The objective of this study is to find strategies for extending the load range of premixed charge compression ignition-based combustion while improving thermal efficiency and reducing combustion noise and exhaust emission levels. Experiments were performed using a single-cylinder direct-injection diesel engine equipped with a common-rail injection system and a cooled EGR system. First, experiments were carried out with single-stage injection. The results indicated a notable improvement of NO x and smoke emissions by selecting lower EGR rates and later injection timings according to the increase in injection quantity. However, the problems of high pressure rise rates and levels of unburned species emissions developed. To solve these problems, two-stage injection was applied. These additional experiments started with injection and EGR conditions that were based on the results of the single-stage injection tests, and modifications were made to mitigate the increased emissions and decreased thermal efficiency. As a result, judicious selection of injection and EGR conditions for two-stage injection provided a drastic improvement in exhaust emissions with a sufficiently low pressure rise rate to be equivalent to pilot-diesel operation.

Abstract:We have characterized several dynamic aspects of a simple chemical system capable of self-movement: An oil droplet in water system. We focused on spontaneous mode switching and collective behavior of droplets as emergent properties of the system. Droplets demonstrated spontaneous mode switching by changing speed, direction and acceleration over time, and collective behaviors of droplets resulted from such autonomous characteristics. In this paper, we quantitatively measured those characteristics to show that droplets did not act completely independently in the same system, but tend to be attracted to one another and interact with each other by adjusting their motion.

Accurate and quick prediction of spray characteristics such as spray penetration is paramount for the understanding and quantitative analysis of the combustion process in diesel engines, in order to perform parametric study on advanced combustion process in diesel engines, zero-dimensional diesel spray model is often used for the prediction of the spray evolution. In this study, a previous zero-dimensional diesel spray model applied for the spray penetration prediction including the part after the end of injection with a constant injection rate was extended to the cases with varying injection rate. The effective injection velocity was introduced into the previous spray model, which is defined as the ratio of the momentum flux and fuel mass flow rate over the spray tip cross-sectional area. Combined with this definition, the analysis of effective injection rate and its response time was performed during and after the end of injection. After that, the fuel mass flow rate and momentum flux over the spray tip cross-sectional area were derived for varying injection rate even after the end of injection based on the momentum and fuel mass conservation along the spray axis, and further the spray penetration. Finally, the developed model was validated by comparing with the experimental data.

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