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Increasing injection pressure and decreasing nozzle hole diameter have been proved to be two effective approaches to reduce the exhaust emissions and to improve the fuel economy. Recently, the micro-hole nozzles and ultra-high injection pressures are applicable in commercial Diesel engines. But the mechanism of these two latest technologies is still unclear. The current research aims at providing information on the spray and mixture formation processes of the micro-hole nozzle (d=0.08mm) under the ultra-high injection pressure (Pinj=300MPa). The flat wall impinging sprays were focused on and the laser absorption-scattering (LAS) technique was employed to obtain the qualitative and quantitative information at both atmospheric and elevated conditions. The spray parameters were collected, the mixing rate was discussed, and the effects of various parameters on mixture formation were clarified.

Three-dimensional behaviors of direct injection (D.I.) gasoline sprays were investigated using 2-D and 3-D particle image velocimetry (PIV) techniques. The fuel was injected with a swirl type injector for D.I. gasoline engines into a constant volume chamber in which ambient pressure was varied from 0.1 to 0.4 MPa at room temperature. The spray was illuminated by a laser light sheet generated by a double-pulsed Nd:YAG laser (wave length: 532 nm) and the succeeding two tomograms of the spray were taken by a high-resolution CCD camera. The 2-D and 3-D velocity distributions of the droplet cloud in the spray were calculated from these tomograms by using the PIV technique. The effects of the swirl groove flows in the injector and the ambient pressure on the structural behavior of the droplet cloud in the spray were also examined.

Effects of split injection, with a relatively short time interval between the two sprays, on the spray development process, and the air entrainment into the spray, were investigated by using laser induced fluorescence and particle image velocimetry (LIF-PIV) techniques. The velocities of the spray and the ambient air were measured. The cumulative mass of the ambient air entrained into the spray was calculated by using the entrainment velocity normal to the spray boundary. The vortex structure of the spray, formed around the leading edge of the spray, showed a true rotating flow motion at low ambient pressures of 0.1 MPa, whereas at 0.4 MPa, it was not a true rotating flow, but a phenomenon of the small droplets separating from the leading edge of the spray and falling behind, due to air resistance. The development processes of the 2nd spray were considerably different from that of the 1st spray because the 2nd spray was injected into the flow fields formed by the 1st spray.

Measurements of the droplet and ambient air velocities in and around a D.I. gasoline spray were made by combining the laser induced fluorescence (LIF) and the particle image velocimetry (PIV) techniques. Before the fuel spray was injected into a constant volume vessel, rhodamine B-water solution was injected into the ambient air by a swirl-type injector for dispersing the fine fluorescent liquid particles as tracers for the ambient air motion. The fuel spray was injected into the fluorescent tracer clouds by a D.I. gasoline injector and was illuminated by an Nd:YAG laser light sheet (wave length: 532 nm). The light scattered by the droplets in the fuel spray was the same as the Nd:YAG laser wavelength, whereas the light emitted by the fluorescent tracer clouds was at a longer wavelength.

Previous research has shown that the reduced nozzle hole diameter and elevated injection pressure are effective for preparing a uniform fuel-air mixture in a direct injection (D.I.) Diesel engine. A micro-hole nozzle with a hole diameter of 0.08 mm and an ultra-high injection pressure of 300 MPa have been employed to investigate the mixture formation process under various conditions. The aim of the current work is to clarify the effect of nozzle hole diameter and injection pressure on flame lift-off and soot formation processes. The free sprays from the micro-hole and conventional nozzles were investigated at a high-temperature, high-pressure constant volume vessel. A high-speed video camera system was employed to record the non-vaporizing sprays and combustion. The direct photography of OH chemiluminescence was used to provide information about the high temperature combustion process and to measure the flame lift-off length.

In the previous study of the authors, it was found that some benefits for the mixture preparation of DI gasoline engines can be offered by splitting the fuel injection, such as the phenomenon of high density liquid phase fuel piling up at the leading edge of the spray can be circumvented. In a further analysis, the vapor quantity in the “stable operating” range (equivalence ratio of vapor ϕv in a range of 0.7≤ϕv≤1.3) was significantly increased by the split injection compared to the single injection. In this work, the mechanism of the effect of the split injection on the mixture formation process was studied by combining the laser-sheet imaging, LIF-PIV and the LAS (Laser Absorption Scattering) technique. As a result, it is found that the spray-induced ambient air motion can help the formation of the more combustible mixture of the split injection whereas it played a minus role of diluting the spray by the single injection.

The reduced chemical reaction scheme which can take the effect of major fuel components on auto ignition timing into account has been developed. This reaction scheme was based on the reduced reaction mechanism for the primary reference fuels (PRF) proposed by Tsurushima [1] with 33 species and 38 reactions. Some pre-exponential factors were modified by using Particle Swarm Optimization to match the ignition delay time versus reciprocal temperature which was calculated by the detailed scheme with 2,301 species and 11,116 elementary chemical reactions. The result using the present reaction scheme shows good agreements with that using the detailed scheme for the effects of EGR, fuel components, and radical species on the ignition timing under homogeneous charge compression ignition combustion (HCCI) conditions.

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.