measured at various engine speed and loads. The measured common rail fuel pressure and consumption are used to validate the simulation results. The findings of the simulation show good consistency with the experimental results. At last, some simulation results, which highlight the modelling capability, are illustrated at certain values of engine speed and load.

Effects of nozzle geometries and needle lift on steadier string cavitation and larger spray angle in common rail diesel injector

The cavitating flow in diesel injector nozzles plays a vital role in spray atomization and formation of fuel–air mixture, since vortex-induced string cavitation has recently been found a much more influence on spray compared to the ordinary geometry-induced film cavitation. In this study, in order to investigating string cavitation and its’ enhancement on spray, the visualization experimental platform for the real-size optical tapered-hole nozzle was built based on the high-pressure common rail fuel injection system. Groups of optical nozzles with different geometries were designed for exploring the couple effects of several nozzle geometric parameters, including nozzle sac chamber depth, nozzle-hole position height and needle lift, on the three-dimension vortex flow structure and then on the string cavitation and spray characteristics. Results indicated that the string cavitation characteristics are tightly associated with couple characteristics of the parameters. The stable and strong string cavitation during the whole injection process can be obtained in the Min-sac nozzle with the high hole position under the low needle lift. The string cavitation extends to the nozzle-hole outlet, and subsequently induces the special hollow cone spray with air in the spray center location and corresponding a larger spray cone angle even under not so high injection pressure.