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The continuous improvement of gasoline direct injection (GDI) engine is largely attributed to the enhanced understanding of air-fuel mixing and combustion processes. This work investigates the transient behavior of the ambient flow fields of hexane spray using the combined diagnostics of fluorescent particle image velocimetry (FPIV) and mie scattering. A hybrid analysis approach is proposed to investigate the residual effect of spray injection on ambient flow fields, including flow similarity measurement, entrainment velocity calculation, and vortex strength detection. The work investigates the residual effect under different injection durations, injection pressure, and flash-boiling extent of the spray, and unveils correlation between vortex strength and the endurance of the residual effect.

The high injection pressure and small cylinder volume of direct injection spark ignition (DISI) engines can result in flat-wall wetness on the surface of the piston, increasing fuel consumption and pollutant emissions. The characteristics of microscopic fuel adhesion are observed using refractive index matching (RIM). Fuel adhesion characteristics after wall impingement are evaluated with various cross-flow velocities under triple stage injection conditions. The results indicate that cross-flow has a beneficial effect on the diffusion of fuel spray. Average fuel adhesion thickness decreases with an increase in cross-flow velocities. Furthermore, cross-flow promotes the evaporation of fuel adhesion, which leads to a reduction in the fuel adhesion mass/mass ratio. The improvement of injection strategy has guidance on low-carbon future.

Fuel film deposited on fuel injector tips used in gasoline direct injection engines, otherwise known as nozzle tip wetting, has been identified as an essential source of particle emissions. Attempts have been made to reduce nozzle tip wetting by the optimization design of nozzle geometry parameters. However, relevant investigations are still limited to emission measurements and corresponding indirect analysis. Due to the lack of related visualization research, the mechanism of nozzle tip wetting formation and its link with nozzle internal flow are still unclear. To clarify the influence of spray hole length on nozzle tip wetting and the underlying mechanisms, the dynamic formation process and the fuel film area evolution of nozzle tip wetting were visualized directly using laser-induced fluorescence technique and photomicrography technique.

Fuel spray impingement on the combustion chamber wall cannot be avoid in direct injection gasoline engines, resulting in insufficient combustion and unburned hydrocarbon/soot emissions from the engines. And the microscopic characteristics of the impinging spray have a close relation with the fuel film formation, which has a direct effect on the engine performance and emissions. Therefore, figuring out the droplet behaviors of the impinging spray is significantly important for improving the engine performance and reducing emissions. However, the microscopic characteristics of the impinging spray have not been deeply understood and the differences between the impinging and free spray are seldom mentioned in previous study. Therefore, particle image analysis (PIA) technique was applied to detect the microscopic characteristics at the capture location in order to track the droplet behaviors of the spray tip during the propagation process.

To alleviate the shortage of petroleum resources and the air pollution caused by the burning of fossil fuels, the development of renewable fuels has attracted widespread attention. Among the various renewable fuels, ethanol can be produced from biomass and does not require much modification when applied to practical engines, so it has been widely used. However, ethanol fuel has a higher heat of vaporization than gasoline, it is difficult to evaporate and atomize under cold start conditions. Besides, the catalyst has not reached the conversion temperature at this time, resulting in lower conversion efficiency. These factors all lead to higher pollutant emission levels in ethanol-gasoline blends. To solve the above problems, this research used visualization techniques to compare the effects of flash boiling and high-energy ignition technologies on the in-cylinder combustion process and pollutant emission of ethanol-gasoline blends fuel.

The fuel economy of recent small size DI diesel engines has become more and more efficient. However, heat loss is still one of the major factors contributing to a substantial amount of energy loss in engines. In order to a full understanding of the heat loss mechanism from combustion gas to cylinder wall, the effect of hole size and rail pressure at similar injection rate condition on transient heat flux to the wall were investigated. Using a constant volume vessel with a fixed impingement wall, the study measured the surface heat flux of the wall at the locations of spray flame impingement using three thin-film thermocouple heat-flux sensors. The results showed that the transferred heat was similar under similar injection rate profiles. However, in case of flame luminosity, temperature distribution, characteristic of local heat flux and soot distribution was also similar except the smaller nozzle hole size with higher injection pressure.

Owing to the small size of engines and high injection pressures, it is difficult to avoid the fuel spray impingement on the combustion cylinder wall and piston head in Direct Injection Spark Ignition (DISI) engine, which is a possible source of hydrocarbons and soot emission. As a result, the droplets size and distribution are significantly important to evaluate the atomization and predict the impingement behaviors, such as stick, spread or splash. However, the microscopic behaviors of droplets are seldom reported due to the high density of small droplets, especially under high pressure conditions. In order to solve this problem, a “spray slicer” was designed to cut the spray before impingement as a sheet one to observe the droplets clearly. The experiment was performed in a constant volume chamber under non-evaporation condition, and a mini-sac injector with single hole was used.

In this paper, the Diesel spray characteristics were studied by HS video camera and the Laser Absorbing Scattering (LAS) technique means of the combustion deterioration problem caused by the engine downsizing based on the geometrical similarity was investigated. In the experiments, three Diesel injectors with the hole diameters of 0.07mm, 0.101mm and 0.133mm were used. The injection pressures of the injectors with three different diameters were 45MPa, 93MPa and 160MPa, respectively. The Diffused Background Illumination (DBI) method was employed for the nonevaporating spray experiment to obtain spray tip penetration and spray angle at room temperature. The LAS technique was employed for the evaporating spray experiment to obtain the equivalence ratio distributions, evaporation rate, and vapor phase tip penetration. Moreover, the Wakuri Momentum Theory was applied to analyze the data obtained by both the non-evaporating and the evaporating spray experiments.

An investigation on the effect of dwell time of split injection on a diesel spray evolution and mixture formation process was carried out. A commercial 7-hole injector were used in the experiment to eliminate the possible discrepancies on the spray with single-hole research injector. Laser absorption scattering (LAS) technique was implemented for the measurement of the temporal evolution of fuel evaporation and mixture concentration. The diesel surrogate fuel consists of n-tridecane and 2.5% of 1-methylnaphthalene in volume basis was used. The total amount of fuel injected was initially fixed to 5.0 mg/hole. A split ratio of 9: 1 in mass basis was selected according to the results obtained from a previous study. The dwell time was varied from 120 µs to a negative value of −50 µs. The effects of negative dwell time was not ideal for lean mixture formation when compared to zero or positive dwell time conditions.

Good comprehension of multi-component fuel spray behavior is essential for the improved performance of GDI engines. In this study, the spray characteristics of three distinct multi-component surrogate fuels with various proportions of n-pentane, iso-octane, and n-decane were investigated using multiple diagnostics including macroscopic imaging, planar laser Mie-scattering, and phase doppler interferometry (PDI). These surrogate fuels were used to mimic different distillation characteristics of regular unleaded gasoline with different vaporization behaviors. Test measurements show that under subcooled test conditions, the spray geometry is mainly influenced by dynamic viscosity. On the contrary, under superheated test conditions, spray geometry is controlled by the specific component of fuel which has the highest vapor pressure. A triangular methodology is created to evaluate the influence of component proportion on spray characteristics.

To get a better understanding of the characteristics of the high pressure gasoline sprays impinging on a wall, a fundamental study was conducted in a high-temperature high-pressure constant volume vessel under the simulated engine conditions of in-cylinder pressures, temperatures, and wall temperatures. The injection pressure was varied from 20 to 120 MPa. The spray tip penetration, vapor mass distribution, and vaporization rate were quantitatively measured with the laser absorption-scattering (LAS) technique. The velocity fields of the wall-impinging sprays under vaporizing conditions were measured with the particle image velocimetry (PIV) technique using silicone oil droplets as tracers. The effects of injection pressure and spray/wall interactions on spray characteristics were investigated. The results showed that the increased injection pressure improved penetration, vaporization, and turbulence of the sprays.

In direct injection spark ignition (DISI) engine, it is difficult to avoid the spray impingement of fuel on the cylinder wall and piston head, which is a possible source of hydrocarbons and soot emission. The injector nozzle geometry and injection pressure are essential components for the spray atomization and mixture formation. For better understanding the effects of injector hole diameters and injection pressure, the fuel spray and adhesion on a flat wall by different mini-sac injectors with a single hole was examined in this study. A flat-wall made of quartz glass was used as the impingement plate. Refractive Index Matching (RIM) method was applied to measure the thickness of fuel adhesion on the wall. All the cases performed in constant high-pressure chamber were under high temperature condition considering the real gasoline engine condition. Time-resolved behaviors of the fuel adhesion as well as adhesion mass, area and thickness were discussed.

The effects of split injections of a diesel spray was evaluated in a constant volume chamber under evaporating, non-reacting condition. Laser absorption scattering (LAS) technique was utilized for the mixture concentration measurement, using a diesel surrogate fuel consists of n-tridecane and 2.5% of 1-methylnaphthalene in volume basis. While fixing the total injected fuel mass of 5.0 mg/hole, the effects of split ratio in mass basis and the dwell time (or injection interval) were investigated. Among the split ratios conducted in the current study (3,7, 5:5 and 7:3), the split ratio of 7:3 was the optimum for lean mixture formation regarding the overall distribution of the equivalence ratio at end-of-injection (EOI) timing. The air entrainment wave at the EOI timing of the first injection allowed the fuel at the vicinity of the nozzle to become leaner at a faster rate.

Injection pressure plays a vital role in spray break-up and atomization. High spray injection pressure is usually adopted to optimize the spray atomization in gasoline direct injection fuel system. However, higher injection pressure also leads to engine emission problem related to wall wetting. To solve this problem, researchers are trying to use flash boiling method to control the spray atomization process under lower injection test conditions. However, the effect of injection pressure on the spray atomization under flash boiling test condition has not been adequately investigated yet. In this study, quantitative study of internal flow and near nozzle spray breakup were carried out based on a two-dimensional transparent nozzle via microscopic imaging and phase Doppler interferometery. N-hexane was chosen as test fluid with different injection pressure conditions. Fuel temperature varied from 112°C to 148°C, which covered a wide range of superheated conditions.

A comparison of spray characteristics was conducted between single- and multi-hole injectors. A commercial software (AVL FIRE) was used to investigate the internal flow inside the sac volume, as well as the initial spray behavior at 1 mm downstream of the nozzle exit. Microscopic imaging was applied to observe the spray dispersion angle (spray cone angle) at the vicinity of the nozzle. Laser absorption scattering (LAS) technique was implemented for measuring the mixture concentration. Three injection quantities, namely 0.5, 2.5, and 5.0 mg/hole, were selected to observe the differences between transient and quasi-steady spray. The vapor penetration at the initial stage of the injection was greater for single-hole than that of multi-hole injector due to faster fuel pressure build-up process inside the sac volume.

It has been found that the spray impingement on piston for SIDI engines significantly influences engine emission and combustion efficiency. Fuel film sticking on the wall will dramatically cause deterioration of engine friction performance, incomplete combustion, and substantial cycle-to-cycle variations. When increasing the injection pressure, these effects are more pronounce. Besides, the ambient pressure also plays an important role on the spray structure and influences the footprint of impinging spray on the plate. However, the dynamic behavior of impinging spray and corresponding film was not investigated thoroughly in previous literature. In this study, simultaneous measurements of macroscopic structure (side view) and its corresponding footprint (bottom view) of impinging spray was conducted using a single-hole, prototype injector in a constant volume chamber.

Flash boiling spray has been proven to be a useful method in providing finer fuel droplet and stronger evaporation in favor of creating a homogeneous fuel-air mixture. Combustion characteristics of flash boiling spray are thus valuable to be investigated systematically for aiding the development of efficient internal combustion system. An experimental study of flash boiling spray combustion in a SIDI optical engine under early injection has been conducted. The fuel, Iso-octane, was used across all tests. Three fuel spray conditions experimented in the study: normal liquid, transitional flash boiling and flare flash boiling sprays, within each case that Pa/Ps ratio was set in (>1), (0.3~1), and (<0.3) respectively. A small quartz insert on the piston enables optical access for observing combustion process; non-intrusive measurements on flame radicals has been carried out using a high-speed color camera.

Nozzle length has been proven influencing fuel spray characteristics, and subsequently fuel-air mixing and combustion processes. However, almost all existing related studies are conducted when fuel is subcooled, of which fuel evaporation is extremely weak, especially at the near nozzle region. In addition, injector tip can be heated to very high temperature in SIDI engines, which would trigger flash boiling fuel spray. Therefore, in this study, effect of nozzle length on spray characteristics is investigated under superheated conditions. Three single-hole injectors with different nozzle length were studied. High speed backlit imaging technique was applied to acquire magnified near nozzle spray images based on an optical accessible constant volume chamber. Fuel pressure was maintained at 15 MPa, and n-hexane was chosen as test fuel.

Increasing the injection pressure and splitting the injection stage are the major approaches for a diesel engine to facilitate the fuel-air mixture formation process, which determines the subsequent combustion and emission formation. In this study, the free spray was injected by a single-hole nozzle with a hole-diameter of 0.111 mm. The impinging spray, formed by a two-dimensional (2D) piston cavity having the same shape as a small-bore diesel engine, was also investigated. The injection process was performed by both with and without pre-injection. The main injection was carried out either as a single main injection with injection pressure of 100 MPa (Pre + S100) or a split main injection with 160 MPa defined by the mass fraction ratio of 3:1 (Pre + D160_3-1). The tracer Laser Absorption Scattering (LAS) technique was adopted to observe the spray mixture formation process. The ignition delay/location and the soot formation in the spray flame were analyzed by the two-color method.

The performance of a diesel engine largely depends on the spray behavior and mixture formation. Nozzle configurations and operating conditions are important factors that influence spray development. Using numerical and experimental methods, this study focused on the spray development of multi-hole nozzles under non-evaporating and evaporating conditions to compare the influence of nozzle hole diameter and injection pressure on spray characteristics. High-speed video observation was employed to study the properties of spray development under the non-evaporating condition, while the Laser Absorption Scattering technique was used in the observation and quantitative analysis of evaporating spray characteristics in the evaporating condition. In addition, computational fluid dynamics study results published previously [1] were correlated with the current experimental results to provide more detailed explanations about the mechanism of the characteristics of spray behavior.