An experimental study was performed to analyze the combustion processes of JP-8 and fossil Diesel fuel in an optically-accessible single-cylinder heavy-duty diesel engine equipped with a high pressure common-rail injection system. In terms of emission, JP-8 emitted less smoke with more HC and NOx. Direct imaging and two-color thermometry were applied to verify the emission trend for both fuels. The combustion process was characterized by means of image analysis focusing on the luminosity intensity and its spatial distribution (flame spatial fluctuation (FSF) and flame non-homogeneity (FNH)). The results from the two-color thermometry were analyzed by the flame temperature and KL factor distribution. From the combustion process analysis of the direct imaging, it was verified that JP-8 had a longer ignition delay compared to fossil Diesel fuel regardless of injection pressure. However, flame luminosity of JP-8 was vanished more rapidly. The flame luminosity intensity analysis showed that fossil Diesel fuel had stronger flame luminosity overall and duration of visible flame luminosity was longer than JP-8. This implies that fossil Diesel fuel had more diffusion dominant combustion. From the flame luminosity variation rate analysis, decreasing rate of flame luminosity for JP-8 was higher compared with fossil Diesel fuel, showing that oxidation rate of JP-8 was much higher than fossil Diesel fuel. From FSF and FNH analysis, JP-8 showed lower value for both FSF and FNH in the later stage of combustion, because the later stage of combustion with JP-8 has less jet structure in comparison with fossil Diesel fuel. The flame temperature field from two-color thermometry showed that locally high temperature region existed with JP-8. KL factor distribution of JP-8 was distributed more uniformly with a relatively lower level of KL intensity in comparison with fossil Diesel fuel in the late stage of combustion. (C) 2012 Elsevier Ltd. All rights reserved.