Analysis of Relation between Mixture Formation during Ignition Delay Period and Burning Process in Diesel Combustion

Many technologies for reducing exhaust emissions of wide variety of diesel engines from small size to large size ones have been considered with the improvement throughout the combustion process. To reduce emissions, mixing of fuel and air is still important phenomena.

Purpose of this study is to clarify the relation between mixture formation during the ignition delay period and burning process in diesel combustion that strongly affects the exhaust emissions. In this study, a rapid compression machine was used to simulate actual phenomenon inside the combustion chamber with changing ambient density. In addition, swirl velocity and injection pressure were changed as experimental parameters to improve mixture formation at high ambient density. This study constructed schlieren photography system with a high-speed digital video camera to investigate the detail behavior of mixture formation during ignition delay period. This method can capture spray evaporation, spray interference and mixture formation process clearly. Ignition process and flame development were investigated by direct photography method using a light sensitive high-speed color digital video camera. The sensitive camera can capture flame development clearly with the mixture of dark and bright flames. Moreover, the mechanism and behavior of flame pattern were analyzed by newly developed image analysis technique.

Flame images reveals that spray ignition occurs with dark flame between sprays at the middle and downstream of the spray where combustible mixture is first formed; then bright flame appears near the centerline of each spray where fuel is continuously injected to rich and high temperature atmosphere. Increasing ambient density restrains spray tip penetration, resulting that mixture is only formed at the center region of the combustion chamber. It is necessary for high ambient density condition to improve mixture formation. High-pressure injection is most effective in improvement of mixture formation under condition of high ambient density.