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Stringent emission standards are driving the development of diesel-fuel injection concepts to mitigate in-cylinder formation of particulates. While research has demonstrated significant reduction in particulate formation using micro-orifice technology, implementation requires development of industrial processes to fabricate micro-orifices with diameters as low as 50 μm and with large length-to-diameter ratios. This paper reviews the different processes being pursued to fabricate micro-orifices and the advanced techniques applied to characterize the performance of micro-orifices. The latter include the use of phase-contrast x-ray imaging of electroless nickel-plated micro-orifices and laser imaging of fuel sprays at elevated pressures. The experimental results demonstrate an industrially viable process to create small uniform orifices that improve spray formation for fuel injection.

Gasoline direct injection (GDI) has the potential to greatly improve internal combustion engine performance through precise control of the injection rate, timing, and combustion of the fuel. A thorough characterization of the hydrodynamics of fuel injection has to come from a precise, quantitative analysis of the sprays, especially in the near-nozzle region. A lack of knowledge of the fuel-spray dynamics has severely limited computational modeling of the sprays and design of improved injection systems. Previously, the structure and dynamics of highly transient fuel sprays have never been visualized or reconstructed in three dimensions (3D) due to numerous technical difficulties. By using an ultrafast x-ray detector and intense monochromatic x-ray beams from synchrotron radiation, the fine structures and dynamics of 1-ms GDI fuel sprays from an outwardly opening nozzle were elucidated by a newly developed, ultrafast, microsecond computed microtomography (CT) technique.

Present knowledge of the velocity of the fuel in diesel sprays is quite limited due to the obscuring effects of fuel droplets, particularly in the high-density core of the spray. In recent years, x-ray radiography, which is capable of penetrating dense fuel sprays, has demonstrated the ability to probe the structure of the core of the spray, even in the dense near-nozzle region. In this paper, x-ray radiography data was used to determine the average axial velocity in diesel sprays as a function of position and time. Here, we report the method used to determine the axial velocity and its application to three common-rail diesel sprays at 250 bar injection pressure. The data show that the spray velocity does not reach its steady state value near the nozzle until approximately 200 μs after the start of injection. Moreover, the spray axial velocity decreases as one moves away from the spray orifice, suggesting transfer of axial momentum to the surrounding ambient gas.

By taking advantage of high-intensity and high-brilliance x-ray beams available at the Advanced Photon Source (APS), ultrafast (150 ps) propagation-based phase-enhanced imaging was developed to visualize high-pressure high-speed diesel sprays in the optically dense near-nozzle region. The sub-ns temporal and μm spatial resolution allows us to capture the morphology of the high-speed fuel sprays traveling at 500 m/s with a negligible motion blur. Both quality and quantitative information about the spray feature can be readily obtained. In the experiment, two types of single-hole nozzles have been used, one with a hydroground orifice inlet and the other with a sharp one. Within 3 mm from the nozzle, the sprays from these nozzles behave differently, ranging from laminar flow with surface instability waves to turbulent flow. The sprays are correlated with the nozzle internal geometry, which provides practical information for both nozzle design and supporting numerical simulation models.

A numerical simulation of shock wave generation by high-pressure and high-speed spray jet has been conducted to compare to the experimental results obtained by X-ray radiographic technique. Using the space-time conservation element solution element (CESE) method and the stochastic particle techniques to account for fuel injections and droplet collisions, supersonic-spray-induced shock waves are successfully simulated. Similar to the experimental condition, a non-evaporating diesel spray in a chamber filled with inert gas sulfur hexafluoride (SF6) at 1 atm pressure under room temperature (30° C) is simulated. To simulate the needle lift effect in the single-hole diesel injector, various injection-rate profiles were employed. In addition, the effects of discharge coefficients, with Cd ranging from 0.8 to 1.0, were also considered to simulate the shock generation processes in the leading spray front.

A time-resolved x-radiographic technique has been employed for measuring the fuel distribution close to a single-hole nozzle fitted in a high-pressure diesel injector. Using a monochromatic synchrotron x-ray beam, it is possible to perform quantitative x-ray absorption measurements and obtain two-dimensional projections of the mass of the fuel spray. We have completed a series of spray measurements in the optically dense, near-nozzle region (< 15 mm from the nozzle orifice) under ambient pressures of 1, 2, and 5.2 bar N2 and 1 bar SF6 at room temperature with injection pressures of 500 and 1000 bar. The focus of the measurements is on the dynamical behaviors of the fuel jets with an emphasis on their penetration in the near-nozzle region. Careful analysis of the time-resolved x-radiographic data revealed that the spray penetration in this near nozzle region was not significantly affected by the limited change of the ambient pressure.

Using synchrotron x-radiography and mass deconvolution techniques, this work reveals strikingly interesting structural and dynamic characteristics of the direct injection (DI) gasoline hollow-cone sprays in the near-nozzle region. Employed to measure the sprays, x-radiography allows quantitative determination of the fuel distribution in this optically impenetrable region with a time resolution of better than 1 μs, revealing the most detailed near-nozzle mass distribution of a DI gasoline fuel spray ever detected. Based on the x-radiographs of the spray collected from four different perspectives, enhanced mathematical and numerical analyses were developed to deconvolute the mass density of the gasoline hollow-cone spray. This leads to efficient and accurate regression curve fitting of the measured experimental data to obtain essential parameters of the density distribution that are then used in reconstructing the cross-sectional density distribution at various times and locations.

The fuel spray behavior in the near nozzle region of a gasoline injector is challenging to predict due to existing pressure gradients and turbulences of the internal flow and in-nozzle cavitation. Therefore, statistical parameters for spray characterization through experiments must be considered. The characterization of spray velocity fields in the near-nozzle region is of particular importance as the velocity information is crucial in understanding the hydrodynamic processes which take place further downstream during fuel atomization and mixture formation. This knowledge is needed in order to optimize injector nozzles for future requirements. In this study, the results of three experimental approaches for determination of spray velocity in the near-nozzle region are presented. Two different injector nozzle types were measured through high-speed shadowgraph imaging, Laser Doppler Anemometry (LDA) and X-ray imaging.

A low-pressure direct injection fuel system for spark ignition direct injection engines has been developed, in which a high-turbulence nozzle technology was employed to achieve fine fuel droplet size at a low injection pressure around 2 MPa. It is particularly important to study spray characteristics in the near-nozzle region due to the immediate liquid breakup at the nozzle exit. By using an ultrafast x-ray area detector and intense synchrotron x-ray beams, the interior structure and dynamics of the direct injection gasoline sprays from a multi-orifice turbulence-assisted nozzle were elucidated for the first time in a highly quantitative manner with μs-temporal resolution. Revealed by a newly developed, ultrafast computed x-microtomography technique, many detailed features associated with the transient liquid flows are readily observable in the reconstructed spray.

In order to analyze the effects of nozzle geometry on the structure of fuel sprays, quantitative x-ray measurements have been performed on sprays from nozzles with different degrees of hydro-grinding. The two nozzles were measured at injection pressures of 500 and 1000 bar in an ambient environment of 1 bar nitrogen gas. Time-resolved x-radiography was used to measure the two-dimensional mass distributions of the spray as a function of time for the entire spray event. The initial mass flow through the nozzles was determined from the x-ray data, the nozzles showed no appreciable differences in the early part of the injection event. The transverse mass distributions were fit with Gaussian curves, and the assumption of axisymmetry was used to calculate the volume fraction of each spray. It was observed that the nozzle which had undergone extensive hydro-grinding generated a more dense spray than the sharp-edged nozzle at an injection pressure of 1000 bar.

Propagation-based and phase-enhanced x-ray imaging was developed as a unique metrology technique to visualize the internal structure of high-pressure fuel injection nozzles. We have visualized the microstructures inside 200-μm fuel injection nozzles in a 3-mm-thick steel housing using this novel technique. Furthermore, this new x-ray-based metrology technique has been used to directly study the highly transient needle motion in the nozzles in situ and in real-time, which is virtually impossible by any other means. The needle motion has been shown to have the most direct effect on the fuel jet structure and spray formation immediately outside of the nozzle. In addition, the spray cone-angle has been perfectly correlated with the numerically simulated fuel flow inside the nozzle due to the transient nature of the needle during the injection.

A quantitative and time-resolved technique has been developed to probe the dense spray structure of direct-injection (DI) gasoline sprays in near-nozzle region. This technique uses the line-of-sight absorption of monochromatic x-rays from a synchrotron source to measure the fuel mass with time resolution better than 1 μs. The small scattering cross-section of fuel at x-rays regime allows direct measurements of spray structure that are difficult with most visible-light optical techniques. Appropriate models were developed to determine the fuel density as a function of time.

Synchrotron x-radiography and a novel fast x-ray detector are used to visualize the detailed, time-resolved structure of the fluid jets generated by a high pressure diesel-fuel injection. An understanding of the structure of the high-pressure spray is important in optimizing the injection process to increase fuel efficiency and reduce pollutants. It is shown that x-radiography can provide a quantitative measure of the mass distribution of the fuel. Such analysis has been impossible with optical imaging due to the multiple-scattering of visible light by small atomized fuel droplets surrounding the jet. In addition, direct visualization of the jet-induced shock wave proves that the fuel jets become supersonic under appropriate injection conditions. The radiographic images also allow quantitative analysis of the thermodynamic properties of the shock wave.