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The mixture formation prior to the ignition process is a key element in the diesel combustion because it significantly influences throughout the combustion process and exhaust emissions. Purpose of this study is to clarify the effects of ambient temperature, oxygen concentration and air entrainment into the spray on the heat release process during ignition delay periods. This study investigated diesel combustion fundamentally using a rapid compression machine and high speed digital video camera. The detail behavior of spray evaporation, spray interference and mixture formation during ignition delay period was investigated using the schlieren photography system. Ignition process, flame development and images of the spray ignition with extremely dark flame were investigated by light sensitivity direct photography method. Heat release processes were analyzed by pressure measurement in the chamber. Results show that short endothermic process produces slow heat recovery, leading to gentle increase of initial heat release rate. Low oxygen concentration produces slow heat recovery process rather than long endothermic period, which suggests fuel-air mixing is required to promote chemical reaction for the case of low oxygen-concentration atmosphere. Initial heat release is activated to some extent by increase of air entrainment into spray during ignition delay period. However, excessive air entrainment increases initial heat release little and rather affects diffusion combustion.

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