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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.