Search

[Display omitted] • Wall impingement can shorten ignition delay by stagnation when H / D is larger than 7. • Ignition initiates from near wall region or the jet root among different H / D. • Secondary jet induced by swirl-shape wall is favorable for ignition. Pre-chamber turbulent jet ignition is a promising high-efficiency combustion technique to achieve stable lean-burn operation for spark-ignition engines. Wall impingement by a high momentum turbulent jet could exert a critical influence on jet ignition behavior. In this work, the passive jet ignition characteristics of premixed methane/air mixture under wall impinging condition was investigated based on optical experiments in a constant volume chamber and numerical simulations. Two ignition modes were found for flat wall impinging cases across a range of non-dimensional impinging distances (H / D) from 5.7 to 20. In mode 1, ignition initiates from near wall region. Under lean conditions of λ = 1.3, compared with that in the free jet case, the ignition delay time characterized by a 5 % mass fraction burned (MFB-05), decreases by 3.3 ms at H = 30 mm and increases by up to 6.6 ms at H = 20 mm. In mode 2, ignition initiates from the jet root. The ignition delay times are shorter for all impinging distances (H = 20, 30, 40 mm), with MFB-05 decreasing by up to 3.4 ms. Simulations reveal that when the wall is located in the jet developing zone with H / D below 7, wall impinging deteriorates the ignition, which is attributed by higher turbulent kinetic energy (TKE) and lower Damköhler (Da) number near the stagnation zone, resulting in extended ignition delay. Conversely, as H / D increases, ignition is promoted by the stagnation effect with a shorter ignition delay. Additionally, the effect of wall shapes was analyzed. Using sharp-tip V-shaped wall, the stagnation region with high TKE is minimized. A blunt-tip V-shaped wall shows enhanced impinging but no obvious increase of Da number at stagnation point. Using M-shaped wall, the ignition mode with a "lift-off" flame was found. Besides, ignition is more favorable in the secondary jet zone with active radicals and higher Da number. [ABSTRACT FROM AUTHOR]

• 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]

A comparative study of effect of different biodiesel–diesel blends (B5, B10, B15, B20, B25, B50 and B100) on injection, spray, combustion, performance, and emissions of a direct injection diesel engine at constant speed (1500 rpm) was carried out. The penetration distance increased with increase in percentage of biodiesel in diesel due to enhanced in-line fuel pressure. The simulation results indicate the spray penetration with biodiesel–diesel blend up to B15 does not lead to wall impingement but B20 is to be a critical limit of wall impingement (within uncertainty ±1.3%). However, it is observed clearly from the simulation results that probability of wall impingement is more with higher blends (B25, B50 and B100). The ignition delay period decreased with all biodiesel blends due to higher cetane number resulting in less rate of pressure rise and the smooth engine running operation. The engine torque does not change significantly with biodiesel–diesel blends up to 20% (B20). However, the torque reduction is about 2.7% with B100 at the rated load. Carbon monoxide (CO), hydrocarbon (HC) and smoke emissions decreased with all biodiesel–diesel blends. However, oxides of nitrogen (NOx) emission increased in the range of 1.4–22.8% with all biodiesel–diesel blends at rated load due to oxygenated fuel, automatic advance in dynamic injection timing (DIT), higher penetration and higher in-cylinder temperature. A notable conclusion emerged from this study is the optimum biodiesel–diesel blend based on no wall impingement (B15: 0% and B20 ±1.3% uncertainty limit) and increase in NOx emission (B15: 4.1% and B20: 15.6%) in a conventional (unmodified) diesel engine is up to B15. [ABSTRACT FROM AUTHOR]

• Droplet diameter and velocity along different splashing directions were analyzed. • The velocity after impingement determines the secondary-breakup droplets location. • After impingement, the ligaments at tip tend to breakup into large droplets. 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 (V y). 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. [ABSTRACT FROM AUTHOR]

• SMD and mean velocity decrease with spray development owing to the better atomization and kinetic energy loss. • Md increases with spray development owing to better dispersion. • The periphery location is a special one with best atomization and lowest velocity but moderate dispersion. • High injection pressure increases velocity and Md but decreases SMD at a certain degree. 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. [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]