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• Under non-evaporation condition, high ambient increases the fuel adhesion mass, area and average thickness by decelerating the droplets. • Under evaporation condition, the high ambient pressure inhibits the evaporation of the fuel to a certain degree is another reason for the increased fuel adhesion on the wall. • High ambient pressure improves the uniformity of the adhesion on the wall. 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. [ABSTRACT FROM AUTHOR]

Abstract Spray-wall impingement has been proved unavoidable in direct-injection spark-ignition (DISI) engines, which affects the fuel-air formation as well as combustion and exhaust emissions, making it difficult to meet the regulation of particle number (PN) in the future standards. In this study, the characteristics of fuel adhesion injected by a mini-sac gasoline injector with a single hole were investigated in a constant high-pressure chamber. The fuel spray and adhesion were measured via Mie scattering and refractive index matching (RIM) methods, respectively. The effect of injection pressure on the spray-wall interaction under room and high temperature condition were tested. The results showed that under room temperature, the injection pressure promotes better atomization, resulting in longer spray tip penetration, larger impinging spray height, and more fuel adhesion on the wall. However, when evaporation occurs, higher injection pressure favors the fuel evaporation due to the small droplets size, leading to shorter spray tip penetration, smaller impinging spray height, and less fuel adhesion on the wall. Moreover, under non-evaporation condition, high injection pressure has less effect on the uniformity of the fuel adhesion on the wall, while under evaporation condition, it improves the uniformity of the fuel adhesion. Owing to the different mechanisms of the fuel adhesion formation in the primary impingement (Region I) and secondary impingement (Region II) regions, injection pressure has more influence on the fuel adhesion in Region I, especially under evaporation condition. [ABSTRACT FROM AUTHOR]

• Mie scattering fails to capture the small size droplets. • The droplets impacting on the wall is the main reason for adhesion increase AEOI. • Most droplets impinging to the wall locate in the first quadrant AEOI. • With spray development, Md firstly decreases then increases with better dispersion. 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. [ABSTRACT FROM AUTHOR]

• Fuel adhesion firstly evaporates from periphery, then the "hollow" occurs and develops. • "Hollow" facilitates the fuel evaporation and deteriorates the uniformity of the fuel adhesion. • The fuel adhesion mass and area decrease with enhanced injecting pressure but increases with enhanced ambient pressure. • The fuel adhesion lifetime is shortened by increasing injection pressure and it extends with an increase in ambient pressure. 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. [ABSTRACT FROM AUTHOR]

• Cross-flow/ wall-impingement effects on fuel adhesion studied with split injection. • Fuel adhesion area decline rate rises with cross-flow velocity, stabilizing after 20 m/s. • Increased cross-flow velocity leads to a reduction in fuel adhesion thickness. • Fuel adhesion thickness also decreases with increasing wall-impingement distance. • Higher cross-flow velocity increases fuel adhesion mass reduction rate in later stages. In direct-injection spark ignition (DISI) engines, high-pressure fuel injectors introduce fuel into the cylinder, causing the spray to come into contact with and adhere to the cylinder wall; this is known as fuel adhesion. Over time, fuel adhesion contributes to carbon deposition on the cylinder wall, which affects its thermal conductivity and consequently diminishes engine combustion efficiency. This study investigates the influence of different impingement distances and cross-flow velocities on fuel adhesion under the triple injection strategy. The fuel adhesion propagation and side-view spray are measured using refractive index matching (RIM) and Mie scattering, respectively. The findings demonstrate that high cross-flow velocity promotes the fuel adhesion shape to be elongated strips. In the early stage, the growth rate of the fuel adhesion area increases with an increase in cross-flow velocity. In the later stage, the decrease rate in the fuel adhesion area initially increases with an increase in the cross-flow velocity; however, when the critical velocity threshold (20 m/s) is exceeded, the decrease rate in the fuel adhesion area tends to stabilize. The average fuel adhesion thickness then accordingly decreases with the increase in the cross-flow velocity and impingement distance in the later stage. In addition, the cross-flow promotes the volatilization of spray and fuel adhesion, thereby decreasing the fuel adhesion mass over time. In the context of carbon neutrality, this study underscores the importance of optimizing fuel injection conditions to reduce emissions and fuel consumption. [ABSTRACT FROM AUTHOR]

• Effect of injection strategies on fuel adhesion are investigated with cross-flow. • The cross-flow can promote the diffusion and evaporation of fuel adhesion. • Triple injection decreases the fuel adhesion characteristics in cross-flow field. • Deformation coefficient I d is proposed to evaluate the distortion of fuel adhesion. The flat-wall wetting phenomenon is inevitable because of the smaller cylinder volume and higher injection pressure inside direct-injection spark ignition (DISI) engines. The fuel adhesion phenomenon after wall impingement negatively affects the fuel spray mixture formation, the fuel consumption, and the pollutant emissions. In this study, the effects of different fuel injection strategies on the wall-impingement behavior were compared under static flow and cross-flow field conditions. The refractive index matching (RIM) method and high-speed video (HSV) camera were adopted to measure the propagation of fuel adhesion and the spray structure of the vertical plane, respectively. Meanwhile, a dimensionless parameter named "deformation coefficient I d ," which is defined as the fuel adhesion length divided by the width, is proposed to evaluate the degree of distortion of the fuel adhesion. Therefore, when I d approaches 1, the fuel adhesion shape approaches a circle. The results show that the cross-flow promotes the diffusion of the fuel spray, which leads to an increase in the fuel adhesion area. The fuel adhesion shape is similar to that of a slender strip under cross-flow field conditions, but almost maintains a circle under static flow field conditions. The cross-flow decreased the average fuel adhesion thickness. Meanwhile, the cross-flow also promotes the evaporation of the adhered fuel on a flat-wall, which leads to a reduction in the fuel adhesion area and mass with time. Additionally, it was found that triple injection can decrease the fuel adhesion thickness, area, and mass ratio under cross-flow field conditions. To achieve carbon neutrality, optimizing fuel injection strategies to reduce emissions is one of the primary purposes of this study. [ABSTRACT FROM AUTHOR]

• Cross-flow enhances fuel adhesion dispersion showing an ellipse propagation. • Average fuel adhesion thickness can decrease under the cross-flow conditions. • Cross-flow promotes the volatilization and decreases the lifetime of fuel adhesion. • The fuel adhesion can be divided into thin and thick fuel adhesion region. • The thin and thick fuel adhesion regions can transform into each other. The impingement of fuel spray on the piston surface significantly affects the mixture formation, combustion performance, and emissions of a direct-injection spark-ignition (DISI) engine. Therefore, the fuel adhesion characteristics of wall-impingement spray were investigated under cross-flow conditions for in-depth understanding the fuel adhesion behavior in this work. Mie scattering and refractive index matching (RIM) were used to observe the propagation of fuel spray and fuel adhesion, respectively. Subsequently, the area, mass, and thickness of fuel adhesion were evaluated under various cross-flow velocities. The results demonstrated that the cross-flow enhanced the diffusion of fuel spray and fuel adhesion, exhibiting an ellipse propagation. Additionally, the cross-flow also accelerated the volatilization of fuel adhesion, so that the adhesion area and mass decreased with time. Furthermore, the average fuel adhesion thickness obviously decreased under the cross-flow conditions. The fuel adhesion was divided into thick and thin fuel adhesion regions based on the distribution of fuel adhesion. Moreover, the formation and propagation mechanism of the fuel adhesion under cross-flow conditions were revealed. The experimental results can provide an insight into the operating conditions of air-flow inside the engine. [ABSTRACT FROM AUTHOR]