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In order to improve the ignition stability and reduce the cycle-to-cycle variation, it is necessary to understand the mechanism of the flame kernel development and the local quenching effect during the spark ignition process. In this study, experiments for the spark ignition process in a high-speed lean gasoline-air mixture turbulent flow field were conducted. OH* chemiluminescence measurement and focusing Schlieren photography was applied to observe the development of flame kernel and discharge channel behaviors simultaneously. Results indicated that flame kernel fragments, generated by the restrike and short- circuit of discharge channels, quenched due to the local turbulence, which led to slow flame propagation or misfire. In that cases, the initial flame kernels showed stretched behaviors, along with high curvatures.

Spark ignition experiments were conducted to investigate effects of multi discharges on ignition of a lean-turbulence mixture. Initial flame kernel developments were observed by Schlieren photography, and discharge characteristics were analyzed by high-speed photography and spectroscopic measurements. Results indicated that the MIE (Minimum Ignition Energy) of the single discharge was lower than that of the multiple one at 100 kPa, and higher at 500 kPa. Each discharge was generated independently when discharge interval was set to 1 ms at 500 kPa, while discharge was occurred continuously at 100 kPa. This discharge characteristics might be related to the ignition performance.

1 In this work, a microwave-assisted spark ignition was investigated for premixed lean methane/air mixtures. Experiments were performed in a combustion chamber with a constant volume. The initial pressure of the chamber was set to 0.1MPa and 1Mpa. Equivalence ratio Ø was varied from 0.51 to 0.6 to investigate the ignition behavior near the lean limit. High temperature regions in flames were visualized by the Schlieren photography using a high speed video camera. When the irradiation delay of microwave is 200 μs and the pulse frequency is 10 kHz, the probability of ignition reached maximum. A microwave irradiation to the mixture enable to ignite the mixture of equivalence ratio of 0.52. Table 1Nomenclature

An experimental study was carried out on visualization of liquid phase temperature distributions in high-pressure diesel sprays impinging on a heated wall. Naphthalene/TMPD-exciplex fluorescence method and pyrene-excimer fluorescence method were utilized for the thermometry. The sprays were injected into a high-pressure and high-temperature gaseous environment. The nozzle hole diameter was 0.100 mm or 0.139 mm. The results showed that cool pockets were formed at the tip and in the impinging part of the sprays. The spray for the nozzle with 0.100 mm hole was heated up faster near the nozzle than for the nozzle with 0.139 mm hole.

This paper presents basic combustion behavior of a compressed natural gas directly injected into a cylinder with spark-ignition. Experiments were conducted in a rapid-compression machine (RCM) with the cylinder bore of 80 mm, the stroke of 180 mm and the compression ratio of 10 at TDC. A CNG was injected through specially designed injectors which were installed at the side of combustion chamber with three modes, twin injectors in parallel, twin injectors in opposed and single injector. Combustion products were also measured with an infra-red gas analyzer. Direct photographs were taken with a high-speed video for observation. Effect of fuel injection timing was examined at constant spark timing together with the influence of injection mode. Results show several beneficial combustion characteristics of direct injection combustion using CNG. Combining with the results of combustion products and photographic observation, the combustion mechanism is discussed.

The initial breakup process from liquid fuel to spray droplets in the vicinity of the nozzle tip under high-pressure ambience is analyzed for the isothermal diesel spray injected into the optically accessible high-pressure vessel. The spray was observed both by the use of planar laser light and also by using diffused shadow light. The results obtained in the present study are summarized as follows. The initial breakup of the developing diesel spray could be photographed more clearly in the vicinity of the nozzle tip. The initial liquid jet from nozzle hole is divided into two zones; the intact liquid pillar zone and the umbrella-like thin liquid protrusion zone. The breakup happens mainly in the periphery of the thin liquid umbrella protruding from the tip of the intact liquid pillar. The high pressure ambience break-up mechanism can be analyzed from observation of the internal flow of the liquid pillar and it's protrusive umbrella.

In order to investigate the effects of a DC electric field and its polarity on the knock intensity in a spark-ignition engine, an experimental study was carried out with a rapid compression machine. To get a good understanding of the effect of an electric field on knocking combustion, the high-speed direct photographs were taken. The ionization current measurements were also carried out using the electrode as an ionization probe The major findings of present investigation of the effects of DC electric fields on the knocking combustion process in a spark-ignition engine could be summarized as follows: It was clearly indicated that the knock intensity decreases with the increase of the electric field regardless its polarity. The knock intensity was strongly dependent upon the burned mass fraction at the onset of the end-gas autoignition, and decreased as the burned mass fraction increased.

Exciplex-based planar fluorescence technique was applied for two-dimensional visualization of the fuel spray including the region close to the nozzle tip. A spray doped with small amount of naphthalene and TMPD was discharged from a diesel nozzle into a pressurized gaseous nitrogen inside the test chamber installed with glass windows. The fuel spray was also allowed to evaporate in high temperature gaseous environments produced by combustion of the homogeneous mixture of methane and air in the test chamber. Photographs of the temporally frozen two dimensional image of the fuel spray were processed using an image analyzer. The image in the longitudinal cross section passing through the center axis of the spray demonstrated that the high density portion of liquid fuel appeared almost periodically downstream and that the axial distance between the neighboring high density portion increased with an increase in the downstream distance.

A three dimensional numerical analysis is made of in-cylinder process in a typical four-cycle reciprocating spark ignition engine with an off-center intake valve. The conservation equations of mass, momentum and energy are solved on the basis of the finite volume method. The ordinary two-equation model is employed as the turbulence model. Fuel is injected into the intake port, and fuel vapor, fuel droplets and air flow into the cylinder through the valve clearance during the intake stroke. As the inlet boundary condition, the inflow velocity distribution, mass fractions of fuel vapor and droplets are given around the intake valve periphery. For simplicity, it is assumed that fuel droplets move with the gas and have the rates of evaporation which are estimated by the classical quasi-steady theory of a single droplet evaporation. Calculation is made from TDC of intake stroke to TDC of compression stroke at every 10 degrees crank angle.

Exciplex-based fluorescence was employed for the remote, nonintrusive, instantaneous and point measurements of fuel droplet temperature. A hydrocarbon droplet doped with naphthalene and TMPD was allowed to evaporate in a heated gaseous mixture of oxygen, nitrogen, carbon dioxide and water. The fluorescence emission spectra from a droplet subjected to nitrogen laser excitation were measured with an optical multichannel analyzer. Photographic observation showed that a droplet fluoresced with a green color at room temperature. As the temperature was raised, fluorescence became purple. The ratio of fluorescence emission intensities at two different wavelengths was an appropriate criterion for in situ determination of droplet temperature. Oxygen in the ambient gas was found to be a major quencher for the fluorescence. Droplet velocity relative to the ambient gas did not have an appreciable influence on the fluorescence emission spectra.