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The effects of dwell time on the mixture formation and combustion processes of diesel spray are investigated experimentally. A commercial multihole injector with a 0.123 mm hole diameter is used to inject the fuel. The injection procedure is either a single or split injection with different dwell times, whereas the total amount of injected fuel mass is 5.0 mg per hole. Three dwell times are selected, that is, 0.12, 0.32 and 0.54 ms, with a split ratio of 7:3 based on previous findings. The vapour phase is observed, and the mixture formation pertaining to the equivalence ratio is analysed using the tracer laser absorption scattering (LAS) technique. A high-speed video camera is used to visualise the spray combustion flame luminosity, whereas a two-colour pyrometer system is used to evaluate the soot concentrations and flame temperature. An analysis of the mixture formation based on the spray evaporating condition reveals a more concentrated area of the rich mixture within a 0.32 ms dwell time. In the shortest dwell time of 0.12 ms, the equivalence ratio distribution decreases uniformly from the rich mixture region to the lean mixture region. In the case involving a shorter dwell time, a suitable position for the second injection around the boundaries of the first injection is obtained by smoothly growing the lean mixture and avoiding the large zone of the rich mixture. Therefore, the shortest dwell time is acceptable for mixture formation, considering the overall distribution of the equivalence ratios. Spray combustion analysis results show that the soot formation rate of the single injection and 0.32 ms dwell time case is high and decreases quickly, implying a rapid reduction in the high amount of soot. Consequently, 0.12 ms can be considered the optimal dwell time due to the ignition delay and relatively low soot emission afforded.

Natural gas with H2 is widely used for lean-burn combustion, which leads to NOx emission as the main problem for it. For decreasing NOx emission and increasing thermal efficiency, the investigation on seeking the influence of H2 fractions on the mixture of CH4 and CO2 was conducted. Firstly, the ignition timing was decided through thermal efficiency and brake mean effective pressure (BMEP) for CH4 only. Then, combustion characteristics of CH4, CH4+CO2 and CH4+CO2+H2 were compared with volume percentage of H2 changing from 5% to 30%. Finally, the H2 injection strategy was checked between closed and open valve injections. Among these discussions, thermal efficiency, power output, BMEP and fuel consumption were evaluated. Results show that CO2 addition decreases power output and BMEP, leading to much more fuel consumption and lower thermal efficiency. When H2 is added, at the rich mixture conditions (λ＜1.0), power output and thermal efficiency decrease sharply as the mixture is enriched. However, at the lean-burn conditions (λ＞1.0), the decrease in flow rate of lower heating value (LHV) and increase in power output finally result in the higher efficiency with H2 addition. Moreover, when λ＞1.0, both low fuel consumption and high efficiency can be obtained with H2 addition to achieve the high BMEP. Furthermore, the open valve injection could obtain higher thermal efficiency, power output and BMEP with lower fuel consumption, suggesting that the H2 injection strategy should be well controlled with the ignition timing.

In the present work, effects of nozzle hole size and rail pressure under non-evaporating spray condition are demonstrated. Three single hole injectors with the bore size of 0.101, 0.122, and 0.133 mm are experimented with injection pressures of 140, 45, and 38 MPa respectively to achieve similar injection rate profile. Diesel spray experiments implement Diffused Backlight Illumination Technique where diffused background is obtained for the High Speed Video camera imaging. Experimental results are then validated with computational and analytical studies. The CFD simulation requires the injection rate profile and spray cone angle as a primary input; thus, based on the High Speed Video Camera start of injection frame the 5 kHz Butterworth low-pass frequency filter is applied to the injection rate raw data. While, the spray cone angle is predicted using a simple model obtained from the relationship between the injection velocity, fluctuating velocity at the nozzle exit and total pressure loss factor of the injector. The experimental spray tip penetration of all three injectors is almost identical as the similar injection rate profile is adopted. Although, the mixture characteristics are better for 0.101 mm hole diameter since the smaller hole diameter with highest injection pressure depicts larger spray angle and better atomization. The computational study agrees with experiments qualitatively; however, the quantitative and qualitative agreements are seen in the analytical study.

The effects of the split ratio on the mixture formation and combustion process of a diesel spray in a constant-volume chamber were experimentally investigated. A commercial seven-hole injector was used in this experiment. The effects of the mass-dependent split ratio and dwell time were observed when the total fuel injection was 5.0 mg/hole. Three split ratios were considered: 3:7, 5:5 and 7:3, while the dwell time of 120 µs was fixed for every condition. A laser absorption-scattering technique was adopted to examine the formation of mixtures with regarding to the equivalence ratio. A high-speed video camera was used to observe natural flame luminosity, and a two-colour pyrometer system was employed to evaluate the temperature and soot concentrations in the flame. Among the distribution ratios tested in this study, the 7:3 split ratio exhibited the best performance for the lean mixture formation considering the overall equivalence ratio distribution. The air entrainment wave at the end of injection timing of the first injection caused the fuel near the nozzle to lean at a rapid rate. The soot formation process for the 3:7 and 5:5 split ratios was observed because the second injection fuel caught the flame of the previous injections; this deteriorated the combustion region and influenced soot formation. The result also revealed that for the 7:3 split ratio, accelerated the soot deduction rate to the cycle of soot oxidation during the combustion period.

The effect of split injection on the fuel spray and combustion processes in a rapid compression and expansion machine was investigated using the visualization process. A two-dimensional piston cavity, designed with the cross section of a reentrant piston, was installed in the combustion chamber to observe the combustion process from the lateral side. Combustion experiments were conducted with injection pressures of 80 MPa, 120 MPa, and 180 MPa and an O2 concentration of 15%. The spray/wall interaction, mixture distribution, and ignition location were investigated using the shadow method. Along with natural flame luminescence, different spray impinging behaviors on combustion process were studied. Furthermore, the combustion characteristics of in-cylinder pressure, apparent heat release rate, and combustion phase were recorded and analyzed simultaneously. The results showed that both high injection pressure and split injection with a longer interval effectively improved the combustion performance. In addition, when the pilot injection was advanced further, the injection interval had a larger influence in reducing soot generation, while the effect of high injection pressure on heat release decreased. Flame separation was found to occur at high injection pressures. It was observed that the flame separation caused by the strong spray momentum was beneficial for reducing soot generation owing to the greater fuel-air interaction area. The spray and combustion processes were investigated in detail, and the significant effects of different injection pressures and injection intervals on combustion performance with the split injection method were highlighted.

Effect of spray/wall interaction in a rapid compression and expansion machine on mixture formation, ignition location, and soot generation was investigated. A two-dimensional piston cavity designed as the cross section of a reentrant piston was utilized to observe the spray and combustion process from the lateral side. The experiment was conducted at 120 MPa injection pressure under single and split injection strategies with an ambient gas of 15% O2 concentration. A shadow methodology was applied to investigate the interaction between the fuel spray and the piston cavity. Combined with the natural flame luminosity captured by a high-speed color video camera, the behaviors of the impinging spray and the combustion process were studied. The combustion characteristics of the in-cylinder pressure, heat release and combustion phase were recorded and analyzed simultaneously. The results showed that the split injection strategies effectively softened the heat release trace and promoted the onset of the main combustion. The cool-flame phenomenon was captured by using the high-speed color video camera, and the intense ignition was observed when the pilot spray was controlled to impinge on the lower lip of the piston rim. Moreover, results also showed that further extending the mixing process of the pilot spray is inclined to form a homogeneous mixture which was beneficial for the promotion of low-temperature combustion and the reduction of soot generation. This research provides a detailed investigation on the spray and combustion process and it highlights the significant effect of spray/wall interaction on the subsequent combustion process.

Direct injection is an attractive technology for improving fuel economy and engine performance in gasoline engines. However, the adhered fuel formed on the piston surface has significant influence on the combustion efficiency and emissions. To obtain a better understanding of fuel adhesion, this work involved investigation of the spray and impingement on a flat wall through a mini-sac injector with a single hole. Different impingement distances and injection pressures were investigated. The evolution of the impinging spray was obtained by the Mie scattering method. The refractive index matching method was applied to measure fuel adhesion. The mass, area, and thickness of adhesion under different conditions were compared. The experimental results show that the fuel adhesion on the wall increases significantly with a large impingement distance. Moreover, the maximum thickness increases and the thickness uniformity of the fuel adhesion declines under a large impingement distance condition.

Single-hole (SH) injectors were compared with multi-hole (MH) injectors to reveal the differences in nozzle internal flow and spray propagation. The multi-phase flow inside the nozzles was numerica...

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.

Fuel film on engine walls caused by spray impingement would dramatically cause engine friction deterioration, incomplete combustion, and significant cycle-to-cycle variations. In a previous work, it has been demonstrated that fuel film would break up via wave entrainment induced by the high-speed coflow. Meanwhile, the film breakup dynamics depend on various boundary conditions, such as injection pressure, ambient pressure, and so on. However, such impact on the wall film formation was not investigated thoroughly in existing literature. This work aims to perform a parameter study to investigate possible means to enhance wave entrainment effect as to reduce the amount of impingement fuel mass. In this study, simultaneous measurements of macroscopic structure and its corresponding footprint of impinging spray are conducted using a single-hole, prototype injector in a constant volume chamber. The macroscopic spray structure was captured by high-speed backlit imaging, and the film was obtained using laser-induced fluorescence under different conditions. The laser-induced fluorescence signal is converted to film thickness following a calibration procedure where laser-induced fluorescence signals from a series of known-thickness film are captured. A mathematical processing method is used to analyze both the dynamic behavior of film thickness and amount of droplet detachment caused by high-speed coflow. It is found that at the leading edge of film waves, a remarkable amount of liquid droplets detaches from the liquid film and the quantity of film mass on the wall decreases during this process. Quantitative analysis is conducted and the mass ratio of detached droplets over residual liquid film is estimated. We hold that the film breakup percentage increases with both ambient and injection pressure due to the enhanced high-speed coflow. Then, variation laws for various boundary conditions are obtained based on the observations.

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.

This paper discusses the turbulent flow and heat transfer from a uniform air flow with high temperature to the outside through a 90&deg, curved square pipe. Both conjugate heat transfer (CHT) simulation and experiments of temperature field measurements at cross sections of the pipe are performed. A straight pipe is investigated and compared with the 90&deg, curved pipe. The temperature of the air flow at the inlet of the pipe is set at 402 K, and the corresponding Reynolds number is approximately 6 &times, 104. To obtain the spatial average temperature at each cross section, the temperature fields are measured along the streamwise of the pipes and in the circumferential direction using thermocouples at each cross section from the inlet to the outlet of both the straight and curved pipes. Furthermore, the simulation is performed for turbulent flow and heat transfer inside the pipe wall using the Re-normalization group (RNG) k-&epsilon, turbulence model and CHT method. Both the experimental and numerical results show that the curvature of the pipe result in a deviation and impingement in the high-temperature core and a separation between the wall and air, resulting in a secondary flow pattern of the temperature distribution.

In diesel engines, fuel mixing is an important process in determining the combustion efficiency and emissions level. One of the measures used to achieve fuel mixing is controlling the nature and behavior of the fuel spray by shaping the injection rate. The mechanism underlying the behavior of the spray with varying injection rates before the start of combustion is not fully understood. Therefore, in this research, the fuel injection rate shape is investigated to assess the spraying and mixing behavior. Diesel sprays with different ambient temperatures and injection pressures are modeled using the CONVERGE-CFD software. The validation is performed based on experimental data from an Engine Combustion Network (ECN). The verified models are then used to analyze the characteristics of the diesel spray before and after the end-of-injection (EOI) with four fuel injection rate shapes, including a rectangular injection rate shape (RECT), a quick increase gradual decrease injection rate shape (QIGD), a gradual increase gradual decrease injection rate shape (GIGD), and a gradual increase quick decrease injection rate shape (GIQD). The spray vapor penetrations, liquid lengths, evaporation ratios, Sauter mean diameter (SMDs), distributions of turbulence kinetic energy, temperatures, and equivalence ratios were compared under different injection rate shapes. The results show that the QIGD injection rate shape can enhance mixing during injection, while the GIQD injection rate shape can achieve better mixing after the EOI.

In this study, the combustion and soot formation processes of mid-low level ethanol-gasoline blends (E0, E20 and E50) sprays injected by a hole-type nozzle were examined in a quiescently high-temperature and high-pressure constant volume vessel by various ignition strategies (e.g. ignition position, ignition timing and spark energy). High-speed imaging of OH* chemiluminescence and two-color pyrometry were employed to clarify the flame propagation and soot formation. The results show that a relatively favorable flame area is potential to achieve for both E20 and E50 at the EOI or slightly after EOI timing. Higher ethanol content blends show more sensitivity to the ignition energy due to the physical properties. The soot level tends to decrease with increasing ethanol content at EOI middle ignition, possibly attributed to the dilution effect of ethanol and the increase of oxygen content. At EOI downstream ignition, however, the soot formation increases in both E20 and E50, possibly as a consequence of the downstream fuel heterogeneity consisting of the inhomogeneous mixture and liquid droplets of highly sooting hydrocarbons. Retarding ignition timing can help to decrease the soot formation. Moreover, E20 shows not only comparably intense combustion to E0 but also less soot formation at EOI middle ignition.

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.

Reducing heat loss is one of the most important development concerns for improving the thermal efficiency of the diesel engine. In order to know heat transfer through the combustion chamber wall mo...

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.

The objective of this study is to obtain an enhanced understanding of the effect of split injection on mixture formation and combustion processes of diesel spray. A two-dimensional (2D) piston cavity of the same shape as that used in a small-bore diesel engine was employed to form the impinging spray flame. The fuel was injected into a high pressure, high temperature constant volume vessel through a single-hole nozzle with a hole diameter of 0.11 mm. The injection process comprised a pre-injection followed by the main injection. The main injection was carried out either as a single injection of injection pressure 100 MPa (Pre+S100), or by two types of split injection of injection pressure 160 MPa. The latter two types were defined by mass fraction ratios 1:1 and 3:1 (Pre+D160_1-1, Pre+D160_3-1). In order to observe the spray mixture formation process, the tracer laser absorption scattering (LAS) techique was adopted. Tracer LAS fuel with 97.5 vol% of n-tridecane and 2.5 vol% of 1-methylnaphthalene (α-MN) was employed. The spatial distributions of the vapor and liquid phases and the spray mixture formation characteristics in the 2D piston cavity for the three injection strategies were investigated. The diesel spray combustion and soot formation processes were studied using a high-speed video camera. The flame structure and soot formation process were examined using two-color pyrometry. The experimental results revealed that the split-injection vapor distribution was significantly more homogeneous than that of the single injection. The main injection fuel caught up with the pre-injection fuel and provided the spray tip with substantial additional momentum, enabling it to advance further. A high soot concentration and low temperatures appeared near the cavity wall region under the three injection strategies. The soot reduction rate for split injection was higher than that for single injection. The second main injection caught up with the previous injection’s flame, which deteriorated the combustion and resulted in higher soot generation. The effect of split injection on the process of soot evolution finished at the same time as that of single injection.

Spray combustion characteristics of waste cooking oil biodiesel (WCO) and conventional diesel fuels were simulated using a RANS (Reynolds Averaged Navier Stokes) based model. Surrogates were used to represent WCO and diesel fuels in simulations. N-tetradecane (C14H30) and n-heptane (C7H16) were used as surrogates for diesel. Furthermore for WCO, surrogate mixtures of methyl decanoate, methyl-9-decenoate and n-heptane were used. Thermochemical and reaction kinetic data (115 species and 460 reactions) were implemented in the CFD code to simulate the spray and combustion processes of the two fuels. Validation of the spray liquid length, ignition delay, flame lift-off length and soot formation data were performed against previous published experimental results. The modeled data agreed with the trends obtained in the experimental data at all injection pressures. Further investigations, which were not achieved in previous experiments, showed that prior to main ignition, a first stage ignition (cool flame) characterized by the formation formaldehyde (CH2O) species at low temperature heat release occurred. The main ignition process occurred at high temperature with the formation of OH radicals. Furthermore, it was observed that the cool flame played a greater role in stabilizing the downstream lifted flame of both fuels. Increase in injection pressure led to the cool flame location to be pushed further downstream. This led to flame stabilization further away from the injector nozzle. WCO had shorter lift-off length compared to diesel as a result of its cool flame which being closer to the injector. Soot formation followed similar trends obtained in the experiments.