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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.

An experimental study was conducted to investigate the flame propagation characteristics in the presence of a heterogeneous concentration distribution of a fuel-air mixture in order to provide fundamental knowledge of the effects of gaseous mixture concentration heterogeneity on the combustion process. Different propane-air mixture distributions were produced by the reciprocating movements of a pair of perforated plates in a constant volume combustion chamber. The mean equivalence ratio of the fuel-air mixture was varied from 0.7 on the lean side to 1.6 on the rich side, the turbulence intensity in the combustion chamber was also varied at levels of 0.185 m/s, 0.130 m/s, 0.100 m/s, and 0.0 m/s. By an independent control of the mixture distribution and the turbulence intensity in the combustion chamber, the flame structure and flame propagation speed at various heterogeneous levels of the mixture distribution were investigated in detail.

In this study, air entrainment, fuel evaporation and mixing process of diesel sprays injected by micro-orifices for direct-injection diesel engines were investigated at the end of injection transient and after the end of injection. The mixture formation process was analyzed using a laser absorption scattering (LAS) technique, providing the information of quantified liquid and vapor mass concentration, entrained air concentration and equivalence ratio. The data was obtained at the timings of quasi-steady state, sudden velocity decrease, the end of injection and after the end of injection. Two micro-orifices, which have different orifice diameters, were selected as test nozzles to investigate the end-of-injection characteristics at different nozzle geometries. In case of smaller orifice diameter, the liquid phase regression was observed around the end of injection, while it was not observed at larger orifice diameter due to denser liquid concentration near the nozzle tip.

In order to investigate the effect of split injections on mixture formation processes in Direct Injection (DI) gasoline engine sprays, an experimental study was conducted applying the laser absorption and scattering (LAS) technique to the sprays using double pulse injections with various dwells and mass ratios. The effects of various dwells and mass ratios between the pulsed injections on the spatial concentration distributions in the spray, the penetration of vapor and liquid phases, and the mean equivalence ratios of the vapor phase and overall spray, were clarified. It was found that the phenomenon of high concentration liquid spray piling up at the leading edge of the spray is avoided by the double injections with enough dwell or appropriate mass ratio. The maximum penetration length of the spray significantly decreases, especially for the liquid phase with high concentration.

The breakup and atomization processes of the pre-swirl spray, which is produced before the hollow-cone spray from a high-pressure swirl-type D.I. gasoline injector, were investigated under different ambient pressure conditions. The injector has a press-fitted swirl tip, in which six tangential slots giving the injecting fuel an angular momentum are perforated at an equal space interval. A microscopic imaging technique was applied to get the spatially high-resolution LIF tomograms of the pre-swirl spray. The sprays were illuminated by an Nd:YAG laser light sheet and imaged using a high resolution CCD camera, fixed with a micro lens and coupled with an optical low-pass filter. The droplet size and the individual droplet's velocity were obtained by applying the image processing and the particle tracking techniques, respectively.

Reduction of orifice diameter of nozzle is advantageous to the fuel atomization in a D.I. diesel engine. However, the diameter reduction is usually accompanied with decrease of spray tip penetration, thus worsening fuel spatial-distribution and fuel-air mixing. In this paper, a group-hole nozzle concept was proposed to solve the problem resulting from minimization of orifice diameter. Compared to the conventional multi-hole nozzle, group-hole nozzle has a series group of orifices, and each group consists of two micro-orifices with a small spatial interval and small angle. For examining the characteristics of the spray injected by the group-hole nozzle, the ultraviolet-visible laser absorption-scattering (LAS) imaging technique was adopted to determine vapor concentration and droplets density as well as other spray characteristics such as spray angle and penetration of both vapor and liquid phases.

To get quantitative measurements of liquid and vapor phase concentration distributions in a gasoline spray, a laser-based absorption and scattering (LAS) technique was developed. The LAS technique adopts ultraviolet and visible lasers as light sources and a test fuel, which absorbs the ultraviolet light but does not absorb the visible light, instead of gasoline. The LAS principle is based on the incident light extinction in the ultraviolet band due to both vapor absorption and droplets scattering, whereas in the visible band, the incident light extinction is due only to the droplet scattering. The absorption spectra and molar absorption coefficients of the candidate test fuels including p-xylene, benzene and toluene, all of which have physical properties similar to gasoline, were investigated, and p-xylene was finally selected as a test fuel. Measurement accuracy of the LAS technique was discussed.

Experimental results of a diesel engine have shown that using split-injection can reduce the NOx and particulate emissions. For understanding the mechanism of emissions reduction, mixture formation in split-injection diesel sprays was characterized in the present paper. A dual-wavelength laser absorption-scattering (LAS) technique was developed by use of the second harmonic (532nm) and the fourth harmonic (266nm) of a pulsed Nd:YAG laser as the incident light and dimethylnaphthalene (DMN) as the test fuel. By applying this technique, imaging was made of DMN sprays injected into a high-temperature and high-pressure constant volume vessel by a single-hole nozzle incorporated in a common rail injection system for D.I. diesel engine. The line-of-sight optical thickness of both fuel vapor and droplets in the sprays was yielded from the sprays images.

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.

The spray and the entrained ambient air motions produced by a swirl-type D.I. gasoline injector were simultaneously measured by combining the laser induced fluorescence (LIF) and the particle image velocimetry (PIV) techniques. For the simultaneous measurement of the spray and the ambient air velocities, the succeeding two image pairs of the fuel spray and the ambient air tracer particles were captured by using a Nd:YAG laser light sheet (wave length: 532 nm) and two high-resolution CCD cameras. The light emitted from the fluorescent tracer clouds was discriminated from the light scattered from the droplets in the fuel spray by an optical low-pass filter (>560 nm), and the Mie scattering signals from the spray particles were screened by a band-pass filter ranging from 520 to 545 nm. The spray and the tracer particle images were analyzed by the double frame cross-correlation PIV technique to obtain the droplets and ambient air velocity distributions.

Some experimental investigations have shown that the trade-off curve of NOx vs. particulate of a D.I. diesel engine with split-injection strategies can be shifted closer to the origin than those with a single-pulse injection, thus reducing both particulate and NOx emissions significantly. It is clear that the injection mass ratios and the dwell(s) between injection pulses have significant effects on the combustion and emissions formation processes in the D.I. diesel engine. However, how and why these parameters significantly affect the engine performances remains unexplained. The effects of both injection mass ratios and dwell between injections on vapor/liquid distributions in the split-injection diesel sprays impinging on a flat wall have been examined in our previous work.

Mixture formation processes play a vital role on the performance of a D.I. Gasoline engine. Quantitative measurement of liquid and vapor phase concentration distribution in a D.I. gasoline spray is very important in understanding the mixture formation processes. In this paper, an unique laser absorption scattering (LAS) technique was employed to investigate the mixture formation processes of a fuel spray injected by a D.I. gasoline injector into a high pressure and temperature constant volume vessel. P-xylene, which is quite suitable for the application of the LAS technique, was selected as the test fuel. The temporal variations of the concentration distribution of both the liquid and vapor phases in the spray were quantitatively clarified. Then the effects of injection pressure and quantity on the concentration distributions of both the liquid and vapor phases in the spray were analyzed.

An early formation process of the spray, which was injected by a high-pressure swirl-type injector that is widely used in direct injection spark ignition (DISI) gasoline engines, was investigated through image analyzing techniques. The sprays were illuminated both by an Nd:YAG laser light sheet for getting the spray tomograms and by a tungsten lamp for getting the scattered back light shadow images of the sprays. The sprays were imaged by using a high-resolution CCD camera and a high-speed digital imaging system. The early development aspects of the spray were investigated in detail through the measurement of the tip penetration, cone angle and width of the early spray. At the start of injection, the liquid column emerges first, and it forms the “pre-swirl spray” without the swirl component. Following the liquid column, the liquid sheet emerges, however its radial velocity component is weak to form the complete hollow-cone spray. This spray changes into the “weak-swirl spray”.

Spray penetration for Diesel injectors, where injection pressure varies with time during the injection period, was calculated. In order to carry out this calculation, the discharge coefficients of the needle-seat opening passage and discharge hole in orifice-type Diesel nozzles were investigated separately. Simple empirical correlations were obtained between these coefficients and needle lift. Then, by introducing these correlations, the injection pressure, which is defined as the pressure in the sac chamber just upstream of the discharge hole, was either derived from measured fuel supply line pressure, or predicted by means of an injection system simulation. Finally, based on the transient injection pressure, spray tip penetration was calculated by taking the overall line which covers the trajectories of all fuel elements ejected during the injection period.

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.

The injection amount per stage in a multiple injection strategy is smaller than a conventional single-stage injection. In this paper, the effect of the injection amount (0.27mg, 0.89mg, 2.97mg) under 100MPa injection pressure and the effect of injection pressure (100MPa, 150MPa, 170MPa) under different injection amounts (0.27mg, 2.97mg) on the spray and mixture formation characteristics were studied by analyzing the vapor/liquid phase concentration distributions obtained under various conditions via using the tracer LAS technique. The spray was injected into a high-pressure and high-temperature constant volume vessel by using a single-hole nozzle with a diameter 0.133mm. The higher the injection pressure with a smaller injection amount is, the shorter the spray tip penetration and leaner air-fuel mixture occur. The combustion processes had been examined by a high-speed video camera with the two-color pyrometry method.

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.

The fuel-ambient gas interaction process of the free diesel spray injected from the micro-hole nozzle (0.08 mm) into the quiescent and engine-like ambient gas condition was investigated by means of the laser-induced fluorescence - particle image velocimetry (LIF-PIV) technique in non-evaporating condition. Direct photography with high speed video camera and two color pyrometry were applied to analyze the evaporation spray and flame characteristics. Three injection pressures from 100, 200 to 300 MPa and two ambient gas densities of 11 and 15 kg/m₃ were selected as testing conditions. The entrained mass flow rate of the ambient gas through the whole spray boundary, the ratio of the total ambient gas entrainment rate to the fuel injection rate, etc., were calculated by using the ambient gas velocity data obtained by the LIF-PIV technique and used to correlate the combustion behavior.