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The formation of the fuel adhesion on the wall is demonstrated as the major cause for the efficiency loss and particle number (PN) emission increase in direct-injection spark-ignition (DISI) engines, making it difficult to meet the regulation of the future standards. In this study, experiments were performed in a constant high-pressure chamber to clarify the characteristics of impinging spray injected by a mini-sac gasoline injector with a single hole. The fuel spray and adhesion were measured by Mie scattering and Refractive Index Matching (RIM) methods, respectively. The effects of ambient pressure on the spray-wall interaction under room and high temperature conditions were checked. The results showed that the increased ambient pressure decreases the spray tip penetration but increases the impinging spray height by strong air-fuel entrainment under both room and high temperature conditions. However, for fuel adhesion, under room temperature, the ambient pressure promotes better atomization by stronger air drag force, resulting in more fuel adhesion on the wall. Moreover, the increased ambient pressure decelerates the droplets, leading to the transition of droplets behavior from “splashing” to “spread” or even “stick”, thus increasing the fuel adhesion on the wall. While, when evaporation occurs, apart from these reasons above, higher ambient pressure suppresses the fuel evaporation, leading to more fuel adhesion on the wall.

This study investigated spray mixture formation of ethanol-gasoline blends (E0, E85 and E100) under direct-injection spark ignition-like (DISI-like) condition in a constant volume vessel. An ultraviolet (UV) – visible (Vis) dual-wavelength laser absorption scattering (LAS) technique was used to quantitatively measure the liquid and vapor phase distributions. The toluene with the similar physical properties to gasoline and high absorption for ultraviolet, especially for a boiling point close to the distillation temperature of 50% mass, was employed to determine the gasoline distribution. Likewise, the ethanol distribution was determined by methyl ethyl ketone (MEK). The results show that E100 evaporates faster than E0 under high temperature condition. A similar temporal variation and level of liquid penetration length are observed for E0 and E100. In E85 spray, ethanol component has a faster evaporation than high boiling point (HBP) components in gasoline before end of injection (EOI). Meanwhile, the HBP component vapor mainly presents downstream the spray. Due to lower boiling point and higher vapor diffusivity, the ethanol component shows a larger dispersion at radial distance of the spray, as a result, it covers the inner HBP component vapor. Furthermore, the evaporation ratio of E85 increases higher than E100 at EOI and slightly after EOI, which is probably attributed to the azeotrope mixture yielded by ethanol and gasoline. This effect, however, remains insignificant until the EOI. In addition, the non-uniform distribution of blending fraction in E85 is clearly identified under different injection pressures. In the case of Pinj = 20 MPa, the dense HBP component vapor at tip region becomes significant. Meanwhile, the higher blending ratio of ethanol component presents a narrow region at spray periphery. The injection pressure shows less impacts on HBP component evaporation in E85. Whereas the evaporation of ethanol component remarkably decreases under a lower injection pressure.

The air flow and wall impingement in a direct-injection spark-ignition (DISI) gasoline engine affect the fuel-air mixture formation and the quality of fuel combustion. In this work, a comprehensive experimental investigation on the effect of cross-flow on the spray structure, droplet diameter and velocity distributions was carried out. The transient impinging spray behavior at different cross-flow velocities was recorded using high-speed photography with a continuous wave laser sheet. It was seen that a higher cross-flow velocity significantly increases the spray area, i.e., the cross-flow favors spray dispersion. Moreover, the spray outline distortion caused by cross-flow in the leeward side is larger than that in the windward side. By employing the particle image analysis (PIA) optical diagnostic method, the Sauter mean diameter (SMD) and the droplet velocity components were investigated. The results show that a higher cross-flow velocity causes an increased proportion of large droplets in the windward side of spray, and the enhanced droplet breakup, resulting in a larger SMD in the windward side of spray and smaller SMD in the leeward side of spray. In the leeward side of spray, the droplet horizontal velocity gradually increases along the cross-flow direction, and after it reaches approximately the cross-flow velocity, the droplet horizontal velocity shows a large fluctuation in the downstream region. Moreover, the droplet vertical velocity decreases sharply from the center line of the main spray body to the spray periphery. By comparing the velocities of droplets, we found that compared with larger droplets, the smaller droplets are more easily affected by a cross-flow owing to the effect of drag acceleration.

The objective of this study is to obtain an enhanced understanding of the effect of split injection on the mixture formation and combustion processes of diesel spray. The study focused on the evaporation, mixture, and combustion of diesel fuel under 2D piston cavity sprays with split injection strategies in a constant volume combustion chamber. Three injection strategies with injection pressures of 100 MPa and 160 MPa, respectively, and a single nozzle hole with a diameter of 0.111 mm were employed. The tracer Laser Absorption Scattering (LAS) technique was used to visualize the spray mixture formation process. High speed imaging of OH∗ chemiluminescence and the two-color method were used to visualize the instantaneous spray combustion process. The experimental results reveal that the vapor distribution of the split main injection with a high injection pressure was more homogeneous than that of a single main injection with low injection pressure at the end of injection (EOI). The split injection can decrease the soot mass. Ignition is more easily during the main injection as a result of the pre-injection. High soot concentration appeared near the cavity wall region under the three injection strategies. The results also indicate that the split injection accelerates the soot oxidation process during the post combustion period.

In direct-injection spark-ignition (DISI) engines, spray-wall impingement affects the formation of the air–fuel mixture as well as combustion and exhaust emissions. In this study, the characteristics of fuel adhesion injected by a mini-sac gasoline injector with a single hole were investigated under the evaporation and non-evaporation conditions. The fuel spray was observed using Mie scattering, and fuel adhesion was measured using the refractive index matching (RIM) method under high injection and ambient pressures. The results showed that when evaporation occurs, the thin fuel adhesion evaporates quickly, whereas the thicker adhesion barely evaporates in a short period of time, resulting in a more uniform fuel adhesion on the wall. The maximum adhesion thickness continues to increase even after the end of injection, likely because of the re-deposition of the splashing droplets. Various mechanisms are purposed to explain the spray-wall interaction: fuel droplets primarily impinge on the wall forming primary impingement region, followed by the re-depositing droplets which form secondary impingement region.

The impingement of fuel spray on the wall in direct-injection spark-ignition (DISI) engines affects the fuel-air mixture formation, combustion and exhaust emissions. A detailed understanding of the fuel spray and impingement is required to reduce the undesirable products of combustion whilst maintaining good fuel economy. In this study, the droplet size was measured by particle image analysis (PIA). The microscopic characteristics of impinging spray were obtained using ultra-high-speed imaging. By changing the injection and ambient pressures, the influences of breakup and coalescence on the droplet behavior were investigated. Before impingement, the region near the center of the spray has larger droplets size and lower droplet number density than the edge, which suggests that spray breakup and atomization is poor in the center region. After impingement, the droplet size decreases along the distance from the wall under low ambient pressure. However, large droplets appear in the region far from the wall under high ambient pressure, indicating the existence of a coalescence effect on the droplets.

The fuel spray injected into a direct injection (DI) engine is substantially affected by both the in-cylinder air flow and the piston cavity wall impingement. The combined effect of the air flow and the wall impingement plays an important role on the spray development, mixture formation, and subsequent combustion. In this study, the effects of cross-flow and flat wall impingement on the spray development and dispersion were investigated. The spray was injected by a valve covered orifice (VCO) nozzle under various cross-flow velocities and ambient pressures. Impingement spray images in a vertical plane and several horizontal planes were obtained by a high speed video camera and a continuous wave laser sheet. A high speed video camera connected with a long-distance microscope was employed to obtain the near-field spray images. The results show that cross-flow favors spray dispersion while the high ambient pressure tends to compress the spray profiles. Additionally, under an approximate liquid-to-air momentum flux ratio q, when the ambient pressure and cross-flow velocity were varied, at 2 ms ASOI the outlines of the spray in the windward side agree well, whereas the spray extended further in the leeward side at a lower ambient pressure. At the plane of y = 25 mm, a complex vortex movement was observed that resulted in a non-uniform distribution of droplets in the upper part of the spray in the leeward side. In addition, at the plane of y = 45 mm, an empty belt area occurred in the vortex core region revealing that the density of the droplets in this region was quite low. The quantitative analysis shows that with increasing cross-flow velocity, the spray tip penetration decreases slightly before impingement while the spray tip penetrates further on the wall surface after impingement. The high cross-flow velocity favors the spray breakup and dispersion leading to a larger wall-jet vortex while the high ambient pressure restrains the spray dispersion leading to a smaller spray tip penetration and vortex height. For near-field spray, the spray image at higher ambient pressure shows fewer ligaments. With increasing cross-flow velocity, the whole spray shifted downstream. The spray outline was wider at the initial stage (0.05 ms ASOI) than that at steady stage (2 ms ASOI) of spray evolution.

The characteristics of spray/ambient gas flow and the turbulent mixing process significantly influence the subsequent combustion feature. There are some related studies on the simultaneous measurement of spray liquid/gas phase flow using optical diagnosis techniques. However, the two-phase flow characteristics of the transient Diesel spray have not been discussed in detail, few works have verified the measuring accuracy of the methods. This study proposes a PIV/LIF-PIV measuring method for simultaneously investigating the liquid/gas flow of transient Diesel spray. The experimental process and effects of imaging parameters/digital image processing method on the measuring accuracy are discussed based on the detectability of cross-correlation peak. The characteristics of the local turbulent mixing of liquid/gas flow for Diesel spray are presented. Numerical simulation using the Large-Eddy Simulation (LES) Eulerian-Lagrangian method is also carried out for extending the understanding regarding the liquid/gas phase flow process. The results indicate that the upstream vortex structures of spray flow lead to the random direction of velocity vectors at the tip, which is related to the shot-by-shot variation of spray structure at early timing after SOI. A similar streamline of spray flow and ambient gas flow could be observed in the spray region especially at 2.0 ms ASOI, which implies the gaseous phase is accelerated rapidly by the internal spray droplets as soon as entering the spray boundary, and both phases reach a “balanced state” at the end of the injection period. Similar results could be derived from the simulation results, the position where the maximum spray velocity exists is quite different because of the randomness of the instantaneous flow. An improved quasi-dimensional velocity predicting model that accounts for the spatial variation of velocity distribution is also developed.

Owing to cavitation inside the nozzle, spray breakup, and complex flow fields in the cylinder of a spark ignition engine, a certain degree of cycle-to-cycle variation (CCV) remains in different spray cycles under the same operating conditions, thereby affecting the spark ignition engine significantly. Therefore, the effect of cross-flow on the cyclic variation of the spray characteristics is investigated in this work. The CCV of fuel spray characteristics is evaluated via the statistical analysis of 30 repeated experiments under atmospheric pressure. First, the coefficient of variation (COV) is used to assess variations in the spray penetration, area, and optical thickness. Moreover, the effects of injection pressure and cross-flow velocity on the statistical characteristics of the spray are analyzed at the end of injection (EOI). Finally, a dimensionless “Ins number”, which is defined as the parameter of any spray characteristic at a certain time divided by the average experimental value, is proposed to evaluate the spray variation characteristics under cross-flow conditions. Therefore, when the absolute value of the Ins number approaches 1, the CCV of the spray characteristic parameters is low. Results show that the COV of the spray penetration is the largest in the initial stage of injection and then decreases gradually. Meanwhile, the COV of the vertical penetration can be categorized into two stages. Furthermore, it is discovered that the variation in the horizontal penetration is enhanced by the cross-flow and injection pressure at the EOI. In addition, in terms of the COV of the optical thickness, the variation on the windward side is greater, and the cross-flow enhanced the variation in the spray tip region. Most of the Ins numbers of the spray characteristics range from 0.9 to 1.1 (±10%) in this study, and they can provide data support for improving the accuracy of empirical prediction equations and models.

Microscopic characteristics of the impinging spray have a significant effect on atomization and pollutant emissions. Therefore, understanding the droplet behavior of the impinging spray comprehensively is essential to evaluate the spray atomization characteristics. However, in experiments, the size and distribution of droplets are hardly investigated in direct-injection spark-ignition (DISI) engines because of the high droplet density during the fuel injection period. In this paper, a spray slicer was adopted to cut the dense spray into thin flake with the objective of observing discrete droplets. And the particle image analysis (PIA) technique was used to detect droplet behaviors at different locations under various injection pressures of 10, 20, and 30 MPa. The results showed that the number density and velocity of droplets at the spray tip gradually decreased along the direction of spray development. The droplet away from the wall has a larger Y component of velocity (Vy). Furthermore, with an increase in injection pressure, the droplet velocity at the spray tip increased dramatically, and the atomization was greatly improved. Besides, sauter mean diameter (SMD) significantly declined at the upstream. However, Weber number (We) was almost the same at the fixed downstream locations under all injection pressures. Finally, We distribution of droplets concentrated at significantly smaller values under the fixed pressure. With an increase in injection pressure, We of droplets distributed near a large value range at all fixed locations.

Downsizing the hole diameter and increasing the injection pressure can effectively increase the fuel–gas mixture formation and thermal efficiency of the diesel engine. In this study, the characteristics of the liquid length and combustion processes were studied using diffuse back-illumination and OH* chemiluminescence imaging. The results show that the time for the liquid length to reach a stable stage reduces with the increase in injection pressure and decrease in hole diameter. Although the injection pressure has a slight effect on the liquid length, the liquid length of the micro-hole injector under ultra-high injection pressure conditions is slightly reduced. Increasing the injection pressure and reducing the hole diameter can reduce soot generation per unit fuel mass. Compared with the use of the micro-hole diameter (D = 0.07 mm, Pinj = 100 MPa), increasing the injection pressure to 300 MPa (D = 0.133 mm, Pinj = 300 MPa) can suppress soot generation per unit fuel mass more effectively. The strong oxidation reaction was distributed both in the upstream and downstream areas of the flame only when the micro-hole injector was used under ultra-high injection pressure. When the liquid length is longer than the flame lift-off length, a large amount of soot is generated during combustion. Through the intersection of the liquid length and flame lift-off length trend line, it is estimated that the region of the liquid length is less than the lift-off length under different conditions. The determination of this region provides guidance and suggestions for the selection of the hole diameter of injectors and injection pressure for diesel engines.

It is proved that fuel efficiency promotion is an important method to improve the fuel economy and power output of direct-injection (DI) gasoline engines; moreover, the quality of fuel combustion is a critical factor for fuel efficiency. In DI gasoline engines, the diffusion and atomization of fuel spray are influenced by the airflow in cylinder when the spray is injected into the cylinder. The experimental measurements and theoretical analyses in this study are based on the conditions of cross-flow whose direction is perpendicular to the direction of spray. The spray images under various velocities and pressures of cross-flow were recorded using high-speed photography; besides, the spray area and spray volume were calculated from the spray images. To understand the movements of spray under the cross-flow conditions, the movements of single droplets with various diameters were deduced using aerodynamics methods. Under a constant velocity of the cross-flow, the diffusion of spray under a low ambient pressure is more distinct than that under a high ambient pressure.

A V-type intersecting hole nozzle, in which each hole is formed by the coalescence of a pair of sub-holes, has been developed to improve fuel-air mixing for diesel engines. In this paper, a numerical study has been conducted to examine the effects of the V-type intersecting hole structure on the internal flow of a nozzle and the initial stage of the fuel-air mixing processes. With this aim, a multi-phase flow calculation has been implemented on seven V-type intersecting hole nozzles with impact angles ranging from 20° to 50° and a referenced cylindrical hole nozzle under injection pressures varying from 60 MPa to 240 MPa. The comparison was made in terms of mass flow, momentum flux, effective velocity, discharge coefficient, area coefficient and velocity coefficient. The three-fluid model, which was validated using X-ray experimental data of an Engine Combustion Network (ECN) Spray A injector, was employed to calculate the associated multi-phase flow. The main results show that use of a V-type intersecting hole in a nozzle eliminates cavitation, leading to higher mass flow and momentum flux. Correspondingly, for these V-type intersecting hole nozzles, the discharge coefficients are insensitive to injection pressure, but decrease with an increasing impact angle, and are 20–30% higher than those of the cylindrical hole nozzle. These higher discharge coefficients mainly result from very high area coefficients that are approximately 0.98 at all injection pressure conditions because of the non-cavitating nature of the in-nozzle flow. Moreover, the impact effect of a V-type intersecting hole results in a fan-shaped jet with a very large spreading angle (25–40°) at the dispersion plane and a relatively narrower angle (approximately 12°) at the impact plane. The jet spreading angles at both planes are wider than that of the cylindrical hole nozzle (approximately 7°), indicating a noticeable improvement in the initial fuel-air mixing. Additionally, an increase in the impact angle of a V-type intersecting hole nozzle promotes initial mixing in terms of yielding a wider jet spreading angle, in spite of a slightly lower effective velocity.

Cavitation in nozzle orifices plays an important role on atomization and spray. In the industrial manufacturing, the processing method, especially the fluid grinding technology seriously affects the geometry of the orifice, such as the inlet rounding and roughness of inner orifice, which may result in the cavitation and turbulence of the internal flow. Therefore, the effect of grinding nozzle orifice on fuel spray was studied in this paper. Two mini-sac injectors with the same single orifice were used to compared. One was electrical discharge machining (EDM) nozzle orifice with Hydro-Erosive (HE) Grinding, and the other was EDM orifice without grinding. The spray experiments were conducted in the constant volume chamber. Firstly, the discharged coefficient (Cd) was calculated by controlling the injection duration and quantity. Secondly, non-evaporating characteristics were obtained through Mie scattering method. The spray tip penetration, spray area, spray cone angle and spray angle were defined and analyzed. Then, evaporating behaviors were characterized by laser absorption scattering (LAS) measurement as well as vapor mass and evaporation ratio. Finally, microscopic behaviors of near-nozzle spray and droplets were compared with the help of particle image analysis (PIA) technology. Results showed that owing to the cavitation without grinding process, Cd decreased sharply, leading to less spray area and angle. But the evaporation was promoted by the cavitation and turbulence. More vapor mass and evaporation ratio can be obtained without HE Grinding. Furthermore, droplet size becomes smaller without grinding due to the better atomization by turbulence. However, the mean velocity decreases sharply, and the coefficient of variation (COV) increases without grinding, which should be considered carefully in the injector design.

Flash boiling spray is considered a potential solution for addressing spray impingement related issues such as pool fire caused by spray-wall interactions, in addition to its benefit of improving fuel-air mixing in engine cylinders. In comparison to typical sub-cooled spray impingement, flash boiling spray impingement can be affected by more complicated heat transfer and the mass transfer processes, including fuel vapor condensation near the wall when the temperature is very low. This work aims to investigate the wall film formation when flash boiling sprays impinge on a cold surface. Laser-induced exciplex fluorescence (LIEF) technique was used in this investigation to separate the fluorescent signals from the liquid phase and the gas phase during spray impingement. The simultaneous measurement of the thickness and temperature distributions of the wall film on a transparent plate were conducted, and corresponding heat and mass transfer of the film were analyzed. Generally, flash boiling impingement on cold surface would deposit less film mass on the wall than sub-cooled sprays, however it was still much larger than flash boiling spray impinging on room temperature plate. It was found that under flash boiling conditions, the combination of fine droplets and a large quantity of vapor impinged on the wall to form much less film. As wall temperature decreased, evaporation suppression and condensation effects became increasingly significant to dominate the wall film formation and development.

In order to provide comprehensive information on gasoline spray characteristics under ultra-high fuel injection pressure for direct-injection spark-ignition engines, the macroscopic characteristics of gasoline sprays for both liquid and vapor phases, including overall spray structures, spray tip penetrations, and spray angles, were determined simultaneously via Laser Absorption-Scattering technique under non-vaporizing and vaporizing conditions. Empirical correlations of spray tip penetration for transient gasoline sprays were obtained and compared with those for diesel sprays. Experimental results showed that under non-vaporizing conditions, spray tip penetration was approximately proportional to time. Under vaporizing conditions, the spray plumes were compressed axially and expanded radially; fuel vapor was concentrated in the spray head, while fuel droplets in the tail. Vapor phase spray tip penetration shared similar values and temporal variations with liquid phase but with a subtle and regular difference. In contrast with diesel sprays, gasoline spray penetration of liquid phase showed larger dependence on ambient gas density and smaller dependence on fuel pressure drop through injector. Compared with liquid phase, vapor phase penetration showed a larger growth rate with time and presented a smaller falling slope against ambient gas temperature.

The air flow and ambient pressure in the cylinder affect the spray significantly when the fuel is injected into the cylinder in direct injection gasoline engine. To investigate the effects of air flow at ambient pressure on the fuel spray, a critical air flow called cross-flow was tested. The cross-flow is the air flow whose direction is perpendicular to the fuel injection direction. A special wind tunnel was designed to provide the controllable cross-flows with various pressures and velocities, and the spray was injected into the cross-flow field by a mini sac injector using dry solvent as the test fuel. The high speed photography and particle image velocimetry technology were employed to observe the fuel spray and measure the velocity distribution of the spray. When the ambient pressure is increased, the bended spray profile is compressed; namely, the penetrations decrease. The cross-flow provides horizontal momentum for the droplets of the spray. When the ambient pressure is increased the velocity distribution of fuel spray becomes more uniform; furthermore, the magnitude of the vertical velocities can exceed the cross-flow velocity. This study is an experiment investigation which is aimed at understanding the fundamental phenomena and accumulating the theoretical supports for the real engine conditions.

It is well known that the liquid adhesion by impingement in the direct-injection spark-ignition (DISI) engine hinders the engine combustion efficiency and increases the particular matter (PM) emissions. Although numerous investigations were done on it, it is still blur for the scholars owing to the complicated droplets-wall dynamics. Due to the dense liquid near the wall, it is difficult to observe the impinging droplets in this region, let alone analyzing it on the micro level. Therefore, the “multiple droplets producer” was applied to cut the spray for clear observation in this study. Tslicer was finally determined at 40 μm to make the multiple droplets impacting on the wall at the near-wall region under various injection pressure from 10 to 30 MPa during the quasi-steady state. Four different locations of (15, 15), (17, 15), (20, 15), (22, 15) were selected along the jet development after impingement. Particle image analysis (PIA) technology was applied to capture the micro behaviors. The diameter and velocity of droplets were calculated and analyzed as well as the Weber number. Results show that droplet behaviors near the wall can be recorded at Tslicer = 40 μm during the injection. Droplet size becomes larger with spray propagation at the near-wall region. Observations demonstrate that the coalescence phenomenon of the secondary droplets as well as the splashing crown structures occurrence leads to the larger droplet and lower velocity. Moreover, non-dimensional parameter We was used to further prove the splashing transition occurring from (17, 15) to (20, 15). Additionally, the experimental results can provide strong evidence and verification basis for computational fluid dynamics (CFD) simulations.

It is demonstrated that the fuel deposit has a significant influence on the NOx and particular matter (PM) emissions, especially with the consideration of the more and more strict emission regulations issued by the governments, which results in that the near-nozzle spray has been received the increased attention, recently. In this study, the near-nozzle spray characteristics at the initial and end stages were investigated by a high-resolution camera with a laser as the light source. At the initial stage, microscopic behaviors of the near-nozzle spray were observed at upstream, midstream and downstream, respectively. Moreover, the injection frequency was changed to check the effect on the near-nozzle spray morphology. Next, the micro spray length, width and angle were obtained from the processed images. While, at the end stage, the near-nozzle spray evolutions were observed and characterized at 0.5, 0.3, 0.2 and 0.1 ms before the end of injection (BEOI), the end of injection (EOI), 0.05, 0.1, 0.15 and 0.2 ms after the end of injection (AEOI), respectively. Furthermore, the droplet area, spray area without droplets, droplet number and averaged diameter were calculated. Results show that three structures of the near-nozzle spray at the initial stage can be classified as mushroom, steeple and cylinder shapes. And the injection frequency has an influence on their formations owing to the residual fuel and sucked air AEOI. Moreover, although more droplets can be identified at the end stage with time, it does not indicate the better atomization. On the contrary, these droplets AEOI are responsible for the injector deposit in the real engine.

Wall-impingement is a serious problem for engine research owing to the possible source of hydrocarbons and soot emission. Therefore, impinging spray becomes the hotpot for investigation, especially for the microscopic characteristics. Droplet behaviors are significantly important to evaluate the atomization for predicting the impingement phenomena, such as stick, spread or splash. Many studies on it for both gasoline and diesel engines were published. However, these researches failed to capture the droplet behaviors clearly in the dense region owing to the overlap of the droplets. Only periphery or far field of the spray was tested. Therefore, a special cone structure named “spray slicer” was designed to cut the spray before observation with the consideration of good observation and less effect on droplet velocity. The experiment was conducted in a constant volume chamber under increased injection pressures between 10 and 30 MPa. Heptane was used as the surrogate fuel injected by a mini-sac injector with a single hole. Microscopic behaviors were recorded by particle image analysis (PIA) method to obtain the droplets size, velocity and minimum distance (Md). Results showed that microscopic behaviors can be observed clearly at different locations. Sauter mean diameter (SMD) and mean velocity decrease with spray development, and the smallest values occur at the periphery. Md increases with spray development owing to better dispersion. Moreover, high injection pressure decreases the droplet size but increases the velocity and Md at a certain degree.

Increasing the injection pressure and reducing the nozzle hole diameters can increase the thermal efficiency of diesel engines (DI). To understand the spray characteristics under ultra-high injection pressure, the spray characteristics of multi-hole injectors with different diameters were investigated under different injection pressures. The results show that the injector with a larger hole diameter has a larger injection delay and a longer injection rate rise and fall time. The injection pressure has a small effect on the injection delay. As the injection pressure increases, the injection rate rise and fall time decreases. Increasing the injection pressure and the hole diameter increases the instability of the spray. The penetration of the injectors with different hole diameters is similar in the initial stage of injection. The penetration increased with the increase of the injection pressure. However, the effect of the high injection pressure on the increase of the penetration becomes weak. The injectors with a larger nozzle hole diameter displayed a larger penetration, spray angle, spray cone angle, and spray area. The micro-hole injector under the ultra-high injection pressure, the expansion of the middle of the spray caused a larger spray angle and spray area. A larger nozzle hole diameter and injection pressure promotes an increase in the gas mass rate. But the injector with the micro-hole diameter has a better M A / M F ratio and a lower average spray equivalent ratio under ultra-high injection pressure. Naber and Siebers’ penetration model and Inagaki and Mizuta's spray angle model with modified parameters showed the better prediction effect under all experimental conditions.

During the post-processing of PIA images, correctly pairing the particles in two frames is the basic premise underlying the accurate measurement of particle velocity. The traditional particle pairing algorithm is designed according to the characteristics of particle morphology, but this method could result in a considerable number of wrong pairings when dealing with areas having particles of large density and similar shape, thus, calculate the particle velocities incorrectly. In this paper, the behavior characteristics of particles are analysed, and a new particle pairing algorithm based on multiple characteristics of a particle is proposed. In this algorithm, the similarities of basic particle features, adjacent particle motion directions and particle neighbourhood space are considered and analysed. A brand-new formula is established to calculate the overall feature similarity of the pairing particle and each searching particle. After the particles in two frames are paired, the particle velocities can be calculated. In this paper, the new algorithm was used to measure the particle velocity under high ambient pressure and cross flow respectively. The particle velocity is similarly to normally distribution. In D(40), the particles with the velocity of 0.8–0.9 m/s are the most, and in D(60), the particles with velocity of 1.6–1.7 m/s are the most, The mean velocity of particles at C(10,25) is 60.07 m/s. In C(20,50), the mean particle velocity is 52.01 m/s. The experimental results show that the new algorithm has a high precision rate of particle pairing, and can accurately measure particle velocities under different experimental conditions.

Many investigations on droplet behaviors during the injection were published. However, studies after the end of injection (AEOI) were seldom reported. Owing to the fuel adhesion on the wall still increases even AEOI, it is significantly important to observe the droplet behaviors for understanding the mechanisms. In this study, bottom view of fuel adhesion and side view of impinging spray were recorded through refractive index matching (RIM) method and Mie scattering observation, respectively. Morphologies of the fuel adhesion with impinging spray were shown, suggesting that even AEOI, fuel adhesion still increases on the wall. The incoming droplets in the air is supposed to be the main reason for it, although the fuel droplets were difficult to be detected through Mie scattering images. Therefore, quantitative measurement for fuel droplet was conducted by particle image analysis (PIA) technique. The microscopic behaviors of fuel droplets were obtained at 3, 5, 10 and 20 ms ASOI. Results showed that due to the small size of the droplets, Mie scattering fails to capture small droplets moving in the air AEOI. However, these small-size droplets really exit and still move towards the wall until impacting on it, which proves that after injection, droplets in the air could adhere on the wall is the main reason for adhesion increase. Furthermore, the droplets size, velocity and minimum distance distributions at different locations with different timings ASOI were analyzed in detail.

The purpose of this paper is to further understand the effect of split injection ratio on the diesel impinging spray combustion characteristics. Diesel combustion formed in the two dimensional piston cavity was studied in a high ambient pressure and high ambient temperature chamber. The injection pressure was 160 MPa and the diameter of single nozzle hole was 0.111 mm. Three split injection ratios were used: 25:75, 50:50 and 75:25. The KL factor and temperature distributions of the three injection strategies were analyzed and discussed. On this basis, the influence of the split injection ratio on the integrated KL factor and the flame temperature is discussed. The experimental results show that extending the splitting interval and decreasing the 2nd/1st ratio are beneficial to avoid the “catch-up” effect and reduce soot. In the absence of a “catch-up” effect, the domain process of soot formation and oxidation zones can be divided into six phases. However, the split injection influence on the flame is completed simultaneously. Due to the pre-injection, ignition is easier during the main injection. The relationship between the spatial KL factor and temperature is also discussed.

The spray evaporation and combustion characteristics of ethanol–gasoline blends, which consisted of E0 (100% gasoline), E85 (85% ethanol and 15% gasoline) and E100 (100% ethanol) were investigated using a hole-type nozzle in a high-temperature and high-pressure quiescent constant volume vessel. A dual-wavelength laser absorption scattering (LAS) technique was employed to differentiate the vapor distribution from the liquid phase. The vapor distribution of gasoline was determined by p-xylene, which had similar physical properties to gasoline and high absorption for ultraviolet. Likewise, the vapor behavior of ethanol was determined by methyl ethyl ketone (MEK). To analyze the flame development and combustion intensity, OH* chemiluminescence was examined by using real ethanol–gasoline blend under the same calorific value. The result shows that the ethanol evaporates faster than the gasoline. Due to different boiling points, a distinct stratified vapor ambient-gas mixture is formed in the E85 spray. It implies that the low boiling point (LBP) components in gasoline probably evaporate similar to ethanol which is distributed homogeneously along the spray axis. While those of high boiling point (HBP), which are the heavy components in gasoline, would locate from the middle to downstream of the spray, and hence a dense gasoline vapor region is formed at the spray tip after the end of injection (EOI). The analysis of the combustion intensity indicates this phenomenon is one possible reason that leads to the different combustion behaviors between the E85 and E100. After ethanol is added, the combustion becomes more vigorous due to the increase of oxygen content. In addition, the combustion stability is improved by using the ethanol–gasoline blends, especially when the ignition is started at the advanced timing and the middle region of the spray.

In direct-injection spark-ignition (DISI) engines, spray-wall impingement affects the mixture formation as well as combustion and exhaust emissions, making it difficult to satisfy the regulation of particle number (PN) in the future standards. In order to understand the mechanisms deeply, not only the formation but the evaporation characteristics of fuel adhesion should be investigated in detail. In this study, the fuel adhesion thickness was measured using the refractive index matching (RIM) method, then mass, area and thickness of the fuel adhesion were calculated. The evaporation evolution of fuel adhesion on the wall was discussed. Moreover, the lifetime of fuel adhesion was compared at the fixed conditions. The results showed fuel adhesion firstly evaporates from periphery, then the “hollow” occurs and develops, which facilitates the fuel evaporation. The increased injection pressure favors the droplets evaporation owing to the better atomization, leading to the decreased fuel adhesion mass and area on the wall. However, due to the strong momentum exchange between fuel droplets and air, the increased ambient pressure increases the fuel adhesion mass and area on the wall. Moreover, high injection pressure shortens the fuel adhesion lifetime by increasing evaporation rate, but high ambient pressure prolongs it due to the liquid/vapor phase changing.

Spray atomization and evaporation play extremely important roles in mixture formation and combustion processes of direct injection (DI) gasoline engines. In this study, the fundamental characteristics of a swirl spray injected into a constant volume vessel are investigated by means of several laser diagnostic techniques including the laser diffraction-based method for droplet size distribution, the laser induced fluorescence–particle image velocimetry for velocity distributions of droplets and spray-induced ambient air flow, and the two-wavelength laser absorption–scattering technique for concentration distributions of liquid and vapor phases in the spray. The results show that the droplets at outer zone of the spray exhibit larger diameter than those at inner zone under both ambient pressures 0.1 and 0.4 MPa. While this can be partially attributed to the effect of spray-induced ambient air flow, the strength of ambient air flow become small when increasing the ambient pressure from 0.1 to 0.4 MPa, indicating the strong influence of spray dynamics on the droplet size distribution. In the evaporating spray, there are higher vapor concentrations near the spray axis than at peripheral zones. At 4.0 ms after start of injection, spray droplets almost completely evaporate under ambient temperature 500 K and pressure 1.0 MPa, but there are significantly amount of fuels with equivalence ratio below 0.5 in the spray. Reduction in ambient pressure promotes the air entrainment and droplet evaporation, but lowered ambient pressure results in more fuel vapor of equivalence ratio above 1.3 along the spray axis.

A gas jet superposition model has been recently developed for computing group-hole nozzle sprays in computational fluid dynamics (CFD) simulations. The objectives of this study are: (1) to perform a systematic validation of the comprehensive spray model for group-hole nozzles using a broad range of experimental data; (2) to analyze the dynamics and physical insight of group-hole nozzle sprays based on the simulation results; and (3) to further clarify the impact of included-angle on spray/mixture properties of group-hole nozzle sprays. An updated version of the KIVA-3V Release 2 code, which employs the Lagrangian-Drop Eulerian-Fluid (LDEF) methodology for numerical calculation of two-phase flows, was used in the simulations. Diverging group-hole nozzles with various included-angles were considered. The test conditions included non-evaporating and evaporating ambient conditions, free sprays and sprays impinging on a flat wall. Detailed comparisons were made between the experiments and computations in terms of spray/mixture characteristics. The results show that numerical parameter dependencies are significantly reduced with the new models, and good levels of agreement are obtained in terms of spray structure, liquid/vapor penetration, overall SMD and cumulative vaporized fuel mass. Both experimental measurements and simulations reveal the importance of included-angle in group-hole nozzle sprays. In particular, some important features of group-hole nozzle spray are captured in the computations by the present models: compared to the equivalent single-hole nozzle, smaller local droplet size can be achieved in the near nozzle field, indicating an enhanced fuel primary atomization; the ambient gas entrainment rate is increased during the injection period, implying the better mixing; the spray axis deflection is identified in the case of group-hole nozzles with smaller angles, which is caused by a negative relative pressure region that exists between the sprays; in addition, the asymmetric structure of wall-impinging group-hole nozzle spray is well predicted by the present models through applying the gas jet superposition model in the entire computational domain.

In this study, gas entrainment characteristics of a diesel spray injected by a group of closely spaced two-orifices (group-hole nozzle) were investigated. Both free and wall-impinging sprays were considered. The gas entrainment characteristics of the group-hole nozzle spray were compared to those of single-hole nozzle sprays: one has the same total hole area with the group-hole nozzle, and the other has the same hole diameter. The gas entrainment characteristics of diesel sprays were investigated using a particle image velocitmetry technique coupled with a laser induced fluorescence technique (LIF-PIV technique). The spray tip penetration of the group-hole nozzle was the shortest among the applied nozzles in a free spray condition, while it was the longest in a wall-impinging condition. In the free spray condition, the gas entrainment of the spray was enhanced by the group-hole nozzle due to extensive momentum exchange with surrounding gas and superposed gas entrainment motion of the two-jets injected by the group-hole nozzle. After wall-impingement, the group-hole nozzle spray showed a stronger wall-jet vortex and increased gas entrainment compared to the single-hole nozzle sprays due to enhanced spray/wall interaction caused by the momentum interaction of the two-jets from the group-hole nozzle. Asymmetric shape of the group-hole nozzle spray resulted in an asymmetric gas velocity distribution of the spray both in the free and wall-impinging conditions.

The objective of this study is to investigate the mixture formation process of sprays injected from two-hole nozzles for direct-injection spark-ignition (DISI) engines. Spray characteristics were examined for vapor and liquid mass distributions, spray tip penetration and spray angle using the laser absorption–scattering (LAS) technique and the computational fluid dynamics (CFD) simulation. Other characteristics including: the vapor phase concentration distribution, droplet spatial distribution and pressure distribution were acquired from the CFD simulation results. Comparison of measured and calculated results showed that as the hole-axis-angle (HAA) of the two-hole nozzle decreased, the droplet coalescence increased and vapor mass decreased. The spray with the HAA of 10° had the longest spray tip penetration and the spray with the HAA of 15° had the shortest one. The two jets of the two-hole nozzle spray had a tendency of offsetting to the central region of the spray. From 10 to 15 MPa, the increase in the injection pressure increased the vaporization rate and the spray tip penetration rate very much, but no such tendency was found when the injection pressure increased from 15 to 20 MPa.